As mentioned in our previous article on rotating magnetic fields of AC motors, this article will review how a magnetic field actually creates torque and rotates the load. If you’re new to this series, you may want to start at our article on the construction of AC motors. Otherwise, we’ll jump right into rotor rotation.
PERMANENT MAGNET
To illustrate how a rotor operates, imagine mounting a magnet to a shaft as a replacement for the squirrel cage rotor. As detailed in our last article, when energy passes through the stator windings, a rotating magnetic field is formed. The rotating magnetic field formed by the stator windings will then interact with the separate magnetic field produced by the shaft-mounted magnet. This interaction between magnetic fields follows the fundamentals of motor magnetism and polarity.
For example, the south pole of the magnet is attracted to the north pole of the rotating magnetic field. Likewise, the north pole of the magnet is attracted to the south pole of the rotating magnetic field. As a result, the magnet is able to rotate as it gets pulled along by the rotating magnetic field. Used on some motors, this design is known as a permanent magnet synchronous motor.
INDUCED VOLTAGE ELECTROMAGNET
Now let’s bring back the squirrel cage rotor in place of the shaft-mounted magnet. They basically behave in the same way. If electricity is applied to the stator, the current will flow through the winding and expand the electromagnetic field. This expanded field will cut across the rotor bars.
A voltage (or electromotive force [emf]) is induced when a rotor bar, or another type of conductor, enters a magnetic field. In the rotor bar, the induced voltage creates a current flow. The current flows through the rotor bars and around the end ring. As the current flows, more magnetic fields are produced around each rotor bar.
In an AC circuit, current flow regularly changes in direction and magnitude. That’s why current flow also creates a regular change in the magnetic field polarity of the rotor and stator. As a result, an electromagnet with alternating north and south poles is formed by the squirrel cage rotor.
The figure below represents a moment in time when current flow through Winding A1 creates a north pole. The increasing magnetic field spreads across a neighboring rotor bar, which induces a voltage. As a result, a south pole magnetic field is created in the rotor tooth. The rotor then follows the rotating magnetic field of the stator.
SLIP
As the rotor follows the rotating magnetic field of the stator, there needs to be a distinction in speed. The reason for this is because if both rotated at the same speed they wouldn’t share relative motion. Without relative motion, no lines of flux would be cut, nor would the rotor receive induced voltage. The distinction in speed is known as “slip.” SLIP IS REQUIRED TO CREATE TORQUE. The load amount determines slip. If load amount is increased, slip will increase or slow down the rotor. If load is decreased, slip will decrease or speed up the rotor. Slip is shown as a percentage and is calculated by the formula below.
As an example, imagine that a 60 Hz four-pole motor features a synchronous speed (NS) of 1800 RPM. Suppose that the rotor speed (at full load) is 1765 RPM (NR). If you follow the formula, the slip equates to 1.9%.
WOUND ROTOR MOTOR
Now let’s move away from the more common squirrel cage rotor to examine a wound rotor. One way a wound rotor differs from a squirrel cage rotor is that it consists of coils instead of bars. These coils are connected to external variable resistors through brushes and slip rings. Voltage is induced in the rotor windings by the rotating magnetic field. Motor speed can be manipulated by increasing or decreasing rotor winding resistance:
- Motor speed can be decreased by increasing the resistance of the rotor windings, which causes less current flow.
- Motor speed can be increased by decreasing the resistance of the rotor windings, which allows for more current flow.
SYNCHRONOUS MOTOR
A third type of AC motor is the synchronous motor, which is not an induction motor. One type is built similar to a squirrel cage rotor; however, it features coil windings AND rotor bars. Brushes and slip rings connect coil windings to an external DC power supply. Once an alternating current is applied to the stator, the synchronous motor starts up much like a squirrel cage rotor. After the motor attains maximum speed, DC is applied to the rotor coils. This creates a strong and ongoing magnetic field in the rotor that matches the rotating magnetic field. As a result, the rotor rotates at the same speed as the rotating magnetic field (or synchronous speed). Therefore, there is no slip. Different types of synchronous motors feature a permanent magnet rotor. In this case, an external DC source isn’t necessary because the rotor is a permanent magnet. These types can be found on small horsepower synchronous motors.
LEARN MORE ABOUT AC MOTORS
We hope this guide on the rotor rotation of AC motors has helped you to better understand part of how electric motors operate. If you want to learn more, check out our other resources on AC motor terminology and how to read electric motor nameplates.