How do shaded-pole motors achieve starting torque in a single-phase AC circuit?
1 Answer
Shaded-pole motors are a type of single-phase induction motors that are commonly used in low-power applications where low starting torque is acceptable. They achieve starting torque through a unique design feature known as the shading coil or shading ring.
The shading coil is a copper or aluminum loop or ring that is placed around a portion of one of the motor's pole pieces. This shading coil is typically made of a material with higher electrical resistance than the rest of the pole. Here's how shaded-pole motors achieve starting torque:
Asymmetric Magnetic Field: When the single-phase AC voltage is applied to the motor's main winding, it creates a rotating magnetic field. In a regular single-phase motor, this rotating field alone is not sufficient to generate significant starting torque because it alternates in direction and cancels itself out over time.
Phase Shift: The shading coil is strategically placed in such a way that it creates a small phase shift in the magnetic flux produced by the main winding. As a result, the magnetic field produced by the shading coil lags behind the main field. This phase shift is crucial for the motor's operation.
Unbalanced Magnetic Field: The combination of the main winding's magnetic field and the slightly delayed magnetic field produced by the shading coil creates an unbalanced magnetic field around the motor's rotor. This unbalanced field causes a rotational force, known as a starting torque, on the rotor.
Rotation: Due to the starting torque, the rotor begins to rotate in the direction of the resultant force. Once the rotor starts moving, it generates its own induced current, which further strengthens the magnetic field and allows the motor to continue running.
It's important to note that shaded-pole motors have relatively low starting torque compared to other single-phase induction motors. They are often used in applications where the load is light and the starting torque requirement is not very high, such as small fans, electric clocks, and other low-power devices.
In summary, shaded-pole motors achieve starting torque by creating an unbalanced magnetic field through the use of a shading coil, which introduces a phase shift and generates a rotational force on the rotor.
The shading coil is a copper or aluminum loop or ring that is placed around a portion of one of the motor's pole pieces. This shading coil is typically made of a material with higher electrical resistance than the rest of the pole. Here's how shaded-pole motors achieve starting torque:
Asymmetric Magnetic Field: When the single-phase AC voltage is applied to the motor's main winding, it creates a rotating magnetic field. In a regular single-phase motor, this rotating field alone is not sufficient to generate significant starting torque because it alternates in direction and cancels itself out over time.
Phase Shift: The shading coil is strategically placed in such a way that it creates a small phase shift in the magnetic flux produced by the main winding. As a result, the magnetic field produced by the shading coil lags behind the main field. This phase shift is crucial for the motor's operation.
Unbalanced Magnetic Field: The combination of the main winding's magnetic field and the slightly delayed magnetic field produced by the shading coil creates an unbalanced magnetic field around the motor's rotor. This unbalanced field causes a rotational force, known as a starting torque, on the rotor.
Rotation: Due to the starting torque, the rotor begins to rotate in the direction of the resultant force. Once the rotor starts moving, it generates its own induced current, which further strengthens the magnetic field and allows the motor to continue running.
It's important to note that shaded-pole motors have relatively low starting torque compared to other single-phase induction motors. They are often used in applications where the load is light and the starting torque requirement is not very high, such as small fans, electric clocks, and other low-power devices.
In summary, shaded-pole motors achieve starting torque by creating an unbalanced magnetic field through the use of a shading coil, which introduces a phase shift and generates a rotational force on the rotor.