Application of thyristors for induction motor speed control

Abstract

The world of the variable speed drive has been dominated by direct current motors because of their adequate speed-torque characteristics and simplicity of control by varying either the field current or armature voltage. The trouble with direct current motors lies at the commutator which requires periodic maintenance and cannot be used in a submerged motor or in an inert atmosphere. Moreover, a commutator limits also the speed of a motor, since as the speed of the motor increases the weight of the commutator increases at the rate of about 10 lb/h.p. However, the arrival of the thyristor has revolutionized the world of variable speed by providing the means of giving the squirrel cage induction motor the torque-speed characteristics similar to those of a shunt direct current motor. The commutator-less alternate current motor meets the mechanical requirements often needed. It is robust, simply constructed and can be designed to run up to 20,000 r.p.m. or even higher. However, when operated from a constant voltage, constant frequency source the induction motor runs at practically constant speed. The reason for this behaviour can be easily seen from the relation the rotor velocity bears to the supply frequency at a given voltage. If nr is the rotor r.p.m, n0 the ratio of supply frequency to the number of pole-pairs, and s the slip-frequency between rotor and stator currents; nr=(les)n0. Since during normal operation, in order to maintain a high efficiency, the slip is kept small and constant, the rotor velocity is proportional to the supply frequency. There are several conventional methods of speed variation for an induction motor, some of which are mentioned below. (a) Pole-changing: when the number of stator poles is changed the synchronous speed is altered, and so the range of rotor velocity at low slip is varied. One way of changing the effective number of poles is to cascade two slip ring induction motors, where the main motor feeds the stator of the secondary machine through its slip rings. If the motors are connected in cumulative cascade the effective total number of poles is the sum of the poles of the two motors, while the effective number is the difference if the motors are connected in differential cascade. When the effective pole number is changed the speed-torque characteristics is shifted bodily to a new synchronous speed. This does not provide continuous speed variation but alters the working range to regions near multiples of 1500 r.p.m., according to the change in pole pairs. Variable rotor resistance: a change in rotor resistance will altar the slip at which maximum torque occurs although it does not change its magnitude. As the rotor resistance is varied the motor can supply the same load at a different speed. If the lead has the characteristic shown, a change in rotor resistance from r1 to r2 will give a corresponding change in speed from n1 to n2. The main disadvantage of this system is the loss of power in the added resistance and the increase in slip with decrease of speed. This increases the motor heat dissipation and lowers the efficiency. Also, the rotor resistance can only be varied in a slip ring motor which needs periodic maintenance and produces sparking due to the rings. (c) The Schrage motor: variable speed operation of an induction motor cannot be complete without a reference to the Schrage motor. The Schrage is basically an inverted motor where the supply is fed to the rotor through slip rings. The rotor also carries an armature winding which is connected to a commutator on the same rotor. The stator ends are connected to this commutator through three sets of brushes per pole pair. Therefore, the voltage in the stator windings is made up of an induced voltage and an injected voltage. The load torque is produced by the current due to the difference in these two voltages. The running speed is achieved when these two voltages are equal. Alteration of the brush separation on the commutator alters the value of the injected voltage and as a result the running speed is varied above and below synchronous speed. (d) a variation of stator voltage alters the induction motor characteristics so that a given load can be operated at different speeds. Variation of line voltage can be obtained by the use of two thyristors back to back on each phase of the motor supply. Control of the thyristors firing angle will give control of the voltage input to the motor. As the voltage is increased or decreased the rotor speed changes until the load torque is equal to the produced torque. (e) If the input frequency is varied, while the voltage is kept proportional to the frequency the motor characteristics are shifted bodily along the speed axis without any loss in design properties. This needs basically two units. A controlled rectifier which will be able to give a controlled output voltage and a static inverter which can generate a three-phase voltage at a desired frequency. This frequency is controlled from the inverter triggering unit which operates at six times the desired frequency. These last two methods use thyristor control units.