Applications and How-Tos

Using the HMI LC as an Intelligent Network Gateway

Historically, connecting two different communication networks was handled by a dedicated network gateway.  We worked on an application a couple months ago where we were able to implement this gateway functionality in an HMI LC.  This is what it looked like.


The customer’s main PLC was an Allen-Bradley communicating with EtherNet/IP (protocol 1).  The AB PLC connected to a machine panel that used a KEB HMI LC.

EthernetIP drive to HMI PLC

AB PLC communicating to KEB HMI LC via EtherNet/IP



The HMI LC used EtherCAT (protocol 2) to control a 200Hp F6-K EtherCAT VFD.

EtherNetIP to EtherCAT

The HMI LC provides an EtherCAT master to the VFD




EtherCAT for the VFD

EtherCAT was chosen because it provides a high-speed CAT5 connection to the drive.  Also, programming the drive communication is made easy through the KEB’s EtherCAT communication handler Function Block.




A group of parameters was defined for the EtherCAT process data (PDO).  The PDO gives cyclical and high-speed updates on the critical parameters.


Additionally, we implemented the DIN66019 II protocol (protocol #3) which can be used to read/write individual drive parameters.  Beyond the fast-channel EtherCAT PDO parameters, this gives the ability to access every KEB drive parameter including fault codes, warnings, etc.  It also provides the functionality to do a full upload and download of drive parameters.


The HMI LC can then relay any drive parameter back to the upstream PLC via EtherNet/IP.  In short, the HMI LC is acting as a gateway to handle the 3 different communication protocols.



HMI LC connections


Mix-and-Match other Communication Protocols

This same concept can be extended to the other 40+ protocols supported by the HMI products.  This includes Modbus, Profinet, etc.  Even serial fieldbus networks Data Highway, DH+, and Profibus are possible.

HMI network gateway

The HMI LC can act as a gateway and interface different communication protocols





So why would somebody prefer to implement this solution with the HMI LC instead of a communication gateway?  It’s a fair question.  The reason is that the HMI LC gives you a lot more flexibility and functionality than the traditional network gateway hardware.



It’s obvious but worth highlighting  – The HMI LC adds visualization.  In this application, it served as a drive remote operator, displaying speed, load, temperature, drive status and error history.

energy conversion system screen

Adding an HMI screen improves usability


Additionally, it offers the possibility to display PDF manuals, diagnostics, logs, error history, trending information, etc.  You won’t get that with a gateway-only device.


Secure Remote Access with CONNECT™

Each HMI LC ships with a Combivis CONNECT™ Runtime.  This allows a secure VPN connection to be made to the machine which improves troubleshooting and diagnostic gathering.


Logic Control

The HMI LC offers the possibility to handle the entire machine control including EtherCAT remote I/O.  From that standpoint, it becomes a very good value.


Automatic Drive Download

If a replacement drive needs to be installed at some point in the future, the HMI LC can automatically download the correct program.  Again, this functionality is made easy with an included KEB Function Block in the Combivis Studio software.  This functionality makes long-term support for the machine easier.




With over 40 communication drivers, the HMI LC can act as a gateway between two different networks.  But the HMI LC is much more than a gateway.  The HMI LC offers a lot of value and is a scalable product that increases a machinery OEM flexibility and maintainability.


If you are interested in more information on the HMI LC contact a KEB engineer today.


Email KEB America

Related posts


EtherCAT servo drive

EtherCAT drive technology

EtherNet IP for packaging machinery

Video: EtherNet IP for packaging machinery

Modbus RTU for VFDs


Selecting and Sizing Geared Motors

In this post, I’ll describe the process I use for sizing gearboxes and geared motors.


To make the selection, I am using KEB’s software sizing program called KEB-DRIVE.  KEB-DRIVE is free and easy to use.  If interested to follow along, you can download a copy of the software.


gearmotor sizing




1.  Consider the application requirements

Before you begin selecting a geared motor, you first need to consider and know the application requirements.  This is not a comprehensive list but gives a general idea of the common considerations.


Torque & Speed

  • What torque and speed are required at the output of the gearing?
  • What does the typical torque profile look like?
  • Is the loading more or less steady – or will the gearing experience shock loading?
  • What electrical power do I have (three phase, 50 or 60Hz, Voltage)?



  • What is the duty of the application?
  • If I need more motor overload – can I rate for a reduced duty like S2 or S3?



  • Does the application require position, torque or speed control?
  • Will the motor be run across the line or with a VFD?
  • Will the motor hold the load at 0 speed indefinitely (e.g. hoisting application)?
  • Does the motor need a spring-set brake on it?
  • Does it need a feedback device like an encoder?



  • How will the gearmotor be mounted to the machine  – foot mounted, shaft mounted, flange mounted?
  • Are there space constraints?
  • Does the output axis need to be inline or at a right angle
Parallel offset gearmotor hoist

This hoist uses an offset output which allows for the drum to be compactly installed.



