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.
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.
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.
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
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.
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.
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.
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.
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.
With higher speed inherently comes higher power in terms of both the energy put into elevator systems (drives) as well as the potential energy returned (regen). As the number of high-rise and super-tall buildings applications increase and higher speeds become more frequent and push new bounds, there will be the need for OEMs to supply equipment in these corresponding high power ranges that serve the elevator market.
High Power Elevator Drives
As an industrial drives manufacturer, KEB is not only active in the elevator market but also supplies inverters for applications such as plastics machinery (extruders, etc.), high-speed compressors and blowers, and shredders. All of these applications typically require drives with high-power ranging from 200 to 800kW. But, simply having a power hardware overlap with other industries does not necessarily make for a solution that is suitable for elevator applications since the requirements are different.
Often, other high-power drive applications do not require high peak currents as they are constant run applications and low, but audible, output switching frequencies (e.g. 2 kHz) is suitable when equipment is installed in a remote or industrial environment. Additionally, the low switching frequencies minimize the heat dissipation and thus the physical size of the drive and its enclosure.
Whereas, the drives needed for elevator applications require high peak current in comparison to the nominal rated motor current and a switching frequency nearing the audible spectrum limit (e.g. 8 – 10 kHz is generally acceptable).
The comparatively high peak current requirements of elevator applications are based on typical acceleration and deceleration rates. Currently, overload requirements are pushed even higher when specified floor-to-floor times require more aggressive speed profiles.
What differentiates high rise and high speed elevator applications in terms of higher nominal and peak power demand can be exemplified by the equations for kinetic energy, power and torque.
The equation for linear kinetic energy equation is KELINEAR = ½ mv2 where m = mass and v = velocity. This indicates that there is a linear relationship between mass and the energy required to accelerate it. Beyond a given load capacity of an elevator, there is also additional mass from ropes, traveling cables, etc, which is proportional to rise.
Therefore, independent of elevator load capacity, as rise increases the system mass will increase and thus the required energy and power increases. In terms of high speeds, if this is achieved by increasing the motor speed, then the equation Power ≈ (Torque x RPM) / 5,252 indicates that for a given sheave diameter based on speed and a given torque requirement based on load, that as the motor speed increases the required motor power will also increase.
On the other hand, if high speeds are achieved by increasing the sheave diameter, then the equation Torque = Force x Radius indicates that the required power from the higher required torque will also increase.
While a drive could simply be oversized to meet the peak motor requirements, a better solution would be a nominal drive rating that is in-line with the motor rating to minimize the physical size and cost, but also have a high peak current based on the requirements of the application.
From the existing variety of high power hardware developed from other industry applications and taking into consideration the needs of the elevator market, KEB has recently added to its portfolio of elevator drives a design solution which meets the high power demands of high speed and high-rise elevator applications. The resulting solution is an F5 model elevator drive rated for 250HP with a rated current of 332A (480V mains voltage supply) or 370A (400V), a 30 second peak current rating of 740A, and up to an 8 kHz output switching frequency. Furthermore, design variants are cost-optimized by eliminating redundant input rectifiers and braking transistors when line regeneration or active front end (AFE) units are supplying the drive via a DC bus connection.
High Power Line Regen Systems
With high power being put into the system, there will be a corresponding high power regenerated in overhauling conditions. Given that high rise applications will almost definitely be gearless, the system is very efficient in terms of capturing regenerated energy and prime candidate as a line regen system to regenerate this energy back to the main line to be consumed by other loads and thus reducing the total system operating costs.
When considering line regeneration for elevator applications, there are three options: 1.) line regeneration unit with commutation choke (<50% THDi), 2.) line regeneration unit with passive harmonic filter (<8% THDi), or 3.) an active front-end system (<5% THDi). Each has increasing performance in terms of harmonic current distortion mitigation and power factor correction, but also at increasing price points. The type of system needed may depend on the need to meet specific IEEE-519 harmonic distortion limits, reduce building power consumption as part of a building green certification initiative, or long-term return on investment considerations.
Similar to inverter applications, KEB has already developed high-power regen solutions relevant to elevator applications based on other global applications such as crane applications and storage and retrieval systems. The corresponding high-power solutions for each type of line regen technology include: modular R6 model line regen sizes including 150 HP units rated for 184A at 480V or 221A at 400V and 331A peak, which can also be also paralleled for additive capacity; harmonic filters up to 400 HP rated for 400A at 480V and include designs in dedicated NEMA enclosures; and H6 model active front end units up to 250HP.
