Overload relays are required in a number of applications to prevent damaging conditions from arising. Overloads are mounted directly under the contactor in most circumstances.
There are basically three types of overloads:
- Traditional thermal overloads are either bimetallic or melting alloy. These traditional overloads were originally built using bimetallic strips that actually warped when heated to break contact and trip the devices. This created problems when ambient temperatures fluctuated greatly. Melting alloy overloads were added to overcome this temperature problem until ambient compensated bimetallic overloads were introduced. These overload devices are still slow and inaccurate and to not protect the motor against single phase or unbalance conditions.
- Solid state overloads combined electronic componentry to measure temperature rise in a conductor, thereby adding reliability to the equation. The dimension of adjustability was added so that trip points were precalculated and the user simply sets the adjustment at the FLA level of the motor. The limiting factor is that set points are still extreme. Settings are precalculated at the manufacturing level and attempts are made to generically protect yet still avoid nuisance tripping. The device is still not designed to protect against single phase or unbalance conditions. The question arises as to price vs. protection.
- Electronic overloads are the newest form of overloads. They replace thermal based protection with electrical measurements. Electronic overloads can be simple and inexpensive or quite sophisticated, premium priced devices. SymCom overloads combine the sophistication with affordability. The difference with SymCom overloads is the fact that they are programmable, have a digital readout, and allow the user to accurately set trip levels, trip classes, and time delays. In the case of Symcom Model 777 series, additional features include voltage protection, ground fault protection, and communication interface.
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