  • What environment is the gearmotor going into (e.g. caustic washdown, saltwater, sensitivity to noise, etc.)?
  • What are the ambient temperature ranges during operation?
  • Does it require special ingress protection (washdown, outdoors, etc.)?
  • Is this a food processing application that will require food grade lubricants and grease?




2. Select the correct gear technology for the application


Configurations in KEB-Drive start at the top left.  On the left, you’ll see drop-downs to select different gear types and sizes.

gearmotor selection software

Start by selecting the gear type and size


KEB offers 4 main types of gearing:


Table 1
4 Types of KEB Gearing

“Style” Gear Type Output Efficiency (%)
G Helical Inline 94-96%
F Helical Parallel Offset 94-96%
S Helical-Worm Right Angle 50-90%
K Helical-Bevel Right Angle 94-96%


Helical-Worm and Helical-Bevel will provide right angle outputs. Helical-worm can be cost advantageous for larger reduction ratios.  Helical-bevel has the advantage of better efficiency, which can possibly equate to a smaller motor.


There is also an option available to leave off the gearing if are just interested in selecting a servo or induction motor.


vector induction motor encoder

KEB-DRIVE also allows standalone motor selection




3.  Motor Selection (Size, voltage, frequency)


Working to the right, I then select the size of the motor I want. Options for both Induction motors and AC Servo motors are listed.  Here is a comparison of the advantages between servo and induction motors.

gearmotor motor winding

Next, select the motor size and winding


There is a tab that allows you to select the stator winding information – specifically, the rated voltage and frequency.  KEB has the ability to offer special windings outside of what is listed.  Last year I worked on a 380V/60Hz installation in South Korea – who knew!


Different IE3 windings are available for the Global market


Larger motors will provide more torque.  As you increase the motor size, you’ll see the torque information is updated in torque/speed drop-down.




4. Adjust the Torque/Speed selection


Is it a speed reducer?  Or a torque Increaser?  It’s both – higher gear ratios will provide lower output speeds and higher torques.  Use the drop down to see all the different possible configurations with the selected gearbox/motor combo.


There is a limit to the options that KEB-Drive will show.  Only options with a service factor of 1.0 or greater are displayed.

gearmotor torque speed ratios

Adjusting the ratio affects the output torque and speed


Note: If you have selected a motor winding that is rated for both 50 and 60Hz, you will see two values listed for the speed and torque.  The 50Hz rating will be reflected with the slower rpm output.



5.  What is the gearing Service Factor and why is it important?

The gearing service factor (SF) is the ratio between the:

gearing sf equation


In other words, Service Factor provides a relative comparison to how much capacity the gears have in the current configuration.


A SF of 1.0 means the gears will have a nominal output torque equal to that of their rating.  Selecting a motor/gear configuration with a SF of less than 1.0 is not advised.  This means the gears will be undersized when operated at the nominal point.  This could also indicate that the motor selected is too large.


Sometimes, the application is very difficult with regards to Duty, Shock loading, Temperature, etc. (look at step 1 – application requirements).  In this case, it is advisable to select a very high SF which compensates for the factors that will stress, wear, and possibly damage the gearing.


Manufacturers typically provide a table of typical applications.  Many also list a multiplication factor based on duty.  It is advised that difficult applications like a Rock Crusher have a SF in the 3.0 range or higher.  Relatively easy applications like fans with light duty might have a SF closer to 1.0.  It often becomes a trade-off between safety factor and cost.


At this point, if the selection is not able to meet the required torque, speed, and SF of the application, you’ll need to return to step 2 and select a different gear and motor combination.  Or conversely, if the motor or gear appear to be oversized, then you can return to step 2 in order to optimize.



6.  Select gearmotor options (mounting style)


This section allows a user to select how the geared motor will be mounted.  The flexibility of mounting is one reason that the KEB integral gearmotor solution has been so popular.  Users can select a unit with an output shaft.  Or a shaft mounted unit with a hollow bore.  Mounting feet and mounting flanges can also be selected.


F series gearbox

Hollow shaft gearmotors avoid using an intermediate coupling


For shaft mount applications, a nice option is a shrink disk mounting.  KEB’s shrink disc mounting provides a zero backlash connection between the gearbox and machine shaft.   Assembly and disassembly are very easy.


torq loc gearmotor

Shrink Disk mounting makes shaft mount assembly easy


To go along with the flexibility-theme, users can select either English or Metric units on the shaft/bores.  If you don’t see the exact option you want in the configurator, then I suggest contacting a KEB engineer to explore what is possible.  We are able to offer a lot of customer flange and bore designs if the customer wants.


This section also includes the lubrication used in the gearbox.  A number of different lubricants are possible depending on the application requirements.  Consult the KEB gearmotor manual but the temperature ranges for the lubricants is listed.  For food and packaging applications, a USDA food-grade lubricant is available.


Below that is a list of checkboxes that can be selected.  Gearing options like low backlash and protection covers are possible.



7.  Choose the motor options

This is where a user selects the motor options.  Options like a motor on the brake.  Or, the type of encoder for closed loop applications.  Even the type of motor fan can be selected here.


