Single Phase Induction Motors Prof. T. H. Panchal Asst. Professor Department of Electrical Engineering Institute of Technology Nirma University, Ahmedabad
Introduction As the name suggests, these motors are used on singlephase supply. Single phase motors are the most familiar of all electric motors because they are extensively used in home appliances, shops, offices etc. It is true that single phase motors are less efficient substitute for 3-phase motors but 3-phase power is normally not available except in large commercial and industrial establishments. Since electric power was originally generated and distributed for lighting only, millions of homes were given single-phase supply. This led to the development of single-phase motors. Even where 3-phase mains are present, the single-phase supply may be obtained by using one of the three lines and the neutral.
Types of Single Phase Motors Single-phase motors are generally built in the fractionalhorsepower range and may be classified into the following four basic types: 1. Single-phase induction motors (i) split-phase type (ii) capacitor type (iii) shaded-pole type. A.C. series motor or universal motor 3. Repulsion motors (i) Repulsion-start induction-run motor (ii) Repulsion-induction motor 4. Synchronous motors (i) Reluctance motor (ii) Hysteresis motor
Single Phase Induction Motors A single phase induction motor is very similar to a 3- phase squirrel cage induction motor. It has (i) a squirrel-cage rotor identical to a 3-phase motor and (ii) a single-phase winding on the stator. Unlike a 3-phase induction motor, a single-phase induction motor is not self starting but requires some starting means. The single-phase stator winding produces a magnetic field that pulsates in strength in a sinusoidal manner. The field polarity reverses after each half cycle but the field does not rotate. Consequently, the alternating flux cannot produce rotation in a stationary squirrel-cage rotor.
However, if the rotor of a single-phase motor is rotated in one direction by some mechanical means, it will continue to run in the direction of rotation. As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below the synchronous speed. Once the motor is running at this speed, it will continue to rotate even though single-phase current is flowing through the stator winding. This method of starting is generally not convenient for large motors.
Fig. (1) 1-phase supply Stator winding Squirrel cage rotor
Fig. (1) shows single-phase induction motor having a squirrel cage rotor and a single phase distributed stator winding. Such a motor inherently does not develop any starting torque and, therefore, will not start to rotate if the stator winding is connected to single-phase a.c. supply. However, if the rotor is started by auxiliary means, the motor will quickly attain the final speed. This strange behavior of single-phase induction motor can be explained on the basis of double-field revolving theory.
Double Field Revolving Theory This theory makes the use of idea that an alternating uni-axial quantity can be represented by two oppositely rotating vectors of half magnitude. Accordingly, an alternating sinusoidal flux can be represented by two revolving fluxes, each equal to the value of the alternating flux and each rotating synchronously in opposite direction. As shown in fig. (a), let the alternating flux have a maximum value of ϕ m. Its component fluxes A and B will each be equal to ϕ m / revolving in anticlockwise and clockwise directions respectively.
After some time, when A and B would have rotated through angle +θ and θ, as in fig. (b), the resultant flux would be m cos cos m After a quarter cycle rotation, fluxes A and B will be oppositely directed as shown in fig. (c) so that the resultant flux would be zero. After half a cycle, fluxes A and B will have a resultant of - x ϕ m / = ϕ m. After three quarter of a cycle, again the resultant is zero as shown in fig. (e) and so on. If we plot the values of resultant flux against θ between limits θ = 0 to θ = 360, then a curve similar to the one shown in fig. () is obtained. That is why an alternating flux can be looked upon as composed of two revolving fluxes, each of half the value and revolving synchronously in opposite directions.
If the slip of the rotor is s w.r.t. the forward rotating flux (i.e. one which rotates in the same direction as rotor) then its slip w.r.t. the backward rotating flux is ( - s). Each of the two component fluxes, while revolving round the rotor, induces an emf and this produces its own torque. The two torques (called forward and backward torques) are oppositely directed, so that the resultant torque is equal to their difference as shown in Fig. (3)
Power developed by a rotor is If N is the rotor r.p.s. then torque is given by 1 R I s s P g s R I k s R I N T s N N Now R I s s N T s g s g 1 ) (1 1 1
Hence the forward and backward torques are given by Total Torque watt synch s R I T watt synch s R I T s R I K T and s R I K T b f b f.. T f T b T
Fig. shows both torques and the resultant torque for slips between zero and +. At standstill, s = 1 and (-s) = 1. Hence, T f and T b are numerically equal but, being oppositely directed, produce no resultant torque. That explains why there is no starting torque in a single phase induction motor. However, if the rotor is started somehow, say, in the clockwise direction, the clockwise torque starts increasing and, at the same time, the anticlockwise torque starts decreasing. Hence, there is a certain amount of net torque in the clockwise direction which accelerates the motor to full speed.
