A synchronous Motor is found more economical when the load is above

A synchronous Motor is found more economical when the load is above

Right Answer is:

100 kW


A synchronous Motor is found more economical when the load is above 100 kW.

A synchronous motor is electrically identical to an alternator or A.C generator. The synchronous motor speed can be controlled by varying the frequency of its source. Due to the non-availability of economical variable frequency sources, speed control of this method was not used in past. These motors are mainly used in constant speed applications.

Synchronous motor drives are close competitors to induction motor drives in many industrial applications and their application is growing. Synchronous motors are generally more expensive than induction motor drives, but the advantage is that the efficiency is higher, which tends to lower the life cycle cost.

The development of semiconductor variable frequency sources, such as inverters and cyclo-converters, has allowed their use in variable-speed applications such as high power and high-speed compressors, blowers, induced and forced draft fans, mainline traction, servo drives, etc. Most synchronous motors are rated between 150 kW and 15 MW and run at speeds ranging from 150 to 1800 RPM.

Synchronous motors are made at speeds from 1800 (two-pole) 150 rpm (48-pole). They operate at a constant speed without slip an important characteristic in some applications. Their efficiency is 1-2 to 5% higher than that of induction motors, the higher value at the lower speeds. They are the obvious choice to drive large low-speed reciprocating compressors requiring speeds below 600 rpm. 

Why do, above 100 kW, synchronous motors predominate over induction motors?

Because the reactive power control through excitation is possibly easier with synchronous motors, and thus the requirements in this regard from the full power PECs (power electronic converters) are lower. Consequently, the PECs are simpler and less expensive and the overall costs of the drive are lower, though the synchronous motor, for comparable power and speed, is more expensive than the induction motor. The efficiency of the synchronous motor above 100 kW is also, in general, higher than that of the cage-rotor induction motor.

Also, the cost per horsepower is generally higher than that of a 3-phase induction motor. Hence synchronous motors are rarely used below 100 kW. because of their much higher initial cost compared to 3-phase induction motors. However, their unique characteristics of constant-speed operation, power-factor control, and high operating efficiency make them highly suitable for heavy industry, particularly for applications requiring low speed and high horsepower.

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