KEB offers a variety of incremental and absolute encoder options


8.  General options

Next, the general options can be defined.  These are special requirements that can be easily selected in the configurator.  This includes a second nameplate, condensation drain hole, etc,   The conduit box orientation and the fitting location are also defined here.


Finally, the paint treatment can be selected here.  The number of applications and thickness of paint is defined.  Consult the KEB gearmotor catalog for the specifications.  Added protection is possible with the P1, P2, or P3 options. These would be good options to consider for washdown, outdoor, and maritime gearmotor applications.

IP65 gear motor used in marine application

IP65 gear motor used in marine application


The paint color is also defined here.  Gray and black are most commonly requested so they are standard options.  But we can paint to whatever color the customer specifies.  In the case of a special, contact KEB.


9.  Gather 3D models and dimensional drawings

The last option is the mounting position of the gearmotor.  This is important as it determines the oil fill level of the gearmotor and venting.


At this point, the user can explore additional tabs in KEB-DRIVE in order to get more information.  Of note, the customer can get extra motor information including the efficiency values, nominal torque, and inertia values.

motor inertia values

An exported summary includes detailed info like motor rotor inertia


The Dimensions tab includes a drawing with critical dimensions and 3D .step models.

geared motor 3D models

3D and Dimensional drawing outputs




KEB-Drive is a useful tool because it is easy to use and selections can be made very quickly.  However, please note that these are only the commonly requested features.  There are many more possibilities KEB can offer like absolute or safety encoders, quiet brakes for theatre or elevator applications, custom flanges, etc.


If you have specific needs feel free to consult with a KEB gearmotor engineer and we’d be happy to help with your application.


Contact Us


Related posts

VIDEO KEB Gear motors: German Engineered. American Assembled

Parallel offset gearmotor hoist

Theatre Gearmotors for Hoists and Winches

Helical Bevel Gearmotors

IE3 gearmotor winding

IP65 Washdown Integral Gearmotors

7 tips for Long Encoder Cable VFD applications

There are a number of issues that arise when a VFD and motor are mounted far apart.  A previous post went over voltage spikes due to dV/dt of the drive’s PWM switching.  The answer to that problem was a dV/dt choke or sinewave filter.


For closed loop applications, another problem that can arise when a drive and motor are located far apart has to do with the encoder signals.  Specifically, the voltage drop experienced on encoder signals.


This post will outline the problems of long feedback cable runs and how to address them.


Voltage Drop of Encoder Signals

The encoder cable is basically a transmission line carrying the encoder signal from the encoder device to the encoder card on the VFD.  The encoder cable has some impedance which is a characteristic of the cable design.


The cable resistance should be listed on the manufacturer’s cable data sheet.  The resistance is usually listed as some value per length – (e.g. Ohms/meter or Ohms/ft.).  The longer the cable the more resistance it will have.  And the more cable resistance, the more voltage drop the signal will have.



encoder cable resistance

Standard KEB encoder cable resistance is called out in our manual



Let’s look at an example.  Assume a worst-case scenario of a 200mA current draw.  Using the data above for a 75-meter application, a TTL signal will lose 1.05V due to the voltage drop.


Encoder cable voltage loss

A long encoder cable run will cause the signal voltage level to drop


The most commonly used incremental encoders are the TTL type and have a target “on” voltage level of 5V.  However, the drive’s encoder card will have an acceptable range on the voltage level that the drive or encoder input will accept.  For example, KEB’s encoder cards recommend a minimum of 4.75V for the “High” TTL signal.


If there is too much voltage drop the TTL signals, you will get erratic operation and nuisance trips – the worst kind and a pain to troubleshoot.  A common error is that the A or B channel compliment does not match the respective channel.  With KEB Combivert F5 inverters, this will result in the E.EnC1 fault.


So if you are experiencing random encoder faults and suspect too much voltage drop, here are 7 things you can do to address the issue.


1. Use a Shorter cable

This one is simple but is worth mentioning because it might be the easiest to implement – if possible, use a shorter feedback cable.  I’ve seen many applications where they use a “standard” cable or something they have on the shelf.  The result is they use a much longer encoder cable than they really need.


An excessively long feedback cable can also couple unwanted noise and introduce other issues.  So, use the shortest possible encoder cable that is practical for the application.



2. If powered from the VFD – Increase the supply voltage


Some encoders receive power from the drives control/encoder card.  The Push-Pull type TTL encoders will output a signal amplitude that is proportional to its supply voltage (i.e. there is no regulated supply onboard).  With these types of encoders, there are effectively two conductor voltage drops – the supply, and the return.


encoder card supply voltage

The voltage drop will be highest when the encoder is powered from the VFDs encoder card.


KEB’s F5 drive uses a fixed 5.2V supply to power the encoder.  The idea is that the extra .2V compensates for some voltage drop.  Some other encoder cards have means to increase the supply voltage.


If possible, you can adjust the encoder card supply voltage up.  Just be sure to use a voltmeter to measure the supply voltage and check the encoder specs to make sure a higher voltage is permitted.



