Rotor Rotation

Permanent Magnet

To see how a rotor works, a magnet mounted on a shaft can be substituted for the squirrel cage rotor. When the stator windings are energized, a rotating magnetic field is established. The magnet has its own magnetic field that interacts with the rotating magnetic field of the stator. The north pole of the rotating magnetic field attracts the south pole of the magnet, and the south pole of the rotating magnetic field attracts the north pole of the magnet (Picture 1). As the magnetic field rotates, it pulls the magnet along. AC motors that use a permanent magnet for a rotor are referred to as permanent magnet synchronous motors. The term synchronous means that the rotors rotation is synchronized with the magnetic field, and the rotor’s speed is the same as the motor’s synchronous speed.

Picture 1: The rotating magnetic field of the stator

Induced Voltage Electromagnet

Instead of a permanent magnet rotor, a squirrel cage induction motor induces a current in its rotor, creating an electromagnet. As the following illustration on Picture 2 shows, when current is flowing in a stator winding, the electromagnetic field created cuts across the nearest rotor bars.

Picture 2: Rotor as electromagnet


When a conductor, such as a rotor bar, passes through a magnetic field, a voltage (emf) is induced in the conductor. The induced voltage causes current flow in the conductor. In a squirrel cage rotor, current flows through the rotor bars and around the end ring and produces a magnetic field around each rotor bar. Because the stator windings are connected to an AC source, the current induced in the rotor bars continuously changes and the squirrel cage rotor becomes an electromagnet with alternating north and south poles (Picture 3).

Picture 3: Squirrel cage rotor as electromagnet


The following illustration on Picture 4 shows an instant when winding A1 is a north pole and its field strength is increasing. The expanding field cuts across an adjacent rotor bar, inducing a voltage. The resultant current flow in one rotor bar produces a south pole. This causes the motor to rotate towards the A1 winding. At any given point in time, the magnetic fields for the stator windings are exerting forces of attraction and repulsion against the various rotor bars. This causes the rotor to rotate, but not exactly at the motor’s synchronous speed.

Picture 4: Rotation of the motor

Slip

For a three-phase AC induction motor, the rotating magnetic field must rotate faster than the rotor to induce current in the rotor. When power is first applied to the motor with the rotor stopped, this difference in speed is at its maximum and a large amount of current is induced in the rotor. After the motor has been running long enough to get up to operating speed, the difference between the synchronous speed of the rotating magnetic field and the rotor speed is much smaller. This speed difference is called slip. Slip is necessary to produce torque. Slip is also dependent on load. An increase in load causes the rotor to slow down, increasing slip. A decrease in load causes the rotor to speed up, decreasing slip. Slip is expressed as a percentage and can be calculated using the following formula:

% Slip = (NS - NR)/NS x 100

For example, a four-pole motor operated at 60 Hz has a synchronous speed (NS) of 1800 RPM. If its rotor speed (NR) at full load is 1775 RPM, then its full load slip is 1.4%.

Wound Rotor Motor

The discussion to this point has been centered on the more common squirrel cage rotor. Another type of three-phase induction motor is the wound rotor motor. A major difference between the wound rotor motor and the squirrel cage rotor is that the conductors of the wound rotor consist of wound coils instead of bars. These coils are connected through slip rings and brushes to external variable resistors (Picture 5). The rotating magnetic field induces a voltage in the rotor windings. Increasing the resistance of the rotor windings causes less current to flow in the rotor windings, decreasing rotor speed. Decreasing the resistance causes more current to flow, increasing rotor speed.

Picture 5: Wound rotor motor

Synchronous Motor

Another type of three-phase AC motor is the synchronous motor. The synchronous motor is not an induction motor. One type of synchronous motor is constructed somewhat like a squirrel cage rotor. In addition to rotor bars, coil windings are also used. The coil windings are connected to an external DC power supply by slip rings and brushes (Picture 6). When the motor is started, AC power is applied to the stator, and the synchronous motor starts like a squirrel cage rotor. DC power is applied to the rotor coils after the motor has accelerated. This produces a strong constant magnetic field in the rotor which locks the rotor in step with the rotating magnetic field. The rotor therefore turns at synchronous speed, which is why this is a synchronous motor. As previously mentioned, some synchronous motors use a permanent magnet rotor. This type of motor does not need a DC power source to magnetize the rotor.

Picture 6: Synchronous motor