Systems in Application
KEB recently had the opportunity to put one of these high-power solutions to the test on an 8 m/s (nearly 1,600 FPM) elevator. The system components provided by KEB consisted of a 250HP F5 Elevator Drive, a 250HP H6 Active Front End, and an EMC high-frequency noise filter. Additionally, we were given the opportunity to visit the jobsite. Although I had personally not worked with such high power equipment in the past, when it came to starting up and adjusting the elevator, it was same user interface, parameters and control performance I am familiar with in North America…just on a much higher-power and faster scale!
This article originally appeared in Elevator World Magazine, January 2017.
Tony is the Business Development Manager of the Elevator Applications group at KEB America. He has been with KEB since 2005 and has a BEE from the University of Minnesota – Twin Cities.
Sometimes elevator installers will run into the problem that three phase power is not available in a certain location. Because three phase is needed to run most electric motors efficiently, an answer to this problem has to be found. One commonly used solution is a rotary converter.
A rotary converter is used to convert single phase input into a three phase output. This is accomplished by using what is called an idler motor. An idler motor is simply an induction motor that is being powered by a single phase. Powering an induction motor from a single phase is possible as long as the motor has been spun up by some means. The rotating flux produced by the motor generates a voltage on the third terminal of the motor that is shifted 120 degrees from the voltage between terminals one and two. A three phase load can now be connected along the three terminals of the motor. This type of converter has a number of drawbacks. It is bulky and requires the purchase of an induction motor with a larger power rating than the load motor it is supposed to supply. It also often does not supply balanced power across all three legs.
Luckily there is a better solution. KEB F5 inverters are capable of taking a single phase input and still powering a three phase motor. However, special considerations need to be taken when sizing KEB inverters for single phase input. Note: These special considerations are not needed when using KEB inverters rated for one phase input (typically smaller drives in the KEB B/D housing).
Step 1 – Size input rectifiers
Since the current through a single input rectifier is going to be higher through one phase rather than through three phases (this is because a system supplied by a single phase input needs the current of a three phase system times √3 to produce the same amount of power), it is necessary to upsize the input rectifier stage of the drive to match the higher current requirement. This can be done by selecting an inverter in the correct voltage class that has a rated output current of at least 2x the motor full load current.
Step 2 – Size DC bus capacitors
Rectifying power through a single phase produces more current ripple on the DC bus capacitors, so additional bus capacitance is needed to smooth the associated ripple. To ensure that the drive selected in step 1 has the necessary DC bus capacitance, find the drive that has a rated output current equal to or slightly greater than the full load current of the motor. Refer to the capacitance chart and see if the drive selected in step 1 has a DC bus capacitance that is approximately 2x the drive selected here. If this is the case then the drive selected in step 1 can be used for your single phase application. If the capacitance is below the 2x requirement, find the next largest drive that has double the capacitance and use that one for your application.
|Voltage||Housing||Size||DC Bus (μF)|
Step 3 – Install 5% reactor
Single phase supplies have extremely high current spikes and will cause more mains distortion and DC bus capacitor heating (heat = shorter VFD lifetime). Install a 5% choke at the input of the inverter. Below is a chart that provides a drive size based on voltage class and motor full load current for single phase applications. It should be possible to simply match your voltage class and FLA to the chart and select the correct drive for your application.
|Voltage class||Motor FLA||Correct drive size|
This article has been written with elevator and hydraulic pump applications in mind. But the concept of using a KEB VFD with single phase input to operate a three phase motor can be extended to other applications as well. Interested in knowing more? Contact a KEB engineer today.
When referring to VFDs and individual energy consumers, why is current distortion more applicable than voltage distortion?
The harmonic voltage distortion on a system is affected by all operating loads on the utility. Unless all other electrical loads are turned off when the voltage measurement is taken, it is practically impossible to use this measurement to isolate the distortion effects caused by an individual VFD load.
A more relevant measurement for an individual user or component manufacturer is a current measurement. A harmonic current distortion measurement taken at the equipment disconnect isolates the drive’s individual harmonic performance. In theory, if the power quality of the individual load is in compliance with the desired utility distortion levels, then it could not cause the utility to fall out of compliance.
