An overload in a motor circuit can take several forms. For example, an overcurrent fault due to a short circuit or earth fault, or motor overload. It raises the question “Does my motor need an overload relay and VSD?” Each fault type needs different handling, and the type of protection depends on the needs of the installation. Another consideration may be emergency fans or pumps where the motor must not stop, regardless of what is happening.

The primary function of a variable speed drive (VSD) is to control motor speed, not to act as a circuit protection device. But most drives can detect motor overloads and limit the output conditions to protect the motor. It assumes motor drive installations and commissioning are to the manufacturers’ instructions. Additionally, most drives will accept an input from a thermistor in the motor windings. The drive may also output a pre-alarm for signalling an overload and avoid unnecessary tripping. Importantly, all VSDs are different, and if in doubt, RTFM (read the flipping manual).

Overcurrent Protective Device

According to Schneider Electric, using upstream circuit breakers and fuses meets the electrical installation standards for overcurrent protection. Circuit breakers can interrupt the high current that can flow in the event of a short circuit. Likewise, fuses have a similar capability. Although a VSD can detect a short on the output IGBTs and disconnect it fast, this provides no protection upstream of the IGBT output section. Thus, it is not suitable for use in meeting the requirements for an overcurrent protection device. Over-current/short circuit protection should be provided by an upstream magnetic only circuit breaker of fuses.

Earth faults

According to Gambica, if an earth fault occurs in the motor circuit of a VSD, then the VSD overcurrent protection will remove its output within a short time, less than 1 ms. All VSDs include functions to protect them from damage caused by short circuits. The VSD’s internal protection comes from its powerful semiconductor and complex circuitry.

Overload relay and VSD?

Extra motor overloads are not needed, in addition to the overload protection provided by the VSD. Overcurrent protection needs an upstream breaker or fuses with the correct drive installation and commissioning. The exception being multiple motors controlled by a single inverter.

Multiple motors

In multi-motor applications where one VSD is controlling several motors, each motor should have an overload device that both removes the supply to the motor, but also powers down the drive. Users should avoid disconnecting the load side of a running drive as it can cause other problems.