Single Phase Motors

Single Phase Motor

(A) Introduction- A motor with only one set of winding wound on the stator to work at a low voltage A.C. of upto 250 volts is called a single phase motor. It is light in weight; small in size and cheap in cost, but its working characteristics are fewer in comparison to polyphase induction motors.

Drawbacks of Single Phase Induction Motors In Comparison To Polyphase Induction Motors- 

• These have a low efficiency.
• These produce a small torque.
• These have a low power factor.
• Their actual capacity is low in comparison to the rated capacity (upto 60% only).
• These may be used only for light jobs.

Fundamentally a Single Phase Motor is Not a Self-Start Motor- The stator of a single phase motor has only one set of winding, but it may be wound on one or several pairs of poles. When it is connected to a.c. mains, the polarity of its poles will change alternately N and S, but no rotating magnetic field will be produced as that produced in two or three phase motors. The construction of the rotor of a single phase motor is identical to that of a squirrel cage polyphase rotor, but it is not self- starting. If the rotor is slightly rotated by hand, then the motor will set into rotation in the direction in which it was rotated by hand. But, the motor can not be started with load under such condition. Sometimes, a loose pulley is used with the motor, and the belt is turned over a fixed pulley with the help of a lever, provided the motor is running at full speed in the right direction. A friction clutch may also be used to put the load on the running motor.

Starting Methods of Single Phase Motor- A single phase induction motor is wound with a single phase winding. When it is connected to A.C. mains, it will produce a pulsating flux. i.e. the polarities of the poles will change with a rise or fall in the flux intensity. The induced current will tend to flow in both directions alternatively and the rotor will remain at standstill. Therefore, a starting device is necessary for starting the motor. The following methods are used to make a single phase motor, self-starting :

1. Phase Splitting Method- In this method one phase is splitted into two parts which are supplied to the main and starting windings. Thus, both types of winding are connected in parallel and a rotating type. of magnetic field is obtained for rotating the rotor by itself. 2. Series Motor Method- In this method the motor is constructed in a manner similar to that of d.c. series motor, but with some special arrangements, which will be discussed later on. 3. Repulsion Method-In this method a repulsion effect is created between the magnetic fields of the rotor and stator.

Types of Single Phase Motors- The single phase motors are classified as follows : 

(1) Split phase induction motor 

(2) Capacitor induction motor 

(i) Capacitor start motor 

(ii) Permanent capacitor motor 

(iii) Capacitor start capacitor run motor 

(3) Shaded pole type induction motor 

(4) Universal motor 

(5) Repulsion motor 

(i) Repulsion start motor 

(ii) Repulsion start, repulsion run motor 

(iii) Repulsion induction motor 

(iv) Slip ring induction motor.

Split Phase induction motor- 

(A) Construction- A phase is splitted into two parts so as to make the motor seif-starting. For this, two windings are wound on the stator-(1) Main or running winding, (2) Starting auxiliary or split phase winding. The running winding consists of a thick enamelled copper wire so as to have a low resistance with high inductance, while the starting winding consists of a thin enamelled copper wire so as to have a high resistance with low inductance. The running winding is directly connected across the main supply line, while the starting winding is connected through an automatic change over switch or a centrifugal switch, The rotor used with this motor is of squirrel cage type. It is made of laminated iron cores having closed slots all around the periphery of the rotor, in which copper bars are driven and their ends are welded to conductor rings on either side. 

Phase splitting method

(B) Working Principle- When the stator windings are connected to a.c. supply, then the main winding current will lag behind the starter winding current by 90°, because the inductance of the main winding is much higher than that of starting winding. In this way a phase difference will be produced between the two windings, resulting in the production of a rotating magnetic field, which will rotate the rotor. Phase difference between splitted phases The purpose of this winding is to start the motor by itself. But, once the motor is started and it has, attained 75% to 80% of the synchronous speed, the starting winding is disconnected by the operation of an automatic change over switch or a centrifugal switch. The motor will then run at full speed. The connections of the main and starting windings are shown in Fig. 18.3. If the starting winding is not disconnected then it will produce the following defects in the motor :

Connections of split-phase induction motor

1. The starting winding will draw more current and will cause heat to be developed in the motor, which will be able to damage the stator windings. 
2. The efficiency of the motor will be reduced. 
3. A humming sound will be produced in the motor.