With this in mind, the IEEE 519 standard defines acceptable THDI levels to be taken at a point of common coupling (PCC). The exact location of the PCC is open to interpretation and needs to be selected for each application. In practice, the PCC is usually taken to be the disconnect switch supplying the elevator control room.
What is Power Factor (PF) and how does it related to harmonics?
Power factor (PF) is sometimes used to refer to the amount of harmonics in a system. The total PF represents the ratio between the power being put into useful work (active power) at the fundamental frequency and the total electrical power being transferred. A unity PF (1.00) is ideal and indicates that all power being delivered is being usefully consumed by the load. A low power factor is to be avoided because it requires electrical components to be oversized to accommodate the transfer of unused reactive and/or harmonic power.
The IEEE 519 standard describes two components that make up the total power factor – the displacement factor and the distortion factor.
The PFDisplacement is the ratio of the used active power (measured in watts) and the reactive power (measured in VoltAmps) at the fundamental frequency. The displacement factor is affected by the impedance of the load and calculated by measuring the phase shift (expressed as ø) between the applied voltage and load current. A standard six-pulse VFD will have a relatively high PFDisplacement , usually greater than 0.9.
PFDistortion is the component of power factor which is affected by harmonics. It represents the ratio between the rms current at the fundamental frequency and the total rms current at all frequencies.
In short, a PFDistortion value close to 1.00 represents a system with very little harmonic distortion. The further the distortion PF value moves away from 1.00 the more harmonic distortion is present. However, a PF measurement by itself is somewhat lacking in that it gives no information to the magnitude of contributing harmonic distortion relative to the size of the utility supply.
KEB engineers answer a few frequently asked questions about elevator drive harmonics and why it’s an important factor to consider in your elevator application.
1. How does a VFD create harmonic distortion?
The input rectifier stage of a 6-pulse VFD draws non-linear (non-sinusoidal) current from the power supply. A VFD’s total rms current draw contains current components at both the fundamental frequency (60Hz) and at frequencies at higher harmonic orders (300Hz, 420Hz, etc.).
An electrical load (i.e. VFD or a computer power supply) which draws a significant amount of current harmonics relative to the size of the power supply can create voltage distortion on the supply.
2. Why should I care about harmonic distortion?
Significant voltage distortion can create operational problems for other electrical equipment connected to the utility. Certain types of electrical equipment are more susceptible to voltage distortion issues – especially sensitive computer equipment found in laboratories, hospitals, airports, and universities.
Besides affecting the operation of other electrical equipment, harmonics also create larger RMS currents which increase conductor heating, stress the system’s electrical and mechanical components, and increase the likelihood of system resonances.
3. What is IEEE 519-2014?
The IEEE 519 standard defines acceptable power quality levels for both the utility and consumers and outlines the procedures needed to test power quality. The standard states that a utility power company has a responsibility to provide its customers with a quality power supply with low voltage distortion. A utility customer also has a responsibility to not draw significant harmonic currents which introduce voltage distortion on the utility and problems for other clients.
Specifically, chapter 10 describes “the current distortion limits that apply to individual consumers of electrical energy”.
|Maximum Harmonic Current Distortion In Percent Of IL
Individual Harmonic Order (Odd Harmonics)
|ISC/IL||< 11||11 ≤ h ≥ 17||17 ≤ h ≥ 23||23 ≤ h ≥ 35||35 ≤h||TDD|
Table 10.3 – Current Distortion Limits for General Distribution Systems (120V through 69,000V)
4. THD vs. TDD
As defined by the IEEE 519 standard, the total harmonic current distortion (THDI) is the ratio between the amount of harmonic current and the current at the fundamental frequency. Because the exact loading and utilization of an elevator application are not known up-front, the THDI is a relevant metric to component manufacturers. Using some basic assumptions, a manufacturer can guarantee a given THDI performance level at a rated current level.
The THDI is an instantaneous measurement and will not remain the same across all loading levels. Specifically, when a VFD is operating at reduced load the THDI value will increase. However, it is important to remember that while the THDI value might increase as the loading level decreases, the harmonic level is expressed as a percentage of fundamental current which is at a reduced level – therefore, the actual amount of harmonic current at partial load is less than the amount at full load.
According to the IEEE 519 standard, the total demand distortion (TDD) is the ratio between the amount of harmonic current and the maximum demand load current as measured over a 15 or 30 minute period. However, a 15 or 30 minute sampling is likely not relevant in an elevator application because of the sporadic nature of loading levels and duty.
