Saving a Motor's Life

There are many ways to kill a motor. There are also measures that can be taken to protect motors from the harm of poor power quality. Harmonics are the main culprits that can damage a motor and are generally produced by inverters within a variable-frequency drive (VFD). The VFD's inverters initially convert—or rectify—incoming AC voltage to DC voltage.

12/01/2001


There are many ways to kill a motor. There are also measures that can be taken to protect motors from the harm of poor power quality.

Harmonics are the main culprits that can damage a motor and are generally produced by inverters within a variable-frequency drive (VFD). The VFD's inverters initially convert—or rectify—incoming AC voltage to DC voltage. Then, the DC voltage is inverted back into an AC waveform of varying magnitude and frequency. These higher-order frequencies cause current or voltage waveform distortions that can ruin a motor.

Along with nuisance tripping of circuit breakers, these harmonics can cause overheating in power cables, motors and transformers. They are amplified by resonance in power-factor correction capacitors—with consequences for an entire electrical system—but the focus here is their harmful effects on motors.

The motors that are hurt most by harmonics are those directly connected to VFDs. In these motors, overheating and reflected voltage pulses can stress motor-winding insulation and lead to failure. Additional problems result when reflected voltage pulses cause overvoltages on the feeder cables between VFD and motor.

Inverters in pulse-width modulated (PWM) VFDs produce an output voltage composed of a series of pulses of the same amplitude but varying widths, which over time average to approximate a sinusoidal waveform (see Figure 1, p. 22).

On the leading edge of these pulses, there is a certain amount of overshoot beyond the desired voltage amplitude, which eventually oscillates or settles downward to the desired level.

The formula dv/dt—slope of the pulse—refers to the change in voltage on the voltage pulse's leading edge divided by the time that it takes to peak. There are variations on this calculation. Sometimes, only the rise from 10% to 90% of voltage pulse and the corresponding time duration are used.

Motors can be damaged if dv/dt is too high, because voltage peaks are unevenly distributed across the windings. Up to 85% of the voltage can appear between the winding's first and second turns.

These pulses typically are reflected back from the motor onto the feeder cable, as the motor's impedance is normally greater than that of the feeder cable—similar to waves of water striking a wall and being reflected back as ripples on the incoming waves.

A portion of the reflected pulses is added to the initial pulses, resulting in overvoltages at the motor. In fact, smaller motors, which have characteristic impedance greater than that of large motors, have a more significant problem here. For a 480-volt motor, this overvoltage could be as high as 1,400 volts.

A motor's insulation may initially withstand short-duration 1,400-volt pulses, but when repeated continuously, the pulses will overstress insulation and cause it to fail. When this occurs, the result is either a fault to ground or a turn-to-turn fault that will severely damage a motor.

Putting Up a Defense

Fortunately, there are ways to shield a VFD-controlled motor from the harmful effects of poor power quality, including the following:

  • Thermal protective devices that detect overheating and shut down motors before the damage is done.

  • Load reactors that reduce harmonics and produce smoother voltage waveforms to the motors.

  • Inverter-duty motors, dv/dt filters and shielded cables that can mitigate the effects of reflected voltage pulses and the overvoltages they cause on motors that are controlled by VFDs.

Three types of devices can detect overheating in VFD-driven motors: thermostats, thermistors and resistance-temperatures detectors (RTDs). However, all of these devices have to be built into a motor, so they can be applied only to new or newly rewound motors.

A motor thermostat is a temperature-activated switch to which the VFD control circuit is wired. If motor temperature exceeds a certain value, then the thermostat opens, turning off both VFD and motor.

The thermistor is a resistive device that changes its resistance in response to temperature changes. Built into the motor winding, it is wired to a control module on the motor's exterior. The module contains a relay that opens a contact if the temperature exceeds a certain value. As with the thermostat, the contact is wired into the VFD's control circuit and shuts the VFD and motor off in response to overheating.

An RTD is essentially the same as a thermistor but is applied differently. It is used with a temperature monitor or motor-protection relay and is embedded in a motor's stator winding, usually one pair in each phase for a total of six in a three-phase motor. Specifications normally call for RTDs to be embedded in a motor winding's anticipated hot spots. A temperature monitor or motor-protection relay is connected to the RTD using shielded wire and indicates a corresponding temperature for the RTD's resistance.

Output from an RTD is a very low-level, continuous signal that is subject to noise interference from external sources. This signal can also be used to display motor temperature through the temperature monitor. The monitor or the motor-protective relay is set to detect a maximum allowable temperature and open or close a contact when a maximum temperature is reached.

