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Q and A about Adjustable Frequency Drives

The purpose of this page is to provide complete answers to questions concerning adjustable frequency drives and related topics.
Questions and Answers are listed under various topics

Power Factor Correction and AFDs
AFD Harmonic damaging
Reflected Waves and the AC motor

Power Factor Correction and AFDs

Question:
Can power factor correction capacitors be applied to an adjustable frequency drive controlled motor?

Answer:
In general, power factor correction capacitors should not be used with AFDs. Power factor correction capacitors are used to create a shorter, alternate electrical path for the reactive current component found in inductive loads, such as motors and transformers. This alternate path reduces the electrical current demand placed on the utility or supply source. As the load on the motor decreases, the power factor of the motor also decreases however, since the total current is decreasing, the reactive component of the current also decreases. In this way, the current through the capacitors remains fairly constant. The capacitors are sized (selected) as a KVAR device and as such are rated for only a small portion of the total user load.

The capacitors do not control the amount of current flowing through the capacitor. The inductive load controls the amount of current flowing through the capacitor. The capacitor creates an alternate path for the reactive motor current. If the capacitors are located close to the motor, the reactive current electrical path is short. If the capacitors are located at the utility/facility interface, the reactive current path is longer but still reflects a higher power factor to the utility and allows the distribution and transmission capabilities of the utilities to handle more real power transfer.

When capacitors exist on a distribution system they are a source of energy. The capacitor voltage will supply current to any load connected to the capacitor. If a resistor was connected across a capacitor, current will flow between the capacitor and the resistor load. Real power will exist during this transfer since there is no reactive current component. The capacitor current is controlled by the capacitor voltage and the value of the resistor. In a similar manner, when PWM (pulse width modulation) drives are used to control a motor, the capacitor current is controlled by the capacitor voltage and the input impedance of the PWM drive. Since PWM drives transfer real power through the drive to the motor, the current demand on the power factor correction capacitor is real and is a larger portion of the load compared to the reactive portion of the motor for which the capacitor was originally selected. Thus, the current is forced to carry more current than it was selected to handle. Some "experts" will state that harmonic currents cause power factor correction capacitors to overheat, capacitor fuses to open. Although it is true that capacitors can be damaged and fuses will open, the real reason this occurs is due to the change forced upon the capacitors. Having been selected for KVAR loads, the capacitor must handle the KVA load and essentially are undersized. Dependent on how effectively the distribution source (i.e. transformer) works with the power factor correction capacitors will determine how much extra work the capacitors must do. If the capacitors are located close to the transformer source, the capacitors are less likely to experience the full change from KVAR to KVA. However, if the capacitors are located closer to the PWM AFD, they will experience a greater demand for load current and experience more problems.

The solution to potential problems between PWM AFDs and power factor correction capacitors is to remove the capacitors from the distribution system, since they are no longer needed to improve the power factor of the controlled motor. If fixed speed motors exist on the same distribution system with PWM AFD controlled motors, then locating the capacitors at least 250 electrical feet from the PWM AFD will reduce the stress on the capacitors however, this adds losses into the system.

It is important that the combination of capacitors and reactive components connected to a distribution system does not create resonance (i.e. the combination is tuned to resonate at some frequency). The current pulses associated with PWM AFDs may excite a distribution system that is tuned to harmonic frequencies contained within the current pulses. If the distribution system is not a tuned circuit prior to adding PWM AFDs, then the addition of AFDs will not create a tuned circuit. If the distribution system is tuned (i.e. resonant) then components can be added to either retune or filter the frequencies which can prevent sustained resonance. The simplest solution may be to retune by relocating the capacitors thus adding or subtracting inductance in the system circuit.

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AFD Harmonic damaging

Question:
How damaging are AFD harmonics to a motor?

Answer:
Harmonics are used to describe the voltage and current waveforms that exist in a distribution system. The harmonics associated with AFDs are not likely to have any destructive impact on the motor. You can answer your own question by asking how can voltage or current damage the motor. When harmonics or the waveform shape exceed values which are destructive to motors then problems will occur. With AFDs, there are two locations where the voltage waveforms are different than the sinusoidal waveform expected from the utility.

When the AFDs draw current from the utility, current pulses rather than sinewave exist. These current pulses caused voltage drops in the system, similar to the sinewave voltage drops except there are other frequencies (i.e. harmonic) which cause the shape of the voltage to become slightly distorted. With a sinewave voltage drop, a loss in voltage (i.e. voltage regulation) would occur. With current pulses, the voltage regulation turns into voltage distortion. If the amount of distortion were to cause large voltage changes, motors connected directly to that changing voltage would be affected. Fortunately, the harmonic content of the current pulses is small. This results in small voltage distortion at frequencies which would impact motors. Typical harmonic voltage distortion that exists on the distribution system is too small to impact motors connected direct on line. Typical harmonic values of 20 volts when compared to the 460 volts of 60 Hz will be ignored by the motor. Recall that the force or torque experienced in the motor is a function of the square of the applied voltage. At low harmonic voltage values, the possible damaging forces on the motor results in less than a few degrees of motion on the shaft of the motor while the motor is operating at its nameplate speed. It would takes large amounts of harmonic voltages to seriously impact the motor. Which leads us to the next location where harmonics exist.