A measurement taken at off-peak times or when the motor is not fully loaded will not provide an accurate picture of the worst-case distortion values. A more relevant demand distortion test would be to run the elevator at rated load and rated speed. This provides a meaningful measurement of the worst-case scenario – when the elevator would contribute the most voltage distortion to the utility.
6. What is ISC/IL ?
ISC refers to the available short circuit current of the supply. IL refers to the rated current of the load. So, the ISC/IL is a relative comparison between the size of the utility power supply and the size of the connected load. Large loads have a greater effect on the voltage distortion of a power supply than small loads. Therefore, larger loads have stricter requirements to the amount of acceptable harmonic distortion. The IEEE 519 standard defines acceptable TDD levels at given ISC/IL ratios.
It is difficult for a VFD or component manufacturer to guarantee a level of harmonic performance without knowing the ISC/IL ratio, pre-existing voltage distortion of the supply, and actual loading levels. Therefore, harmonic mitigation performance is often specified using assumptions of the relative size of the load with restrictions placed on the amount of baseline voltage distortion.
KEB harmonic filters are designed to provide ≤ 8% THDI at rated load, assuming ISC/IL ≥ 20 and the amount of baseline voltage harmonic distortion (THVD) is < 2%.
Do you have any questions on harmonics or KEB harmonic filters? Contact a KEB elevator application engineer to discuss.
This is the last of 3 posts on escalators. Post 1 discusses the operational advantages using a VFD with escalators. Post 2 describes the energy savings that can be achieved with VFDs in escalator applications.
This post focuses on using line regenerative drives with escalators. Specifically, which escalator applications are prime candidates for regen and will provide the best payback.
During periods where there is an overhauling load (e.g. passengers riding an escalator down), an induction motor acts as a generator and returns energy from the mechanical system to the VFD.
Traditionally, braking resistors were used with VFDs to dissipate the regenerated energy as heat. As an alternative to braking resistors, a KEB R6 Line Regenerative drive can be added to the escalator VFD through a DC bus connection.
The KEB R6 unit operates by measuring the line voltage and frequency. When the escalator motor regenerates the DC bus voltage rises. Once the DC bus voltage exceeds a predetermined threshold, the R6 matches the line frequency and opens and closes its IGBTs, allowing current to flow back to the utility where it is consumed by other electrical loads.
Good Escalator Candidates for R6 Line Regen Units
Systems with a high gravitational potential energy will provide the best regen energy savings. The amount of gravitational potential energy in a system can be explained by the equation:
Gravitational Energy = mass * gravity constant * height of travel
So, the escalator characteristics that lead to relatively high regen returns will be:
Mass – High capacity escalators that move large amounts of traffic are prime candidates for line regen. These escalators will have larger motors (20Hp +) which are sized to move the large escalator mass and people.
Height of travel – Escalators with long travel in the downward direction are also good candidates for line regen. Longer travel means more gravitational potential energy is being converted into electrical energy.
Other system parameters which will affect line regen payback are:
Efficiency – The efficiency of the escalator drivetrain will have a dramatic effect on the amount of regenerated energy. High efficiency (>90%) helical or bevel gearing is ideal. Low-efficiency worm gearing (<60%) provides significantly less savings with an R6 line regen unit as a sizeable amount of energy is lost in the power transmission. The efficiency of induction motors increases with larger sizes, so again, large motors (>20Hp) are well suited to be used with R6 regen units.
Usage Profile – The amount of traffic on the escalator and the duty of the escalator will have also have a large effect on the returned energy. Subway stations and airports conveying large numbers of passengers and operating at 24/7 duty are ideal and provide the best payback. Minimally loaded escalators or those that are only used for a small amount of time will not provide a good return on investment for line regenerative systems.
Using a line regen unit instead of braking resistors provides a secondary benefit. A line regen unit creates much less heat compared to that of a braking power resistor.
Depending on where the brake power resistor is located, the extra heat might require extra cooling or possibly a larger enclosure for the escalator controller and drive. The larger enclosure, in turn, might require that the controller panel is located in a separate machine room.
