
- Electrical Machines - Home
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- DC Machines
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- Induction Motors
- Introduction to Induction Motor
- Single-Phase Induction Motor
- 3-Phase Induction Motor
- Construction of 3-Phase Induction Motor
- 3-Phase Induction Motor on Load
- Characteristics of 3-Phase Induction Motor
- Speed Regulation and Speed Control
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- 3-Phase Induction Motor Working Principle
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- Single-Phase Induction Motor Performance Analysis
- Linear Induction Motor
- Single-Phase Induction Motor Testing
- 3-Phase Induction Motor Fault Types
- Synchronous Machines
- Introduction to 3-Phase Synchronous Machines
- Construction of Synchronous Machine
- Working of 3-Phase Alternator
- Armature Reaction in Synchronous Machines
- Output Power of 3-Phase Alternator
- Losses and Efficiency of an Alternator
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- Working of 3-Phase Synchronous Motor
- Equivalent Circuit and Power Factor of Synchronous Motor
- Power Developed by Synchronous Motor
- More on Synchronous Machines
- AC Motor Types
- Induction Generator (Asynchronous Generator)
- Synchronous Speed Slip of 3-Phase Induction Motor
- Armature Reaction in Alternator at Leading Power Factor
- Armature Reaction in Alternator at Lagging Power Factor
- Stationary Armature vs Rotating Field Alternator Advantages
- Synchronous Impedance Method for Voltage Regulation
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- Significance of Short Circuit Ratio in Alternator
- Hunting Effect Alternator
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- Excitation System of Synchronous Machine
- Equivalent Circuit Phasor Diagram of Synchronous Generator
- EMF Equation of Synchronous Generator
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- Voltage Regulation of Alternator
- Synchronous Generator with Infinite Bus Operation
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- Short Circuit Ratio Calculation of Synchronous Machines
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- Potier Triangle for Voltage Regulation in Alternators
- Parallel Operation of Alternators
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- Slip Test on Synchronous Machine
- Constant Flux Linkage Theorem
- Blondel's Two Reaction Theory
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- Ampere Turn Method for Voltage Regulation
- Salient Pole Synchronous Machine Theory
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- Hunting Synchronous Motor
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- Output Power of Synchronous Motor
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- V Curves & Inverted V Curves of Synchronous Motor
- Torque in Synchronous Motor
- Construction of 3-Phase Synchronous Motor
- Synchronous Motor
- Synchronous Condenser
- Power Flow in Synchronous Motor
- Types of Faults in Alternator
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- Solid State Motor Starters
- Characteristics of Single-Phase Motor
- Types of AC Generators
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- Electrical Machines Basic Terms
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- Discussion
Split-Phase Induction Motor
A split-phase induction motor is a type of single-phase induction motor in which the stator is provided with a starting or auxiliary winding (S) and a main or running winding (M). The starting winding is displaced by 90° from the main winding as shown in the figure.

The starting winding operates only during the brief period when the motor starts up. The starting and the main windings are so designed that the starting winding (S) has a high resistance and relatively low reactance while the main winding (M) has relatively low resistance and high reactance so that the currents flowing in the two windings have reasonable phase difference (α) of about 25° to 30° as shown in the phasor diagram.

Operation of Split-Phase Induction Motor
When the starting winding of the motor is connected to the source of single-phase AC supply, the starting winding carries a current Is while the main winding carries a current Im as shown in the connection diagram.

As the starting winding is made highly resistive whereas the main winding highly inductive. Therefore, the currents Is and Im in the two windings have a reasonable phase difference of about 25° to 30° between them. As a result, a weak revolving field is produced which starts the motor. The starting torque of the split-phase motor is given by,
$$\mathrm{\tau_{st} \:=\: kI_{s}I_{m} \: \sin \: a}$$
Where, k is a constant of proportionality whose value depends upon the design of the machine.
When the motor speed reaches about 80% of the synchronous speed, then the centrifugal switch isolates the starting winding from the circuit. Now, the motor operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. The normal speed of the motor is less than the synchronous speed and it depends upon the mechanical load on the shaft of the motor.
Characteristics of Split-Phase Induction Motor
The characteristics of split-phase induction motors are given as follows −
- The split-phase motor has the starting current about 7 to 8 times of the full load current.
- The starting torque of a split-phase induction motor is about 1.5 times of the full-load torque.
- The maximum or pull out torque is about 2.5 times of the full-load torque at about 75% of synchronous speed.
- Split-phase induction motors are less expensive, thus, they are very popular motors in the market.
- Split-phase induction motors are suitable for the applications where the starting period is small. Since the starting winding of the split-phase motor has high resistance, it heats up quickly. If the starting period exceeds 5 seconds, the starting winding may burn out unless the motor is protected by a thermal relay.
- The power rating of split-phase induction motors lies between 60 W and 250 W.
Applications of Split-phase Induction Motor
The split-phase induction motors are suitable for the applications where a moderate starting torque is required and the starting period is small such as −
- To drive fans
- In washing machines
- Oil burners
- Smalls machine tools
- Blowers and centrifugal pumps
- Food mixers and grinders, etc.