3. Apply power at the encoder


Another option (if supported by the encoder) is to apply the supply power directly to the encoder.  Since the supply is directly applied to the encoder, this allows only one voltage drop on the feedback signals – effectively cutting the total voltage drop in half compared to the initial example.


TTL encoder local supply

Applying power at the encoder can help reduce the voltage drop


Check your encoder datasheet to see how the encoder can be powered.  Some encoders can only be powered from the drive’s encoder card.  However, other encoders are designed to allow a direct power source.  Some might even allow a higher input voltage like 24V but then regulate the output voltage accordingly (e.g. 5V).


So do check the encoder data sheet and see what the input power options are.


4. Consider using HTL logic

A “High” or “On” level for HTL logic is defined between 15V and 30V, with a target of 24V.  KEB F5 HTL encoder cards will have a regulated 24V supply that can be used to power the encoder.  Because the HTL signal is higher, it can support more of a voltage drop before hitting the lower “On” threshold.


HTL provides much more room for voltage drops – 9V from the regulated supply to the lower threshold (24V-15V = 9V).  Compare that with TTL’s range of only 3V from supply to lower “High” threshold (5V-2V).


HTL encoder waveform

HTL logic provides a higher voltage compared to TTL


Just be sure that the drive’s encoder card supports an HTL input or the card could be damaged.



5. Use an encoder cable with lower resistance

KEB offers special encoder cables for long distance runs.  The cables use larger conductors so they have less resistance.  Less Ohms/meter will result in less voltage drops on the feedback signal.


TTL long encoder cable

KEB offers feedback cables for long encoder runs


6. Use a signal repeater

Another option is to use a signal repeater.  Signal repeaters were commonly used before real-time fieldbuses like EtherCAT were available.  They function by splitting, amplifying and conditioning the encoder signal.


KEB offers a signal repeater like this (00F4072-2008) that can be used for long run and multi-follower applications.



7. Use Fieldbus I/O – Like KEB’s Counter module

Another option is to use a fieldbus encoder module to transfer the data.


KEB’s encoder module allows the input of up to 2 TTL encoder signals.  The encoder signal is wired to the module where it is converted and transferred via the EtherCAT bus.


ethercat encoder module

It’s possible to transfer the encoder feedback via the EtherCAT bus


One big advantage with this solution is the low cost of a CAT5/6 cable.  The cabling cost will be much less than a long encoder cable.  The second advantage is that the position is now on the bus and available for use by the control without any other handling.


One consideration with this implementation is the input delay.  The listed value for the input delay on the KEB module is 1ms.  This will be suitable for most motion applications but you’ll want to verify it with your individual application.



I’d be interested to hear of other solutions or problems you’ve encountered.  Leave your comments below.


Have questions? – Contact a KEB Application Engineer today.



Related Posts

dvdt choke

dV/dt Chokes for long VFD runs


How Pulse Width Modulation in a VFD works

Variable frequency drives (VFDs) used on industrial applications provide an efficient way to vary the speed and torque of the connected motor.  The VFD consists of three main parts:  The input converter section, the intermediate DC bus, and the output inverter section.


VFD topology

Figure 1 – A VFD is made up of 3 main parts


The converter section uses a diode bridge rectifier to convert the AC input voltage to a DC voltage.  The DC bus section consists of a capacitor bank, which is used to smooth out the DC voltage from the converter section and provide some voltage storage capacity.  The inverter section of the VFD takes the DC voltage from the DC bus and inverts it back to a variable voltage and variable frequency AC voltage used for the motor control.


What is Pulse Width Modulation (PWM)?


The process involved in inverting the DC voltage to the variable voltage variable frequency (VVVF) AC voltage in the inverter section of the VFD is called pulse width modulation or PWM.


Pulse width modulation uses transistors which switch the DC voltage on and off in a defined sequence to produce the AC output voltage and frequency.  Most VFD’s today utilize insulated gate bipolar transistors or IGBT’s.  The typical configuration of the IGBT’s in the inverter section of a VFD is shown below in figure 2.


vfd output igbt schematic

Figure 2 – Schematic of output IGBT stage of a VFD



The transistors act as a switch connecting the DC bus across the windings of the motor.  A VFD with a 480VAC input will have a DC bus of approximately 678VDC.  Thus the ‘pulse’ refers to the switching on and off of the transistors producing a pulse of voltage with an amplitude of approximately 678VDC.


The goal of the PWM control is to create a sine wave current waveform output to produce torque in the motor.

In order for current to flow between two phases of the motor above, at least one transistor in the top portion of the diagram and one in the bottom of the diagram must be activated.  By utilizing specific combinations of transistors, current can be induced in either direction between phases.



For example, if T1 and T6 are open, current will flow from DC bus positive through the U to V phase of the motor and then to the DC bus negative.  If T3 and T4 are open, then current will flow from the DC bus positive through the V to U phase of the motor to the DC bus negative.


One of the advantages of using a VFD with PWM technology is the ability to control the amount of current going through the motor windings, which when running a rotary industrial motor, translates into controlling the amount of torque at the motor shaft.