- 208-230VAC & 460-480VAC
- 50/60 Hz operation
- Up to 135kVA
Performance & Features
- 150% current overload (for 60 seconds)
- Block commutation control scheme
- Programmable I/O
- Minimal parameter adjustment required
- Automatically detects 230 or 460V connections
- Includes internal DC fuses
- Can be used with 3rd party drives
R6 Line Regen Unit
|Voltage Class||208-230V & 480V|
|Mains Frequency||50 or 60Hz|
|Overload Capacity||150% for 60 sec.|
|Approvals||cULus and CE|
|R6-NCM Regen Units and Chokes|
|Part Number||Rated Supply Current (A)||Rated Regen Current (A)||Choke Part Number|
Do you want to discuss if your escalator application would be a good candidate for regen? Contact a KEB application engineer today to discuss.
More posts in this series
This post (2 of 3) describes how using a VFD to control an escalator motor can reduce energy consumption compared to conventional line start control.
The first post in this series described how using a drive to control an escalator can provide operation and performance advantages.
The third post shows the energy savings that can be achieved with an R6 line regenerative drive.
The benefits from operating an escalator motor with a VFD generally come in two forms: 1) operational or performance advantages and 2) energy savings. KEB has created these posts to help customers, consultants, and users understand the benefits of applying KEB’s energy saving accessories to their escalator VFD.
Reduced Speed Operation and Standby Mode can create energy savings in your escalator VFD
Depending on the installation and usage profile, some escalators can operate unloaded for long periods of time. During periods of inactivity, it is possible to reduce the speed of the escalator motor with a KEB drive in order to save energy.
A typical implementation has the PLC or VFD actively monitoring escalator usage. After the escalator has been unused for a defined period of time (e.g. 5 or 10 minutes), the KEB VFD reduces the speed of the escalator motor – for example, to 20% of rated contract speed.
The VFD continues to operate in Reduced Speed Mode until the escalator is needed. When a passenger approaches the escalator they trip an input (e.g. light curtain or pressure mat) to the controller and the VFD smoothly ramps the motor up to contract speed.
Taking the concept of reduced speed operation even further, it is possible to completely stop the escalator motor during periods of inactivity. In this scenario, there is no current output to the motor and, therefore, no associated motor losses. The only motor/controller losses are due to the control electronics which are minimal.
Figure 1 below shows the typical energy usage of an unloaded 15kW squirrel cage induction motor in various control modes. The largest incremental savings results from using a VFD to operate the motor at a reduced speed during periods of inactivity. Based on the measured savings from a 15kW motor, Table 1 shows the estimated annual energy savings a user could expect when using a KEB VFD to reduce escalator speed.
Estimated Annual Energy Savings using Reduced Speed Operation
Data based on a 15kW induction motor
*Assume 365 days of operation
**Assume energy cost of $0.10/kWHr
Reduced speed operation has a secondary benefit. Lower operating speeds will increase the lifetime of the mechanical wear components on the escalator like bearings and chains.
This additional cost savings comes in two forms: fewer replacement parts and less labor costs.
One thing to note is that local governing escalator code might not allow for the escalators to operate at reduced speeds or to be placed into Standby Mode. If the code does allow for these operating modes, the escalator passengers still might need to be educated that they need to approach the escalator before it ramps up to normal contract speed.
Power Factor Improvement
Power Factor (PF) is a unitless number that represents the ratio between the power being put into useful work (measured in Watts) and the total electrical power being transferred (measured in Volt-Amps). A unity PF of 1.00 is ideal and indicates that all power being delivered is being usefully consumed by the escalator motor.
A low power factor will result in a higher input current for a given amount of power output at the motor shaft. This higher input current is to be avoided because it requires electrical components to be oversized in order to accommodate the transfer the unused reactive power. Also, there will be more resistive conductor losses due to the transfer of the additional reactive power. Finally, a low power factor will cause higher heating in electrical components which will negatively affect the operating lifetime of the components.
Using a VFD dramatically improves the power factor of an induction motor compared to a motor that is being operated from the utility line. Figure 2 shows the difference in PF between an induction motor that is operated by a VFD and one that is line fed. The positive effects on the PF are most noticeable when the motor is partially loaded – a scenario common in many escalator applications. However, even when the motor is at full load the PF is improved by 10% when using a KEB VFD.
VFD effects on Input Current
As described previously, the input current is related to the PF. A low PF will result in a higher input current for a given output power. Figure 3 shows the effect of a VFD on the escalator system’s input current.
Again, a VFD provides the largest benefit when the escalator motor is operated at partial load. In this case, the no load RMS current with a VFD is over 50% lower than the case of the line fed motor.
Do you want to discuss the benefits of using KEB VFDs with your escalators? Contact an application engineer today!