Torque Slip Characteristics of Induction Motor

Last Updated : 08 Apr, 2024

The torque slip characteristics of an induction motor are the plot that shows the relationship between the torque produced by the motor and the slip. Where slip is the difference between the speed of the rotating magnetic field produced by the stator and rotor the speed. The torque slip characteristics are essential for optimizing the motor’s performance. The torque slip curve provides essential characteristics of the motor including its capabilities, efficiency and also operating range.

It is used to design and analyze motor systems for various applications. These are used in applications where high starting torque and high pull-up torque are required. It is used to predict motor behaviour under different load conditions and facilitates the design of control strategies to enhance performance.

Here in this article, we will discuss the definition of torque slip characteristics, torque slip characteristics in three-phase induction motors as well as single-phase induction motors, the effect of rotor resistance on torque slip curve, advantages, disadvantages and applications of induction motors.

Table of Content

Torque Slip Characteristics?
Single-Phase Induction Motor
Three Phase Induction Motor
Effect of Rotor Resistance
Advantages
Disadvantages
Applications

What are Torque Slip Characteristics?

Torque-slip characteristics, also known as the torque-speed characteristics or torque-speed curve, are Used in understanding the behavior of an induction motor which are under different load conditions. It is the relationship between the developed torque of an induction motor and its slip, which is the difference between synchronous speed.

Now we will go through Torque Slip Characteristics of Single Phase and Three Phase Induction motor

Torque Slip Characteristics in Single-Phase Induction Motor

In single phase induction motor , two revolving fields produce torques in opposite directions. Field no-1 rotates in a clockwise direction ,generating torque in that direction . Whereas field no-2 rotates in counterclockwise ,producing torque accordingly. The torque produced by the clockwise field is considered as positive , while that of the counterclockwise field is negative.

Torque slip characteristics in single phase induction motor

Initially both fields have slip =1 .When field no-1 reaches synchronous speed ,its slip becomes zero while the slip of field no-2 becomes two and vice versa. As a result, the resultant torque curve passes through zero torque indicating zero starting torque. However , once the motor starts rotating , torque is consistently present as seen in positions 1-1^‘ and 2-2^‘ , where one fields torque dominates ,causing the motor to accelerate in the corresponding direction .That means , once started ,the motor continues to rotate in the direction it is initiated.

Torque Slip Characteristics of Three Phase Induction Motor

The torque slip characteristics of three phase induction motor gives the relationship between torque and slip . These characteristics are divided into mainly three regions according to slip . They are :

Low slip region
Medium slip region
High slip region

Low Slip Region

In this region slip is very low and the motor speed is close to synchronous speed . Mainly the torque is developed due to the difference between synchronous speed and rotor speed. So, the torque developed in this region is proportional to the square of the slip and inversely proportional to the square of the stator frequency. The torque remains relatively constant with a slight decrease as slip increases . The starting torque is high in this region.

T = 3SE₂²R₂ / 𝜔(R₂² + S² X₂² )

T = KS

T ∝ S

Medium Slip Region

In this region slip is moderate and the motor operates below synchronous speed. Compared to low slip region, the torque developed decreases rapidly with increasing slip but remains stable. The curve looks like rectangular hyperbola that indicates the maximum torque that motor can produce. This torque also called as breakdown torque .This is commonly used in variable torque applications like pumps and fans etc.

T = 3SE₂² R₂ / 𝜔(R₂² + S²X₂² )

T = K’ / S

T ∝ 1/ S

High Slip Region

In this region, the slip is high and the motor operates significantly below the synchronous speed .The torque developed in this region is less compared to medium slip region . The torque drops sharply and the motor’s efficiency decreases. In this condition ,the motor may damage if it runs continuously. This is used in applications where constant torque is demanded .

Torque slip characteristics

The above figure shows different toque slip characteristics of a typical three phase induction motor for a slip range from S = 0 to S =1 for various values of rotor resistance .

Hence , the torque is now inversely proportional to the slip. Thus , the torque slip curve is a rectangular hyperbola .Thus , with increase of slip beyond the maximum torque , torque value decreases due to increase in load. So, the motor could not pick up the load and eventually stops . This results in blocked rotor .