In the case of a VFD that utilizes PWM technology, this is done by varying the RMS voltage to the motor.  By controlling the amount of time each pulse is on and off, the resulting RMS voltage across the motor phases can be controlled.  The ‘width’ of the pulse factors into the resulting RMS voltage output.


A longer ‘ON’ time of the pulse results in a higher RMS voltage across the phases.


pwm long graphic

PWM representation with longer “ON” time



A shorter ‘ON’ time of the pulses results in a lower RMS voltage across the motor phases.

pulse width modulation vfd

PWM representation with shorter “ON” time



So by modulating the pulse width over each successive half wave, the RMS voltage across the motor phases can be controlled.  The resultant variable RMS voltage allows the VFD to vary the amount of current flowing between motor phases.  The current waveform produced through the PWM process is also influenced by the IGBT switching frequency.


Switching Frequency

The IGBT switching frequency refers to the rate of the on/off switching if the individual IGBT’s.  Typical switching frequencies used are 4kHz, 8kHz, and even up to 16kHz.


A higher switching rate will provide a cleander waveform to the motor as there will be more pulses over each half wave.  See the KEB blog post on the switching frequency for more information.


In addition to the motor torque (current), the motor speed (frequency) can also be controlled by utilizing PWM.  By changing the period of the voltage pulses which induce the current in the motor phases the resulting output current waveform frequency can be changed.


pwm for different frequencies

Different PWM waveforms results in different output frequencies




By coupling the control of the pulse width and the pulse group period, PWM drives provide a means to control both the voltage and frequency output to an AC motor.


The ability to control the torque and speed of an AC motor opens up the application possibilities for machine designers.  Motor speeds can be optimized for the application to achieve higher system efficiency (i.e. fan control).   Motor speeds can be increased above the nominal motor speed to increase production rates.  Motor torque can be limited to help protect mechanical components of the system.  Controlled starting and stopping of motors can eliminate mechanical components that may wear over time.


Contact a KEB Application Engineer today to discuss your application requirements.


Short Circuit Current Rating (SCCR) and Fuse Selection

Protecting electrical components from short circuit current is essential when designing an electrical panel. But what exactly is the short circuit current rating (SCCR) of electrical components? More specifically, how is the SCCR calculated?


This post will go through a detailed procedure for determining the SCCR of a system, focusing on the differences between direct and isolation transformer fed systems.


What is the SCCR of a device and/or system?

The short-circuit current rating is the maximum amount of RMS (Root-Mean-Squared) current an electrical component can handle when using an overcurrent protection device, such as a fuse, or for a given amount of time at a specified voltage. The SCCR rating applies both for individual electrical components and for entire electrical assemblies or systems.


Figure 1 below shows an AC current waveform after a fault current situation occurs.

short circuit fault chart

Figure 1: AC current waveform behavior after a short circuit fault (Source: Littelfuse, 2007)


When selecting the proper fuse, it is important to look at the peak let-through current, Ipeak   (Ampere) and nominal melting, I2t (Ampere Squared Seconds), ratings.


The Ipeak value is the peak amount of current a fuse will let through before clearing. The I2t value is the amount of thermal energy required to melt the fuse element.


Figure 2 below shows the Underwriters Laboratories (UL) published maximum allowable Ipeak  and I2t values for fuses at 100kA and 200kA fault current levels.


ul table maximum allowable fuse

Figure 2: UL maximum allowable Ipeak and I2t values for various types of fuses (Source: Littelfuse 2010)


Determining the available short circuit current allows for proper fuse selection within the entire system. Knowing how much current a fuse will let through during a fault current situation helps engineers design safe electrical panels, while at the same time protects connected equipment from fire.


The primary role of the fuse is to prevent equipment fire, not to protect the equipment itself from damage. The available short circuit current at a given point in the electrical system can be determined by reviewing the components which are upstream from the electrical supply point.



Direct Feed Requirements


The SCCR rating of the motor control system is determined by examining the maximum ratings of the individual devices connected to the individual branch circuits. The device with the lowest rating becomes the limiting factor.


Figure 3 below shows an example of a branch circuit diagram fed by a 460V three phase supply.


three phase branch circuit diagram

Figure 3: Example Three phase branch circuit diagram


The SCCR of Branch 1 is 10kA due to the SCCR value of the molded case circuit breaker (MCCB). Whereas the SCCR of Branch 2 is 15kA resulting from the MCCB. Therefore, Branch 1 is the limiting factor. This motor control system can be fed by a circuit having a maximum capacity of 10kA.


The feeder circuit is comprised of a fusible switch having a rating of 200kA which is intern connected to a distribution block having only a 10kA rating. Therefore, the distribution block limits the combination to a 10kA rating for the feeder.


However, the fuses which are placed in the fusible switch can be selected such that they limit the available peak current. The combined FLA of the system requires a 60A fuse. As an example, selecting an RK5 60 amp fuse would limit the peak let-through current to 21kA when connected to a system capable of 100kA as defined by Figure 2.


