AutoQuiz: What is “Slip” in an AC Induction Motor? - ISA Interchange

Slip is usually expressed as a percentage, and varies by motor, from nominally 0.5 percent for very large motors to about 5 percent for small, specialized motors.  If ns is the stator electrical speed and nr is the rotor's mechanical speed, the slip, S, is defined by:

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S = (ns - nr) / ns

Motor rotation is developed in an AC induction motor through the effects of a moving magnetic field. As the speed of the rotor drops below the stator speed, or synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the rotor's windings and creating more torque.

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Slip is required to produce torque. Under load, the rotor speed drops, and the slip increases enough to create sufficient additional torque to turn the load. A very efficient way to control slip is to use a variable frequency drive

The correct answer is B, "difference between speed of stator field and rotor speed.”

If you want to learn more, please visit our website 3 Phase Slip Ring Induction Motor Diagram.

Why High Slip is Needed in Oil Well Pump Applications
Design D motors, also called "high slip" motors, are often used to power hoists and cycling loads, such as oil well pump jacks and low-speed punch presses. The most commonly recommended motor for oil well pumping units is a rpm. It has a normal starting current, a high starting torque (275% or more of full-load torque), and a high slip (5-8%). Because Design D specifications are not drawn as closely as they are for other designs, manufacturers have developed several designs with variations in slip that still fall within Design D specifications, i.e. >5%. 
What is Slip? Motors are designed to operate at one fixed speed, which is called synchronous speed. Measured in RPM, synchronous speed is determined by the number of poles and the driving frequency of the motor. Induction motors exhibit a phenomena known as slip where they have speeds below synchronous speed. This difference in speed occurs when the motor begins to deliver torque to the load. Standard NEMA motors - Design A, B or C - are designed to operate between 1-5% slip. (The more efficient the motor, the less that slip occurs).  As percent slip increases for these motors, torque will quickly drop off and the motor will produce less torque per amps as shown in the corresponding chart. Motors that are specifically designed with high slip, usually between 5-13%, fall into the NEMA Design D category. As slip increases for these motors, the torque will continue to increase until we have maximum torque at 100% slip. As a result, NEMA D motors have increasing torque per amps as they move from synchronous rotation to a locked rotor state (i.e. high starting torque). 


Why is slip important for oil well pump jacks and similar applications?  NEMA A, NEMA B, and NEMA C are also "very responsive" with the torque they deliver. What this means is that as soon as the motor begins to change its slip, it delivers high torque. This is good for many applications, but not for pump jacks. Pump jacks can experience mechanical failure, valve ware, fluid inconsistencies, and many other events that can change the load on the motor. Motors that are "very responsive" can provide too much torque and break components like the sucker rod. NEMA D motors on the other hand provide "sloppy torque" where their slip can change without the motor delivering large amounts of torque to the load. This prevents damaging pump mechanical components. Additionally, a NEMA D motor is often used to account for the cyclical nature of the load in beam pump applications. The high slip motor is able to provide torque across the full range of speeds -i.e. "sloppy torque." And as such, it is able to "iron out" the inherent load peaks and valleys that are specific to pump jack applications. 
How high the motor slip should be for pumping installations is debatable. A slip greater than 7 to 8% offers no additional advantages from the overall pumping efficiency standpoint, and which is why the Design D motor with a 5 to 8% slip is the most recommended for sucker-rod installations.