The principle of pulse-width modulation (PWM), as applied to variable-speed drives (vsd's), was discussed here last month. In today's drives, insulated gate bipolar transistors (IGBTs) are used to produce high-speed pulse switching to simulate the voltage sine wave to the motor.

This switching rate is referred to as the carrier frequency. In today's drives, these frequencies can be in the range of 4 to 20 kilohertz (kHz). For instance, at 4 kHz, the transistors switch on and off 4,000 times per sec (Figure 1).

In applying these high-frequency switching devices in drive technology, it has been found they can produce voltage spikes downstream from the vsd, transmitting them to the motor. The amplitude of the voltage spikes and corresponding stress on motor insulation is proportional to the distance from the vsd to the motor.

Traditional motor insulation systems have been unable to withstand these spikes. In recent years it has been common to hear of manufacturers rating their motors for inverter duty. We'll cover several aspects of this classification here.

Motors rated for inverter duty typically have higher-voltage insulation systems and should be used when applying vsd's. Most vsd manufacturers also specify the maximum cable distance from the drive to the motor, but generally, the shorter the distance, the better. If long motor lead lengths are required, line reactors and terminal filters can be applied on the vsd output to mitigate voltage spikes.

Figure 1: Schematic representation of pulse-width modulation.

More Possible Problems

Because of the high-output switching frequencies, vsd's can be a source of radio frequency interference (RFI). This may cause problems with communications circuits near the drive or its associated power cables. The two types of RFI include radiated and conducted.

Conducted RFI results from the transmission of high-frequency "noise" back onto the vsd power supply cables, output cables, and the motor itself. A properly designed vsd includes RFI suppression circuitry, which prevents RFI noise from being conducted back to the power lines. This is an important consideration when specifying a vsd.

Radiated RFI field intensity drops off rapidly with increasing distance from the source. Interference from motor power cables with nearby control and communication circuits can be minimized by enclosing both in grounded steel conduit, and by maintaining a distance of at least 12 in. between them. Cables from the vsd to the motor can also interfere with each other, so the cable for each drive and motor should be installed in dedicated conduits.

Another potential concern, especially in larger motor sizes, is premature bearing failure. In some cases, it has been observed that a voltage differential can develop between the rotor and stator.

When the voltage differential is large enough to break down the electrical resistance of the grease film in the bearings, current will flow, causing bearing damage. Some of the solutions to this problem have included the use of insulated bearings or conductive brushes to ground the rotor shaft.

Figure 2: Example of constant torque vs. variable torque.

Starting And Stopping

The torque that is required for starting and accelerating a load is an important consideration, as well as the amount of time it takes to get to full-load speed. In fan and pump applications, accelerating torque is most dependent on inertia. Large fans, for which the combination of diameter and mass could produce relatively high inertia, require more accelerating torque.

For any load with high breakaway or accelerating torque, the motor and drive combination must provide enough torque to begin rotating the load and bring it to the proper speed within an acceptable amount of time.

Vsd manufacturers often distinguish between two types of loads, constant torque and variable torque. (See Figure 2 for a graphical representation of load types.)

The variable-torque category consists of fans and centrifugal pumps. Constant-torque applications include the majority of the remaining applications. Most manufacturers apply a dual rating to their drives to take the type of load into account. For instance, a drive rated for a 10-hp variable-torque load may only be rated 7.5 hp for a constant-torque load.

The constant-torque rating enables the drive to provide the higher levels of current required in accelerating and operating a motor at various speeds in constant-torque applications. While the variable-torque drive rating is intended for use with fans and pumps, the effect of inertia may make it necessary to consider constant-torque drives for some variable-torque applications.

It is possible that the drive can also be used for braking if, based on system operation requirements, the load must be stopped more quickly than would result from coasting. When voltage is removed from the motor input terminals, it acts like a generator, returning energy to the source. This is called regeneration. The drive can use this energy to decelerate the load, returning it to the line. It must be specifically built with this feature.

Another deceleration method that's commonly employed takes the regenerated energy and connects it to a resistor bank within the drive. This simply converts the energy into heat.

Motor Protection

It's important to remember that motor size should be based on the horsepower requirement of the load to be driven at full speed. The vsd should, in turn, be matched to that motor. For the motor circuit design, it is important to consider that the drive full-load current rating may be slightly higher than the motor full-load current due to the drive efficiency.

A vsd must, and generally does, include the protective functions that would otherwise be provided by an "across-the-line" motor starter. These keep the motor from overheating due to excessive current flow. As outlined in the first article of this series, these protection parameters include short-circuit and overload protection.

Short-circuit protection is usually provided by the upstream circuit breaker, or fusing within the starter. Devices called overload relays provide the overload protection, and are generally sized to open at not more than 125% of motor's full-load ampere (FLA) rating.

Traditionally, overload relays consist of a pair of bi-metallic elements that open when they reach a certain temperature, similar to the operating principle of a traditional thermostat. The vsd provides these functions as features of the drive control.

Airflow through the motor also helps keep the motor within proper operating temperature. Recall that a cooling fan is generally mounted on the rotor shaft to circulate cooling air in the motor housing. At lower speeds, this fan may not produce enough airflow to cool the motor. The motor manufacturer should be consulted in specific applications where the anticipated operating range is well below the rated speed of the motor.

Manufacturers may include the motor speed operating range among the criteria for rating them as inverter-duty motors. For instance, a motor with a 10:1 variable-torque speed range means that it is rated to operate on a 6- to 60-Hz source. It's also important to compare the drive output current capability with the FLA rating of the motor to avoid a situation where the motor FLA could exceed the drive's continuous-current rating. The motor FLA is available from the motor manufacturer, and is required by NEMA to be engraved on the motor nameplate.


The problems that could be encountered in employing vsd's should not discourage their use in applications where they are desirable. Drive technology has become quite reliable, and properly installed motor-drive combinations are reliable as well.

However, technical representatives of both vsd and motor manufacturers should be consulted for proper application of their products in specific situations. ES