Variable frequency drives (VFDs) are typically added to a system for economic reasons. While the realized economic benefits may be drastic throughout the life of the motor, several factors must be addressed to ensure the motor’s long-term reliability.
Non-Inverter Duty Motors
The use of a VFD adds several areas of stress that non-inverter duty motors are unable to cope with long term. The most common type VFD is pulse width modulated (PWM). A PWM drive uses short bursts of electricity to simulate a sine wave. This approximation of the sine wave adds several types of stresses to the controlled motor. Shaft grounding rings, insulated bearings and line filters are some of the most common ways customers protect their motor investment from negative VFD attributes.
While VFDs are not 100 percent efficient, they do make economic sense when the operating speed of a motor needs to be lower than its rated speed. Therefore, choosing the correct motor for the application becomes crucial because motor efficiency drops as speed and load decrease. VFD efficiency also declines with motor speed, so it too needs to be correctly sized to the application. The motor and drive efficiencies combine to form part of the system efficiency. The system’s efficiency is the product of:
- VFD’s efficiency
- Motor’s efficiency (average motor efficiency when operated on a VFD is reduced by one NEMA band versus sine wave operation)
- Driven equipment’s efficiency (for example, pumps and fans)
When VFDs are used, steps must be taken to protect the motor and ensure efficient system operation. These steps are discussed in this section.
VFDs are used to run motors at speeds other than their nameplated speed. When a motor is subjected to a constant torque load, slower speeds may require an additional cooling method to avoid degrading the motor’s electrical insulation. However, if the load is a variable torque load, motor cooling is usually a non-issue when running at slower speeds.
Another problem that typically arises is the resonant frequency of the system. If a motor is run at the system’s resonant frequency, many components of the system can be damaged because of excessive vibration. However, once this frequency is known, the VFD can be programmed to skip the resonant area during operation.
The switching frequency of a drive is another area for concern. When the frequency is too fast, voltage overshoot will occur. This overshoot is a result of the capacitance of the motor charging and discharging at a rate that is different from the drive. This large voltage spike experienced by the motor in a short time period can cause an uneven distribution of voltage in the windings. When a motor is repeatedly exposed to overshoot, it is normally seen in the first few turns of the windings after failure.
The cables between the VFD and motor should also be considered. When a mismatch in impedances happens, reflections occur in the lines that add with the nominal voltage signal sent to the motor. This phenomenon can be intensified by the switching frequency of the drive.
Several downsides exist when starting an induction motor on full line voltage. Large magnetic fields are created that lead to movement in the motor’s windings while large inrush currents cause rapid heating. The movement of the windings can eventually lead to abraded insulation and shorted conductors. The rapid heating becomes an issue because the windings expand faster than the stator core, leading to eventual insulation damage. VFDs reduce this extreme mechanical stress by ramping up the frequency and voltage to bring the motor to operating speed, which lowers the motor’s inrush current.