Thermal overload relays are protecting devices. they’re designed to chop power if the motor attracts an excessive amount of current for an extended amount of your time. To accomplish this, thermal overload relays contain a unremarkably closed (NC) relay. once excessive current flows through the motor circuit, the relay opens because of augmented motor temperature, relay temperature, or perceived overload current, betting on the relay type.
Thermal overload relays are the same as circuit breakers in construction and use, however most circuit breakers dissent there in interrupt the circuit if overload happens even for an immediate. Thermal overload relays are manage the design to live a electrical motor’s heating thus overload should occur for an extended size before the circuit is interrupted.
In spite of being comparatively easy and cheap, thermal overload relays are terribly effective in providing motor running over current protection. This can be attainable as a result of the foremost vulnerable a part of most motors is that the winding insulation and this insulation is extremely liable to injury by overly heat.
Square D manufactures 3 kinds of overload relays, the melting alloy, the bimetallic, and solid state. In some varieties, the bimetallic is obtainable in each non-compensated and close temperature-compensated versions. In each melting alloy and bimetallic, single component and 3 component overloads are on the market.
Bimetallic Thermal Relay
Bimetallic thermal relays include 2 strips of various metals. These dissimilar metals are for good joined. Heating the electrical device causes it to bend as a result of the metals expand and contract at different rates. The electrical device conjointly applies tension to a spring on a contact. If heat begins to rise, the strip bends and therefore the spring pulls the contacts apart, breaking the circuit.
Eutectic Alloy Thermal Relay
Eutectic (melting alloy) thermal relays include a heater coil, a mixture alloy and a mechanical mechanism that activates a tripping device once an overload happens. To live the temperature of the motor, the relay monitors the quantity of current being drawn. this can be done indirectly, through a heater coil.
Solid State Overload Relay
Solid-state overload relays don’t have moving, mechanical components. Instead, they calculate the typical temperature at intervals the motor by observance its beginning and running currents. Solid-state overload relays are a kind of over current relay.
Temperature Control (Probe) Thermal Relay
These relays shield the motor by using semiconductor unit or resistance temperature detector (RTD) probes to directly sense the windings’ temperature. The motor should have one or a lot of positive temperature constant (PTC) semiconductor unit probes embedded in its windings. once the probe’s nominal operational temperature is reached, the resistance will increase speedily. This increase is detected by a threshold circuit that controls a collection of relay contacts.
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