Solid State Overload Relay
There are some substantial advantages to using the solid state electronic
device. The device does not require thermal units. The customer does not
have to exercise skill at the selection procedure nor is there a need to stock
Product Description
thermal units for repair or replacement. Not having to install thermal units can save from 20-30% of the
installation time for a starter or separate overload relay, as compared to the traditional NEMA devices.
The solid state device, when operated within its operating temperature range, does not require ambient
compensation. Only the level of current being drawn by the motor affects the trip of the device.
Solid state devices are typically available as part of a starter or as a separate component. This adds to
the flexibility of their application and mounting. Some solid state devices are designed to retrofit melting
alloy or bi-metal devices from the same manufacturer. This flexibility provides the user a migration path
to the new technology. Product selection and application are not dramatically different from the
traditional melting alloy or bi-metal devices. The mounting and “look” are also similar to the traditional
devices. Backward compatibility can also be useful if the decision is made to standardize on the new
technology and the user wishes to upgrade the existing installed base.
The most important feature offered by a solid state overload relay is phase loss protection. While a
phase loss causes a significant current increase in the remaining phases of the motor circuit, there is a
major increase in rotor current that can cause motor damage.
The time it takes for a melting alloy device to trip is determined only by the level of current in the
remaining phases. The majority of the motors installed (world-wide) are run at about 70% of their full
load capability. In these situations, the phase loss condition may result in a level of current in the
remaining phases just slightly above the actual FLA of the
