Microprocessor Control of Synchronous Motor Drives:
Variable speed drives employing Microprocessor Control of Synchronous Motor Drives are becoming very popular in industrial applications. They are an immediate solution for high power reversible drives and are becoming competitors to dc and induction motor drives. The Microprocessor Control of Synchronous Motor Drives operates at leading power factors when overexcited. The armature voltages can be used to commutate the inverter thyristors on the machine side. This is possible over a reasonably wide range of speeds. However, at very low speeds up to about 10% of base speed, the machine commutation is ineffective and therefore forced commutation is required. The inverter may be equipped with forced commutation circuit which operates up to 10% of the base speed, and above this speed the machine commutation takes over. The assistance in commutation may be provided by means of in-terrupting the de link current.
A thyristor across the dc link is used. When the commutation is required the current is diverted to this thyristor, so that the current in the inverter thyristors falls to zero. The line side converter is forced into inverter operation so that the polarity of voltage changes. After the thyristors in the machine side converter have attained their positive block-ing capability, the next thyristor pair in the sequence is fired. The line side converter is forced back to rectifier action. The current is now transferred to the inverter and the thyristor across the inductance stops conduction au-tomatically. One of the starting schemes must be employed. The machine commutation is possible only with current fed operation. The cycloconverter may also be used to control the speed of the Microprocessor Control of Synchronous Motor Drives. Machine commutation is possible here also. However, in the low speed range the line voltages may be used for commutation. The starting problem does not arise here.
The control of the inverter or cycloconverter feeding a synchronous motor can be accomplished using the position of the rotor with respect to stator. A rotor position sensor on the shaft senses the position of the rotor with respect to the stator and sends the firing pulses to the thyristors of the inverter. The six inverter thyristors are fired in a sequence once, by the time the rotor moves by two pole pitches or 180°. This provides synchronism between the frequency and rotor speed. The motor is said to be in self controlled mode.
The self control of the motor is also possible by sensing the stator induced voltages which effectively eliminates the mechanical rotor position sensing. In one cycle of stator voltages all the six thyristors arc fired once in a sequence. Self control is possible with VSI, CSI and cycloconverter.
A Microprocessor Control of Synchronous Motor Drives in its self controlled mode has a performance similar to that of a dc motor, both in steady-state and dynamic conditions. It, there-fore finds application where dc motors cannot be employed. It is also called commutator less motor (CLM).
When a naturally commutated converter operates in the inverter mode the overlap limits the range of firing angle. The upper limit of firing angle called inverter limit is decided by the turn off time of the thyristors and the overlap. The limit is set such that there is enough margin for (tq) the turn off of the thyristor taking overlap into consideration, so that the commutation failure does not occur. A margin angle is defined as the difference between the lead angle of firing and overlap angle. A control is evolved to see that under no circumstances does this margin angle go below the turn off angle of the thyristor, so that commutation failure does not occur.
- Commutation failure is prevented.
- The maximum power output can be increased by simultaneous control of the field current to compensate for armature reaction. This improves the overload capacity of the motor.
- There is improvement in the power factor
- The torque pulsations under the load conditions are reduced. The ripple content of the dc current also decreases.
However, there are certain disadvantages. The upper speed is limited and hence the range of speed control is limited. To overcome this disadvantage, margin time control is employed, which improves the performance also.