Heat is a major factor in the performance and life that a motor, and also one of the primary sources that motor heater is current running through the engine windings. Due to the fact that heating is one unavoidable problem of engine operation, it’s essential to defend the motor from overheating, or heat overload.
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In a ahead post, we defined several varieties of sensors that deserve to measure the temperature of motor windings directly. But in some cases — an especially for AC induction motors — motor heating have the right to be measured indirectly by thermal overload relays, which identify motor temperature by surveillance the quantity of present being ceded to the motor.
Thermal overload relays space wired in series with the motor, therefore the current flowing to the motor additionally flows through the overload relay. When the present reaches or exceeds a predetermined limit for a specific amount the time, the relay activates a system that opens one or more contacts to interrupt existing flow to the motor. Heat overload relays space rated by their expedition class, which defines the quantity of time for which the overload deserve to occur before the relay responds, or trips. Common trip classes space 5, 10, 20, and also 30 seconds.
Taking time, and current, right into account is necessary for AC induction motors due to the fact that they attract significantly much more than their full rated current (often 600 percent or more) throughout startup. So if the relay tripped automatically when the overload existing was exceeded, the motor would have challenge starting.
There space three types of heat overload relays — bimetallic, eutectic, and electronic.
Bimetallic thermal overload relays (sometimes referred to together heater elements) are do of 2 metals, with various coefficients of heat expansion, that space fastened or bonded together. A winding, wrapped approximately or placed near the bimetallic strip, carries current.
As the present running through the relay (and, therefore, through the motor) heats the bimetallic strip, the 2 metals increase at various rates, causing the piece to bend towards the side v the lower coefficient of thermal expansion. Once the strip bends, that actuates a typically closed (NC) contactor, bring about it to open and stopping the existing flow to the motor. As soon as the bimetallic relay has actually cooled and also the metal strips have actually reverted come their regular state, the circuit is immediately reset and also the motor deserve to be restarted.
Eutectic heat overload relays use a eutectic alloy (a combination of steels that melts and solidifies at a characterized temperature), housed in a tube and also connected come a heater winding. The supply present to the engine flows with the heater winding and heats the alloy. Once the alloy will a enough temperature, that transforms promptly to a liquid.
As a solid, the alloy stop a mechanical device, such as a spring or ratchet, in place. Yet when the alloy melts, the mechanical machine releases, opening the overload contacts. Like the bimetallic design, a eutectic heat overload relay cannot be reset until the alloy has sufficiently cooled and also returned to its original, hard state.
Electronic thermal overload relays measure present electronically, quite than relying top top a heater mechanism, and also so space insensitive to alters in ambient temperature. They’re additionally less prone to “nuisance,” or false, tripping. Digital overload relays can provide data such together the percent of thermal capacity utilization (%TCU), portion of full-load amps (%FLA), time-to trip, existing RMS, and also ground fault present — information that can assist operators command diagnostics and also predict once the relay is at threat of tripping.
Electronic design can likewise protect motors against phase ns (also referred to as phase failure), i m sorry occurs when one step of current equals zero amps, often due to a short circuit or blown fuse. This causes the engine to draw excessive current on the staying two phases and leader to significant motor heating.
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Thermal overload relays space typically part of the motor starter, which contains the overload relay plus contacts. It’s essential to note that thermal overload relays are just designed to safeguard the engine from overheating, and won’t expedition if yes sir a quick circuit, so extr fuses or circuit breakers are necessary to protect the circuit.