Preface for 3-Phase Induction Motors

Hello everyone, I hope that you all are doing in your lives. The topic for today’s is AC Motors which are popularly in use nowadays due to its wide range of operation and flexibility. For example ovens, pumps, exhaust fans, hairdryers, etc. In AC Motors we are going to study the most important type of AC Motors I.e 3-phase induction motors. Besides we are also going to study the classification of AC Motors, the Advantages, Disadvantages of the type, and also the difference in the types of rotors.

AC Motors

An electric motor that works on AC supply is called an AC Motor. The motor mainly consists of two parts one the stator and the other the rotor. AC motors can be broad of two types induction and synchronous AC motors. We are studying induction motors; in which the electric current at the rotor requires to obtain the necessary torque is obtain by the electromagnetic induction of the stator. And its working principle is that; whenever a current-carrying conductor or a wire is place in a magnetic field it faces a force. Which in turn helps the motor to get in motion.

Types

According to the principle of operation

  1. Synchronous Motors
    1. Plain
    2. super
  2. Asynchronous Motor
    1. Squirrel Cage
    2. Slip Ring

According to type of current flow

  1. Single-Phase
  2. 3-Phase

According to speed

  1. Firstly Constant Speed
  2. secondly Variable speed
  3. And lastly Adjustable Speed

3-Phase Induction Motors

Certainly out of all the types mention above 3-phase induction motors are widely in use for industrial as well as low-power applications in domestic equipment. More than 80% of all motors used in the world are induction motors. This is because they are cheap and most robust. These motors also go by the name asynchronous motors because their operating speed is slightly less than the synchronous ones.

Construction of 3-Phase induction Motors

However, an induction motor consists of two main parts;

  1. Stator
  2. Rotor

Apart from these two main parts, it also requires bearings, bearing covers, end plates, etc. for its assembly. As the name says the stator is the stationary part and the rotor is the moving part.

Stator

  1. The stator of a 3-Phase induction motor has three main part namely- stator frame, stator core and stator winding.
  2. The stator frame is made up of number of stamping, which consist of slots to receive the stator windings. However these stamping are isolated from each other.
  3. The stator is carries a 3-phase winding and is fed up with 3-phase supply.
  4. Besides, the stator core is made up of high silicon-sheet steel lamination of 0.4 to 0.5 mm thickness. similarly these are also separated from each other by varnish or paper
  5. However the lamination for the small machines are in the form of complete ring, but for large they are made in sections.
  6. The stator windings may be in star or delta connection. These are wound for a certain number of poles. The exact number of poles is determine on the bases of the require speed.
  7. Both of these variables are inversely proportional to each other. Thus increase in number of poles means decreasing speed.

Rotor

The rotor also has laminations and is of the material as the stator. These laminations have slots on their outer surface to house the rotor conductor. However, there are two types rotor

Squirrel Cage rotor

The motor in which these types of rotors are used, are usually know as squirrel cage IM. Almost 90% of IM are squirrel cage type as this type of rotor has the simplest and most rugged construction. For example:- Water Pump. Diagram Squirrel cage rotor

These rotors consist of a core that is made up of cylindrical laminations with parallel slots to carry the rotor conductors. The rotor conductors are heavy bars of copper, aluminum, or alloy. One bar is placed in each slot, rather the bars are inserted from the end when semi- close slots are used. The rotor bars are brazed or electrically welded to two end rings to form connectivity. These end rings are short circuits to each other. Thus rotor bars are also permanently short circuit on themselves. Hence it is not possible to add external resistance in series with the rotor circuit for starting purpose. The rotor slots are usually not parallel to the shaft but slightly askew.squirrel cage 3-phase induction motor

Advantages
  1. The rotor is of robust and cheap construction
  2. The absence of brushes decreases the risk of sparking
  3. Squirrel-cage rotors require less maintenance
  4. Lastly these motors have higher efficiency and better power factor.
Phase Wound or Slip Ring Rotor