Braking Region

In this region, motor operates as a generator. The slip in this region is slip <1 . It indicates that rotor speed is higher than the synchronous speed that causes stator to induce a current in the rotor windings. This current produces retarding torque that opposes rotor motion. Consequently, the motor speed reduces and it stops at some point of time. The torque produced in this region is negative.

Motoring Region

It is the operational region where the motor operates as a motor that converts electrical energy into mechanical energy .In this region, slip is positive i.e. slip is between 0 to 1. It indicates that motor is rotating at a speed lower than synchronous speed. The torque produced in the motoring region is positive. The torque produced in motoring region is directly proportional to the slip.

Generating Region

In this region , slip in this region is negative . It indicates that the motor speed is higher than the synchronous speed and reversal in the power flow direction. The motor receives mechanical energy by dispensing electrical energy. The torque in this region is negative. As the low slip values the generated torque rises steeply due to high rotor impedance. As the slip increases , the torque also increases but at a decreasing rate.

Torque Slip Curve of Three Phase Induction Motor

Effect of Rotor Resistance on Torque-Slip Curve

By increasing the rotor resistance in a slip ring induction motor alters the torque slip curve .The maximum torque produced by the motor remains unaffected by the rotor resistance but is inversely proportional to the rotor standstill reactance. As a result adjusting the rotor resistance modifies the slip value at which maximum torque is reached. When rotor resistance increases , the slip at which maximum torque occurs also increases.

With greater rotor resistance ,the curve shifts to the right , indicating a higher slip value at which maximum torque is achieved. This occurs because higher rotor resistance reduces the rotor current , which in turn reduces the slip required to develop maximum torque. As a result , the motor operates closer to synchronous speed for higher rotor resistances .

S = R₂ / X_2s

torque slip characteristics with addition of various external resistors

The above torque slip curves are drawn by keeping rotor reactance X_2s constant for various values of rotor resistance R₂ .

When R₂ is 0.1 times of X₂ , the maximum torque will occur at slip

S = R₂ /X_2s = 0.1 X_2s / X_2s = 0.1

Now , if we increase the rotor resistance value by adding some external resistance then the maximum torque would occur at a slip is also changes.

Advantages of Torque Slip Characteristics of Induction Motor

It provides straightforward way to understand the behavior of an induction motor .
Induction motors exhibit high starting torque at low slip. It helps in determining the starting torque of the motor .
It helps engineers in optimizing the efficiency of induction motors under different operating conditions.
It helps in design of speed control systems for induction motors that enable precise speed regulation where it is required.
It helps in enabling stable motor operation by analyzing torque slip characteristics .
It allows the motor to adapt to varying load conditions without significant loss of performance.

Disadvantages of Torque Slip Characteristics of Induction Motor

They have limited speed control capabilities due to their limited torque-speed relationship .
At low slip values , induction motors operate inefficiently leads to wastage of energy .
They draw high starting currents that leads to voltage drop and power quality issues .
They may struggle with sudden overload conditions, as their torque output is limited by the slip characteristics.
The efficiency is dependent on slip. So, the efficiency varies according to operating conditions .

Applications of Torque Slip Characteristics of Induction Motor

Induction motors are used in fans and blowers in manufacturing plants and HVAC systems.
Induction motors are used in mining equipment like conveyors and crushers .
Induction motors are used in electric vehicle propulsion systems to optimize motor efficiency .
Induction motors are used in pumps and compressors in HVAC systems and agriculture.
These are used in wind turbines to efficiently design of turbines.
Induction motors are used in paper and textile manufacturing process.

Conclusion

Thus, the torque slip characteristics of both three phase and single phase induction motors plays a crucial role in understanding their performance and applications. For three phase motors, the curve typically exhibits a high starting torque and a nearly constant torque at rated speed , making them suitable for various industrial applications requiring constant torque. Single phase motors are having low starting torque and exhibit more valuable torque characteristics that limits their applications to lighter loads and used where three phase power is unavailable.

By adding external resistance ,the starting torque of induction motors can be increased . However , this comes at low cost of reduced efficiency and increased heating due to the energy dissipated across the added resistance. And the applications of induction motors span across various industries including manufacturing, agriculture, transportation and household appliances. Three phase induction motors are used in heavy duty applications such as pumps, compressors, conveyors and industrial fans due to their robust performance and efficiency.