Maintenance of Electrical Motors

A motor is an apparatus or a machine in which electrical energy is converted to mechanical motive energy. It has two different kinds of coils which are located around its iron cores. One of the coils, or windings, as is the most usual expression concerning a motor, is located in fixed part of the motor, the so-called stator. The electric cable is connected to this winding. The other windings is located on the moving part of the motor, the so-called rotor. When the power supply is switched on and flows through the stator windings, a magnetic field is created which activates the rotor winding and makes the rotor start rotating.

The more electric current that flows through the stator winding, the more power can be taken out from the rotor shaft, or on the contrary, the more the machine, which is driven by the motor, is loaded, the more current will flow through the stator winding and consequently also the motor cable. We know that when a too high electric current flows through the transformer, we will get problems with the heat produced in the coils. The same applies to the motor – if we load the motor so much that excessive current flows through the stator windings, the motor can become so warm that painting, oil and wire insulation may catch fire, “the motor is on fire”. In order to prevent such accidents, each motor is provided with two different safety devices, the usual fuse and the thermal overload relay or overload relay as it is called. Both these safety devices are located in the contactor boards. The fuses are located in the contactor boards, and the overload relay is located below the contactor. On each motor, there is a nameplate with motor data printed on it. The highest current rating that may flow through the stator winding is also stated on this nameplate and it is very important the current rating on which the scale of the overload relay is set is not higher than the rating stated on the motor. If, for instance, 10A is printed on the motor, the scale of the overload relay must also be set at 10A or if the operation – the load on the motor shaft – allows it, a few amperes lower, e.g. 8 or 9A there will then be better safety margin against overheating of the motor. The nameplate of the motor is in addition to data about full load current provided with data about effect in KW (kilowatt) or HP (horsepower) and voltage in V.

A motor is always manufactured in such a way that it can be connected to two voltages by means of a simple reconnection and these are then stated on the nameplate. It reads, e.g. 45 415/240V. Then we also have two different figures concerning the permissible load current and care should be taken that the correct figure is read off. The rule is that the first figure concerning volt refers to the first figure concerning current and thus the second figure concerning volt refers to the second figure concerning current. E.g. 415/240V and 10/15A means for our part, as we have a voltage of 415V, a current rating of 10A. There is only one figure concerning power, e.g. 7.5HP. It is important to keep the motor clean and it must absolutely not be covered by bags or any such things, as the cooling must be as efficient as possible in order to prevent overheating. The motor is cooled by means of a built-in fan blowing air over cooling flanges of the motor. If this air flow is blocked by e.g. a bag, it is of no matter how low the overload relay is set. The motor will be overheated without the thermal relay tripping.

Warning

Remember always to switch off the safety switches as soon as a motor or machine has to be checked, no matter how small a check-up. If an electric operation is to be made in the motor, say disconnecting the motor cable it is not enough to switch off the safety switch. The operator in the control room must also be informed, and a padlock must be attached to secure the safety switch. The key should then be kept by the electrician doing the work.