The rotor here is wound for as many poles as the number of stator poles. it always has 3-phase winding even when the stator is having two-phase winding. The rotor winding is always in the form of a star connection. The remaining ends of the winding are together connected to the slip ring. With the help of these slip rings and brushes, extra resistance can also be added in series with each rotor phase. Diagram Slip ring rotor of a 3-phase induction motor

This will increase the starting torque obtain by the motor and will also help in reducing, the starting current. Whenever running beneath normal conditions, the slip-ring is automatically short by means that of a metal collar, that is push along the shafts and connects all the shafts along. The brushes are automatically raising away the slip-ring to cut back the frictional losses and therefore the wear and tear. Hence under normal conditions, slip-ring rotors are short on themselves like squirrel-cage rotors.

Advantages
  1. High starting torque and low starting current
  2. Additional resistance and the rotor circuit can be connection to control the speed

Principle Of Operation

Induction Motor works on the principle of EMI. The 3-phase IM consists of a stator that is stationary and a rotor that is in motion. However, the rotor is not in the form of excitation because of any source and only has magnetic coupling to the stator.

Certainly, under normal running conditions, the rotor winding (squirrel cage or slip cage) is always short to each other the allow induce current to flow in the rotor winding. The flow of 3-Φ current in the winding produces a rotating magnetic field of non-variable amplitude. The speed of this rotating magnetic field is synchronous speed, where Ns is;

Ns = 120f / p

However; f= stator supply frequency & p= number of poles of the motor

Slip of 3-phase Induction Motor

So Let,

  1. Ns= Speed of rotating field is rotation per minute.
  2. N= Speed of rotors
  3. Ns-N= Relative speed between the two i.e rotating magnetic field and the rotor.

The difference between the synchronous speed [Ns] and the real speed [N] of the rotor is known as slip. It is usually express as a percentage of the synchronous speed. This also goes by the name absolute slip or fractional slip and ‘s’ is use to denote it.

% Of slip (s) = [(Ns – N) / Ns] * 100

Ns – N is sometimes also known as slip speed.

However, the actual speed of the motor can be express in terms of slip as;

N = Ns(1 -S) r.p.m

At the start, the motor is at rest and hence its speed N is 0.

S = 1 (at start)__________(Max slip)

Whenever S = 0 gives N = Ns which is not possible for an induction motor. So slip of the induction motor cannot be 0 under any circumstances.

Slip Range = 0.01 to 0.05 i.e 1% to 5%

The slip corresponding to the full-load speed of the motor is the full-load slip. Thus the value of slip at full-load is about 4 to 5% for small and 2 to 2.5% for large motors. The slip at zero loads is nearly 1%.

Advantages and Disadvantages of 3-phase induction motors

Advantages

  1. It has very simple and extremely rough, almost unbreakable construction.
  2. So its cost is low and it is very reliable.
  3. It has sufficiently high efficient. In normal running condition, no brushes are require, hence friction loss is reduce
  4. It has good power factor.
  5. Maintenance require is quite less because of its easy structure.
  6. It can produce sufficient torque.
  7. Lastly they can operate at much higher speed as compare to dc motors.

Disadvantages

  1. Its speed cannot vary without sacrificing some of its efficiency.
  2. Its speed decreases with increase in load.
  3. Lastly its starting torque is inferior to that of a DC shunt motor.

Application

  1. Fans
  2. Pumps
  3. Chemical industries
  4. Sugar and paper industries

Conclusion

Here we are at the last part of the blog. I hope that you liked the content above and all your doubts are clear. If you do like then please do share it with others and also comment down below the part which you like the most. Besides if all are having any doubts regarding the topic then feel free to ask below.

Have a nice day 🙂

Regards.

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1 Comment

  • Kushal April 22, 2021 at 5:58 pm Reply

    Excellent . Your explanation is always best

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