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.
For further assistance, call a Galco Sales Representative today at