However, this is 11kA more current than what the branch circuits are capable of handling.


A better choice would be a Class J fuse with a peak let-through current of only 10kA. Using the 60A Class J fuse, the available peak current on the load side of the fuse is only 10kA, which then satisfies the 10kA limitation of the distribution block and the MCCB in Branch 1. The result is that the complete system can be connected to a supply having a 100kA rating.


It is important to note that fuse manufacturers often publish lower peak let-through values for their fuse products than what is listed in the UL table in Figure 2. However, when designing the distribution system, the UL values must be utilized rather than the manufacturer specific values as these are considered worst case.


AC motor control systems being fed by an isolation transformer, the fuse type and size is dictated by the available short circuit current of the isolation transformer. To select the proper fusing, the available short-circuit current of the transformer must first be determined.



Feeders with Isolation Transformers  


Maximum permissible fuse sizing is dictated by UL and further by the National Electric Code, NEC. According to the National Electric Code (NFPA-70 or CSA 22.1), per Article 450-3(B) of NFPA-70 (similar statements can be found in CSA 22.1), the maximum fuse size is defined in table 450.3(B) with a rating not greater than 125% of the rated secondary current.


In the case of multiple secondaries, it is the rated value of the winding with which the unit is supplied from. The fusing of the transformer supersedes the fuse rating of the AC motor control; because the value required for the transformer is often lower than the maximum value the AC motor control was tested with.


One more step must also be taken to ensure proper fuse selection. An isolation transformer limits the available short circuit current of the system. With a limited available fault current, the peak current may not be enough to clear the fuse in a fault situation.


For adequate protection, the fuse must clear instantly, with the upper clearing time limit within the first half cycle of an AC current wave (i.e., approximately 10mSec). Therefore, it is necessary to determine the amount of short-circuit current available on the secondary winding of the isolation transformer.


Sizing Example

Consider the following example for proper fuse sizing. A 460V application is using an isolation transformer rated for 34kVA, a KEB Harmonic Filter, KEB R6 regen unit and KEB F5 elevator drive.


Since the available short circuit current of the isolation transformer is unknown, it must be calculated. To calculate the transformer secondary rated current, along with the collective available short circuit current, the following equations can be used…


secondary current calc - 1a

1a – Secondary current calculation example


The secondary current can be calculated using the 34kVA power rating of the isolation transformer fed with a 460V line. Equation (1a) becomes the following…


secondary current calculation - 1b

1b – Secondary current calculation example


The fuse size must not be greater than 125% of the secondary rated current. From the Eq. (2) below, 125% of the secondary rated current is 53.3 amps.


transformer secondary - 2

2 – Secondary current calculation example


Next, the available short-circuit current at the transformer must be determined. This can be calculated by determining the collective available short circuit current.


Collective Short Circuit Current


To protect against short circuit currents, limited by an isolation transformer, the required current a fuse needs to clear instantly needs to be evaluated. To do this, the collective available short circuit current of the mainline and isolation transformer must be determined.


Table 2 below provides a summary of the short-circuit current calculations followed by detailed calculations for determining the available short circuit current of the isolation transformer.


To calculate the available short circuit current of the mainline and transformer, the following equations can be used.


Transformer secondary 3a

3a – Secondary current calculation example


Assuming the mainline has a current of 400 amps and an impedance of 0.5% Eq. (3a) becomes the following…


transformer secondary 3b

3b – Secondary current calculation example


Next, the current of the transformer can be calculated by the following equation…


transformer secondary calculation 4a

4a – Secondary current calculation example


Assuming the transformer has a rated power of 34 kVA Eq. (4a) becomes the following…



4b – Secondary current calculation example


Next, the available short-circuit current of the transformer can be calculated by the following equation…


transformer secondary 5a

5a – Secondary current calculation example


Assuming the transformer has a rated impedance of 5% and a current of 43 amps Eq. (5a) becomes the following…


secondary current 5b

5b – Secondary current calculation example


Now that the available short circuit current of the transformer has been calculated, the proper fuse must be selected. To select the proper fuse size, consult with the manufacture’s published fuse blow curves.


Fuse Blow Curves


Fuse manufacturers typically publish self-tested performance data for their fuses to help customers select the proper size. This performance data typically varies between manufacturers, so be sure to consult the manufactures’ data before selecting a fuse.


For this application, Class J fuses are to be used. Figure 4 below shows the performance data for Mersen Time Delay Class J fuses (Mersen Electrical Power, 2002).


mersen class J fuse melt

Figure 4: Time Delay Class J Fuse Melting Time – Current Curve (Source: Mersen Electrical Power, 2002)


Figure 5 below shows the performance data for Mersen High-Speed Class J fuses (Mersen Electrical Power, 2003).

Mersen High Speed J fuse melt

Figure 5: High Speed Class J Fuse Melting Time – Current Curve (Source: Mersen Electrical Power, 2003)


Looking at the Figure 4, Time Delay Class J Fuses rated for 50A and 60A require 800A and 950A, respectively, for 10ms to clear. In comparison, as seen in Figure 5, High Speed Class J Fuses rated for 50A and 60A require 400A and 500A, respectively, for 10ms to clear.


