Electrical Engineering ⇒ Topic : Starting of Synchronous Motors

It has been mentioned in the beginning of this chapter that a synchronous motor has no self-starting torque. This is one of the serious disadvantages of this type of motor. Some special methods are used for starting synchronous motors. The most commonly used methods are:

(1) using damper windings, and

(2) using wound rotor in the synchronous-induction method

Using Damper Windings 

 Synchronous motors are generally provided with a damper winding consisting of copper or bronze bars embedded in slots in the pole faces and short circuited at both ends by means of conducting rings. The damper winding serves not only to damp out the oscillations of rotor (hunting) during normal motor operation, but is also useful for the purpose of starting when suitably designed.

    The line voltage is applied to the stator terminals with the field circuit left open and unexcited. The motor then starts as an induction motor and when it attains nearly 95% of its synchronous speed, the rotor circuit is excited. The rotor poles slip backwards through the synchronously rotating armature field. Alterntely, the rotor poles will be aligned and de aligned with the magnetic axis produced by the stator rotating field as the stator mmf slips faster with respect to the motor. The chances of the rotor pulling into synchronism depend upon the instant of closing the rotor circuit. The rotor circuit must be closed when the rotor magnetic axis (poles) is about to align with the statormagnetic axis so that the rotor may be pulled into synchronism effectively.The disadvantage of this method is that at the instant of starting, the rotorwill be stationary and the rotating field due to the stator current will induce very large voltages in the rotor at supply frequency, even though this induced voltage and its frequency reduce as the rotor picks up speed. The rotor field winding must be provided with a suitable insulation to withstand these induced high voltages.

  The above disadvantage can be eliminated by applying a reduced voltage to the stator at the time of starting by using either auto-transformers or a star/ delta starter. This reduces the starting current even though the starting torque is reduced considerably. This method is useful for starting synchronous motors when there is no initial load on the motor. The reduced voltage can be as much as 50 to 75% of the normal voltage for satisfactory operation.

Using the Wound Rotor in the Synchronous Induction Method

    For this method, the rotor of a synchronous motor must be a wound-rotor of 3-phase windings, similar to the stator windings of a synchronous or induction motor. The machine is started in a manner similar to that of a slip-ring induction motor by inserting variable resistances in the rotor circuit as shown in Figure (a). If the synchronous motor has a built-in excitor on its shaft, then it can be connected in one of the phases of the rotor as shown in the figure.The resistors are included in the rotor as shown in the figure. The resistors are included in the rotor circuit at the time of starting and thereafter, they are

                 FIGURE (a) Starting of a synchronous motor with wound rotor (synchronous induction motor)

slowly cut out as the rotor picks up speed. When the speed exceeds the critical value, the exciter builds up its dc voltage and the dc current from the exciter circuit magnetises the rotor of the synchronous motor with the same number of poles as the stator. With the arrangement shown in the figure, the R-phase will be carrying the full dc current and the other phases will be carrying half the current. This depicts the instant of the distribution of ac currents in which the R-phase is carrying the maximum of the ac positive value while the othertwo phases are  carrying one-half of the maximum of the ac negative value, as discussed in Figure. Consequently, by the time the starting resistors are short-circuited the machine will be running at full speed as an induction motor. The rotor is acted upon by two torques, namely, one unidirectional torque due to the induction motor action, and the second, a pulsating torque at slip frequency due to the rotating magnetic field produced by the stator currents. The slow relative motion of the stator mmf with respect to the motor enables the rotor to pull into synchronism, as discussed in the previous section at the instant when the rotor magnetic axis is about to align or coincide with the stator magnetic axis. This type of starting is usually known as the synchronous-induction motor method of starting, and is useful when large starting torque is required at low starting currents.

STARTING OF SYNCHRONOUS MOTORS

A synchronously rotating magnetic field is set up when three-phase supply is applied to the stator.

The relative speed between rotating stator field and standstill rotor is equal to the synchronous speed when the rotor is stationary. Therefore, the torque angle between rotating stator poles and rotating rotor poles is a function of time. Since electromagnetic torque varies sinusoidally with time, the average value of the electromagnetic torque over a complete cycle is zero. Therefore, the net starting torque of a synchronous motor is zero. The following two methods are generally used to start synchronous motor:

• Auxiliary motor starting
• Induction motor starting

The main purpose of auxiliary motor is to bring the speed of the synchronous motor near to the synchronous speed. Induction motor or DC motor is used as the auxiliary motor