Presentation

on

Synchronous motor Drive

Self controlled drive empoying load commutated inverter

• Each phase of a synchronous motor is represented by an internal voltage in series with the motor inductance assuming a non-sailent pole motor.

• The dc output voltage of the controlled converter can be controlled by changing the firing angle α.

• The load inverter is a converter operating in the inverter mode with α greater than 90°.

• The three phase motor voltages are measured using the measurement transformer,to calculate the position of the rotor field current as a function of time as shown in fig...

• These voltages are then rectified to provide a dc signal proportional to the instantaneous rotor speed.

• Keeping If and toff constant,the actual speed is compared with the reference speed.

control circuit for load commutated inverter based synchronous motor drive

• Show in the fig. At the time of starting and at low speeds the induced emf in synchronous motor is not sufficient to ensure the current commutation of the load inverter thyristor.

• The frequency and phase current should be changed in synchronization with the rotor position.

• The fillter inductance reduces the current harmonics and ensures that the input to the load inverter and hence to the motor appears to be a current source.

• The actual DC link current is sensed by a current sensor.

• If the actual value of Id is less than the reference value,then the line converter increases the dc voltage applied to the link thus increasing Id and the torque produce by the motor.

• The torque producing of the motor is decreases,but the motor can supply the rated power .

• The rated speed ,the field flux needs to be weakened by reducing If ,this region is called as flux weakening region.

closed loop control of load commutated inverter synchronous

motor

• Shown in the fig..there are two feedback loops.• The outer loop is the speed loop while inner loop

is the current loop.

• The inner control loop are some circuits are emplay only the speed feedback.

• The motor is required to operate at a desired speed but it has to meet the load torque requirement ,which depends on motor current Idc.

Speed sensing

• As shown in the fig..the terminal voltage sensor senses the stator voltage and generates a pulsed waveform the frequency of which is same as the machine induced voltage.it is proportional to the speed of the rotor.

• The speed error signal is applied to the speed controller as shown in figure..

Current sensing

• The feedback signal is applied to a comparator which compares it with the current reference signal produced by the speed controller ,to produce Vc to the control the firing angle of the source side converter shown in fig ..

1.speed controller:

• The speed controller is a (P+I) type controller which produces an output voltage proportional to the error between the set or desired speed and the actual speed.

• The output of the speed control is called as torque reference.

2.current controller• This is also a controller which generates the

control voltage that decides the firing angle of the converter.

• The input to the current controller is the error signal between the torque reference signalat the output of speed controller and the current feedback signal.

Properties

• Used for the synchronous motor in very large HP rating.

• The load commutated inverter is simple and has lower compared to the inverter used in CSI -induction motor drives .

• The power switches used in the LCI may not have the self commutation capacity.

Thank you…