Split Phase Induction Motors - Principle and Types

This presentation describes Single phase Induction Motor operation and types

1. Split Phase Starting Methods of Single Phase Induction Motors

2. Starting Methods of 1-phase Induction Motors: As single phase Induction Motors do not possess inherent self starting torque, one has to employ some external means to make it self starting. Commercial single phase induction motors are classified in accordance with the methods of starting employed. The types and methods usually employed in industry are as follows: Split phase method Shaded pole method

3. Split phase method: When a motor is provided with two windings, even though these are excited from the same voltage, the currents in the two windings can be made out-of-phase by adjustment of the impedance of the auxiliary winding in relation to the main winding. As a result main m.m.f. F m and auxiliary winding m.m.f. F a constitute an unbalanced field set with 90° electrical space phase relationship. The two symmetrical components, now being unequal F f ≠ F b (F f > F b is desired); r esults in the net production of starting torque. This is how a single phase motor is made self starting.

4. Phase Splitting Methods: Two of important methods of phase splitting are: Resistance Split phase method. Capacitor Split phase methods.

5. Resistance Phase Splitting: Schematic diagram is as shown in Figure. The motor employs an auxiliary winding with a higher R/X ratio as compared to the main winding. This difference in the R/X ratio causes the auxiliary winding current I a to lead the main winding current I m by angle α, as shown in Phasor diagram. The fields created by the two currents also have a phase difference of α, thereby constituting an unbalanced system. The result is the production of the starting torque. V Iaa I m α

6. When motor attains 75 or 80% of synchronous speed the auxiliary or starting winding is taken out of circuit. Otherwise it will result in overheating and burning out of auxiliary winding owing to its low current carrying capacity & noisy performance. The starting torque is 150 to 200% of full-load running torque T he starting current is 6 to 8 times full-load current. Because of low starting torque, this motor is rarely used in sizes larger than 1/4 kW. Standard sizes are from 1/8 kW to 1/4 kW with a speed of 1440 rpm. Applications: fans blowers centrifugal pumps refrigerators, duplicating machines, wood working tools, grinders, oil burners other low starting torque applications.

7. Capacitor Split phase Motor: B y using a capacitor in series with the auxiliary winding high starting torques can be obtained. The auxiliary winding along with the capacitor may be disconnected after starting. However, generally the capacitor and auxiliary windings are allowed to remain connected. It helps in improving the overall motor performance and in particular the power factor. Thus, two types of capacitor split-phase motors (also known as capacitor motor) exist. Capacitor Start Motor Capacitor Start and Run Motor.

8. Capacitor start motor: The motor uses the capacitor only for the purpose of starting. The capacitor value is usually chosen as to give α = 90° elect The range of capacitance is 250µF or above.

9. As the motor picks up the speed, the starting winding is to be disconnected, because: The capacitance that gives the largest starting torque is four times too big for the best running conditions if the capacitor is used only for starting, it can be electrolytic type of capacitor which is much larger than smaller and cheaper paper capacitors required for continuous operation. This type of motor has become popular because of development of cheap and reliable electrolytic condensers. The usual size range is from 1/10 to 1/2 kW but larger sizes are also available.

10. A typical torque-speed characteristic is shown in Figure. Applications: Refrigerators C ompressors O ther applications involving a hard starting torque such as in small portable hoists. The direction of rotation of motor may be reversed by interchanging the connections to the supply of either main or auxiliary winding.

11. Capacitor Start Capacitor Run Motor: Its also called Two-value capacitor motor. The schematic diagram is shown in Figure. As the name suggests, the motor uses a capacitor not only for starting but also continuous (run) operation. The capacitor used permanently is called the “run capacitor”.

12. The run capacitor C R small value oil impregnated paper continuous rating capacitor. Start capacitor C S much larger value electrolytic short duty capacitor. When the motor attains 70% of synchronous speed the starting capacitor C S is taken out of the circuit by the operation of centrifugal switch. Capacitor C R remains permanently in series with the starting or auxiliary winding. These motors besides their ability to start heavy loads, they are extremely quiet in operation, have better efficiency and power factor when loaded and develop up to 25% greater overloaded capacities. The disadvantage of these machines is only high cost.

13. The Speed- Torque characteristic is as shown in figure. These motors are often employed requiring a quiet operating motor. This type of motors are becoming increasingly popular and are now manufactured in the larger sized single phase motors. The direction of rotation may be reversed by interchanging the connections to the supply of either main or auxiliary windings.

14. The End