Slip in an Induction Motor : Working, Importance and Its Formula

In a 3-Φ Induction motor, the stator of the motor will generate a rotating magnetic field or RMF because of the phase shift with 120 degrees within the 3- Φ supply input. So the RMF revolves with the stator of its own speed which is known as synchronous speed and it is denoted with ‘Ns’. The rotating magnetic field (RMF) converses with the rotor; because the change in flux can induce an emf. So the rotor in the motor begins revolving with a speed which is known as actual speed (N). The main disparity between the synchronous & actual speed is known as the SLIP. The slip value is equal to ‘1’ as the rotor in the motor is at rest & it will not equivalent to ‘0’. So while operating the motor, the synchronous speed is not equivalent to ‘N’ i.e, actual speed in a given time. This article discusses an overview of the slip in an induction motor.

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What is Slip in an Induction Motor?

Definition: In Induction Motor, a slip is a speed among the rotary magnetic flux as well as rotor expressed in terms of for every unit synchronous speed. It can be measured in dimensionless & the value of this motor cannot be zero.

If the revolving magnetic flux’s synchronous speed & the speed of rotor are Ns & Nr in the motor, then the speed among them can be equivalent to (Ns – Nr). So, slip can be determined as

S = (Ns – Nr) / Ns

Here, both the rotor’s speed and synchronous speed are not equivalent (Nr < Ns) and the slip value is constantly < 1. So for this motor, it will be like (0

In this motor, if the power supply given to the 3-phase stator winding is a 3-phase, then a revolving magnetic field can be generated within the air gap so this is known as the synchronous speed. This speed can be determined with the no. of poles as well as the frequency of power supply. Here poles and frequency are denoted with P & S.

Synchronous Speed (N) = 2f/Prps (Here, rps is the revolution for each second).

This magnetic field which rotates will cut the inactive rotor conductors to produce e.m.f. Because the circuit of the rotor will be short-circuited, and the emf which is generated will raise the current supply of the rotor.

The interface between the rotor current & revolving magnetic flux can generate torque. Thus, according to Lenz’s law, the rotor starts to turn in the direction of the rotating magnetic field. As a result, the relative speed is equivalent to (Ns – Nr) and it is arranged among them to give rise to slip within the motor.

Importance of Slip in an Induction Motor

The importance of slip in the induction motor can be discussed below based on the values of a slip because the motor behavior depends mainly on the slip’s value.

When the Value of Slip is ‘0’

If the slip value is ‘0’ then the speed of the rotor is equivalent to revolving magnetic flux. So there is no motion among the coils of the rotor as well as revolving magnetic flux. So, there is no flux cutting act in the rotor coils. Therefore, emf will not be generated within rotor coils for generating rotor current. So this motor will not work. So, it essential to have a positive slip value in this motor and due to this reason, the slip will never become ‘0’ in an induction motor.

When the Value of Slip is ‘1’

If the slip value is ‘1’ then the rotor in the motor will be stationary

When the Value of Slip is ‘-1’

If the slip value is ‘-1’ then the speed of the rotor in the motor is more comparable with the synchronously revolving magnetic flux. So, this is possible only when the rotor within the motor is turned in the revolving magnetic flux direction using the prime mover

This is only possible when the rotor is turned in the direction of revolving magnetic flux by some prime mover. In this condition, the motor operates as an induction generator.

When the Value of Slip is >1

If the slip value of the motor is greater than one then the rotor will turn in the opposite direction to the revolution of magnetic flux. So if the magnetic flux is revolving in the direction of clockwise, then the rotor will turn rotating in the anti-clockwise direction. So, the speed among them will be like (Ns + Nr). In braking or Plugging of this motor, the slip is greater than ‘1’ is attained to rapidly bring the rotor of the motor at rest.

Formula

The formula of the slip in the induction motor is given below.

