By

Mike Glampe

VFD switching frequency

VFD switching frequency refers to the rate at which the DC bus voltage is switched on and off during the pulse width modulation (PWM) process.  The switching on and off of the DC voltage is done by Insulated Gate Bipolar Transistors (IGBTs).   The PWM process utilizes the switching of the IGBT’s to create the variable voltage and variable frequency output from the VFD for control of AC induction, permanent magnet synchronous or DC motors.  The switching frequency, sometimes called the “carrier frequency”, is defined using the unit of hertz (Hz) and is typically in the kHz (Hz*1000) range, typically ranging from 4 to 16khz, or 4000 to 16000 switches on/off per second.

 

To determine what switching frequency would work best for your application, it is beneficial to look at the advantages and disadvantages as the switching frequency is increased.

 

Switching frequency – Effect on current distortion

The harmonic content in the current waveform generated by the PWM process is reduced as the switching frequency increases.   The ‘cleaner’ waveform results in higher efficiency by reducing the current ripple, which results in lower motor losses.   This benefit of a higher switching frequency is more pronounced as the output frequency to the motor increases.

 

The effects of using different switching frequencies can be seen in the application below.  A surface mount permanent magnet (SMPM) motor is run by a KEB high speed VFD.  The operating point of the system is 50kW @ 10000rpm (333hz).  Figure 1 shows the current waveform (green) and the PWM output of the VFD (red) at a 4khz switching frequency.   In this application, the required current to reach the operating point is 178amps.

 

The formula for the total harmonic current distortion (THD(I)) is:

Current distortion formula for VFD

where

I1 = r.m.s. current at the fundamental frequency

In = r.m.s current of nth harmonic

n(max) is the number of the highest measurable or significant harmonic.  In this case, the highest harmonic used was n = 50.

 

 

For this application, running at 50kW @ 10000rpm, using a 4khz switching frequency, the THD(I) = 12.69%.

THiD at 16kHz switching frequency - higher output

50kW @ 10000rpm, using a 4khz switching frequency, the THD(I) = 12.69%

 

 

Running the system at the same operation point (50kW at 10000rpm) and increasing the switching frequency to 8khz gives the current waveform at the motor shown in figure 2 and the resulting THD(I) = 6.27%.

VFD switching frequency - THiD at 8kHz switching frequency - higher output

50kW @ 10000rpm, using a 8khz switching frequency, the THD(I) = 6.27%

 

The reduced current distortion translates into lower rotor heating of the motor and a higher motor efficiency.  The reduced rotor heating is of great concern when motors utilize bearing technology that requires very small clearances (air foil or magnetic).  Excess rotor heating can cause the rotor to expand or elongate, which could result in the rotor impacting the bearing surface.

 

Switching Frequency – Effect on high frequency outputs

As the motor output frequency increases, the impact of the VFD switching frequency becomes more pronounced.  Using the same motor as above, the operating point was increased to 100kW at 20000rpm (666hz).  Again, the required output current from the VFD to reach this operation point was 178amps.

 

At 4khz switching frequency (figure 3), THD(I) = 17.27%.

VFD switching frequency - THiD at 4kHz switching frequency - higher output

50kW @ 20000rpm, using a 4khz switching frequency, the THD(I) = 17.27%

 

At 8khz switching frequency (figure 4), THD(I) = 8.47%.

VFD switching frequency - THiD at 8kHz switching frequency - higher output

50kW @ 20000rpm, using a 8khz switching frequency, the THD(I) = 8.47%

 

At 16khz switching frequency (figure 5), THD(I) = 4.05%.

VFD switching frequency - THiD at 16kHz switching frequency - higher output

50kW @ 20000rpm, using a 16khz switching frequency, the THD(I) = 4.05%

 

The higher switching frequency decreases the audible noise that can be heard from the motor.  The audible noise from the motor is a result of the stator laminations vibrating at the carrier frequency rate.  As the carrier frequency is increased, the pitch of the noise from the stator laminations is increased moving the levels farther out of the normal hearing range of humans.  Motor noise levels may be of concern based on the application requirements (elevator motors, theater equipment, etc.).  In these cases, a VFD with a higher carrier frequency may be an option.

