Synchronous vs induction motors

A lecture by Brandon Kim, sales application engineer specializing in MV Motors at TMEIC, at the 2018 Turbomachinery Pump Symposium in-part talked about differences between synchronous and induction motors.

Induction motors are the “standard” industrial motors. More than 99% of motors used are induction motors. It is an induction motor if it runs less than the “synchronous” speed. If the synchronous speed, the induction motor would run at 1785 rpm. The reason is the power is “induced” on the rotor.

MV = High Efficiency +1~2%. While synchronous motors have higher efficiency and no slip, induction motors have good starting torque. The synchronous motor has no starting torque. Power factor is lagging in induction motor and unity in synchronous motor. Capex is lower for induction while Opex is lower for synchronous. The induction motor is easy to start whereas synchronous motor needs VSD, pony motor or damper bars.

As a rule of thumb, sync motor is preferred for over 10 MW if slower than 12 pole. In case of 2-4 pole size, choose sync motor for powers greater than 20 MW.

The induction motor is limited in size to about 30,000HP, less efficient, power factor is less than 1.0.

The sync motor is common for larger machines. Thirty years ago, large machines were 5,000 HP, today they are 30,000 HP. In sync motors, rotor is magnetized to the stator. These motors need magnets such as permanent magnets on the rotor.

Both have: • Near identical Stator! • Insulations systems • Enclosures Common accessories: • Space Heaters • Bearing and Winding RTDs • Differential protection

The sync motor has a complex design with two flux circuits. A DC current flux circuit provided by field poles. The level is set by external control. The second flux circuit is the induction flux circuit. Damper bars or amortisseur winding can be used for starting.