There are various technologies available that aid in correcting these issues. This article focuses on applying surge protective devices (SPDs) to a drive system to mitigate the damage that can occur due to the voltage surges while considering the effects of the harmonics on the surge protective device.

ProtectionOfDrives

Application of SPDs
To aid in the description of the application of SPDs to a drive system, please refer to Figure 1.

This figure illustrates a typical drive layout. The incoming power is usually delta configured (3 phases and ground). Often the incoming voltage is 480 V, but other voltages may be used. The incoming power is usually stepped down to a lower voltage (typically 120 Vac) that provides power to the control circuit. The control circuit contains sensitive electronics. Once the power is acted upon by the drive the output is fed to the motor.

As noted, there are four opportunities for protecting the typical drive system – each are labeled with a circled number and are described below.

(1) Drive Input*. Protecting the drive input is an essential step in protecting the drive system. Providing protection at this location prevents surge damage due to events propagated on the electrical system from upstream sources, external events such as lightning and switching surges created by the utility, and the interaction of multiple drives on the same system.

(2) Control Circuit. The control circuit contains sensitive electronics that can be damaged by the environment created  by the drive or by surges from external sources. Protection at this location is essential. Since this circuit is isolated by a step down transformer and it feeds sensitive electronics, a series connected SPD with Frequency Responsive Circuitry is recommended for this location.

(3) Drive Output*. Protecting the immediate drive output is recommended when the length (labeled “L” in Figure 1) of the connection between the drive and the motor is longer than 50 ft (15 m) or if the connection is routed along an external wall or outdoors.

One reason for protecting at the immediate output when the length of the connection to the motor is long is due to reflected waves that can occur as the signal (often higher frequency) from the output of the drive reaches the motor and is then reflect back and forth between the drive and the motor. This action can create “voltage piling” – the reflected voltage adds to the nominal voltage and other reflected waves. The SPD will aid in reducing the voltage peaks of the reflected waves.

More importantly, if the connection between the drive and the motor extends outdoors, along a path that is exposed to the environment or close to the building’s steel structure, protection at this location is important to diminish the effects of direct lightning or induced voltage surges due to nearby lightning. These surges can cause damage to the drive, even if protection is provided at the motor input.

(4) Motor Input*. Protecting the motor input is an essential step in protecting the drive system. Providing protection at this location prevents surge damage due to events propagated from the drive output to the motor input. Providing protection at this location aids in extending the life of the motor as the SPD helps to prevent damage to the windings and bearings of the motor due to surges. Further, if the connection between the drive and the motor extends outdoors, along a path that is exposed to the environment or close to the building’s steel structure, protection at this location is important to diminish the effects of direct lightning or induced voltage surges due to nearby lightning. These surges can cause damage to the motor, even if protection is provided at the drive output.

* At this location, a parallel connected, an SPD with only Voltage Responsive Circuitry is appropriate. Voltage Responsive Circuitry is recommended here due to the potential for high harmonic content of the signal due to the normal operation of the drive and/or to avoid the possible interaction with capacitive devices such as harmonic mitigation circuits and power factor correction devices (that can lead to failure of an SPD that includes Frequency Responsive Circuitry) and possibly disruption of the operation of the drive. Utilizing an SPD with only Voltage Responsive Circuitry at this application point will eliminate this possibility.
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