The Time Delay Class J fuses do not provide adequate short-circuit protection as the available short-circuit current may not be high enough to clear the fuse within one-half cycle and therefore should not be used in this application. The High-Speed Class J fuses are a much safer option by providing the required clearing currents down to 1ms According to Figure 5, the 50A and 60A fuses require 675A and 850A, respectively, for 1ms to clear.


Recall that the available short circuit current from the isolation transformer is 860A. The 60A High Speed Class J Fuse is marginally close, (850A) to the available short circuit current from the isolation transformer (860A). Given this information, the 50A High Speed Class J fuse is the best choice to provide adequate short circuit protection.


For the best protection of electrical components, KEB recommends using Mersen High Speed Class J fuses. These fuses can take the place of a standard Class J fuse; however, they perform like a semiconductor fuse.



Figure 1: Littelfuse. (2010). AC current waveform behavior after a short circuit fault. [Digital image] Retrieved Sept. 17, 2017.

Figure 2: Littelfuse. (2010). UL maximum allowable Ipeak and I2t values for several types of fuses. [Digital image] Retrieved Sept. 17, 2017.

Figure 4: Mersen Electrical Power. (2002). High Speed Class J fuse melting time – Current curve. [Digital Image] Retrieved Sept. 17, 2017.

Figure 5: Mersen Electrical Power. (2003). High Speed Class J fuse melting time – Current curve. [Digital Image] Retrieved Sept. 17, 2017.

Electromagnetic Tooth Clutches – How they Work

KEB Tooth clutches are electrically engaged and provide three times more torque than friction clutches.  This video shows how they work.






KEB is a leading manufacturer of electromagnetic tooth clutches.  Compared to the typical friction clutch, a tooth clutch has the advantage of offering roughly 3 times more torque in the same diameter.


This is particularly advantageous for medical and printing machinery, where space is of concern.  This video describes how a KEB tooth clutch operates.


First, lets get acquainted with the main clutch components: The primary drive shaft typically contains a stationary electromagnet which is fixed with a flange or torque tab.  A rotor is fixed to the primary driving shaft with a key or other connection.


Next, an armature is riveted to a flat spring.  The flat spring is then riveted or screwed to a hub – or in this case, a pulley which is connected to a secondary shaft with a belt.  Both the rotor and armature face feature a fine tooth profile, which mates them together.


Here’s how the tooth clutch works: With the primary shaft stopped, the clutch can be engaged by supplying the electromagnet coil with the nominal DC voltage.  This creates a magnetic field that passes through the clutch rotor and armature.


The magnetic field’s force is large enough to deflect the flat spring and pull the armature across a small pre-set airgap towards the rotor face.  The toothed profile of the armature and rotor mate allowing the pulley to turn synchronously with the primary shaft.


It should be noted that the tooth clutch should be engaged when the shafts are stationary or the tooth profiles could be damaged.


When electrical power is removed from the coil, the flat spring returns and pulls back the armature.


Without the toothed connection, the two shafts rotate independently.



You can read more about KEB’s Tooth Clutches at our Tooth Clutch product page.


More posts in this series


Electromagnetic Clutch

This video describes how KEB electromagnetic clutches operate. KEB clutches are electrically engaged and allow a friction connection between two shafts.

Servo Motor Brakes

KEB offers power-off spring-set servo motor brakes designed specifically for servo motor applications that require fail-safe braking.

Permanent Magnet Brakes

KEB’s permanent magnet brakes are commonly used in servo motor and robotic applications because they are compact and provide zero backlash power-off braking.

VFDs for Single phase applications

VFDs and Single Phase AC motors


My first job out of school was with a motor manufacturer doing technical support.  Being in the midwest, we had a lot of farmer and agricultural customers.  


Their applications ranged from running fans, pumps, elevators, agitators, augers, conveyors, etc.  The farm installations often didn’t have access to three phase power and had to make do with single phase 230V.  We sold a lot of single phase motors into these installations.  


Many of the single phase motors were relatively large – between 5 and 15Hp.


single phase farm duty motor

Farm Duty Single Phase motor


There are a number of challenges to operating large single phase motors.  A frequent question from these customers was, “Can I add a VFD to my single phase motor?”.


This post outlines the use of VFDs in single phase applications  – why a person would want to add a VFD, sizing considerations, a rough cost comparison, and the advantages a VFD offers.


The problem with line feeding single phase motors

One challenge to operating large single phase AC motors from the line is the starting current.  A 10HP single phase motor will pull 38A nominal (at 230V).


But that motor (NEMA B design) will pull 6-8 times the nominal current when starting up – or 234 Amps!  

10HP single phase motor data

A 10HP single phase motor will pull 234 Amps starting current @ 230V.




This is enough to make the power companies take notice, especially if there are multiple motors starting up at the same time or the electrical service to the remote farm is near capacity.


