SINGLE PHASE INDUCTION MOTOR

The single phase induction machine is the most frequently used motor for refrigerators, washing machines, clocks, drills, compressors, pumps and so on. The single phase motor stator has a laminated iron core with two windings arranged perpendicularly. One is the main The other is the auxiliary winding or starting winding

This single phase motors are truly two phase machines. The motor uses a squirrel cage rotor, which has a laminated iron core with slots. Aluminum bars are molded on the slots and short-circuited at both ends with a ring. Figure 1: Single phase induction motor

Figure 2: Squirrel cage rotor

The single-phase induction motor operation can be described by two methods: Double revolving field theory Cross field theory Double revolving theory is perhaps the easier of the explanations to understand.

Double revolving field theory A single phase ac current supplies the main winding that produces a pulsating magnetic field. Mathematically, the pulsating field could be divided into two fields, which are rotating in opposite directions. The interaction between the fields and the current induced in the rotor bars generates opposing torque.

The interaction between the fields and the current induced in the rotor bars generates opposing torque. Under these conditions, with only the main field energized the motor will not start. However, if an external torque moves the motor in any direction, the motor will begin to rotate. Figure 3: Single phase motor main winding generates two rotating fields, which oppose and counter balance one another.

The pulsating filed is divided a forward and reverse rotating field. Motor is started in the direction of forward rotating field this generates small (0.5%) positive slip. Reverse rotating field generates a larger (1.95%) negative slip

Starting Single Phase Induction Motor The three major starting technique 1. Split phase windings 2. Capacitor type windings 3. Shaded stator poles Split-phase windings - A split phase motor is a single phase induction motor with two stator winding, a main stator winding (M) and auxiliary winding (A). - These two winding are set 90 electrical degrees apart along the stator of the motor. - The auxiliary winding is design to be switched out of the circuit at some set speed by a centrifugal switch.

Capacitor Start Motors In this motor a capacitor is placed in series with aux winding By proper selection of capacitor size, the magnetomotive force of the starting current in aux winding can be adjusted to be equal to the magnetomotive force of the current in the main winding and current in aux winding lead current in main winding by 90 degree. This motor are more expensive than split phase motor and they are used in applications where a high starting torque is absolutely require Typical application are compressor, pumps and air conditioners

Shaded pole motor Only have main winding. It has salient pole and one portion of each pole is surrounded by short circuited coil called a shading coil. Shaded pole produce less starting torque than any type of induction motor starting system. They are less efficient and have a much higher slip than other types of single phase induction motors.

Speed Control of Single Phase Induction Motor For squirrel cage rotor motor the following techniques are available Vary the stator frequency Change the number of poles Change the applied terminal voltage This can be done by Used an autotransformer Used SCR or TRIAC circuit Used a resistor

The circuit model of a single phase induction motor The equivalent circuit of a single phase induction motor at standstill. Only its main windings are energized The equivalent circuit with the effects of the forward and reverse magnetic fields separated

Circuit analysis with the single phase induction motor equivalent circuit The equivalent circuit of a single phase induction motor running at a speed with only its main winding energized

In term of ZF and ZB, the current flowing in the induction motor’s stator winding is The air gap power for the forward magnetic field The air gap power for reverse magnetic field

Total air gap power Induced torque Rotor copper losses Forward rotor copper losses Reverse rotor copper losses

The power converted from electrical to mechanical Since This equation can be expressed as

Assignment A 1kW, 240V, 50Hz, 4 pole, split-phase induction motor has the following impedances: R1 = 1.52 X1 = 2.10 XM= 58.2 R2 = 3.13 X2 = 1.56 The core losses of this motor are 60W and the friction and stray losses are 32W. The motor is operating at the rated voltage and frequency with its starting winding open and the motor slip is 6.0 %. Find the following quantities in the motor at these conditions: 1. The equivalent circuit 7. Total air gap power 2. ZF and ZB 8. Pconv 3. Speed in rpm 9. Induced torque 4. Stator current 10. Output power 5. Stator power factor 11. Load torque 6. Input power 12. Efficiency