ABSTRACT
Bimetallic thermal elements of class 10 or 20 are still in use for the low-voltage motors, although digital relays providing some of the features of protection described for the medium-voltage motors are available. There is a recent study of the motor thermal overload relays and phase monitors to power quality events. Digital low-voltage motor protection relays are available. Like multifunction relays for medium-voltage motor protection, the motor thermal characteristics can be coordinated with the thermal curves provided in the relay. Accurate thermal models for phase unbalance are available. The controllers consist of magnetic contactors for starting and stopping of the motors. The controllers are equipped with thermal overloads and loss of voltage is inherent with contactors when used with integral control supply and three-wire control circuits. In a three-wire control, the contactor is maintained through an auxiliary contact in parallel with the start push button.