Making Single Phase IM Self-starting A single phase IM is not self starting. To make the motor selfstarting, it is temporarily converted into a two phase motor during starting period. For this purpose, stator of single phase IM is provided with an extra winding, known as starting( or auxiliary) winding, in addition to the main or running winding. The two windings are spaced 90 electrically apart and are connected in parallel across the single phase supply. It is so arranged that the phase difference between the currents in the two stator windings is very large. Hence, the motor behaves like a two phase motor. These two currents produce a revolving flux and hence make the motor self-starting.
Main Winding Rotor Starting Winding Stator Air Gap
There are many methods by which the necessary phase difference between two currents can be created. 1. Split Phase Induction Motor In split phase machine, the main winding has low resistance but high reactance whereas the starting winding has high resistance, but low reactance. The resistance of the starting winding may be increased either by connecting a high resistance R in series with it or by choosing high resistance fine copper wire for winding. Hence, as shown in fig. the current I s drawn by the starting winding lags behind the applied voltage V by a small angle whereas current I m taken by the main winding lags behind V by a very large angle.
Single Phase Supply Im Main Starting Winding Winding Is R S Rotor
Phase difference between I s and I m is made as large as possible because the starting torque of this motor is proportional to sinα. A centrifugal switch S is connected in series with the starting winding and is located inside the motor. Its function is to automatically disconnect the starting winding from the supply when the motor has reached 70 to 80% of its full load speed.
As seen from the torque/speed characteristic of this motor, the starting torque is 150 to 00% of the full load torque with a starting current of 6 to 8 times the full load current. These motors are often used in preference to the costlier capacitor start motors. Applications: Fans and blowers Centrifugal pumps Washing machines Small machine tools Domestic refrigerators Size ranges from 40 to 50 W with speeds ranging from 3450 to 865 rpm.
. Capacitor-start Induction-run single phase IM In these motors, necessary phase difference between I s and I m is produced by connecting a capacitor in series with the starting winding. The capacitor is generally of electrolytic type and is mounted on the outside of the motor as a separate unit. Starting Winding Im Main Winding Single Phase Supply Is C S Rotor
Capacitor Is S Im Supply Running Starting
The capacitor is designed for extremely short duty service and is guaranteed for not more than 0 periods of operation per hour, each period not to exceed 3 seconds. When the motor reaches about 75% of full speed, the centrifugal switch S opens and cuts out both starting winding and the capacitor from the supply, thus leaving only the running winding across the supply. Current Im drawn by the main winding lags the supply voltage V by a large angle whereas Is leads V by a certain angle. The two currents are out of phase with each other by about 80% as compared to nearly 30 for a split phase machine. Their resultant current I is small and is almost in phase with V.
Since the torque developed by a split phase motor is proportional to the sine of the angle between I s and I m, it is obvious that the increase in the angle alone increases the starting torque to nearly twice the value developed by a standard split phase induction motor.
Capacitor Start and Run Motor In this motor, starting winding and capacitor are connected in the circuit at all times. Advantages of leaving capacitor permanently in circuit are Improvement of over load capacity of the motor Higher power factor Higher efficiency Quieter running of the motor Motors which start and run with one value of capacitance in the circuit are called single-value capacitor run motors Other which start with high value of capacitance but run with a low value of capacitance are known as two-value capacitor run motors.
Single Value Capacitor Run Motor Run Is Im Supply Start C Rotor
Single Value Capacitor Run Motor
Single Value Capacitor Run Motor Forward Reverse A B Supply
Two Value Capacitor Run Motor A B Low S Supply Running Starting