This option—RTD with monitor or relay—tends to be the most expensive solution, due to the added expense of embedding RTDs in the motor stator winding and the cost of the monitor or relay. There is also the additional expense of the RTD's six shielded cables.

There are, however, undeniable benefits. Not only does the RTD option offer the ability to monitor motor temperature and to implement a protection scheme that includes alarm and trip settings, but also, the relay can use the RTD to develop a thermal model of the motor and shut it down if thermal capacity is exceeded.

Unlike the RTD, a motor thermostat doesn't require a controller, and the thermistor only needs a simple controller. The only wiring required is a single pair of American wire gauge No. 14 wires from the thermostat, or the contact in the thermistor control module, to the motor starter's control circuit. This wiring can be run in the same conduit as the motor's power wiring, as it is discrete and is not subject to noise interference.

Thermostats or thermistors with control modules are typically used as a standard on low-voltage (600 volt or less) inverter-duty motors. An RTD with a temperature monitor or motor protection relay would be used on low-voltage motors of several hundred horsepower or higher.

Going on the Offensive

There are also ways to more actively battle harmonics. By actually reducing harmonics, a load reactor —installed on the VFD output—provides smoother voltage waveforms. Like transformers, load reactors are rated in percentage of impedance, with typical ratings of 3% and 5% impedance.

A 5% line reactor offers the same benefits as a 3% model—but with greater effect. Which impedance rating should be used? It depends upon the protection needed.

Load reactors help reduce excessive motor temperature caused by harmonics from the VFD. They also help to reduce audible noise caused by the high carrier frequency and harmonic spectrum. But there are drawbacks to load reactors. They must not be oversized, because they affect the overall efficiency of the VFD and motor combination. An oversized load reactor will unnecessarily increase operating costs.

Another active form of protection is the dv/dt filter , which reduces the effects of reflected voltage pulses and the overvoltages they cause on VFD-controlled motors. These filters are tuned reactive-load (RL) filters consisting of a reactor and a resistor placed at the VFD output. These filters attenuate the magnitude of harmful spikes by slowing the voltage rise time. They are typically used to protect an existing motor that is newly connected to a VFD. A disadvantage with dv/dt filters is that they reduce the voltage to the motor by about 1.5%, but in most cases it should not be harmful.

Another option is the use of inverter-duty motors that are designed to withstand the damaging effects of harmonics. If a motor will be purchased new, the inverter-duty type should be considered.

These motors incorporate insulation that is more corona-resistant than standard motor insulation. Corona is the electrostatic discharge associated with a breakdown of insulation from voltage spikes. The corona discharge in an inverter-fed motor occurs when reflected voltage pulse spikes exceed the insulation's dielectric rating. Repeated occurrence of these spikes—and the partial breakdown they cause—eventually leads to full insulation breakdown. The result is an arc to the motor's grounded frame or from one winding turn to another.

Another way that inverter-duty motors can prevent damage from spikes is by employing concentric, instead of random, winding methods. In a concentric-wound motor , turns of magnet wire are installed in the stator's slots in an orderly, sequential manner, dividing the voltage stress from spikes evenly across the winding coil. In a random-wound motor , where winding conductors are randomly laid into the slots, the first turn might be placed in direct contact with the last turn, as shown in Figure 2.

This is a detriment, because the winding's last turn is closest to the next phase in a delta-connected motor or the neutral in a wye-connected motor. The last turn offers a path from the highest potential to the lowest potential for the impulse of spikes breaking through the first turn's insulation. Concentric windings prevent the first and the last turn from being next to each other, making the motor more resistant to insulation breakdown from spikes.

Shielded cables are another measure that can help protect motors and their feeder conductors from the effects of reflected voltage pulses and the resulting voltage spikes. These cables are intended for use on low-voltage systems. They use standard 600-volt insulation, but insulation thickness is much greater than it is for typical cables. Thicker insulation keeps the conductors farther apart, reducing the cable's capacitance and thereby helping mitigate the magnitude of spikes caused by reflected voltage pulses.

In addition, the cable is shielded with an outer layer of conductive copper foil or braid, or a combination of both, and covered by a protective outer PVC jacket. The shield distributes voltage stress equally around the cable insulation's perimeter, guarding against insulation breakdown.

Poor power quality, if left uncorrected, can be very harmful to motors. Of the many options available for protecting motors, one or more may be required to remedy a motor failure problem. But it is important not to overdo it. Some of these applications can affect motor efficiency and increase operating costs. If an option is not really needed, it will be a waste of limited capital.

From Pure Power, Winter 2001.





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