Torque of synchronous motor: It is important to understand the characteristics of the torque required to operate the synchronous motor at each moment from the initial state to final shutdown. Also to use synchronous motor for different applications, the torque types associated with the motor should be known. The various types of torques associated with synchronous motors are given below:
List of different torques of a synchronous motor
Torque refers to the torque without which the motor cannot rotate. The torques produced in a synchronous motor are:
1. Starting torque (low and high)
2. Running torque
3. Pull in torque
4. Pull out torque
Let us discuss in detail the different 4 types of torques associated with synchronous motors.
4 types of torque of a synchronous motor are described
1. Starting torque of synchronous motor?
Most synchronous motors today are designed to be self-starting. For this reason, squirrel cage winding is done on the pole paces of its rotor. So that it can start automatically like an induction motor. So, the torque or twisting force generated by the motor when full voltage is applied to the stator of a synchronous motor is called starting torque.
This torque can be low and high. If a synchronous motor has low-resistance squirrel cage windings on the pole faces of the rotor, then the starting torque is low. For example, the starting torque of brewers, pumps, grinders, air compressors, centrifugal and reciprocating pumps, etc. is low, i.e. 30% to 50%.
Again, when high-resistance squirrel cage winding is used in the pole face of the rotor of synchronous motor or insulated winding in the groove (slot) of the pole face (combined with slip ring) then the starting torque is high. In this case, the starting torque is 100% to 250%.
For example, such high starting torque synchronous motors are used in ball, rod and tube mills, mining machinery, line shafts, flour mills, etc.
2. Running torque of synchronous motor?
The torque produced in a synchronous motor during running is called running torque. Torque is determined by the horsepower and speed of the drive machine.
That is, the torque developed by a synchronous motor while running is called running torque. Running torque is determined by power rating and motor speed. Peak output power determines the maximum running torque required for the driven machine.
3. Pull in torque of a synchronous motor?
A synchronous motor starts like an induction motor as long as its speed is below 2% to 5%. If the excitation is applied to the rotor poles at this time, the rotating magnetic field of the stator pulls the rotor back.
That is, the amount of torque that the rotating magnetic field of the stator of the synchronous motor pulls back the rotor is called pull-in torque.
4. Pull out torque of a synchronous motor?
The maximum amount of torque that a synchronous motor can generate while maintaining that speed is called the “pull out torque” of that motor. If the motor torque becomes greater than the pull-out torque due to increased load, the motor will stop immediately.
Therefore, the maximum value of load torque that a synchronous motor can develop without losing synchronism at rated voltage and frequency is called pull-out torque or breakdown torque.
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Related Questions and Answers
List the torque of synchronous motor?
The torque lists are: (a) Starting torque low and high (b) Running torque (c) Pull in torque (d) Pull out torque.
What is the starting torque of synchronous motor?
The torque or twisting force produced by the motor when full voltage is applied to the stator of a synchronous motor is called starting torque.
What are the types and what is the starting torque of synchronous motor?
The starting torque of a synchronous motor is of two types, namely: (a) low-starting torque and (b) high-starting torque.
When is the starting torque low in a synchronous motor?
When a synchronous motor has a low-resistance squirrel cage winding on the pole face of its rotor, the starting torque is poor.
What is the value of “low-starting” torque in a synchronous motor?
Low-starting torque values range from 30% to 50%.
What is the value of “high-starting” torque in a synchronous motor?
High-starting torque values range from 100% to 250%.
In what cases are synchronous motors with low-starting torque used?
Areas of use are: (a) Blowers, (b) Pulp grinders, (c) Air compressors, (d) Centrifugal pumps, (e) Reciprocating pumps etc.
In what cases are high-starting torque synchronous motors used?
Areas of use are : (a) Ball color and tube mills, (b) Mining machinery, (c) Line shafts, (b) Flour mills etc.
When does synchronous motor draw less current from AC busbar?
A synchronous motor at unity or single power factor draws the lowest current from the AC busbar.
What is the power input when the excitation is reduced in a synchronous motor when the load is constant?
The power input remains the same in all cases.
Why does voltage increase in a synchronous motor under excitation at steady load?
To maintain the same speed under excitation, the value of counter EMF decreases, thereby increasing the gain voltage.