 

As the switching frequency increases, motor heating due to higher harmonic content in the current waveform decreases.  At the same time, the heat generated internally in the VFD due to the IGBT switching is increased.  Each switching action of the IGBT produces a relatively fixed amount of heat loss.  So as switching frequency increases so does the overall heat loss of the VFD.  The heatsink of the VFD must be designed to provide sufficient cooling of the VFD to operate during the maximum rated ambient conditions.  KEB drives are rated based on a specific switching frequency.  It may be possible for a specific drive to operate at a higher switching frequency than rated, but the output may have to be reduced in order to keep the drive from overheating.  If the system requires a higher switching frequency, then the heatsink may need to be increased in size, have increased air flow or liquid cooled.  All of these options are available on KEB drives.

 

Because of the higher heat loss due to the higher switching frequency, and the fact that the heatsink may need to be larger (if air cooled), this results in a larger physical size VFD and consequently higher initial component costs.

 

KEB has an option for liquid cooling of the VFD for situations where space may be an issue.  While this does potentially reduce the physical size of the VFD housing, it does introduce the requirement for access to a cooling fluid.

 

The VFD ratings are based on maximum ambient conditions.  In most cases, the VFD is mounted inside an electrical enclosure.  There for the maximum ambient temperature around the VFD becomes the temperature inside the enclosure.  With a higher heat loss from the VFD due to the higher switching frequency, this introduces more heat loss into the enclosure.  The higher enclosure heat load may require additional cooling (fans, air conditioner) to be added to the enclosure itself, depending on the application requirements.

 

The VFD switching frequency used for a specific application depends heavily on the application requirements and components in the system.   Motor design, noise levels, required system efficiency, cooling requirements and component costs all need to be considered when determining the optimal VFD switching frequency for the application.

 

KEB VFD’s can operate at switching frequencies up to 16khz.  KEB has a strong background in applications requiring high switching frequencies.  Because of our extensive application experience in these types of applications, KEB drives have been developed to deal with the inherent challenges that come with the higher switching frequencies.  Whether your application requires a high switching frequency or not, KEB has the options and expertise to help you meet your application requirements.

 

If you are unsure what switching frequency is the best for your application, contact a KEB Applications Engineer today.

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

 

 

Summary

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.

 

KEB’s Harmonic Filters Reduce Harmonic Distortion

For this article, we will discuss the harmonic distortion created by the input stage of a VFD and how KEB harmonic filters can be implemented to reduce these harmonics.

KEB Harmonic Filters reduce frequency distortion

Variable frequency drives (VFD) used in industrial applications provide many benefits including energy savings, better process control, extended component lifetime and increased machine safety when utilizing functional safety features.

Variable frequency drives consist of three sub-systems. The diode bridge converter, DC bus, and IGBT output inverter. The diode bridge input rectifier typically consists of a full-wave 6-pulse rectifier which rectifies the incoming AC voltage into a DC voltage. The DC bus system utilizes DC capacitors to help smooth the rectified AC voltage and provide voltage storage for the system. The IGBT output system utilizes the DC bus voltage and creates the variable frequency and voltage output to the motor using a pulse width modulated (PWM) control.

VFD topology

 

 

Any VFD which uses a 6-pulse rectifier on the input stage may be one source of current and voltage harmonics on the main line (Other sources of main line harmonics include: Arc furnaces, any power supply using a static power converter, inverters for distributed generation). This is due to the non-linear load produced by the 6-pulse rectifier. A load connected to an AC input voltage is a linear load if the current draw is in the same form as the voltage (resistive, inductor or capacitive load for example). A non-linear load is then one that draws current that is not in the same form as the as the AC voltage waveform – non-sinusoidal.

