What normally causes motor overload?

Answer 1

Few electric motors have a heat related shutoff, or fuse.

If the motor's bearings overheat - usually through drying out because of no lubrication (grease or oil) - they will eventually seize up so that the motor is overloaded and draws too much current. In that condition the circuit protective device (circuit breaker or fuse) should act to stop supplying the motor with current.

***Original answer***

Many electric motors have an internal thermal shut-off to protect against overheating.

Overloading will cause a motor to overheat and shut off. Check the amp draw and the condition of the bearings.

Answer 2

4 things that come to mind; 1) All mechanical parts wear out eventually. 2) Insulation breaks down over time due to heat exposure. 3) Some motors are intended to be lubricated from time to time. 4) Anything that is powered by electricity will last longer in a cooler environment. Usually from an overload condition. The overload protection equipment has not been set up properly to reflect the full load amps of the motor or not having any overload protection at all. In an overload condition the motor builds up heat. Over time this heat breaks down the wire insulation in the motor. The wiring inside a motor is covered with a varnish insulation which is very thin and becomes brittle with heat. Once the insulation breaks down due to the heat the windings will short open or short circuit. At this point there are two options, get a new motor or get the damaged motor rewound. On smaller motors, the cost of a rewind could be more than that of a new motor.

Answer 3

When an inductive circuit is energized by an alternating emf, the resulting current is changing in value continuously and therefore is continuously inducing an emf of self -induction. Because an induced emf opposes the continuous change in the current flowing, its effect is measured in ohms.

This opposition in an inductive circuit to the flow of alternating current is called inductive reactance and is represented by the symbol XL.

I = V / XL

Where I = effective current, amperes

V = effective voltage across the reactance, volts

XL = inductive reactance, ohms

The value of inductive reactance in any circuit depends on the inductance of the circuit and on the rate at which the current through the circuit is changing. The amount of any induced emf in a circuit depends on how fast the flux that links the circuit is changing.

In the case of self-induction, a counter emf is induced in a circuit because of a change of current and the flux in the circuit itself. This counter emf, according to Lenz's law, opposes any change in current, and its value at any time depends on the rate at which the current and the are changing at the time.

When a motor stalls it slows down the rotation and decreases the number of flux links that are cut by the coils. This will lower the counter emf that is induced into the circuit and lower inductive reactance and thereby increase current towards infinity.

The heating effect of an alternating current (ac) over a period of time depends on the average of the squares of the instantaneous values of current over that same period of time, so use of ac in any motor causes heat to be generated in its windings.

Apart from the very simple ones used inside small electric clocks, most single phase and three-phase motors must have ventilation systems built into them to keep them as cool as possible under normal operating conditions. Such systems range from a simple fan - mounted on the rotor shaft to blow the hot air away through ducts in the motor housing - to much more complicated cooling systems involving coolant fluids and external heat exchangers, similar to the radiators used with vehicle engines.

If the heating effect of current passing through the coils - increasing towards infinity - is not stopped by the overload breaker, the motor will burn up.