Crouzet Solid State Relay Knowledge Base

Motors pose unique problems for solid state relays. The power factor of the motor will determine whether zero-voltage turn-on or random turn-on is preferred. Generally, the rule is that if the power factor is 1.0 to 0.7, zero-voltage is preferred. If the power factor is < 0.5, random must be used. Between 0.7 and 0.5, either type SSR could be used.

The mechanical load placed on the motor will determine the starting torque and duration, which in turn determines the starting current and duration. For large inertial loads, the first several cycles of load current will equal stalled or locked rotor current. Based upon ambient temperature and the heat sink, the solid state relay may be operating at an elevated temperature, making it more susceptible to current induced failures during start up.

External over current protection is advised for motor applications to protect both the motor and the solid state relay. The best choice of an SSR for these types of loads is to select a current rating equal to or greater than the locked rotor load current, and an over current protection device less than the locked rotor current, but more than the nominal full load running current. Careful coordination of the relay and over current protection device is required. This is especially true if the motor is started and stopped regularly and frequently.

Voltage selection for SSRs used to control motors is more important than for resistive loads. Back EMF generated by the wind-down of a rotating motor can be 2x or more of the applied line voltage. This voltage will appear across the solid state relays output and it must therefore be rated to block at least that much voltage.

Commutation of the load may also be an issue. A characteristic of semiconductor relays is that if commutating dv/dt is too great, the relay will re-gate itself and continue to conduct load current despite the absence of an input control signal. It is advisable to incorporate solid state relays with back-to-back SCR's in the output rather than Triacs. This is due to the significantly increased commutating dv/dt specification for an SCR.

Solid State Relay Motor Applications;

Cranes & Hoists

UPS Systems & Generators

Conveyors & Sorting Equipment

Packaging Equipment

Copy Machines

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