 

In the VFD system, once the DC bus capacitors are charged, input current to the capacitors will flow only when the incoming AC voltage is greater than the DC bus voltage. This is near the top of the arc of sine wave voltage waveform. As the voltage of the sine wave drops below the DC bus level, the current will stop flowing as shown in the diagram below. This non-linear capacitor current results in a current pulse on the main incoming voltage line.

 

Harmonic filters from KEB - harmonic volts over time

 

Typical industrial systems have both linear and non-linear loads. The non-linear loads cause the main line voltage and current waveforms to become distorted. This introduces harmonics into the waveform. The harmonics will be at an integer multiplier of the fundamental frequency. For example, if the input voltage is 60hz, harmonics may result with frequencies of 120hz, 180hz, etc. The harmonics produced by a static power converter rectifier stage can be determined by:

 

h = n x p ± 1

where

h = harmonic
n = 1 to ∞
p = number of pulses

 

Therefore, with a 6-pulse full wave rectifier, we can expect to see harmonics at the 5th, 7th, 11th, 13th… orders. The amplitude of each harmonic typically diminishes as the harmonic value increases.  As shown in the diagram below, the distorted waveform is made up of the fundamental (work producing) component and of the harmonic components (non-work producing) components.

 

Harmonic filters - waveforms

 

Harmonics on the main line can have the following effects:

  • Cause interference with communication circuits
  • Transformer overheating
  • Nuisance circuit breaker trip
  • Neutral overloading
  • Capacitor bank failure

 

How much harmonic content on the main line is too much? To assist with this question, a standard for harmonic levels was developed by the Institute of Electrical and Electronics Engineers (IEEE). The current standard for acceptable harmonic content is IEEE519-2014.

 

The main components of this standard define the acceptable levels of current and voltage harmonics based on the short circuit current rating (SCCR) of the incoming power distribution system. The amount of distortion that a system can tolerate is dependent on the input impedance level of the voltage distribution system. By determining acceptable harmonic levels based on the system SCCR, the standard takes into account the distribution system impedance. Lower impedance or stiffer distribution networks can tolerate a higher level of harmonics. Where higher impedance or softer distribution networks can tolerate a lower level of harmonic content.

 

The IEEE519-2014 standard also defines the point of common coupling (PCC) which is where the harmonic levels are to be measured. This provides a standard measurement location for the harmonic content of the facility to be measured. Since most industrial environments consist of linear and non-linear loads, even if an individual system produces some harmonic content, the overall effect at the PCC may be negligible. The IEEE519-2014 standard provides some clarification on the PCC over earlier versions of the standard:

 

Point of common coupling (PCC). IEEE519-2014 defines the PCC as:

 

…the PCC is usually taken as the point in the power system closest to the user where the system owner or operator could offer service to another user. Frequently for service to industrial users (i.e., manufacturing plants) via a dedicated service transformer, the PCC is at the HV side of the transformer. For commercial users (office parks, shopping malls, etc.) supplied through a common service transformer, the PCC is commonly at the LV side of the service transformer.

 

The IEEE519-2014 standard provides guidelines for the allowable harmonic content on a voltage distribution system. Standards are provided for both voltage and current harmonic levels.

 

KEB has developed a line of harmonic filters designed to be used on the input of a 6-pulse full wave rectifier. The standard KEB harmonic filter line is designed to reduce the total harmonic distortion (THD) of both current and voltage on the main input line to a maximum of 8%. This level is valid for an Isc /IL ≥ 20.

 

As an example of the effectiveness of the KEB harmonic filter, the following waveforms show the advantages of using a KEB harmonic filter.

 

Voltage and current at the main line – no line choke

Without a line choke high current peaks (5 to 10 times the rms value) occur resulting in a high ripple current in the DC bus of the motor control. This also means higher ripple voltage on the DC bus which can lead to torque ripple or reduced torque output from the motor.

