Speed control of synchronous machine

Speed control of synchronous machine

 Synchronous machine is a constant speed Machine and independent on load. In some industrial applications speed control is necessary. The synchronous machine speed is controlled by varying the frequency. Speed control of synchronous machine is achieved by voltage fed inverters, current fed inverters or cyclo converter. Speed control is of two types I.e. scalar control and vector control. A scalar control synchronous machine consists of two different modes of operation.

1. Separate control or open loop

2. Self control or closed loop

1.Separate control or open loop:

              This is the simple method. In this method all the machines are connected in parallel to the same inverter. So every motor move in synchronism corresponding to the command frequency. The below figure shows open loop speed control scheme of synchronous motor.

     The figure consists of delay circuit, the applied command frequency passes through the delay circuit and output of delay circuit is fed to voltage source inverter. 

        The inverter v/f ratio is kept constant to ensure that the motors operate at full flux at all speed which ensures constant torque capability. A flux control block changes stator voltage with frequency v/f to maintain a constant flux below rated speed and a constant terminal voltage above rated speed. By varying the frequency the speed must be varied from zero to full value.

  

We know synchronous speed Ns = [120*f]/p

                                    Ns α f

                                     f α 1/t

From the above relation torque decreases by increasing the speed is determined by the inverter frequency, speed changes must be made slowly to avoid exceeding pull out torque angle which results in loss of synchronism.

2.Self control or closed loop:

              Self control of synchronous motor is also known as closed loop operation. In many cases closed loop is preferred because with open loop control some disadvantages occurred such as hunting of motor, stability problems, poor dynamic behaviour and losses. Self control synchronous motor has analogy with a DC motor. The below figure shows self control or closed loop control.

The figure contains a three phase controlled rectifier connected to the supply and this controlled unit attaches to DC link and inverter. 

* The inverter may be CSI or VSI, when inverter is employed, the drive is controlled from a DC source ( may be current source or voltage source). The rotor speed determines the inverter frequency. 

* An encoder is mounted in the shaft which emits signal to express the rotor position either ON, OFF condition. The inverter acts effectively on rotor at every instant of time (I.e. torque production) due to this if rotor slows down the required frequency us supplied by the inverter and makes the rotor in synchronized with rotating field.

          If DC input of inverter is kept constant, the motor starts from rest, at first motor current will be large but it will decrease when stator emf equal to applied voltage. The synchronous motor speed is controlled by controlling DC link voltage which depends on firing angle of controlled rectifier. Four quadrant operation is possible as long as inverter is supplied from a fully controlled converter. At low speed the motor is at open loop control due to insufficient generated emf.

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