Speed Control by variable stator voltage

Upon the completion of this topic, you should be able to:-

4.1 study the three phase squirel cage induction motor control method by variable stator voltage

4.1.1 draw the voltage control circuit by means of thyristor 4.1.2 Sketch the characteristic curve of torque versus speed at

variable voltage value4.1.3 explain thyristor triggering sequence to control voltage 4.1.4 draw current and phase voltage waveform at rated

voltage4.1.5 explain thyristor switching sequence to control forward

and reverse

Speed Control by variable stator voltage

• AC motor drives can be broadly categorized into two types, thyristor based and transistor based drives.

• Thyristors posses the capability of self turn-on bymeans of an associated gate signal but must rely upon circuit conditions to turn off

• Transistor devices are capable of both turn-on and turn-off.• Because of their turn-off limitations, thyristor based drives

must utilize an alternating EMF to provide switching of the devices (commutation) which requires reactive volt-amperes from the EMF source to accomplish.

• The drives are categorized according to switching nature (natural or force commutated), converter type and motor type

the characteristic curve of torque versus speed at variable voltage value

Thyristor

• To produce rated voltage across the motor, the respective thyristors are fired with a delay ϴ equal to the phase angle lag that would exist if the motor were directly connected to the line.

• The valves in phase B and C are triggered the same way, except for an additional delay of 120° and 240° respectively.

• To reduce the voltage across the motor, the firing angle ϴ is delayed still more. For example, to obtain 50 percent rated voltage, all the pulse are delayed by about 100°.

•

current and phase voltage diagram at rated voltage

current and phase voltage diagram at 50% rated voltage.