A Split Phase Induction Motor is a type of single-phase motor that uses two windings—main winding and auxiliary (starting) winding—to produce the phase difference required for motor starting. It is commonly used in domestic and light industrial applications due to its simplicity and moderate starting torque.
Construction of Split Phase Induction Motor
A split phase induction motor consists of two windings:
Main Winding:- Connected directly to the supply.
- Has low resistance and high reactance.
- Connected in parallel with the main winding.
- Has high resistance and low reactance.
Working Principle of Split Phase Induction Motor
- When the motor is started, both windings are energized:
- The starting winding current (Is) lags behind the applied voltage (V) by a small angle.
- The main winding current (Im) lags behind the applied voltage (V) by a larger angle.
- As a result, a phase angle is created between the two currents (Is and Im), and the starting torque (Tst) is developed.
- This torque is proportional to the sine of the angle between Im and Is:
Where:
- K is a constant
- θ is the angle between the main winding current (Im) and starting winding current (Is)
- To generate a high starting torque, the phase angle is made as large as possible.
Role of Centrifugal Switch in Split Phase Motor
A centrifugal switch (C.S.) is connected in series with the starting winding as shown in the diagram. This switch is designed to automatically disconnect the starting winding when the motor reaches 70% to 80% of its full load speed.
- For motors rated at 100W or above, a centrifugal switch is used.
- For small motors, a relay is typically used instead of a centrifugal switch.
Characteristics of Split Phase Induction Motor
The typical torque-speed characteristic of a split-phase motor shows the following behavior:
- Starting torque is about 150% to 200% of full load torque.
- Starting current is 6 to 8 times the full load current.
Reversing the Direction of Rotation
The direction of rotation of a split phase induction motor can be reversed by reversing the line connections of either the main winding or the starting winding, but not both.
Applications of Split Phase Induction Motor
The split-phase motor is widely used as a cheaper alternative to capacitor-type motors for applications requiring moderate starting torque.
Typical Applications Include:
- Fans
- Blowers
- Centrifugal pumps
- Washing machines
- Oil burners
- Small machine tools
Commonly Available Motor Ratings:
- Power Range: 0.05 HP to 0.3 HP or 40W to 250W
- Speed Range: 865 RPM to 3450 RPM
Key Features Recap:
- Two windings: Main winding and Starting winding
- Starting winding current (Is) lags voltage slightly, main winding current (Im) lags significantly
- Starting torque (Tst) = K × sin(angle between Im and Is)
- Centrifugal switch (C.S.) disconnects starting winding at 70–80% speed
- Torque-speed characteristic shows 150–200% starting torque and 6–8 times starting current
- Rotation can be reversed by changing winding connections
- Used in cost-effective domestic and light industrial applications
⚡ Must to Know About Split Phase Induction Motor
- The main winding has low resistance and high reactance, whereas the starting winding has high resistance and low reactance.
- The resistance of the starting winding can be increased or decreased by connecting a series resistance, which is generally high in value and made of fine copper wire to enhance its resistive property.
- The starting winding current (Is) lags behind the applied voltage (V) by a small angle, while the main winding current (Im) lags behind the voltage by a very large angle.
- The phase angle between Is and Im is made as large as possible, because the starting torque of the split phase induction motor is directly proportional to the sine of the angle between the starting winding current (Is) and the main winding current (Im).
Conclusion: Creating a large phase difference between the two winding currents is essential to generate a higher starting torque in a split phase induction motor.
❓ What is a Split Phase Induction Motor?
A Split Phase Induction Motor is a single-phase motor that uses two windings—main winding and starting (auxiliary) winding—to produce a phase difference and generate the torque required for motor starting. It is commonly used in domestic appliances and light industrial machines.
❓ How does a Split Phase Induction Motor work?
The motor works on the principle of phase difference between the currents in the main and starting windings. This phase difference creates a rotating magnetic field, which generates starting torque. Once the motor reaches around 70–80% of its rated speed, a centrifugal switch disconnects the starting winding.
❓ What is the function of the starting winding in a Split Phase Induction Motor?
The starting winding in a split phase motor is designed to create a phase difference with the main winding. It has high resistance and low reactance, causing its current to lag less than the current in the main winding. This phase difference generates the initial torque required to start the motor.
❓ Why is a centrifugal switch used in a Split Phase Motor?
A centrifugal switch is used to disconnect the starting winding once the motor reaches about 70% to 80% of its full speed. This improves efficiency and prevents overheating of the starting winding, which is not meant for continuous operation.
❓ Can we reverse the rotation of a Split Phase Induction Motor?
Yes, the direction of rotation of a split phase induction motor can be reversed by changing the line connections of either the main winding or the starting winding (but not both).
❓ What is the starting torque of a Split Phase Motor?
The starting torque of a split phase motor is moderate, typically ranging from 150% to 200% of full load torque. It is sufficient for applications like fans, blowers, and small tools.
❓ What is the starting current of a Split Phase Induction Motor?
The starting current of a split phase motor is relatively high, approximately 6 to 8 times the full load current. This is due to the low impedance during motor startup.
❓ What are the main parts of a Split Phase Induction Motor?
The key components of a split phase induction motor include:
- Main Winding (low resistance, high reactance)
- Starting Winding (high resistance, low reactance)
- Centrifugal Switch or Relay
- Rotor
- Stator
- Shaft and Bearings
❓ What is the typical power rating of a Split Phase Induction Motor?
Split phase motors are usually available in the range of 0.05 HP to 0.3 HP (or 40W to 250W) and operate at speeds between 865 RPM to 3450 RPM.
❓ What are the applications of Split Phase Induction Motors?
Split phase induction motors are used in applications requiring moderate starting torque, such as:
- Ceiling Fans
- Blowers
- Washing Machines
- Oil Burners
- Centrifugal Pumps
- Small Machine Tools
❓ What is the difference between the main and starting winding in a Split Phase Motor?
Main Winding: Low resistance, high reactance; carries current during entire operation.
Starting Winding: High resistance, low reactance; used only during startup and disconnected afterward by a centrifugal switch.
❓ Why is the phase angle important in a Split Phase Induction Motor?
The phase angle between the currents in the main and starting windings determines the starting torque. A larger phase difference results in higher torque. The torque is given by the formula:
Starting Torque (Tst) = K × sin(θ)
where θ is the angle between the two winding currents.