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Slip = (Ns-Nr/Ns)*100

In the above equation, ‘Ns’ is the synchronous speed in rpm whereas the ‘Nr’ is the rotational speed in the rpm (revolution for each second)

For Example

If the synchronous speed of the motor is and the actual speed is then please find the slip in the motor?

Nr = rpm

Ns = rpm

The speed difference can be calculated as Nr-Ns = - = 50

The formula to find a slip in the motor is (Nr-Ns)*100/Ns = 50*100/ = 3.84%

While designing the induction motor, measuring the slip is essential. For that, the above formula is used to understand how to get the difference as well as the percentage of slip.

The Relation between Torque and Slip-in an Induction Motor

The relation between torque and slip in an induction motor provides a curve with the information regarding the difference of torque using the slip. The deviation of slip is attained with the difference of speed changes & the torque equivalent to that speed will also differ.

The curve is defined in three modes like motoring, generating braking and the characteristics of torque slip are divided into three regions like a low slip, high slip, and medium slip.

Motoring Mode

In this mode, once the supply is given to the stator, then the motor starts turning under the synchronous. The torque of this motor will change when the slip changes from ‘0’ to ‘1’. At no-load condition, it is zero whereas, in load condition, it is one.

From the above curve, we can observe that the torque is directly proportional to the slip. When the slip is more, then the more torque will be generated.

Generating Mode

In this mode, the motor runs higher than the synchronous speed. The stator winding is connected to a 3-Φ supply where it provides electrical energy. In fact, this motor gets mechanical energy because both the torque as well as slip is negative and provides electrical energy. Induction motor works by using reactive power so it is not used as a generator. Because, reactive power must be provided from outside and it works under the synchronous speed, then it uses electrical energy instead of providing at the output. So, generally, induction generators are avoided.

Braking Mode

In this mode, the voltage supply polarity is altered. So the induction motor begins to revolve in the opposite direction so motor stops to rotate. This kind of method is applicable whenever it is necessary to discontinue the motor in a less time period.

When the motor starts rotating, then the load accelerates within a similar direction so the motor’s speed can be increased above synchronous speed. In this mode, it works like an induction generator to provide electrical energy to the mains so that it reduces the motor speed compare with synchronous speed. As a result, the motor stops working. This kind of breaking principle is known as dynamic breaking otherwise regenerative breaking.

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slip = current? :?

For a common 'squirrel cage induction motor', when the rotor is rotating in perfect synchronism with that magnetic field, than it 'sees' a DC field. No current is induced in the rotor, and there is no rotor magnetic field to interact with the stator field to produce torque.

As the rotor slows down, the 'sees' an AC magnetic field. Current is induced in the rotor, which produces a rotor magnetic field, which allows torque production.

'Slip' is just one way of describing the difference between rotor speed and synchronous speed. It goes from 0 (synchronous speed with zero torque production) to 1 (locked rotor). (Actually slip can range from below zero to above 1...below zero means a 'plugging' motor, meaning one spinning against the drive direction. Above 1 means a motor operating as a generator. Normal operating region is very near 1.)

The greater the slip the greater the induced rotor current, at least in the limit of less than 'breakdown' torque. Motor current consumption goes up as rotor speed goes down, but I don't know if rotor current goes up monotonically.

-Jon When I teach my class, I explain slip this way. The stator is a set of electromagnets created by coils of wire in the steel frame. The magnets created in the stator have expanding magnetic fields that then cut the conductors of the rotor bars and induces current to flow in them. The flow of current in the rotor bars, combined with the steel core, becomes another electromagnet. The repulsion interaction of those opposing magnetic fields is what creates torque. I explain slip as the resulting lag in the creation of the electromagnetic field in the rotor, because it has to be CREATED by the stator magnets FIRST.

- EST

Strathead:

I don't understand what you were trying to say.