Capacitor Induction Motor- A split phase motor has a low starting torque, thus it does not start at load. If a capacitor is used in it, then it will be able to produce more torque. It consists of running and starting windings and its rotor is cage wound like a split- phase rotor. There are three types of capacitor motors, which are as follows : 

1. Capacitor Start Motor- In this motor, a capacitor is connected in series with the starting winding, The capacitor improves the power factor and a leading current flows in the starting winding. Hence, the torque of the motor will be increased and it will be able to start at load. A centrifugal switch disconnects the starting winding as in the case of a split-phase motor. 

2. Permanent Capacitor Motor- The construction of this type of motor is similar to that of a capacitor start motor. But, it has no centrifugal switch and the starting winding remains connected permanently to the supply line through a capacitor. Both the windings remain in parallel across the supply line, as shown, in  (b). The thickness of wire for both types of windings remain the same and the capacitor produces the required phase difference for starting the motor by itself. This type of motor has a low starting torque, hence it is suitable only for table and ceiling fans. 

Permanent Capacitor Motor

3. Capacitor Start, Capacitor Run Motor- Two capacitors are used in this type of motor, out of which C, (2 to 2.5 µF) is connected directly and the other C,(10 to 20 µF) through a centrifugal switch. The two capacitors are connected in parallel and this group of C, and C, winding, see Fig. 18.6. In the start, both the capacitors remain in the circuit, thus they produce a high starting torque for starting the motor at load. As the motor attains its full speed, one capacitor is switched out by the operation of the centrifugal switch. Thus, one capacitor remains in the circuit, so that the motor can drive the load easily. This type of motor is suitable for such Jobs which require a high starting and running torque; e.g., air- conditioner, pump blower etc, 

Capacitor Start, Capacitor Run Motor

Reversal of Rotational Direction of Single Phase Motors- The rotational direction of a single phase motor can be easily reversed by changing the connections of the starting or running winding, as shown in. If, with normal connections, the rotor rotates in clockwise direction, (a), then after changing the connections of running winding. (b) or of starting winding. (c) the rotor will rotate in anticlockwise direction.

Reversal of Rotational Direction of Single Phase Motors

Characteristics of Split Phase and Capacitor Motor- The torque-speed curves of split-phase and capacitor induction motors are (a) and (b) respectively. On comparing both the curves, it becomes clear that a capacitor induction motor has more starting torque. The torque-speed curves of three types of capacitor induction motors are shown in (a), (b) and (c). On comparing the three curves, it becomes clear that- 

(i) A capacitor start motor has a high starting torque. As the motor speeds up and continues to run only on running windings, the torque will be reduced. 

(ii) The torque of a permanent capacitor induction motor increases with a rise in speed, but its starting value is comparatively low. 

(iii) The "capacitor start, capacitor run' motor has a high starting and low running torque, but the torque increases with a rise in speed.

Torque-speed curves

Shaded Pole Induction Type Motor- the 2/3rd parts of stator poles and shading windings, having low resistance copper rings, wound on the 1/3rd part of each stator pole, The 1/3rd part of each pole is called a shaded pole. The shading winding is also termed as starting winding. Shaded poles are made on one side of the main poles in a definite sequence. A squirrel cage type rotor is used in this motor. This type of motor consists of one main winding, wound When the main winding is supplied with A.C. then an alternating magnetic flux is produced in the main poles. 

Shaded Pole Winding System

The flux will increase with a rise in current. When the flux reaches its maximum value (due to peak current), the rate of change will become zero, and a part of flux will pass through the shaded poles. Thus a current will be induced in them, which will oppose the main flux as per Lenz's Law. way, a phase difference is developed between two fluxes, since they reach at their peaks at different moments. The magnetic field will thus act as a rotating magnetic field and will produce a torque in the rotor. The starting torque of the motor is very low as shown in The torque increases with a rise in speed. 

Torque-speed curves of Shaded Pole Motor

Thus a small load should be driven in the starting. shaded pole The rotational direction of the motor cannot be changed. It will be in the direction of shaded poles. For the reversal of rotational direction, the sequence of shaded poles will have to be changed, which is not easy. This type of motor is suitable for small fans, hair dryer, electric wall clocks, phonographs, etc., because these instruments require a low wattage (15-20 watts) motor. 

Universal Motor- It is designed to work on both a.c. and d.c. supplies. Its construction and working principle is the same as that of a d.c. motor. It consists of a field winding of thick enamelled copper wire and a commutator and brushes for connecting the armature to the source of supply. The field winding is connected in series with the armature winding It has a high torque and high speed, but its speed is lower than that of a d.c. motor. 

Universal Motor

The speed is above the synchronous speed but below the dangerous value of the speed. 

1. Laminated Field- Laminated iron cores are used for the construction of the stator and rotor, so that the eddy currents produced by the alternating flux may not produce excessive heat in the motor. 

