Partial Discharge Generated by Inverter Drives
Inverter drives usually called Variable Frequency Drives, Variable Speed Drives or Adjustable Speed Drives are very popular and growing in use. They have a long list of advantages, and they make it possible to replace DC motors with AC motors that are typically smaller in physical size and reduce operating cost.
A key disadvantage is that if the power system is not applied correctly, there can be large voltage spikes or voltage “overshoot” on the motor terminals. These spikes can cause Partial Discharge and eventually a failure in the motor insulation system. Several users have told us they “threw a VFD on it”, meaning they just bought a VFD, and powered an older motor with it. Usually, this would be done with existing power cables, with little regard to cable lengths, cable type, and other important issues. Result: the motor lasted a short time.
The inverter drives used to control speed come in many varieties with different types of outputs. Pulse Width Modulated control, or PWM, is the most commonly used. It takes a fixed AC input voltage, converts it to DC with rectifiers, and from the DC voltage produces square waves or rectangular pulses with variable pulse width, variable frequency, and sometimes variable voltage. See picture.
The rectangular pulses are typically produced by IGBTs (Integrated Gate Bipolar Transistors). The on/off state of the IGBTs is controlled, and they essentially chop up the DC input voltage into DC output pulses. Since the voltage pulses have different width, they produce a sinusoidal looking voltage and current in the load they are connected to. How close to smooth and sinusoidal the wave will depend on several factors including the number of pulses produced per cycle. This is called the carrier frequency or the switching frequency.
To provide “clean” power with an overshoot voltage the motor can handle, the system which includes the inverter drive, the motor, and the cable between the two, must be matched properly from an impedance point of view. If this is not done, there will be large voltage spikes at the motor terminals. These spikes can be up to about two times the intended voltage, and in severe cases much higher than that.
NEMA Standards Publication ICS 7.2-2015 states that the peak voltage at the motor terminals has a typical maximum of twice the VFD’s DC bus voltage, provided the pulses are far enough apart to allow the ringing that is produced to decay before the next pulse comes. When this is not the case, the peak voltage can be higher. The pulse output voltage, called the DC bus voltage, is typically equal to or less than the peak rated voltage for the motor, RMS x 1.414 Volts. Some VFDs can reduce the pulse output voltage when full power is not required by the motor.
Example of 460V general-purpose and definite-purpose motor voltage overshoot tolerance:
- DC bus voltage and peak sinusoidal voltage: 1.414 x 460V
- The spike voltage for a 460V motor with a power cable length of 50ft/15m or more can be: 2x (1.414 x 460 V) ≈ 1,300 V.
- NEMA MG1 Part 30 states that a general-purpose motor operating at this voltage should be capable of withstanding a repetitive peak voltage of 1,000 volts. This will not be good enough in the example above.
- Furthermore, NEMA MG1 Part 30 states that definite-purpose, inverter-fed motors are designed to withstand maximum repetitive voltage peaks at the motor terminals equal to 1.1 x 2 x 1.414 x VRated = 3.1VRated. This is two times the peak sinusoidal voltage plus a safety factor of 10%.
In this example, 460V x 3.1 = 1,426V. The definite-purpose motor should hold up if the system is set up correctly. If not, spikes can be more than 2x the DC bus voltage + 10% and cause problems.
Reasons for the voltage spikes
The spikes are a response to the excitation of the RCL circuit that includes the cable and the motor. The rise-time of each pulse exciting the circuit can be as low as about 100nsec, so very fast. Transmission line theory says that under these circumstances reflected waves will occur at transition points, where there is an impedance mismatch. These transition points may occur at several locations in the system. The reflected waves are high-frequency ringing as indicated in the picture. If the impedance in the system, or the C and L components of the impedance, have certain values, the voltage of the initial spike can be high and added to the pulse voltage.
In reality, the fast rise-time rectangular pulses produced by an inverter drive are a combination of a broad spectrum of frequencies. As the cable length between the inverter drive and the motor increases so does the distributed capacitance and inductance along the cable. This causes the resonant frequency of the cable to drop. When this frequency and the frequencies contained in the inverter drive pulses are in the same range, they can become additive and result in high voltage spikes. As the cable length increases above 50ft/15m, the overlap in frequencies may start to happen depending on the setup and cable type.
Inverter drive carrier frequencies can be as high as 20kHz. This means that each pulse lasts a very short time. In some setups, the ringing of the reflected wave will not have enough time to dissipate to zero before the next pulse comes. When that happens, the reflected wave will build on top of what is left of the previous reflected wave, and the voltage spikes can get very high.
Partial Discharge Effect on the motor
The front-end voltage spikes or overshoot come at a very high rate, one per pulse produced by the inverter drive. As mentioned, PWM drives can produce pulses with a frequency from 500Hz to 20kHz. When the voltage spikes are high enough they cause partial discharge in the motor windings because of the strength and concentration of the electrical fields produced by the spikes. The inverter drive may also trip. At even a few hundred pulses per second, let alone 20,000 pulses per second, the stress on the winding insulation can be high enough to damage the insulation in short order.
There are many ways to cut down the size of the spikes and their effect on the motor. These include filters and reactors, lower cable lengths, different types of cable, lower carrier frequency, slower rise time pulses, and more. Each has advantages and disadvantages.
Motor analyzers with PD measurement like the iTIG III can help diagnose inverter drive power system problems.