What is split Phase Induction Motor?
Split-phase induction motors are single-phase motors that are commonly used in small appliances and tools. They are called "split-phase" because they have two windings: a main winding and a starting winding. The main winding is responsible for the motor's running operation, while the starting winding is used to get the motor up to a certain speed, after which it is disconnected.
Construction Split Phase Induction Motor:
The split-phase induction motor consists of a stator, which is the stationary part of the motor that houses the windings, and a rotor, which is the rotating part of the motor. The stator consists of a main winding or running winding, which is responsible for producing the main magnetic field, and an auxiliary winding or starting winding, which is used to produce an additional magnetic field during starting. The rotor is usually made of a laminated iron core with slots to hold the windings, and is supported by bearings at each end.
The auxiliary winding is connected in series with a centrifugal switch, which is located inside the motor. The purpose of the centrifugal switch is to disconnect the auxiliary winding from the main circuit once the motor reaches a certain speed. This is necessary because the auxiliary winding is only intended to be used during starting, and would overheat if left connected during normal operation.
The split-phase induction motor uses a phase shift between the main and auxiliary windings to produce a rotating magnetic field. This is achieved by using narrow conductors in the auxiliary winding, which have a higher resistance to reactance ratio compared to the main winding. The increased resistance of the auxiliary winding causes the current in the auxiliary winding to lag behind the current in the main winding, creating a phase shift between the two currents. This phase shift produces a rotating magnetic field, which causes the rotor to rotate and follow the field.
Because the auxiliary winding is only used during starting, the split-phase induction motor has a significantly lower starting torque compared to other types of motors.
Working of Split phase induction:
Split-phase induction motors are single-phase motors that are commonly used in small appliances and tools. They are called "split-phase" because they have two windings: a main winding and a starting winding. The main winding is responsible for the motor's running operation, while the starting winding is used to get the motor up to a certain speed, after which it is disconnected.
The main winding and the starting winding are both connected to the single-phase AC power source, but they are designed to have a certain phase difference between them. This phase difference creates a rotating magnetic field within the motor, which is what causes the motor to rotate.
When the motor is first started, the starting winding is energized and the main winding is not. This creates a magnetic field around the starting winding, which causes the motor to start rotating. As the motor reaches a certain speed, a centrifugal switch or a relay is activated to disconnect the starting winding from the power source and connect the main winding instead. At this point, the main winding takes over and provides the necessary power to keep the motor running.
The main winding has a current that is lagged behind the supply voltage by about 90 degrees, while the starting winding has a current that is roughly in phase with the line voltage. This phase difference between the two windings is what creates the rotating magnetic field within the motor.
Phaser Diagram of Split Phase Induction Motor:
In the phasor diagram of the split-phase induction motor, the current in the main winding (IM) lags behind the supply voltage (V) by approximately a 90-degree angle. This means that the main winding current lags the supply voltage by 90 degrees in phase.
The current in the auxiliary winding (IA) is approximately in phase with the line voltage. This means that the auxiliary winding current is almost in phase with the supply voltage, with a phase difference of around 0 degrees.
The time phase difference (Ï•) between the main winding current and the auxiliary winding current is not 90 degrees, but of the order of 30 degrees. This phase difference is enough to produce a rotating magnetic field.
The rotating magnetic field is produced by the time phase difference between the main winding current and the auxiliary winding current. The rotation of the magnetic field is what causes the rotor of the induction motor to rotate, which in turn causes the shaft of the motor to rotate. This is how the induction motor converts electrical energy into mechanical energy.
Torque Speed Characteristic of Split phase Induction Motor:
The torque-speed characteristic of a split-phase induction motor is a graph that shows the relationship between the torque produced by the motor and its speed of rotation. The graph typically has two regions: the starting region and the running region.
In the starting region, the motor is operating at low speeds and is producing high torque. This is the region where the motor is starting up and is accelerating to its full operating speed. The starting torque of the resistance start motor is about 1.5 times the full load torque. This means that the motor produces 1.5 times more torque than it does when it is running at its full load condition.
In the running region, the motor is operating at its full speed and is producing lower torque. The maximum torque that the motor can produce is about 2.5 times the full load torque, which occurs at about 75% of the synchronous speed. The synchronous speed is the speed at which the motor's rotating magnetic field is rotating at the same speed as the rotor.
The starting current of the resistance start motor is high, about 7 to 8 times the full load value. This means that the motor draws a high current when it is starting up, which is necessary to produce the high starting torque required to get the motor up to speed.
The direction of the resistance start motor can be reversed by reversing the line connection of either the main winding or the starting winding. However, this can only be done at a standstill condition, as reversing the direction of the motor while it is running can cause damage to the motor.
The advantages of a split-phase induction motor include:
- They are economical and can be replaced easily once they wear out.
- They are available in different frame sizes, making them easy to install in most machines.
The disadvantages of a split-phase induction motor include:
- They have low starting torque, so they are not suitable for applications with power ratings above 1 KW.
- They have lower power output and efficiency compared to 3-phase motors.
- They rely on the different resistance and inductance of the starting winding, which can limit their performance.
The applications of a split-phase induction motor include:
- General purpose loads such as AC units, grinders, lathes, drilling machines, washing machines, AC fans, drill presses, centrifugal pumps, floor polishers, blowers, mixer grinders, heating blowers with belt-driven, and conveyors with small belt-driven loads.
- Applications where three-phase power is not available or not required.
- Loads that do not require high starting torque, such as fans and grinding wheels, and where mechanical advantage can be used to help the motor start.