Types of single phase induction motors

 

Single-phase motors run on a single-phase AC supply. Single-phase motors are classified into four main types, Induction motors, Repulsion motors, A.C series motors,s, and Unexcited synchronous motors. The classification of these four types is explained in this article.

Single phase Induction Motor

The construction of a single-phase induction motor is similar to that of a three-phase induction motor, with the exception that the stator is coiled for a single-phase winding. The flux produced by this single-phase winding is variable but not rotating. Fluctuating flux in the rotor cannot provide unidirectional beginning torque. However, if the rotor is given an initial start in another fashion, unidirectional torque will emerge instantly, and the motor will accelerate until it reaches its rated speed. These motors may be divided into the following categories based on how they are started :

 

1.     Split phase motor.

2.     Capacitor motors.

3.     Shaded pole motors.

4.     Repulsion Motors:

·       Compensated repulsion motor.

·       Repulsion start - induction run motor.

·       Repulsion induction motor.

5.     5.Unexcited Synchronous Motors:

·       Reluctance motor.

·       Hysteresis motors.

The following sections go into greater detail about each of these types.

Single phase Induction Motor

Single-phase motors are made to work with a single-phase AC source. Single-phase motors are widely used in modern life, including in homes, workplaces, workshops, and commercial organizations. As a result, several types of single-phase motors are being created to fulfill specific needs. The following are the different types of single-phase motors :

·       Split-phase winding:

In this type main winding has low resistance and high reactance and starting winding have high resistance but low reactance. The purpose of starting winding is to produce a phase difference between the main current and supplied voltage. A centrifugal switch is connected in series with the starting winding. The function of this switch is to disconnect starting winding from the circuit when the motor reaches 70 to 80% of its full load speed.

·       Capacitor motors: 

To provide the required phase difference, an electrolytic capacitor is connected in series with the beginning winding. When the motor gets near its rated speed, a centrifugal switch disconnects the starting winding and capacitor from the field circuit. The primary disadvantage is that these capacitors are intended for very low duty cycles and are only warranted for 20 uses. This kind can be further subdivided into the following categories.

1.     Capacitor start - induction run.

2.     Capacitor start - capacitor run motors.

Even when the motor is functioning regularly, the capacitor in the second kind does not get unplugged.

·       Shaded coil: 

The stator of these motors has prominent poles, while the rotor is of the squirrel cage kind. A slit has been carved into the laminated poles one-third of the way from the edge. A short-circuited Cu coil, sometimes known as a shading coil, is installed in this slot. Shading coil has a lot of inductive properties. As a result, when the current passes through the field, the magnetic axis moves towards the shaded region, and the rotor begins to rotate in that direction.

Single phase repulsion motor

The stator winding in single-phase repulsion motors is dispersed and non-salient pole type. These windings are placed in the smooth cored stator's slot. In most cases, the stator is wrapped for four, six, or eight poles. The rotor is wound similarly to a D.C. motor's rotor. You can use an axial commutator (bars parallel to the shaft) or a radial commutator (bars radial). Carbon brushes with short circuits are utilized. When electricity is supplied to the field winding, transformer action induces a current in the rotor winding.

 

Lenz's law, which is dependent on the location of the short-circuited brushes, may be used to identify the direction of this current. This induced emf creates a magnetic field surrounding the armature, and we may change the rotor poles by moving the brushes such that the rotor's N pole is under the stator's N pole. As a result of the repulsive forces between two similar poles, the motor will begin to operate. These motors can be further classified as follows:

·       Compensated repulsion motor: 

This motor has compensated winding, which is an extra stator winding. This compensated winding is designed to increase power factor and enhance speed control. This winding is smaller than the main winding and is wound in each main pole's inner slot. The compensated winding is linked to the armature in sequence.

·       Repulsion start - induction run motors:

This motor operates on the repulsion principle and short circuits the commutator with a centrifugal short circuiting switch when it achieves 75 percent of its rated speed. As a result, when the motor is functioning normally, it uses the induction principle. Because brushes don't carry any current, they may be raised to reduce frictional losses.

·       Repulsion induction motor:

This motor, sometimes known as a squirrel cage repulsion motor, is a mix of repulsion and induction motors. The stator winding is comparable to that of conventional repulsion motors, but the rotor winding is made up of a squirrel cage winding and a commutated winding similar to that of a d.c. armature. Both of these winding functions are active throughout the process.

A.C. series motor

AC series motors have the same architecture as DC series motors. Or, to put it another way, it's a DC series motor with a few tweaks. Laminating poles and a yoke reduces eddy current loss. To enhance the power factor, the magnitudes of field and armature winding reactances are decreased. The number of turns in the field winding is likewise lowered, but the armature winding needs to be increased to maintain the same torque.

·       Universal motor:

A universal motor is one that can operate on both a D.C. and an A.C. source. A universal motor is essentially a smaller version of an AC series motor. The starting torque of a universal motor is high, and it has variable speed capabilities. Because these motors operate at dangerously high speeds even when there is no demand, they are generally incorporated within the device they power.

In general, there are two types of universal motors:

1.     Concentrated pole: Non compensated type and low power rating.

2.     Distributed field:  Compensated type and high power rating.

Unexcited single phase synchronous motors

These motors rotate at a constant speed, which is the same as the synchronous speed of the rotating flux. Because their rotors do not require dc excitation, they are referred to as unexcited single-phase synchronous motors. These motors are classified into two categories:

·       Reluctance motors:

At typical running circumstances, a centrifugal switch disconnects the auxiliary winding on a reluctance motor with a split phase wound stator. Revolving flux is produced by the stator. The rotor is of the squirrel cage type and is magnetically asymmetrically built. When the stator winding is activated, a rotating magnetic field is created, which causes the rotor to experience reluctance torque. The rotation of the rotor is caused by this torque.

·       Hysteresis motors:

Split phase windings are used in the stator, and these two windings are linked to a single phase supply. The rotor is made of a smooth chrome steel cylinder with a high retentivity, resulting in a significant hysteresis loss. The rotor has no winding and is extremely smooth, resulting in extremely quiet operation. The stator and rotor poles lock up in this motor, resulting in synchronous rotor rotation.