2. Large Armature Winding- A field supplied with a.c. will have more inductance and thus will cause a greater voltage drop. Since, the inductive reactance  X_L=2. Π f. L, therefore, in order to reduce its value, JA either the inductance or the frequency should be reduced. The reduction in frequency is not easy since  Indian standard frequency is 50 c/s. Also the reduction in inductance is not advisable, because it will reduce the overall power of the motor. Hence, the torque and power factor of the motor will be reduced. The above defect is eliminated to some extent by increasing the number of turns in the armature winding.

 3. Compensating Winding- A large-armature winding will produce more armature reaction, which will reduce the power factor and efficiency of the motor. Therefore, a compensating winding is placed in the stator. Two types of compensating winding connections are shown in Fig. 18.13. In conductively compensated series motor, the compensating winding is connected in series with the armature, The ampere turns of this winding are kept equal to that of armature winding but of opposing nature. The armature reaction effect can not be eliminated completely, because certain amount of flux leakage is always left between the windings. This method is suitable for a.c./d.c. universal motors. It Increases the speed regulation and speed- torque. In inductively compensated series motor, the compensating winding is a short-circuited winding and it is not connected to any other winding or the source of supply, as shown in  This winding works as the secondary winding of a transformer. The ampere turns of this winding are also kept equal to that of armature winding but of opposing nature. This winding neutralizes the armature reaction to great extent, but does not eliminate it completely. If the compensating winding is kept open circuited then it will increase the inductance of the motor which is undesired. This method of compensating provides more speed regulation, power factor and efficiency in comparison to conductively compensated motor.

Compensated Universal Motors

4. Elimination of Commutation- When A.C. is supplied through the brushes to the armature then it causes more sparking at the commutator. At the instance when a brush touches two commutator segments at a time, the brush will work as secondary winding of a transformer. More current will flow in the brush and it will produce more sparking. The sparking opposes the main flux and reduces the torque. 

Elimination of Commutation

This defect is eliminated by inserting resistors between the armature coils and the commutator segments, as shown in Fig. 18.14. The magnitude of short-circuited current may rise upto 4 times of the normal current value. These high currents are dissipated by the inserted resistors upto a few seconds and the sparking is eliminated.

Connection of Interpoles in compensated series motor

Application of Series Motors. 

1. Electric Trains- 200 to 600 volts, 15-25 c/s, 2200 H.P. motors are used in, railway engines. 
2. Domestic and Industrial- Single phase, 230 volts motors are used in hair dryers, sewing machines, blowers and kitchen appliances.

Single Phase Slip-ring Induction Motor. The rotor of a single phase slip-ring induction motor is exactly similar in construction to that of a 3-phase slip- ring induction motor as shown in Fig. 18.16. It has a 3-phase, star-connected winding, which is connected to 3-phase external rheostat through three slip-rings. The stator has a single phase running winding along with resistor and two inductive coils connected across it. The inductive winding produces a phase difference, and the resistor and inductor splits one phase into two parts so as to produce a rotating magnetic field.

Single Phase Slip-ring Induction Motor

When single phase A.C. is supplied to the stator, then a phase difference of 90° is produced due to the reactance of inductive coil and resistance, and thus a rotating magnetic field is produced. The rotor rotates with a high starting torque. On reducing the resistance at the starter, the speed of the rotor will increase, and at full speed the rotor windings become short-circuited.n The speed of the motor remains below the synchronous speed, and the motor is capable to drive a load at different speeds. The motor is suitable for refrigerators, grinders, lathes etc., but its capacity is limited upto 1 H.P. only.

Principle of Repulsion Motors. We know that the like poles of the two magnets repel and unlike poles attract each other. This very principle is utilised in the operation of a repulsion motor. The construction of the stator of a repulsion motor is identical to that of a single phase universal motor. It has two or four stator poles having stator windings would in the stator slots. The construction of the rotor is quite similar to that of a d.c. armature. A commutator is mounted on the rotor shaft. Two brushes are placed in such a way that they make a sliding contact with the commutator. The brushes are advanced by 20° in the rotational direction, so as to meet the requirements of a.c. supply. The rotor has no connection with the stator, and it works as the secondary winding of a transformer while the stator works as primary. When A.C. is supplied to the stator, then an e.m.f. is induced in the rotor winding due to mutual induction. It is necessary to short-circuit the brushes to start the flow of induced current in the rotor. The induced current will energise the rotor poles. The polarities of the rotor and stator poles will be the same, and thus they will repel each other and will set the rotor under rotation.

