Hp range: 0.5 – 500 hp.
Starting current: 6 to 10 times full-load current. Good running efficiency: 87% - 89.
Good power factor: 87% - 89%. Low rated slip: 3 –5 %.
Starting torque is about 150% of full load torque.
Maximum torque is over 200% but less than 225% of full-load torque.
Typical applications: constant speed applications where high starting torque is not needed and high starting torque is tolerated.
Design B motor
Hp range: 0.5 to 500 hp
Higher reactance than the Design A motor, obtained by means of deep, narrow rotor bars.
The starting current is held to about 5 times the full-load current.
This motor allows full-voltage starting.
The starting torque, slip and efficiency are nearly the same as for the Design A motor.
Power factor and maximum torque are little lower than class A, Typical applications: constant speed applications where high starting torque is not needed and high starting torque is tolerated.
Unsuitable for applications where there is a high load peak Design C motor
Hp range: 3 to 200 hp
This type of motor has a "double-layer" or double squirrel- cage winding.
It combines high starting torque with low starting current. Two windings are applied to the rotor, an outer winding having high resistance and low reactance and an inner winding having low resistance and high reactance.
Operation is such that the reactance of both windings decrease as rotor frequency decreases and speed increases.
On starting, a much larger induced currents flow in the outer winding than in the inner winding, because at low rotor speeds the inner-winding reactance is quite high.
As the rotor speed increases, the reactance of the inner winding drops and combined with the low inner-winding resistance, permits the major portion of the rotor current to appear in the inner winding.
Starting current about: 5 times full load current. The starting torque is rather high (200% - 250%).
Full-load torque is the same as that for both A and B designs. The maximum torque is lower than the starting torque, maximum torque (180-225%).
Typical applications: constant speed loads requiring fairly high starting torque and lower starting currents
Design D motor
Produces a very high starting torque-approximately 275% of full-load torque.
It has low starting current, High slip: 7-16%
Low efficiency.
Torque changes with load
Typical applications: used for high inertia loads
The above classification is for squirrel cage induction motor
3.22. Torque of squirrel cage induction motor
In order to perform useful work, the induction motor must be started from rest and both the motor and load accelerated up to full speed. Typically, this is done by relying on the high slip characteristics of the motor and enabling it to provide the acceleration torque.
Induction motors at rest, appear just like a short circuited transformer, and if connected to the full supply voltage, draw a very high current known as the "Locked Rotor Current". They also produce torque which is known as the "Locked Rotor Torque". The Locked Rotor Torque (LRT) and the Locked Rotor Current (LRC) are a function of the terminal voltage to the motor, and the motor design. As the motor accelerates, both the torque and the current will tend to alter with rotor speed if the voltage is maintained constant.
The starting current of a motor, with a fixed voltage, will drop very slowly as the motor accelerates and will only begin to fall significantly when the motor has reached at least 80% full speed. The actual curves for induction motors can vary considerably between designs, but the general trend is for a high current until the motor has almost reached full speed. The LRC of a motor can range from 500% Full Load Current (FLC) to as high as 1400% FLC. Typically, good motors fall in the range of 550% to 750% FLC.
The starting torque of an induction motor starting with a fixed voltage, will drop a little to the minimum torque known as the pull up torque as the motor accelerates, and then rise to a maximum torque known as the breakdown or pull out torque at almost full speed and then drop to zero at synchronous speed. The curve of start torque against rotor speed is dependent on the terminal voltage and the motor/rotor design. The LRT of an induction motor can vary from as low as 60% Full Load Torque (FLT) to as high as 350% FLT. The pull-up torque can be as low as 40% FLT and the breakdown torque can be as high as 350% FLT. Typical LRTs for medium to large motors are in the order of 120% FLT to 280% FLT.
Figure above graph shows that starting torque known as locked rotor torque (LRT) is higher than 100% of the full load torque (FLT), the safe continuous torque rating.
The locked rotor torque is about 175% of FLT for the example motor graphed above. Starting current known as locked rotor current (LRC) is 500% of full load current (FLC), the safe running current. The current is high because this is analogous to a shorted secondary on a transformer.
As the rotor starts to rotate the torque may decrease a bit for certain classes of motors to a value known as the pull up torque. This is the lowest value of torque ever encountered by the starting motor. As the rotor gains 80% of synchronous speed, torque increases from 175% up to 300% of the full load torque. This breakdown torque is due to the larger than normal 20% slip.
The current has decreased only slightly at this point, but will decrease rapidly beyond this point. As the rotor accelerates to within a few percent of synchronous speed, both torque and current will decrease substantially. Slip will be only a few percent during normal operation. For a running motor, any portion of the torque curve below 100% rated torque is normal.
The motor load determines the operating point on the torque curve. While the motor torque and current may exceed 100% for a few seconds during starting, continuous operation above 100% can damage the motor. Any motor torque load above the breakdown torque will stall the motor. The torque, slip, and current will approach zero for a “no mechanical torque” load condition. This condition is analogous to an open secondary transformer.
There are several basic induction motor designs (Figure below) showing considerable variation from the torque curve above. The different designs are optimized for starting and running different types of loads. The locked rotor torque (LRT) for various motor designs and sizes ranges from 60% to 350% of full load torque (FLT). Starting current or locked rotor current (LRC) can range from 500% to 1400% of full load current (FLC). This current draw can present a starting problem for large induction motors.