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Gar, I don't profess to be an expert on motors, but I do remember from my theory training a synchronous motor is a specific kind of motor. So I took to Google and this came up right away. http://www.electrical4u.com/synchronous-motor-working-principle/

In the first paragraph he says, "Among 3 phase induction motors and synchronous motors are more widely used"

He goes on to say that a synchronous motor is a motor designed such that the rotating magnetic field and the rotor rotate at the same speed and it is a fixed speed motor. I vaguely remember that it can't run slower or faster than the field, or it just fails to have any torque, so I felt it important to point out what I did originally. We are not talking about synchronous motors here.

- EST

Strathead:

The article you referenced is really not very clear on a number of points. It appears to be written in English by a translator or a person that grew up in a non-English speaking country.

But separately in your post you said This made no sense. Why would you imply that a synchronous motor does not run at synchronous speed?

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No, I implied that a synchronous motor is a specific kind of motor and when we are talking about the slip of a motor we are not talking about synchronous motors, because in fact, they actually do run at synchronous speed. Shortly before post another post used the term synchronous motor to reference an induction motor running at a theoretical synchronous speed. I was pointing out they are not the same thing. The best (but not great) analogy I can come up with is if we threw in a shunt motor in to the discussion. It isn't germane to the discussion just like a synchronous motor isn't. Or another way to put it, synchronous motor, shunt motor, wire wound motor, and permanent core motors refer to different styles, designs or types of motors that have different construction and function characteristics.

Does that make sense to you?

I only referenced that article because it was the first one on Google that referred to synchronous motors and induction motors in the same early paragraph. Are you denying that a synchronous motor is not the same thing as an induction motor?

No, I implied that a synchronous motor is a specific kind of motor and when we are talking about the slip of a motor we are not talking about synchronous motors, because in fact, they actually do run at synchronous speed. Shortly before post another post used the term synchronous motor to reference an induction motor running at a theoretical synchronous speed. I was pointing out they are not the same thing. The best (but not great) analogy I can come up with is if we threw in a shunt motor in to the discussion. It isn't germane to the discussion just like a synchronous motor isn't. Or another way to put it, synchronous motor, shunt motor, wire wound motor, and permanent core motors refer to different styles, designs or types of motors that have different construction and function characteristics.

Does that make sense to you?

I only referenced that article because it was the first one on Google that referred to synchronous motors and induction motors in the same early paragraph. Are you denying that a synchronous motor is not the same thing as an induction motor?

A pure induction motor has no electrical connection between the stator and rotor and only one of them is connected to an outside source. One or the other (stator/rotor) has it's current induced into it by the magnetic field produced by the other.

- EST

Strathead:

A synchronous motor is not an induction motor, but both have the same kind of stator. Their rotors are what make a big difference in the characteristics and performance of the motors.

A synchronous motor runs at synchronous speed with or without friction.

A synchronous motor always runs at synchronous speed (except that instantaneous speed can slightly fluctuate) until torque load is great enough to cause breakaway.

An induction motor can never run at synchronous speed because of friction and losses.

.

I sort of know that. Refer to post #7 and the answer to post #7 which is post #9. These two are what prompted me to make the statement in the first place. I have reread my post and it still doesn't confuse me. And it still reads as clarification.

For future reference I wrote, "So a synchronous motor is different than a motor running at synchronous speed." What part of that is confusing and how else should it be said. Another analogy, WE could be talking about the candy "Orange slices" if someone came in in the middle and thought is was slices of an orange clarification would be called for. I don't believe the OP understood the difference.

You have this minor problem:

I don't have a minor problem and this is very frustrating. Read the thread. It was directly discussing the theoretical issue of an induction motor running at synchronous speed. That is exactly why I felt the need to make my statement in the first place. I am not heavy on motor theory, but I know just enough to understand when I need to stop and do research on it. The intention wasn't to get in to the entire theory of eddy currents, magnetism and motor theory, it was to inform the OP that synchronous motors exist and the discussion to that point wasn't applicable to them.