A rotor and a stator are the two key components of an AC induction motor. Currents flow in the rotor bars while a stator contains a rotating magnetic field. When these two elements interact, this produces a torque. There is a lag in the speed of the magnetic field due to the rotor speed. This causes the rotor bars to generate a powerful amount of torque, as the rotor bars cut magnetic lines of force.
Breaking Down Induction Motor Slip
Slip refers to the difference between the shaft rotating speed and the magnetic field’s synchronous speed, which is measured in frequency or RPM. The percent slip is a method that helps calculate an electric motor’s speed performance. A higher load also increases the slip and the amount of torque produced.
A motor slip depends on the parameters linked to the motor performance. Moreover, several factors affect an induction motor slip such as the rotor resistance, voltage frequency of the stator, and the load torque. As the slip increases, this also affects the wounded motor’s induction speed. The rotor circuit needs to obtain maximum resistance to increase the slip. Motors with a lower horsepower also tend to have a much higher slip than those with high horsepower because of the former’s ability to produce much resistance in rotor winding.
Relationship Between Slip and Frequency
When an induction motor slip occurs, the voltage stays at its maximum level. However, the turning of the rotor causes the voltage and the slip to decrease. It is also worth noting that there is a direct proportion to the slip and the frequency. This means that when there is a reduction in a slip, the frequency also decreases.
Both the frequency and the slip also affect the induction motor’s inductive reactance. When the rotor is in its stationary state, this keeps the inductive reactance, slip, and frequency at the highest level. The turning motion of the rotor leads to the reduction of the inductive reactance. Thus, the power factor goes as low as 1. Since the rotor refers to the summation of the changeable inductive reactance and the constant resistance, this leads to inductive reactance changes with the slip.
The rotation of the motor places the inductive reactance at its high points. A further increase in the motor’s speed, however, comes with a different effect. For instance, this results in a reduction in the inductive reactance until it becomes equivalent to the resistance.
Going back to the previous statement on how slip produces torque, it is worth mentioning its effect to induction motors. The slip does not reach zero, and it continues to drive the rotor in electrical induction motors. If the slip goes down as low as zero, the rotor field begins to catch the stator field. Then, the force between the stator and the rotor reaches zero until this comes to the point that the rotor can no longer rotate. This also leads to an attraction between the stator and the rotor, while allowing the slip to become more effective.
How to Reduce Slip
There are ways to minimize slip in induction motors. It is possible to address the issue on slip through reluctance, permanent magnet, and synchronous motors because these motors do not involve a measurable slip.
A synchronous motor is excellent for both low- and very high-power applications. It is not advisable for use in the medium horsepower range that is common with several industrial applications. A reluctance motor is known for a less-than-perfect output and weight ratio. This makes them less competitive and dependable than the squirrel cage induction motor.
Lastly, permanent-magnet motors are common with adjustable speed drives. These are known to providing accurate speed control without having to use gearboxes by enabling you to select a very low base speed. Precise speed control is also made possible without slip. The only limitation of this type of motor is the absence of standardization and the steep price point.
If you want to minimize slip, you may also consider choosing an oversized AC induction motor. Since there is a lower slip value with large motors, it is probable to think of it as a viable solution with slips. The only drawback that may arise with this solution is the mere fact that your operation costs and initial investment will increase because of the increased energy consumption with a larger motor.
Hence, it is best to weigh your options when attempting to reduce the slip. Take into account your industry’s budget and the pros and cons of each solution for minimizing induction motor slip. These steps help you can make a more informed decision.
(Editor’s Note: This post was originally published in March 2013 and has been completely revamped and updated for accuracy and comprehensiveness.)