Starting Methods of Single phase induction motors

STARTING METHODS

Single phase induction motors are named according to the starting Methods (or) device employed. So based on starting methods, the various types of single phase induction motors are

1. Split phase Motor

2. Capacitor Motor

(i) Capacitor Start Motor

(ii) Capacitor run Motor

(iii) Capacitor start and capacitor run Motor

3. Shaded Pole Motor

4. Reluctance Motor

Split Phase Motors

The stator of the split phase induction motor consits of two windings. One is known as starting or auxiliary winding and the another is called as main or running winding. Both the windings are placed in the stator slots but starting winding is displaced in space b 90°, with the main winding on the stator slots, so as to create at start a condition similar to two-phase machine, which will enable the Single Phase motor to produce rotating magnetic field at starts. Connection diagram of both the windings has been shown in Fig.3.39.

Further the windings are designed in such a manner that main winding is highly inductive and the auxillary winding has a greater resistance.

When the motor is connected to single phase supply it trans current, I which is the vector sum of currents flowing through both the windings. Since starting winding has more resistance single of lags of starting winding correct, I_s is less as compared to the??? Winding current, In (See fig 3.39(b)). The angle between the two corrects, e is called torque angle, so starting torque produced is proportional to sing and motor will no have high starting torque if 0 is approaching to 90°. Generally in split phase motor ne auxiliary winding is disconnected, when the motor picks up speed. This is achieved by mi connecting a centrifugal switch or time lag relay in series with auxiliary windings. 90/This switch or device is normally closed type and when motor picks up speed, near to about 80% it opens automatically with centrifugal force in case of centrifugal switch and due to time adjustment in case of TDR (i.e. Time Delay Relay). Thus puts the starting winding out of the circuit.

Split phase motor generally heve starting torque about twice the full load torque of the motor. Its typical torque speed curve has been shown in Fig.5.14. The curve clearly shows, the starting torque, the speed at which centrifugal switch or such device operates and its normal running region. Other important information related to its performance are:

1. Its starting current is about 6 to 8 times of the full load current.

2. Its slip is about 5% to 7% which is higher than 30 induction motor. Because in 1f motors, an average Torque becomes zero little earlier than synchronous speed. See Fig.3.40. Therefore all single phase induction motors have greater slip as compared to three phase induction motors.

3. Its power factor is low, which is about 0.6 at full load.

4. Its full load efficiency is about 60% to 65%.

5. For the same frame size its rating is about 60% as compared to that of polyphase motor.

6. Split phase motors are generally designed for 4 pole, 230V, 50Hz, 1f supply system.

Applications:

i. Drills

ii. Press burners

iii. Fans

iv. Blowers

v. Washing Machines

vi. Centifugal Pumps

vii. Grinders

viii. Wood working tools

Capacitor Motors

The stator of the capacitor motors also has two windings like split phase induction motor i.e. starting winding and running winding. But in this motor phase angle between the currents of man and starting winding is obtained by connecting a capacitor in series with the starting winding. Circuit and not by keeping the difference of resistance and reactance with ???. In fact capacitor motor is also a split phase motor but, it is in the improved forms and may be called as split phase capacitor motor. The use of capacitor has may advantages such as

1. The starting torque is much higher as compared to usual split phase motor as the torque angle is about 90°.

2. Starting live current is less.

3. Power factor of the motors gets improved.

The capacitors may be connected in series with the starting winding in three different ways. There fore, there are three types of capacitor motors.

i. Capacitor start motor

As shown in Fig.3.41 this motor has one dry type electrolytic capacitor which is specially designed for ac and is connected with centrifugal switch in series with the starting winding. As discussed in the case of resistance split-phase induction motor the centrifugal switch of this motor will also disconnect the starting winding, once the motor has picked up speed-say upto 80% of its synchromy speed. The starting capacitors used are designed for definite duty cycle and not for continuous use. So a faulty centrifugal switch ????.

The number of times of the starting winding is more in this motor as compared to starting winding turns used in resistance split-phase motor. This leads to large number of ampere times, providing large rotating flux and high starting torque. The typical torque - speed characteristic of this type of motor has been shown in Fig.3.42

Other important informations related to its performance are:

1. Its starting current is about 4 to 5 times of its full load current.

2. Its slip is about 5%

3. Its power factor is high at starting but low when running. Its full-load factor is about 0.65.

4. Its full-load efficiency is about 65%

5. In this motor angle q between the furo current is nearly 90° at start (see box fig.3.41(b)) so motor has more starting torque as compared to split-phase motors.