The output voltage waveform of the AFD is a series of pulses which change a fixed dc voltage source, internal in the AFD, to a variable width square voltage to simulate an adjustable sinewave of voltage. The motor expected to receive a certain volt-seconds area with a symmetrical change in polarity. Whether the volt-second area is contained within a sinewave or contained within a series of pulses makes no difference to the motor. The motor performs as long as the correct volt-second area exists. Before the days of PWM, the voltage waveform sent to the motor by AFDs were single pulses displaced by 120 degrees to create a simulated 3 phase voltage. The harmonic content of that voltage waveform represented a harmonic voltage distortion of 33% THD. After many years of installation and application, it was proven that as potentially damaging as that types of waveform was, the impact on the motor was minor. General practice for selection and application of motor experiencing that waveform was to select a 1.15 S.F motor and derate that motor to 1.0 S.F. This allows the extra heat that was created to dissipate properly and allowed the motor to operate without problems. As a side note, this poor waveform did not allow motors to operate at low speeds in constant torque applications. Today things have changed. With PWM techniques applied to the motor, less heating, wider operating speed ranges are now possible. The full load motor current resulting from today's PWM AFD applied voltage waveforms creates less heat in the motor when compared against direct on line operation. Today's PWM AFD can be fine tuned to the application eliminated all possible losses from the motor and system.

Note that there are other types of motor controls. Some contribute more harmonics to both the distribution system and the motor. Since most ac motors are controlled by PWM AFDs, concern about harmonics, causes by PWM AFDs, impacted either the ac line or the motor should be minimum. Harmonics on the distribution system, are of concern, however information, not fear should be used to guide in the selection of equipment which can improve performance and increase operating efficiency.

Another topic associated with PWM AFDs and their impact on motor is the Reflected Wave Issue. This is sometimes confused with harmonics. It is a significantly different issue and is covering under the topic of motors and reflected waves.

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Reflected Waves and the AC motor

Question:
What are Reflected Waves and will they damage motors?

Answer:
Reflected waves occur due to a mismatch in impedance between the motor cables and the motor. You may be familiar with the requirement, in audio sound equipment the importance of matching the impedance of your speakers to the output of the amplifier. Matching impedance insures that the full range of frequencies is transferred to the speakers. If a mismatch in impedance exists some of the frequencies do not get to the speaker. They are reflected back to the amplifier. 

The PWM pulses at the output of the 460 volt AFD change in value from zero to 650 volts and contain many high frequencies. The leading edge of each pulse represents the higher frequencies. The faster the voltage changes (i.e. rise time) the greater the number of high frequencies. These pulses are sent to the motor as the modulated voltage supply. The low frequencies are accepted by the motor but the higher frequencies are turned away because of the difference in impedance between the motor cable and the motor. The higher frequencies are turned back because higher frequencies create a greater difference in impedance than do lower frequencies. The frequencies that are turned back or reflected combine with the pulses coming from the AFD to form a different shaped waveform.

Because of the wide variation in impedance between different motor cable and motor ratings, accurate definitions of the exact shape of the new waveform is difficult to predict. What is known is that a minimum length of motor cable is required in order to achieve the maximum possible peak voltage value for the reflected wave. What the means is for short (10 to 30 ft) cable distances, it is unlikely that the full value of peak voltage will occur. However, as the distance of the motor cable increases, the value of the peak voltage will increase until a value of twice normal pulse amplitude is achieved. Increasing the motor cable distance beyond that point will not cause the peak voltage to increase. Whether the motor is impacted by this reflected wave is dependent on how good the insulation is in the motor. With smaller HP rated motors, there is less physical space in the motor so a greater risk for insulation breakdown exists. As the HP rating of the motor become greater than 2 HP, the physical space increases thus decreasing the risk for insulation breakdown.

Of the many motors that have been installed with AFDs, the occurrence of insulation breakdown, due to reflected wave, is small. Solutions to reduce the peak voltage that can be created include adding a reactor between the AFD and the motor or add a path circuit at the motor to absorb the high frequencies that are being reflected. Remember that the high frequencies reflected from the motor will travel in a path that has the lowest impedance. Adding a circuit at the motor which attracts high frequencies will prevent the formation of high peak voltages at the motor. Of all the motors that are manufactured, it is possible that some imperfections may exist in a small percentage of motors.

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