# What is the ratio of no-load speed to full load speed of a 200 kVA, 12 poles, 2200 V, 3 phase, 60 Hz synchronous motor?

What is the ratio of no-load speed to full load speed of a 200 kVA, 12 poles, 2200 V, 3 phase, 60 Hz synchronous motor?

#### SOLUTION

• The synchronous motor has the special property of maintaining a constant running speed under all conditions of load up to full load.
• This constant running speed can be maintained even under variable line-voltage conditions.
• It should be noted that, if a synchronous motor is severely overloaded, its operation (speed) will suddenly lose its synchronous properties and the motor will come to a halt.
• The synchronous motor gets its name from the term synchronous speed, which is the natural speed of the rotating magnetic field of the stator.
• This natural speed of rotation is controlled strictly by the number of pole pairs and the frequency of the applied power.
• Like the induction motor, the synchronous motor makes use of the rotating magnetic field.
• In a synchronous machine, the rotor is magnetized and it runs at the same speed as the rotating magnetic field.
• The principle of operation of the synchronous motor is as follows: a multiphase source of AC is applied to the stator windings and a rotating magnetic field is produced.
• A DC current is applied to the rotor windings and a fixed magnetic field is produced.
• The motor is constructed such that these two magnetic fields react upon each other causing the rotor to rotate at the same speed as the rotating magnetic field.
• If a load is applied to the rotor shaft, the rotor will momentarily fall behind the rotating field but will continue to rotate at the same synchronous speed.
• Once the rotor’s north and south poles line up with the stator’s south and north poles the stator current is reversed, thus changing the south- and north-pole orientation in the stator, and the rotor is pushed again.
• This process repeats until the current in the stator stops alternating or stops flowing. In a three-phase motor, the stator magnetic flux rotates around the motor and the rotator actually follows this rotating magnetic field.
• This type of motor is called a synchronous motor because it always runs at synchronous speed (rotor and magnetic field of the stator are rotating at exactly the same speed).
• Maximum torque is achieved when the stator flux vector and the rotor flux vector are 90° apart.
• Synchronous motors operate at synchronism with the line frequency and maintain a constant speed regardless of load without sophisticated electronic control.
• The synchronous motor typically provides up to a maximum of 140% of rated torque.

Here

Ns = 120 × f/p = 120 × 60/12 = 600 rpm

Speed of the synchronous motor remains constant irrespective of the load of the motor ie speed at no load and full load remains 600 rpm hence ratio of speed from no load to full load will be 1.