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 motor and, therefore, only slightly above the
rating of the thermal unit. Therefore, it could take a substantial amount of time for the melting alloy device
in this application to respond to phase loss.
The bi-metal device offers a limited form of phase loss protection by means of a differential tripping
mechanism where the device will trip somewhat faster when an overload is detected on only two of the
phases. This device contrasts with a solid state overload relay with phase loss protection that would trip
in less than three seconds and alert the user of a potential distribution system problem in advance of
motor failure. Consequently, the problem does not have an opportunity to affect other equipment on the
system.
The solid state device also provides phase unbalance protection where the device will trip if the current
on any phase is 25% greater than the average of all three phases. Phase unbalances are typically
caused by an unbalanced up-stream single phase load that can disturb phase voltages. Such a
condition can similarly lead to excessive rotor currents and motor damage.
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