 

Harmonic filters - no choke

 

Voltage and current with line choke impedance = 3% 480V/60Hz (4% 400V/50Hz)

With inductance (line choke) the peak amplitude of the current is reduced to a range of 3 to 5 times the rms value. This means lower ripple current in the DC bus of the motor control. This reduces heating in the capacitors and effectively extends the lifetime by a factor of two.

 

Harmonic filters - impedance

 

Voltage and current with the harmonic filter THDI < 8%

With the harmonic filter, the line-side current is sinusoidal. The peak amplitude of the current is 1.4 times the rms value. The output of the filter provides a nearly ideal rectangular voltage and current waveform into the motor control resulting in diode conduction of almost 180 degrees. The voltage on the DC bus is practically flat and the DC bus capacitors see minimum ripple current. This results in a further extension of life to 3+ times the nominal. Furthermore the average voltage value of the DC bus does not sag much in comparison to the first two cases. The result is more motor torque at higher speeds.

 

Harmonic filters - filtered

 

The KEB harmonic filters utilize a patented design which incorporates the inductor windings on a common core. This common core design provides the following advantages over competitor harmonic filters:

  • High damping of main line voltage overshoot
  • Minimal oscillations due to rapid load changes
  • Increased lifetime of intermediate DC bus capacitors due the trapezoidal output of the voltage waveform (less DC bus ripple).
  • Negligible DC bus voltage drop at full load
  • Parallel operation of several VFD’s from a single harmonic filter
  • Low capacitive reactive power at idle or low loads

 

KEB harmonic filters utilize three-phase capacitors in delta configuration. The three-phase capacitor design increases the symmetry and helps to protect the VFD from voltage surges in the event of a phase failure. In the event of a capacitor failure, all three phases are disconnected simultaneously. A phase failure on a competitor’s harmonic filters using single phase capacitors can result in a severe voltage imbalance and cause damage to the VFD input stage.

 

Additional KEB harmonic filter design highlights:

  • KEB Harmonic filters are available specifically for applications with a 480VAC, 60hz mains or 400VAC (380), 50Hz mains.
  • KEB Harmonic filters are available in sizes ranging from 23A to 400Amp rated input current.
  • KEB Harmonic filters are UL rated
  • All KEB harmonic filters incorporate a circuit protection device on the capacitor banks of the harmonic filter. The capacitor banks are mounted independent of the inductor core assembly to allow for increased mounting flexibility.
  • KEB harmonic filters utilize lug style power connections.
  • Capacitor contactor option available to disconnect the capacitor bank when the machine is idle.
  • KEB Harmonic filters can be equipped with a line sync module for use with the KEB R6 line regenerative unit.

 

Please contact KEB today to speak with an application engineer.

Optimizing high speed motors with KEB

This is the second part of our short series on how KEB VFDs can be used in high speed applications. In our last post we discussed the benefits of KEB technology in high speed applications.

 

In this post we’ll talk about some of the options you have for optimizing high speed motors by KEB and some commercial considerations to partner with KEB.

 

KEB Sine FilterKEB sine filter technology Maximize system efficiency

 

High speed motors may run with very small rotor clearances on the bearings. Rotor heating becomes a concern as any elongation or expansion of the rotor due to excess heat can cause the rotor to impact the bearing.

 

In applications requiring very low harmonics on the output waveform, KEB produces a line of high speed sine filters to filter the output voltage and current waveform to a near sine wave to keep the rotor heating to a minimum.

 

KEB SCL software is designed to run with an output sine filter. The software also incorporates an electronic filter to avoid resonance issues between the drive/sine filter/motor combinations.

 

 

KEB simulation tool The right system for your motor

 

High speed motors and their bearing systems may have limitations such as resonance and excess heating when operating in certain conditions. The selection of the proper components for the system is paramount to avoid any potential issues.

 

To aid in the selection of the proper components, KEB has developed a simulation program which utilizes the motor design data. The simulation program allows KEB engineers to run a simulation with or without output filters and view the expected output voltage and current waveform.