Applications

1. Pumps

2. Compressors

ii. Capacitor Run Motor

In this motor also as shown in Fig.3.42 (a) a capacitor is connected in series of the starting winding. No centrifugal switch or other such device has been used for disconnecting the starting winding. As the starting winding remains connected with the supply it is designed similar to running winding. The capacitor used in this type of motor is also of full time rating as it will always be in the series of starting winding. Paper speed oil filled type capacitor are used in this type of motor.

In this motor also as shown in Fig.3.42 (b) the time-phase difference between two currents is nearly 90°, so the starting torque of this motor is also higher and the rotating field produced will be a uniform one, thus providing noise less operation.

The capacitor remains in the circuit so resultant line current is less, power factor is high, which is nearly unity.

Its full load efficiency will be higher, about 75%. Fig 3.43 shows its typical forque speed characteristics.

Applications

1. Fans

2. Room Coolers

3. Portable tools

iii. Capacitor Start and Capacitor Run Motor

When high starting torque is required as lampared to capacitor run motor and the advantages of keeping the capacitor in the circuit is also to be achieved then instead of using one capacitor two capacitors of different ratings are employed.

As shown in Fig.3.44(a), one capacitor m series which centrigual switch or other such device and another capacitor is permanently connected in the starting winding circuit.

When motor is connected to single phase supply, similar to Capacitor start motor, centrifugal switch will disconnect the capacitor in its circuit and now the motor will be operating like capacitor run motor. Hence the motor drives the name, capacitor start and capacitor run motor. For Phasor diagram, see Fig.3.44(b)

The value of the capacitor, in the circuit of centrifugal switch i.e., starting capacitor, is large, about 80 μF to 300 μF and it is of electrolytic type. The running capacitor is of low value, about 3 μF to 16 μF and it is an oil impregeated paper type capacitor.

Hence in order to have both, high starting torque and better operating power factor, capacitor start and capacitor run motor can be used.

Fig.3.45 shows the torque-speed (or slip) characteristic. Starting torque of this motor is about 300% of full load torque. Resultant line current is low and power factor is high, but its cost is also high.

Applications

1. Loaded Conveyors

2. Compressors

3. Reciprocating pumps

4. Stokers

This motor is used when high starting torque is required.

The direction of rotation of all the split phase motors, whether resistance split phase motor or split phase capacitor motors, can be reversed by interchanging the connections of the supply to either of the starting winding or of the running winding.

Shaded pole motor

The stator of the motor has salient poles with saw cut at about 1/3rd position of the pole and is surrounded by a coppering usually called as shading band, see Fig.3.46(a). Poles of shaded pole motor carry single phase ac winding known as field winding.

The rotor of this motor is generally squirrel cage type but keeping in mind the cost of the motor, for very small rating shaded pole motors, solid core can also be used. The rating of the motor is so less that more than 50% of power is lost. It will not produce appreciable heat and there is no appreciable increase in the electricity bill.

When ac supply is gives to the station terminals, alternating flux is produced. This flux will induce short circuit correct in the shading band by transformer action. This flux of shading band, according to Lenz's law will not allow the flux to rise at area in this portion, but will take some time to attaining certain value. When current through main winding decreases, again shading band will not allow the field to dk out quickly. Thus, creating time-phase-displacement between the fluxes of unshaded portion and shaded portion.

Let f_s is the flux in shaded portion, f_n represents the flux in unshaded portion of the pole and f_m is the main pole flux. The relation between these fluxes has been shown in Fig.3.46 (a).

In this motor flux does not rotate through 360° but appears to sweep over pole- faces from unshaded portion to shaded portion. Under the influence of this flux small starting torque, as the angle between f_s and f_n is very small, is produced. As soon as rotor starts rotating due to single phase induction motor action, additional torque is produced and rotor rotates continuously with the speed little less than the synchronous speed.

The direction of rotation of the motor is not reversible. Motor always rotates in the direction from unshaded portion to the shaded portion. The typical speed-torque characteristics has been shown in Fig.3.47. Since the value of angle Y is very small so this motor has very low starting torque, about 50% of its full load torque. Its efficiency is also very low. However the motor is very rugged, reliable and low in cost.

Applications

i. Small fans

ii. Electric clocks

iii. Hair dryers

iv. Grama phones