Induction motor or asynchronous motor is one of the most commonly used types of electrical (alternating current) motors which runs at a speed less than its synchronous speed.
This motor doesn’t have brushes or commutator which makes it powerful and efficient. It’s used in many applications as industrial applications, newer diesel trains, small fans, toys, high-speed vacuum cleaners, drilling machines, pumps, dishwasher, mixers, compressors, and many other numerous applications.
Parts of an induction motor:
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It simply consists of two main parts:
- The stator: it’s the stationary part consisted of electromagnet around the outside of the motor. It’s made up of various stamping with slots to carry the windings.
- The rotor: it’s the rotating part sited inside the stator. It may be squirrel cage or wound type. And it comprises a cylindrical laminated core with axially placed parallel slots used to carry the conductors.
And there are also:
- Windings: which made of aluminum or copper wires which are insulated in coils around iron cores.
It’s important to know what is the synchronous speed to know how the asynchronous motor run. Simply the synchronous speed is the speed of rotation of the magnetic field in a rotary machine. This speed depends upon the number of poles of the machine and the frequency.
So why the asynchronous motor runs at a speed lower than this synchronous speed; simply when a rotating magnetic field produced in the stator a flux created in the rotor and this flux allows the rotor will rotate.
Pay attention; the rotor rotates over a period from the stator which means that there is a lagging of flux current. The flux current in the rotor lags the flux current in the stator so the rotor will never reach it’s rotation magnetic field speed (the synchronous speed).
We can classify the induction machine upon the input power supply into two main categories:
1-Single phase induction motor:
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- Split phase motor.
- Capacitor start induction motor.
- Capacitor start and capacitor run motor.
- Shaded pole motor.
2-Three phase induction motor:
It’s a rugged, reliable and economical self-starting motor used as industrial drives; you should also ask yourself why it’s a self-starting motor. We can also subdivide into:
- Squirrel cage motor.
- Slip ring motor.
To make it easy for you; self-starting means that the machine starts running automatically without any external force to it.
We supply the stator windings with Ac supply so alternating flux produced around these windings. And as we say this alternating flux will revolve with the synchronous speed.
The relative speed between the rotor conductors and the stator will cause induced emf in the rotor conductors which are short-circuited so a current would be produced in the rotor depending upon Faraday’s law of electromagnetic induction.
And the rotor current will produce alternating flux around the rotor and we say that this flux lags the stator flux. If we follow Lenz’s law the flux will oppose the cause of its protection.
The rotor rotates in the same direction of the stator to try to reach the synchronous speed but it’ll never succeed.
Induction (asynchronous) motor history:
We present this history for only how interested in the invention of this motor and her you the history:
- In 1820 Joseph Henry and Michael Faraday discovered an experiment with the phenomena of induction.
- Francois Arago formulated the existence of rotating magnetic fields in 1824 which turned switches manually on and off.
- Hippolyte Pixii in 1830 developed an Ac generator.
- In 1870 Elihu Thomson developed the recent understanding of induction in dynamos.
- Galileo Ferraris in 1885 conceived the idea of the polyphase Ac motor; later in the same year, he conceived the idea of two out of phase synchronized currents that was used to produce two magnetic fields which could be combined to produce a rotating magnetic field without using switching or any moving parts.
- He also published his Ac polyphase motor in 1885. This motor worked without a commutator, and this development made the Ac motor more efficient.
- In 1888 Tesla stood to show his polyphase motor.
- Dobrovolsky built in 1889 his first transformer and motor which worked with his 3 phase Ac system.
- In 1892 Charles P. Steinmetz presented his paper on hysteresis (delay effect in a 3 phase Ac system), and he continued to improve and troubleshoot the future of Ac power systems.
- Of course, the induction (asynchronous) motor improved over the time until 1900 when engineers at Siemens find a way to make the motors smaller and lighter but of course with the same output power rate.
- Sure, today the used electrical motors are smaller than in 1990 which are still used in many places.
Advantages of induction (asynchronous) motors:
Both single phase and three phase motors have similar advantages as:
- They are simple, rugged, and robust which help them operate in any environmental condition.
- Also, Induction motors don’t contain brushes, slip rings, or commutators which make them maintenance free.
- They are also cheaper in cost.
- Also, They run off of Ac grid power.
Disadvantages of induction (asynchronous) motors:
There would meet some drawbacks with induction machines as:
- It’s difficult to control the speed of the motors.
- Also, The power factor drops to a very low value at lower loads.
- As the power factor drops a higher current drawn and results in high copper losses which made the efficiency drops.
- Also, The starting torque is low and we can improve in slip ring motors by adding resistors to the rotor windings.
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