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.
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 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 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.
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.
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.
If you’re interested in hearing more about our SCL technology you can read this previous post about its use in PM motor control.
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 gives better 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.