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Single Phase Induction Motor

July 12, 2020

Working Principle of Single-Phase Induction Motor

In this article, we will explain the Working principle of single-phase induction motor and construction of single-phase induction motor.  When you look around you in your home, office, factory, workshop, business establishment, and everywhere, what do you find? Undoubtedly there is a single-phase motor in:

  • Air conditioner
  • Refrigeration systems
  • Ceiling fan
  • Blowers
  • Hairdryers
  • Clocks
  • Pump drive
  • Washing machines
  • Vacuum cleaners
  • Drills
  • Compressors
  • Food mixer

Also, machines supplied by single-phase supply and produce around 1 HP output power. Of course, I don’t need to tell you why you can’t dispense those motors, yes you are right they are:

  • Simple in construction.
  • Cheap in cost and maintenance.
  • Reliable.
  • Lightweight and compact in size.
  • High efficiency.
  • Require very little maintenance.
  • Rarely require repairs, and can easily repair.
  • They will last for years of operation with very little problems.
  • Construction of single-phase induction motor

Of course, it’s not what we need to know about single phase motor we need to deepen in the motor. So we’ll start with the construction of the motor, let’s go.

Construction of single-phase motors

As we know single-phase motors are very simple in everything, particularly in construction as they are a construct of:

A stator

It’s stationary and consists of lamination. We made the stator of stamping and this stamping consists of slots carrying the stator windings. The stator windings (main windings) are excited with a single-phase AC supply and create a set of N.S poles.

And the stator also carries small auxiliary winding operates only during the brief period when the motor starts up and it has the same number of poles in the main winding.

A rotor

It’s a squirrel cage (as at 3 phase squirrel cage induction motor) consists of insulated aluminum or copper bars placed in the slots. The rotor bars are shorted at both sides by end rings.

A terminal box

We use it to protect and provide safety to the electric connections as it is made of metal or plastic which isolates the inner connections from any threat.

A driveshaft

It’s a mechanical component that is used to transmit torque and rotation.

Working principle of single-phase induction motor

This is an important part of the single-phase induction motor. We will work on only one coil in the main and auxiliary winding to help understand this well.

When an AC current is applied to the main winding, a fluctuating magnetic field is produced. Similarly, when an already rotating rotor is placed in this field, the rotor will keep on rotating in the same direction. This is because the fluctuating field equals the sum of two opposite rotating magnetic fields.

If the two rotating magnetic fields are opposite, an equal and opposite torque will be produced and the total torque on the rotor will be zero and the rotor won’t start.

So, single-phase induction motor are not self-starting

But if one torque is greater than the other, the equivalent torque will be in the same direction of initial rotation and the rotor will keep on rotation and work. However, it can’t achieve without using an initial rotation. So we need to know how to add the initial rotation and why it is important. let’s see.

Initial rotation provided to single phase motor

We use an auxiliary winding capacitor arrangement to cancel any of the rotating fields; why? Because the auxiliary winding will produce two oppositely revolving magnetic fields. But what is the benefit of it!.

Well, one of these fields will cancel the RMF of the main winding, and the other will be added up. This will produce a single magnetic field revolving under a specific speed.

The final single magnetic field will give the rotor starting torque, that is, the rotor will be self-started. Hence, when the rotor attached to its specific speed it will keep on rotating even when we dispense the auxiliary winding. It can only stop working with a centrifugal switch.

We have cover two things about single phase induction motor, that is, construction of single-phase induction motor and working principle of single-phase induction motor.

It’s very simple and interesting, but that is not all. We will get more by knowing the types of a single-phase induction motor, their usage, and importance.

Single-phase induction motors, and their starting methods

You will remember that single-phase induction motors are the most common in every place around us. The most used types of single-phase motors are single-phase induction motors.

Also, Single-phase induction motors are AC motors where they can convert electric power to mechanical power to perform some physical tasks. They require only one power phase for operation so they are used in low power applications.

Construction of single-phase induction motors

Of course, single-phase induction motors have the same construction of single-phase motors. The single-phase induction motor consists of:

Stator

It’s the stationary part which has a laminated stamping used to reduce eddy current losses on its external surface. Stamping is also made of silicon steel to reduce hysteresis losses.

The stamping is provided with slots that carry two windings, that is, the main winding and an auxiliary (starting) winding. The latter is placed perpendicular to the main winding. Also, the windings are distributed.

When we apply a single-phase AC supply to the stator winding, it produced a magnetic field and the rotor rotates at a speed somewhat less than the synchronous speed which is

Ns=(120 f )/P

Where:

Ns: is the synchronous speed.

f: the frequency of supply voltage.

P: number of poles of the motor.

Rotor

It is the rotating part that consists of a cheap and low power rating squirrel cage rotor. It has an aluminum, brass, or copper bars (conductors) that are closed or semi-closed slots. The slots have short-circuited aluminum bars shortened by a ring at both ends.

The slots are a bit skewed to each other to prevent the magnetic locking of stator and rotor teeth. They also make the working of the induction motor more smooth and quiet.

