Institute of Technology, Nirma University B. Tech. Sem. V: Electrical Engineering 2EE305: ELECTRICAL MACHINES II Handout: AC Commutator Motors Prepared by: Prof. T. H. Panchal Learning Objective: Introduction AC Series Motor Universal Motor Repulsion Motor Repulsion start Induction run Motor AC COMMUTATOR MOTORS The more common commutator motors are the series motors, universal motors, repulsion motors and the repulsion-induction motors, with various modifications and combinations. AC Series Motor Direct current shunt or series motors rotate in the same direction regardless of the polarity of the supply. Thus, it might be expected that either motor would operate on alternating current. It has been found, however, that the shunt motor develops but little torque when it is connected to an ac supply. The reason of it is that the field winding, owing to its high inductance, causes the field current to lag the armature currents being the same, the main field and armature currents are in phase, therefore, theoretically same torque is
developed with a given alternating currents as with a like amount of direct current in a series motor. If an ordinary dc series motor were connected to an ac supply, it would operate, but not very satisfactorily. The reasons are (i) pulsating torque due to reversal of armature and field current every half cycle, (ii) excessive eddy current losses in the field and yoke due to alternations in the field flux, (iii) heavy sparking due to induced voltages and currents in the armature coils short-circuited by the brushes when undergoing commutation and (iv) abnormal voltage drop and low power factor due to inductance of field winding. Hence some modifications are necessary for satisfactory operation on ac. These modifications are (i) Series field with as few turns as possible in order to reduce reactance. (ii) Large number of armature conductors to compensate for reduced field and develop a given torque. (iii) Compensating winding is provided to neutralize the armature reaction effect. (iv) Large number of poles with lesser flux per pole in order to lower transformer emf in the short-circuited element of the armature winding. (v) Very small armature gap due to weak field. (vi) Laminated all parts of magnetic circuits to reduce eddy current losses. (vii) Design for operation on low voltage and low frequency in order to give less inductance and so to improve operating characteristics.
(viii) Provision of commutating or inter poles in order to have good commutation, increased efficiency and also increased output with a given size of armature core. The single phase ac series motor has practically the same operating characteristics as dc series motors. The torque or tractive effort varies nearly as the square of the current and the speed varies inversely as the current nearly. The inductively compensated ac series motor also operates satisfactorily on dc system and has increased output and efficiency. The speed of the motor while working on ac system can be controlled efficiently by taps on a transformer. The most important application of ac series motor is in electric traction service up to 1600 kw, 200 to 600 volts using 15-25 Hz. Universal Motors For some applications, it is desirable to employ a motor that will operate on either ac or dc supply. By a compromise design, fractional horse power series motors may be built to operate satisfactorily on either 50 Hz or direct current at 15 or 230 volts. Such motors are known asuniversal motors. Universal motor is a series wound motor, which operates at approximately the same speed and output on either dc or ac of approximately same voltage. The armature of universal motors is of the same construction as ordinary series motor. In the small sizes the voltage induced by transformer action in a coil during its commutation period does not tend to produce sufficient current to cause any serious commutation problem. High-resistance brushes are employed to aid the commutation. In large motors compensating winding is provided to improve the commutation. The stator core and yoke are laminated to reduce eddy currents produced by alternating flux.
The motor is simple less expensive and is used for lower rating and higher speeds. The characteristics of universal motors are very much similar to those of dc series motors. These motors are suitable for sewing machines, table fans, vacuum cleaners, portable drills, hair dryers, blowers and kitchen appliance etc. The direction of rotation can be changed by interchanging connections to the field with respect to the armature as in a dc series motor. Repulsion Motors Repulsion motors are similar to series motors except that rotor and stator windings are inductively couple i.e. the rotor current is obtained by transformer action from the stator. The stator usually carries a distributed winding like the main winding of an ordinary single phase induction motor. The rotor or armature is similar to a dc motor armature, with a drum type winding connected to the commutator. However, the brushes fixed directly opposite to each other are not connected to the supply, but are connected to each other or short circuited. The magnetic axis of the rotor is determined by the brush position. If the rotor axis were in line with stator field axis, the short-
circuited brushes would join the points of maximum potential difference, current would flow between the brushes, but no torque would be produced as the torque angle would be zero. If the rotor axis were in quadrature with stator field axis, the torque angle would be at the optimum value of 90 but the emfs induced in the two sections of the windings would be equal and the short circuited brushes would join the equipotential points, therefore, no current would flow resulting there by no torque. Actually, the brushes are placed in an intermediate position, as shown in figure below. The current induced in the rotor winding has component in time phase with the flux and produces a torque. The direction of rotation depends upon the position of the brushes. If the brushes are shifted round the commutator, the direction of rotation will reverse. Speed control is affected by varying the impressed voltage or by changing the position of brushes. The characteristics of the repulsion motor are similar to those of a series motor i.e. high starting torque and high speed at light loads. The motor is used where sturdy motor with the large starting torque and adjustable but constant speed is required. Most common use, of this type of motor is in the coil winders, in which the operator adjusts the speed by shifting the brushes; the motor is equipped with a special lever mechanism that shifts the brushes when a foot treadle is pressed.
Repulsion-Start Induction Run Motor As the name implies, the repulsion start induction motor starts as a repulsion motor and runs as an induction motor. The rotor of a repulsion motor is built like that of a dc motor i.e. the windings are connected to commutator. To convert such a motor to an induction motor, it is necessary to short circuit the commutator segment, which is accomplished by a centrifugal switch that operates as the motor attains nearly synchronous speed to throw a circular conduction disc on the commutator. In some motors the centrifugal device also lifts the brushes from the commutator in order to reduce wear of the brushes and commutator bars and to make the running operation quieter. When the motor is connected across single phase ac supply the current induced in the armature winding is carried by the brushes and commutator resulting in high starting torque. The starting torque is 2.5 to 4.5 times of full load torque and starting current is approximately 3.5 times of full load current. When the motor armature attains nearly synchronous speed, the commutator is short-circuited by the centrifugal switch and the armature acts as a squirrel cage armature. Thus repulsion start induction motor combines the desirable starting characteristics of the repulsion motor with operating characteristics of the induction motor. The direction of rotation can be reversed by changing the position of brushes. Although it is the most costly of the various types of single phase motors, it nevertheless possesses two extremely desirable characteristics (i) high starting torque for periods that are of comparatively long duration and low starting current. In connection with the latter point, these motors draw starting current that is about 60 to 70% of that taken by corresponding sizes in the capacitor-start or capacitor-start-capacitor-run motors. This type of motor is suitable for refrigerators, compressors and other drives, particularly those which have a high inertia and prolonged starting period. The usual range of sizes is from kw to 12 kw but for special applications, ratings as high as 30 kw are available.