Chapter 17: Synchronous Motor Lecture 34 Electro Mechanical System

Electro Mechanical System Introduction The synchronous generators can operate either as generators or as motors. When operating as motors (by connecting them to a 3-phase source), they are called synchronous motors. Synchronous motors run in synchronism with the revolving field. The speed of rotation is tied to the frequency of the source. The motor speed stays constant, irrespective of the load or voltage of the 3-phase line. Synchronous motors are used not so much because they run at constant speed. They possess some unique electrical properties. Synchronous motors are rated between 150 kW (200 hp) & 15 MW (20 000 hp) and turn at speeds ranging from 150 to 1800 r/min. These machines are mainly used in heavy industry Lecture 34 Electro Mechanical System

Electro Mechanical System Construction Synchronous motors are identical to salient-pole ac generators. The stator: It is composed of a slotted magnetic core, which carries a lap winding which is also identical to a 3-phase induction motor. The rotor: Has a set of salient poles excited by a dc current using two slip-rings. It also carry a squirrel-cage winding similar to that in a 3-phase induction motor. This damper winding serves to start the motor. Modern synchronous motors often employ brushless excitation, similar to that used in synchronous generators. Lecture 34 Electro Mechanical System

Starting a Synchronous Motor A synchronous motor can not start by itself The motor is equipped with a squirrel case winding, to start it as an induction motor. When the stator is connected to 3-phase line, the motor accelerates until it reaches slightly below synchronous speed. The dc excitation is suppressed during this starting period. Very large synchronous motors (20 MW and more) are sometimes brought up to speed by an auxiliary motor, called a pony motor. Finally, in some big installations the motor may be brought up to speed by a variable-frequency electronic source Lecture 34 Electro Mechanical System

Electro Mechanical System Pull-in torque As soon as the motor is running at close to synchronous speed, the rotor is excited with dc current. This produces N and S poles around the circumference of the rotor. If the poles on the rotor at the moment the exciting current is facing the poles of opposite polarity on the stator, a strong magnetic attraction is set up between them The mutual attraction locks the rotor and stator poles together. The rotor is pulled with the revolving field. Torque develop at the moment is called pull-in torque. Lecture 34 Electro Mechanical System

Electro Mechanical System Pull-in torque dc current must be applied at the right moment, otherwise a mechanical shock will be produced and the circuit breakers will trip. The starter detects the precise moment when to apply excitation. Once the motor turns at synchronous speed, no voltage is induced in the squirrel-cage winding and so it carries no current. The behavior of a synchronous motor is entirely different from that of an induction motor. Lecture 34 Electro Mechanical System

Electro Mechanical System Motor under Load At no-load conditions, the rotor poles are directly opposite the stator poles and their axes coincide As mechanical load is applied, the rotor poles fall slightly behind the stator poles, but continues to turn at synchronous speed Greater torque is developed with increase separation angle There is a limit when the mechanical load exceeds the pull-out torque; the motor will stall and come to a halt The pull-out torque is a function of the dc excitation current and the ac stator current Lecture 34 Electro Mechanical System

Under load-simple calculations Equivalent circuit represents one phase of a wye-connected motor. It is identical to the equivalent circuit of an ac generator. The flux  created by the rotor induces a voltage EO in the stator. It depends on the dc exciting current Ix. EO varies with excitation. Rotor and stator poles are lined up at no-load. Induced voltage EO is in phase with the line-to-neutral voltage E. If, we adjust the excitation so that EO = E, the motor "floats" on the line and the line current I is practically zero. The only current needed is to supply the small windage and friction losses in the motor, and so it is negligible. Lecture 34 Electro Mechanical System

Under load-simple calculations If we apply a mechanical load. The motor will begin to slow down, so the rotor poles fall behind the stator poles by an angle α. Due to this mechanical shift, EO reaches its maximum value a little later than before. EO is now δ electrical degrees behind E. The mechanical displacement α produces an electrical phase shift δ between EO and E, which produces a difference of potential Ex across the synchronous reactance Xs given by: Ex = E – EO A current I must flow in the circuit, given by: jIXS = Ex I = – j Ex /XS = – j(E – EO)/Xs I is nearly in phase with E, so the motor absorbs active power Lecture 34 Electro Mechanical System