Additionally, the rotor and the mechanical load are connected together through the shaft.

Working principle of single-phase induction motors

The working of a single-phase induction motor requires a lot of focus. Take a breath and let’s start.

When we supply the stator with a single-phase supply, an alternating flux is produced in the stator winding. And this alternating flux causes induced current in the rotor bars.

According to Faraday’s law of electromagnetic induction, this induced current will also produce alternating flux. Despite both alternating fluxes, the motor fails to start.

And this makes us delve deeper into why single-phase induction motors aren’t self- started and types of starting use. The reason why the single-phase induction motor isn’t self -starting can clearly be described by this by two methods: double field revolving theory and cross-field theory.

We will explain the double field revolving theory as it is easier to understand.

Double field revolving theory

This theory states that any alternating quantity can be resolved into two components. And each component has a magnitude equal to half of the maximum magnitude of the alternating quantity.  Also, both components rotate in the opposite direction to each other.

First, we need to know the symbols:

Qm: the pulsating field in the motor.

ω: the speed of rotation of pulsating fields (rad/Sec).

ϕ1,ϕ2: alternating flux produced by the stator.

We can resolve the pulsating flux Qm into two components:

Qm/2 and -Qm/2 (the negative sign only explains the direction of rotation).

Each of these components rotates in the opposite direction.

And as we explained before, when we apply a single-phase AC supply to the stator winding it produced Qm.

And as a double field, this alternating flux is divided into two components Qm/2.

Each component rotates in the opposite direction with the synchronous speed Ns.

If we notice the resultant of these two components of flux at any instant of time, we will have the value of instantaneous stator flux at that particular instant.

Qr=Qm/2+Qm/2.

At the starting of the motor, both components of magnetic flux will be equal in magnitude and oppose each other thus canceling each other. Hence the net torque will be zero and that’s the reason that makes single-phase induction motor, not self-starting motors.

Of course, we have to solve that. It’s what is made with the types of starting of single-phase induction motors. Keep attention.

Types of single-phase induction motors

We illustrated before that a single-phase induction motor has no starting torque. But when it rotates at any other speed, except synchronous speed, there is a resulting torque.

Here we introduced an auxiliary winding in the stator with an addition to the main winding and we put the auxiliary at a space angle of 90° and that’s what produced a starting torque.

And to produce a maximum starting torque, the current at the main and auxiliary windings must be at the angle of 90°.

We don’t only depend upon the auxiliary winding which can be arranged in ways to change the characteristics of the machine and give subdivided types of single-phase induction motors.

These types are classified according to the starting method they use. Thus, types of single-phase induction motors are also starting methods of single-phase induction motors. They are:

  • Split phase induction motor.
  • Capacitor start induction motor.
  • Capacitor start capacitor run induction motor.
  • Permanent split capacitor motor.
  • Shaded pole induction motor.

1. Split phase induction motor

It’s one of the most widely used types of single-phase induction motors. It is also known as resistance start motor.

We use it in markets and domestic applications like fans, blowers, washing machines, grinders, lathes because it is low in cost and has a low starting current and torque.

In addition to the main winding, a centrifugal switch is connected in series with the auxiliary to disconnect the auxiliary winding from the circuit when the motor attains a speed up to 75 to 80 % of the synchronous speed.

The nature of the main running winding is inductive so we add a high resistance to the starting winding to make the phase difference between the two windings. The current for the highly resistive winding (starting winding) is in phase with the voltage or varies by a little small angle.

Also, the current for the highly inductive winding (running winding) lags the voltage by a large angle, so the resultant of these two current produce rotating magnetic field rotates in one direction.

The starting and main current get split from each other by some angle, hence its name- split-phase induction motor.

2. Capacitor start induction motor

In this motor, we add more turns to the auxiliary winding and we placed an electrolytic capacitor serially with the auxiliary winding.

Of course, there is no change in the angle between the main winding and auxiliary winding. It is at 90°.

In this case, we connect a capacitor to the starting winding to keep the current flowing in the capacitor. Since the running winding is inductive in nature, there will be a large phase angle difference between these two currents and they produce a resulting current.

The resultant current will produce a rotating magnetic field which will produce by turn a very high starting torque.

It is high in cost and has a power rating from 120W to 7KW. So, we use this type in applications that require high starting torque.

3. Capacitor start capacitor run induction motor

This is similar to the capacitor start induction type. But here we use two capacitors in parallel and we also use a centrifugal switch. And without controversy, we use the capacitors to improve the power factor and the running conditions of a single-phase induction motor.

This motor is simple in operation, has better efficiency, and able to start large loads. Thus, the domestic and industrial applications achieve high utilization efficiency of this motor.

4. Permanent split capacitor motor

In this case, we provide only one capacitor in series with the auxiliary winding, and this capacitor work in both running and starting conditions because there isn’t a centrifugal switch.

This motor has the advantages of; good efficiency, low start current, large torque, uses simple capacitors, and more importantly, it doesn’t have a centrifugal switch.