Types of Repulsion Motors. There are three types of repulsion motors, which are as follows 

1. Repulsion start motor 

2. Repulsion start-run motor 

3. Repulsion induction motor 

1. Repulsion Start Motor- The construction of a repulsion start motor is the same as explained in Art. In this motor, the brushes are advanced by 20° from the direction of main field or in other words When 230 volts A.C. is supplied to the stator, then 25- 30 volts are reduced in the rotor windings. The brushes are short-circuited, so that a current may flow in the rotor windings which may energise the rotor poles. Since, the stator and rotor poles have the same polarities, therefore, the rotor will start to rotate. The wire used for short-circuiting the brushes should have a low resistance. 

Repulsion start motor

As the motor speed up, the torque will increase. Initially the motor has a low torque and draws small current in the start. Different speeds can be obtained by advancing or retarding the brushes. If the brushes are advanced further, then the motor speed will increase and vice-versa. The speed will be zero if the brushes are adjusted in the neutral axis. (b) shows the speed-torque curve, while 

(a) shows the position of the brushes. If the brushes are further advanced beyond perpendicular axis to the main field, then the speed of the motor will be reduced. For an advance angle of 20°, the motor speed will be maximum, but in opposite direction. The brushes should be moved gently. The motor is suitable for lifting heavy-loads, e.g., cranes, hoists etc.

 2. Repulsion Start-run Motor-The construction and working principle of this type of motor is similar to that of a repulsion start motor. The motor starts in the same manner as a repulsion start motor starts. But, as the rotor attains nearly 70% speed of its full speed value, a copper ring short-circuits all the segments of the commutator. 

Repulsion start-run motor

The copper ring is thrown over the commutator by the action of centrifugal mechanism. Also, the brushes are lifted above from the commutator due to same mechanism, otherwise the brushes will continue to cause sparking. After short-circuiting of the commutator segments, the motor will start to work as an induction motor. In the start, the motor was having a high torque with low speed, but now it will have a low torque with high speed.

Speed-torque curve of a repulsion start-run motor

3. Repulsion Induction Motor- In this motor, the stator winding is similar to that of an- induction motor of a repulsion start motor, but the rotor has two separate winding. The inner winding is of squirrel cage type, while the outer winding is of repulsion start rotor type, which is connected to a commutator. The commutator needs not to be short-circuited, and the brushes also remain to touch with the commutator. In the start, the motor works as an ordinary repulsion motor, and the cage winding has no effect owing to its high inductance. At full speed both the windings . become operative. 

Repulsion Induction Motor

 Moreover, the effect of squirrel cage winding will be more, which will provide a stability to the motor speed. Hence, the motor will run at a constant speed even at varying load. The construction and speed-torque curve of the motor is shown (a) and (b) respectively. The motor has a high efficiency, high starting torque and a power factor upto 0.9. As a disadvantage, there is more sparking at the commutator and the motor will require more maintenance. It is suitable for driving workshop machinery.

 Selection Of Single Phase Motor For Various Jobs

Jobs	Types Of Motor
Pressure pump Presser switch Lathe Centrifugal pump Lift  Gramophone Compressor Refrigerator Air-Conditioner Electric Wall Clock Sewing Machine Vacuum Machine Cooler Shaving Machine Mixer Cum Grinder Cinema Projector	Capacitor run induction motor Split-phase induction motor Capacitor start, capacitor run motor Capacitor start, capacitor run motor Universal Motor Shaded Pole Motor Repulsion start-run motor Capacitor start, capacitor run motor Capacitor start, capacitor run motor Shaded pole motor Universal Motor Universal Motor Capacitor Start Shaded Pole Motor Universal Motor Universal Motor

Types of Motors used in Railway Traction. D.C. series motors are suitable for all types of services, but more particularly for suburban services where high rate of acceleration is required, in view of its simple and robust in construction, giving high starting torque and smooth speed control. Single phase A.C. compensated series motors are not suitable for suburban services where stops are frequent. However such motors which have speed torque, speed current characteristics similar to those of D.C. series Inotors are extensively used for mains lines work. The following items should be remembered. 

(1) The D.C. system is preferred for suburban services and road transport where stops are frequent and distribution distances are small. 

(2) A.C. single phase system is invariably applied for main line service, where the cost of overhead structure is of extreme importance and rapid retardation and acceleration is not so important as for suburban railways. 

(3) Three phase A.C. system is employed in some hilly areas where output power required is high and regeneration on large scale is possible. However this system is no longer likely to be adopted an account of disadvantage of two overhead contact wires and control difficulties.