When the magnetic field in the stator cuts across the poles of the squirrel-cage rotor, a current is induced in the rotor. This current is out of phase with the applied current, but it's strong enough to cause the rotor to start to turn. The speed of the rotor is determined by the number of poles in the stator and the frequency of the incoming AC voltage. A formula is provided to determine the operating speed of the motor:
Operating speed of motor = F x 120/P
where F is the frequency of the applied voltage, 120 is a magnetic constant, and P is the number of poles. It should also be noted at this time that this formula calculates the speed of the rotating field, and the actual speed of the rotor will be slightly less due to slip. The concept of slip will be explained in later sections.
The full rpm is called synchronous speed. From this formula we calculate that a two-pole motor will operate at 3600 rpm, a four-pole motor will operate at 1800 rpm, a six-pole motor will operate at 1200 rpm, and an eight-pole motor will operate at 900 rpm. These speeds don't include any slip or losses due to loads. From this example, note that the only way an A induction motor can have its speed changed is to change the number of poles it has, or change the frequency of the voltage supplied to it.
When power is first applied, the stator field will draw very high current since the rotor is not turning. This current is called locked-rotor amperage (LRA) and is sometimes referred to as inrush current. When LRA moves through the stator, its magnetic field is strong enough to cause the rotor to begin to rotate. As the rotor starts moving, it will begin to induce current into its laminated coils and build up torque. This causes the rotor to spin faster, until it begins to catch up with the rotating magnetic field.
As the rotor turns faster, it will begin to produce voltage of its own. This voltage is called back EMF or counter EMF. The counter EMF opposes the applied voltage, which has the effect of lowering the difference of potential across the stator coils, The lower potential causes current to become lower when the motor is a full load. The full-load amperage is referred to as FLA and will be as much as six to ten times smaller than the inrush current (LRA). The stator will draw just enough current to keep the rotor spinning.
When the load on the rotor increases, it will begin to slow down slightly. This causes the counter EMF to drop slightly, which makes the difference in potential greater and allows more current to flow. The extra current provides the necessary torque to move the increased load and the rotor’s speed catches up to its rated level. In this way, the squirrel-cage induction motor is allowed automatically to regulate the amount of current it requires to pull a load under varying conditions. The rotor will develop maximum torque when the rotor has reached 70 - 80% of synchronous speed. The motor can make adjustments anywhere along its torque range. If the load becomes too large, the motor shaft will slow to the point of stalling and the motor will overheat from excess current draw. In this case the motor must be wired for increased torque, or a larger-horsepower motor should be used.