 

Potential system resonance points can be identified and proper steps taken to minimize any issues with the actual system. Utilizing this powerful simulation technology, KEB engineers can work with customers to provide the optimal solution in technology, efficiency, and cost.

 

KEB Simulation

Detailed simulation software provides optimal sizing and selection of components

 

 

KEB options The best solution for the application

 

High speed motors are used for their high efficiency and high power density. Many high speed, high power motors utilize liquid cooling for maximum power output. In addition to the traditional air cooled VFD heatsink, KEB drives are also available in liquid cooling configurations.

When running at higher output frequencies, there is additional heat that must be dissipated. Liquid cooling provides an efficient way to remove this added heat. KEB has options for utilizing water/glycol cooling or refrigerant cooling.

 

KEB is your partner Flexible, powerful, proven

There are a lot of commercial reasons to partner with KEB for your high speed motor application.  First, KEB uses our standard drive hardware and topologies to control high speed motors.  We are not using a one-off custom drive design which can only be “matched” with one type of motor and applied in a very limited scope.  Because standard hardware is used, both economy and quality benefit from the increased scale.

 

Secondly, KEB is not a boutique drive manufacturer.  KEB is a global manufacturer so users are assured that the product carries the leading global product certifications like UL and CE.  Along with the certifications, up-to-date documentation and an extensive support network are available.

 

Finally, KEB is proven.   With over 100,000 installed drives specifically running high speed machinery, KEB is the leader.  If you want to know more, contact a KEB applications engineer and we’ll help you choose the right options for your installation.

 

Advanced control for high speed motors

One of KEB’s specialties is controlling high-speed motors used on turbo blowers and power generating systems.  High speed motors can operate in excess of 100,000 rpm and typically use air foil or magnetic bearings.  Obviously at these speeds, precise control is required in order to reduce vibration and motor heating.  Enter KEB.

 

This is the first of two parts on how KEB VFDs can be applied in high-speed applications. In this post we’ll talk about KEB’s unique technology and how it applies to operating high speed machinery.

 

The second post discusses some options to optimize performance with high speed motors.

 

KEB Control  – Advanced control technology for advanced technology motors

From high speed spindles and routers to blowers and compressors, KEB’s proven Variable Frequency Drive (VFD) and filter technology provides the control systems to make these applications a success.  One advantage with the KEB drive platform is that one drive can operate a variety of different motor types.

 

High speed motors, whether a surface mount permanent magnet (SMPM), interior permanent magnet (IPM) or induction motor, utilize advanced technology to provide a high power density and high efficiency solution to the application.

 

Such advanced motors incorporate advanced technology such as magnet bearings and air foil bearings. The VFD system to control these motors should also utilize advanced technology to provide the best performance and highest efficiency of the overall system. KEB drives and filters provide the advanced control technology needed to run these motors effectively and efficiently.

 

KEB Sensorless Closed Loop Software (SCL)  – Precise control without feedback

 

KEB’s SCL software provides precise speed and torque control of SMPM or IPM high-speed motors without requiring a feedback device. The high-frequency output required for the typical high-speed applications has the potential to cause adverse effects to the feedback signals, which can affect the control of the motor.

 

KEB SCL software eliminates this issue by utilizing a high-speed processor and direct measurement of the motor characteristics to build a precise model of the motor. The software uses the output current and back EMF(for PM motors) and compares these to the calculated model values and makes adjustments based on this information effectively closing the loop internally in the software.

 

KEB SCL Control

KEB’s proprietary SCL motor algorithms provides extremely precise torque and speed regulation.

 

 

 

KEB Drives  – High output frequency

 

In addition to the advanced software, KEB drives are available with hardware capable of switching frequencies up to 16khz, depending on the drive size. The higher switching frequency allows the current waveform to approach a sine wave which provides superior motor performance and higher system efficiency.

 

Output frequencies up to 1600hz are available. Depending on the motor type and control, motor output speeds of 100krpm have been achieved.

 

If you have any questions about how KEB drives can be used in high-speed applications contact us today and our applications engineers will be ready to help you out.

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