To be fair, the problems associated with high starting currents will also affect a line-fed three phase motor.  But in the case of a three phase motor, a person can easily add a VFD.  One benefit of VFD operation is that while ramping up the motor speed, it will limit the motor current.  


The problem is that a VFD cannot operate most single phase motors – at least not at reduced speeds.



The centrifugal switch in capacitor start Single Phase motors

There are a few different designs of single phase motors.  I’ll highlight the one I have seen most in industrial applications – ones with a capacitor start and centrifugal switch.  The design uses a capacitor network that is in the motor circuit at low speeds.  The capacitors help develop torque at zero speed and get the motor started in the correct direction.


single phase motor wiring diagram

Common single phase motor wiring diagram – with capacitors and centrifugal switch


Once the motor is spinning and has inertia, a centrifugal switch opens and the capacitor network is disconnected from the primary motor windings.  The speed at which the switch opens happens before reaching the nominal slip speed.


For this reason, it is not a good idea to use a motor designed for 60Hz on a 50Hz main.  At least not without swapping out or adjusting the centrifugal switch.  It could be possible, that the switch never opens when running at 50Hz.  This could damage the capacitors or overheat the motor windings.


A similar concern would be using a VFD to control the speed of a single phase motor.  Lowering the speed would effectively keep the capacitors in the circuit and potentially damage the motor.



Single phase input to VFD

So if you can’t use a VFD with this design of single phase motor, what is the solution?  The answer is to input single phase to the VFD.  The VFD can act as a phase converter and output three phase to a three phase motor.  


There are some considerations, particularly with sizing.  Some VFDs are designed and rated to input both single and three phase.  Check with the VFD manufacturer but you’ll see something like this in the manual which denotes both phases.


With larger drives, the ratings tend to only indicate a 3 phase input.  Single phase input is possible but a derating is likely needed.  


Let’s look at a VFD application with three phase input running a 10HP motor.  Let’s assume there aren’t any losses and PowerIN = PowerOUT.  The input current and the output would be the same.  


Three phase vfd input

The input power is distributed through three phases


Now, take that same application running a 10HP motor but with a single phase input.  PowerIN = PowerOUT.  Except all the power at the input is now going through one conductor instead of three.  Effectively, there is a √(3) factor applied to the single phase input current compared to the three phase current.  


single phase VFD input power

All input power (current) flows through one conductor



Again, some drive sizes already have input rectifiers over-dimensioned and can inherently handle the increased single phase input current – this should be reflected in the power stage ratings.  For larger HP applications, the net result is the drive might need to be upsized to handle the larger input current.


As a rule of thumb, we suggest rounding up and assume the single phase input current will be double that of the three phase input current.


Finally, it is also a good idea to use a 5% line reactor when applying single phase input power to a drive.  During power-up, the drive will have an inrush of charging current to the unit.  The 5% reactor will help reduce the peak charging current and protect the VFD’s input rectifier stage.



What about cost

There is a price premium for single phase motors, especially larger HP motors.  Doing a quick calculation of that same 10HP motor from above and the single phase variant is a +60% cost premium.  My guess is that some of the added cost is due to the added parts of the capacitor network and switch.


The other part of the cost is because larger single phase induction motors are more of a specialty compared to the three phase types.  


Add in the additional cost of a VFD/reactor but also subtract out the premium for the single phase motor.  I think you’ll find the cost of adding a VFD is much less than you think.


Why not just use a rotary phase converter instead of a single phase VFD?

A phase converter is certainly an option.  It will convert single phase power to three phase power.  But that is all it does.  It does not offer the many advantages that a VFD will offer.  


There is also a similar argument to be made on the cost of a phase converter.  The phase converter will likely not save much, if any money, compared to a drive.



Advantages of using VFDs in single phase applications

A user will benefit going from a line fed motor to a VFD controlled motor.  They will be able to optimize the motor speed for the process.  Maybe this means slowing the conveyor down during loading instead of completely shutting off the motor.  Lightly loaded motors can also be oversped to speed up processes.


A user will also benefit from the energy savings due to the VFD.  Especially quadratic load applications like fans and pumps.   The higher duty the application, the more savings there will be.  Add some basic feedback to the application like a temp or humidity sensor and the VFD can be wired to regulate a process.  KEB’s F5 even has a built-in PID controller so the entire process can be regulated inside the drive – removing the need for an external PLC or control


One advantage with VFDs that is often overlooked is all the protective functions they have that detect abnormal situations.  

  • Over/Undervoltage – Automatically shutdowns when there is a brownout or power surge.
  • Motor Overheat – This option requires a thermistor or motor temp sensor.  It protects the motor investment and is a good idea for expensive motors, difficult to service motors, and high ambient temperature applications.
  • Overcurrent protection – This could detect an abnormal fault like a shorted motor winding and shutdown.


There are many more protective features for sure, but you get the idea.  


What experience do you have operating VFDs with single phase inputs?  Add comments below.  Contact us, if you want to discuss an application.


1 2 3 5
  • This field is for validation purposes and should be left unchanged.