Synchronous Motors - The Benefits

By Jim Parrish, Lion Oil

Synchronous Motors
The Benefits of Synchronous Motors

Induction motors are often the choice for various industrial production processes. Use of an induction motor, however, will result in increased lagging power factor burdens in the plant. This burden often must be corrected by adding capacitors. If not, the result is the penalty of larger kVA burdens to the interconnected system. For low speed applications, synchronous motors may be a better choice when equipped with accessories that offer power factor control. The synchronous machine, with the aid of an intelligent excitation controller, can control power factor to reduce the plant reactive loading to the connected system.

Lion Oil is a refinery in El Dorado, Arkansas, that processes petroleum products. Environmental mandates that go into effect June 2006 include new clean air regulations requiring a reduction in sulfur emissions for diesel fuel from 500 parts to 15 parts per million. Current refining equipment is not designed to process the petroleum product at the increased pressures and higher temperatures needed to meet the EPA regulations. To meet the refining requirements, a new higher-rated motor connected to a hydrogen-reciprocating compressor would be needed to drive the refining process. Lion Oil currently utilizes induction motors, ranging in size from 100 to 3500 horsepower. With such a large number of induction motors, power factor is an issue at the plant.

Capacitors are used for correction to minimize penalties from the electric utility. The decision to use a synchronous motor instead of an induction motor was based upon the need to have an optimally efficient plant with low operating cost.

Decision for selection of synchronous motor
The induction motor has fixed stator windings that are electrically connected to the AC power source. Current is induced in the rotor circuit via transformer action, resulting in a magnetic field that interacts with the stator field causing rotation. The rotor must rotate slower than the stator field for the induction to occur, thus an induction motor operates at less than synchronous speed using no other power source to excite the rotor’s field.

Induction Motors, especially those operating at 600 RPM and below, have lower operating efficiency and, inherently, a lagging power factor that draws exciting current from the connected source. Since an induction machine draws all exciting current from the AC source, it may take in the range of 0.3 to .6 p.u. reactive magnetizing kVA per HP of operating load. The synchronous motor has fixed stator windings electrically connected to the AC supply with a separate source of excitation connected to a field winding on the rotating shaft. Magnetic flux links the rotor and stator windings causing the motor to operate at synchronous speed. While induction motors can be started and accelerated to steady state running condition simply by applying AC power to the fixed stator windings of the motor, a synchronous motor starts as an induction motor. Then, when rotor speed is near synchronous
speed, the rotor is locked in step with the stator by application of DC voltage and current applied to the rotor of the motor.

Once the synchronous motor is operating at synchronous speed, it is possible to alter the power factor of the motor by varying the excitation supplied to the motor’s field. Since power factor correction could be provided, a synchronous motor was chosen.