Motor Considerations with AC Drives

By Leeson.

One drawback to pulse width modulated drives is their tendency to produce voltage spikes, which in some instances can damage the insulation systems used in electric motors. Pulse Width Modulation, or PWM, is a technique for getting analog results with digital means. This tendency is increased in applications with long cable distances (more than 50 feet) between the motor and drive and with higher-voltage drives. In the worst cases, the spikes can literally “poke a hole” into the insulation, particularly that used in the motor’s windings.

To guard against insulation damage, some manufacturers now offer inverter-duty motors having special insulation systems that resist voltage spike damage. An inverter-duty motor is a much newer concept that became necessary as motors began to be driven by VFDs (inverters or AC drives). An inverter duty motor can withstand the higher voltage spikes produced by all VFDs (amplified at longer cable lengths) and can run at very slow speeds without overheating.

For example, LEESON’s system, used in all three-phase motors 1 HP and larger, is called IRIS™ (Inverter Rated Insulation System).

Leeson IRIS™ – Inverter Rated Insulation System

Particularly with larger drives, it may be advisable to install line reactors between the motor and drive to choke off the voltage spikes. A line reactor is an optional component that can be added to a drive system to protect the VFD and other devices from power surges and transients. A line reactor is essentially an inductor – a coil of wire that forms a magnetic field as current flows through it.

In addition, some increased motor heating will inevitably occur because of the inverter’s “synthesized” AC wave form. Insulation systems on industrial motors built in recent years, and especially inverter-duty motors, can tolerate this except in the most extreme instances. A greater cooling concern involves operating for an extended time at low motor RPM, which reduces the flow of cooling air and especially in constant torque applications where the motor is heavily loaded even at low speeds. Here, secondary cooling such as a special blower may be required.

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