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THEORY
In very basic terms, the reason that the shaft of any motor rotates (when the motor is switched on) can be explained by one law of electromagnetic theory:

Fleming's left hand rule
This states that if an electric current is passed through a conductor which is positioned perpendicular to the direction of a magnetic field, then that conductor will experience a mechanical force tending to cause it to move in a direction that is perpendicular to both the current flowing through the conductor AND the direction of the magnetic field. The relative directions of these three vectors can be deduced by holding one's left hand with the thumb, first finger and second finger all perpendicular to each other.

then: If CURRENT flows along the SECOND finger and the FIELD flows along the FIRST finger then the resultant MOTION will be in the direction of the THUMB.

In a typical ac electric motor, a rotating magnetic field is set up by the current flowing through windings in the stator. This current also causes an "induced" current to flow through the bars in the rotor (hence the term "induction" motor). The resultant force causes the rotor to rotate as it continually "chases" the rotating magnetic field and, since the rotor is firmly fixed to the shaft, the shaft also rotates.

The basic construction of ac induction motors has changed very little over the years and we will now discuss some basic items.

THE STATOR WINDING
The windings in a motor are there to provide a path for the ac current to flow which in turn produces the magnetic field which will cause the rotor to rotate. The windings are insulated copper wire and inserted into slots in the stator laminations. These slots have insulation between the windings and the steel laminations. This is known as the "stator pack". The windings are designed to provide the output and speed required. The stator pack is, in turn, inserted into the motor casing known as the "stator frame". The ends of the winding are brought out through the motor casing to terminals in a terminal box mounted on the frame. This is where the mains leads are connected.

THE ROTOR
This consists of laminations, shaft, bearings and a "winding". The type of "winding" will depend on the type of motor required.

If the rotor has a winding similar to that of the stator it is known as a "wound rotor motor". The wound rotor motor also is provided with copper or brass rings on the shaft and brushes. These transfer the current generated in the rotor to external resistance banks used to bring the motor up to speed or control the speed. When the motor is up to the full load speed the slip rings are shorted together to enable the motor to run continuously at the full load speed.

If the "winding" consists of solid bars that are joined either end by a shorting ring, it is known as a "squirrel cage rotor" motor. This is because the cage of the rotor resembles the cage that squirrels use to play with when in captivity. The bars are generally aluminium but can be copper or any such material. The squirrel cage rotor motor is the most common type in use today as it requires simple control gear and, in most cases, can be used instead of a wound rotor motor.

The bearings are used to support the shaft and to enable it to rotate.

POLARITY AND SPEED
You will remember the earlier reference to the rotating magnetic field and how the rotor "chases" it. In theory, if there were just the one magnetic 'pole', the rotor would rotate at a rate equal to twice the frequency of the supply, that is to say, for a 50 hertz ( or 50 cycles per second) supply the rotational speed would be 100 revolutions per second, or 6000 rpm.

In practice it is not possible to create one magnetic pole without at the same time creating an equal and opposite pole, so the highest achievable speed for an ac induction motor using a 50 HZ supply is 3000 rpm.

It is possible to arrange the stator windings in such formations as to provide any number of PAIRS of poles and so we can offer 2,4,6,8,10,12 pole motors etc. Motors over 12 pole are available if required but are not in common use.

Poles and Synchronous Speed
The number of poles are determined by the number of magnet poles. They should be an even number.

 

Remember that as the number of poles increase, so the speed decreases.

We call the hypothetical speed "synchronous" speed because it is the speed that would be obtained if the rotor rotated in "synchrony" with the magnetic field. In any ac induction motor the synchronous speed is never achievable, since friction losses in the bearings, air resistance within the motor and additional drag imposed by the load combine to cause the rotor to lag slightly behind the rotational speed of the magnetic field. This lagging effect is known as the "slip".

The "synchronous" speed of a motor can be determined by the formula: 

synchronous speed =  120 x f
                                       Poles
where :
Speed is expressed in rpm
f equals frequency in HZ
poles is an even number. (ie 2,4,6 etc)

If the frequency varies, the speed varies in a direct ratio.

The percentage slip varies from one motor to the next (as a general rule of thumb, the larger the motor, the less slip is experienced) and for any given motor the slip will decrease as the load decreases. At no load the slip may be as little as 0.5%, whilst at full load, depending on the size of the motor, it can be high as 5.0%.

Thus typical "Full Load" speeds for , say , 2,4,6 and 8 pole motors on 50 Hertz supply, could be 2950, 1470, 980 and 735 rpm respectively, compared with the synchronous speeds of 3000; 1500; 1000 and 750 rpm.

It is not surprising to find that the "slip" of a motor is closely related to the motor's efficiency and, in fact, the full load speed of a motor is a good guide to the motor's efficiency.

TORQUE
The physical size of a motor is not purely dependent on the kW rating. A 15 kW 6 pole motor, for instance is far larger than a 15 kw 2 pole machine.

If there were a single factor which determined the frame size of a motor it would be the torque. Torque is the rotational equivalent of linear force and for any rotating machine, if the power and speed are known, then the torque is given by the formula:


Torque = kW x 9550 Newton Metres
Poles

When a motor is driving the load at full speed, the torque developed by the motor will always equal the torque required by the load to keep it running at that speed. The more accurate the motor selection, the closer this torque value will approach the rated full load torque (F.L.T.) of the motor.

 

During the starting cycle (or Run Up Time), however , the torque developed by the motor at any given instant must always exceed the torque required by the load at that particular speed, otherwise the load will not continue to accelerate and the motor will stall.

At any given speed during run up, the difference between the motor torque and the load torque is known as the Accelerating Torque and, taken over the complete curve of torque against speed from zero to 100% speed, it is this accelerating torque - together with the load Moment of Inertia - which determines the run up time.

The above curve is typical for a squirrel cage motor.

The initial point is known as the starting torque or locked rotor torque (L.R.T.) , the minium point is known as the Pull Up torque (P.U.T.) and the maximum point known as the Pull Out Torque (P.O.T.)

The above explanation is a very simplified explanation of "How a Motor Works" but we hope it will be of some help to enable a "layman" to understand.

The writer would like to acknowledge assistance from various articles and technical papers in preparation.

 

 

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