These benefits make it the most suitable for fans, blowers, heaters, air conditioners, and so on.

5. Shaded pole induction motor

The difference in this motor is the stator as it consists of salient poles with an exciting coil. We wrapped each pole by a shading coil so we call it the shaded pole.

This motor is very economical, reliable, has a simple construction. But in contrast, it has a low power factor, has a poor starting torque, and has low efficiency and high copper losses.

The advantages and disadvantages of this motor make it the most suitable for small instruments like toys, record players, small fans, electric clocks…etc. Substantially it is available in a range of 1/300 to ½ KW.

Types of single-phase induction motor

Types of single phase induction motor aren’t new to your ears. When we talk about induction motor we illustrated that the single-phase motor can be classified into many types.

Later we said that the single-phase induction motor isn’t a self-starting motor, why? Here is the answer. The AC supply is a sinusoidal wave; which produces a pulsating magnetic field in the uniformly distributed stator winding.

If this pulsating magnetic field becomes a two oppositely rotating magnetic field, we will have a no resultant torque at the starting so the motor doesn’t run and classified as not a self-starting motor.

To solve this problem we made the stator windings into two winding the main winding and the auxiliary winding. We connect a capacitor in series with the auxiliary winding to make a phase difference when a current flow through the two coils.

The phase difference will make the rotor generates a starting torque and the motor starts to rotate.

That’s the theory that leads us to the types of single-phase induction motor.

According to that; we classify the single-phase motor upon the additional means used to make it self-starting to:

1- Split phase induction motor:

It’s the most known type of single-phase induction motor.

It has a running winding, secondary start winding, and centrifugal switch; It’s usually operating at 1/20 HP to 1/3 HP. We can find it in ceiling fans, blower motors for oil furnaces, grinder, lathes, washing machine tubs, air conditioning fans, and small pumps.

We use a normally closed centrifugal switch as a control device to take out the motor start winding from the circuit once the motor reaches 75 to 80% of its rated speed.

To start the split motor we connect the start winding in parallel with the run winding. And when the motor reaches 75% of the full speed the centrifugal switch is opened to disconnect the start winding.

And the motor continues operating with the run winding. When we want to power off the motor we close the centrifugal switch when the motor reaches 40% full-load speed.

And this motor is very economical, reliable, simple in construction, and robust. But in return, it has a low power factor, very poor starting torque, very low efficiency, and high copper losses.

2-Capacitor start induction motor:

This motor has a main winding that is arranged for a direct connection to the power source. And auxiliary winding connected in series with a capacitor and a starting switch. We use this starting switch to disconnect the auxiliary winding from the power source after starting of the motor.

Even if we use a solid-state switch, or current-sensitive and voltage-sensitive relays as a starting switch, we set the switch to stay close to maintaining the auxiliary winding circuit in operation.

When the motor starts and accelerates to approximately 80% of the full-load speed, the starting switch opens to disconnect the auxiliary winding circuit. And the motor continues running with the main winding as an induction motor.

3-Capacitor start capacitor run induction motor:

This two-value capacitor motor has different values of capacitance for starting and running. And we automatically change the value of capacitance from starting to running conditions.

It doesn’t matter what is the switch, we only provide two capacitors.

For starting conditions, we usually use a high (electrolytic type) value of capacitance to provide the needed high capacitance per unit volume. For the running conditions, we usually use a lower (metalized polypropylene unit) value of capacitance that is rated for continuous operation.

We connect capacitors in series with the auxiliary winding. When we open the starting switch it disconnects the starting capacitor from the auxiliary winding circuit. At the same time, the running capacitor connected in series with the auxiliary winding is still connected to the power source.

In this case, when the motor is running, both the auxiliary and main windings are energized and contribute to the motor output.

Adding a running capacitor in the auxiliary winding circuit increase breakdown torque by 5-30%, increase the lock-rotor torque by 5-10%, improve the full-load efficiency by 2-7 points, improve the full-load power factor by 10-20 point, reduce the full-load running current, and reduce magnetic noise cooler running.

This motor has a high starting torque so it is suitable for air conditioners, conveyors, compressors, grinders and other applications that require high starting torque.

4-Permanent split capacitor (PSC) motor:

The permanent split motor also called capacitor motor uses the same value of capacitance for both starting and running operations.

We can find the permanent motor in limited applications as with fans, pumps, heaters, air conditioners, blowers, and other applications which don’t require normal or high starting torque.

At starting, it has only a running capacitor connected in series with the auxiliary winding which reduces the starting torque. The starting torque of this motor is usually 20-30 % of the full-load torque. And we may use a high-resistance rotor to improve stable speed operation and increase the starting torque.

5-Shaded pole induction motor:

The shaded motor is a special type of single-phase induction motor.

It is extremely popular in low starting torque applications because it’s designed in several ways to develop different values of starting torque. We can find this motor in small cooling fans, in computers, inside refrigerators.

The shade windings are additional windings that occupied in each corner of the stator poles. These windings don’t have any electrical connection for starting but they use induced current to produce a rotating magnetic field.