Concept:
Synchronous speed in rpm of a three-phase induction motor is given by,
\({N_s} = \frac{{120f}}{P}\)
Where f is the frequency in Hz
P is the number of poles
A rotor of a three-phase induction motor rotates at a speed close to the synchronous speed but not equal to the synchronous speed.
The difference between the synchronous speed and the rotor speed is known as slip speed. It is given by
Slip speed = Ns - Nr
Slip is defined as the ratio of slip speed to the rotor speed.
Mathematically, Percentage slip, \(\% \;s = \left[ {\frac{{{N_s}\;-\;N}}{{{N_s}}}} \right] \times 100\)
Or fractional slip, \(s = \frac{{{N_s}\;-\;N}}{{{N_s}}}\)
Where s is the slip
f is the supply frequency
Calculation:
We know that \({N_s} = \frac{{120f}}{P} = \frac{{120 \times 50}}{4} = 1500\;rpm\)
Given that \({N_r} = 1400\;rpm\)
Slip (s)\(= \frac{{{N_s} - {N_r}}}{{{N_s}}} \times 100 = \frac{{1500 - 1400}}{{1500}} \times 100 = 6.67\%\)
Slip (s)
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.
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<s<1).
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.
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.
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.
If the slip value is ‘1’ then the rotor in the motor will be stationary
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.
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.
The formula of the slip in the induction motor is given below.
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)
If the synchronous speed of the motor is 1250 and the actual speed is 1300 then please find the slip in the motor?
Nr = 1250 rpm
Ns = 1300 rpm
The speed difference can be calculated as Nr-Ns = 1300-1250 = 50
The formula to find a slip in the motor is (Nr-Ns)*100/Ns = 50*100/1300 = 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 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.
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.
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.
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.
Thus, this is all about an overview of a slip in an induction motor. When the speed of the rotor within the motor is equivalent to synchronous speed then slip is ‘0’. If the rotor is turning at synchronous speed in the rotating magnetic field direction, then there is no cutting action of flux, no emf within the rotor conductors & no flow of current within the rotor bar conductor. Therefore, electromagnetic torque cannot be developed. So the rotor of this motor cannot achieve synchronous speed. As a result, slip is not at all zero within the motor. Here is a question for you, what the function of an induction motor?