Electric Motors

If two magnets are placed close together their magnetic fields will interact. They will either attract or repel each other, depending on the direction of their magnetic fields.

Fig. 13 Opposite magnetic poles will attract each other

Fig. 14 Similar magnetic poles will repel each other

If the magnets are free to move then they will try to line up with each other. Consider the case of one magnet fixed in position and one free to pivot about its centre. If the magnetic fields are out of line then the pivoted magnet will rotate until the magnetic fields line up.

Fig. 15 The pivoted magnet will try to rotate

In this example the interaction of magnetic fields caused rotary motion. This is the principle of operation of electric motors. Electric motors convert electrical energy into rotary motion.

Multiplying the voltage applied to a motor by the current it draws gives the electrical input power to the motor in watts (W). Thus a 240V motor which draws 2 amps has an input power of (240 x 2) = 480 W.

P = E x I

If the armature of a motor is coupled to a load then the rotary action of the shaft tends to turn the load. This turning force is called torque. Multiplying torque and rotational speed gives mechanical power in horsepower (HP).

Kw = Nm x rpm

9550

Nm = Kw x 9550

rpm

Not all electrical input power is converted to horsepower. There are electrical losses due to resistance in the windings and mechanical losses due to friction which reduce the horsepower delivered by the shaft.

There is a relationship between motor speed and torque, and this relationship depends on the type of electric motor being used.

The general symbol for a motor is shown below:

Fig. 16 General symbol for a motor