Understanding Synchronous Rotation in 3-Phase Induction Motors

In summary, the conversation discusses the physics behind the operation of a 3-phase induction motor on a 50Hz power supply. When the motor is loaded, the rotor current has a lower frequency than the magnetizing stator current, causing a demagnetizing effect on the air gap. To cancel out this effect, additional stator current is drawn with a frequency of 50Hz, resulting in both magnetic fields rotating synchronously. This is similar to a transformer, but with a rotating transformer concept.
  • #1
cnh1995
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Suppose a 3-phase IM is operating on a 50Hz power supply. When the motor is loaded, more current is drawn from the stator because of the demagnetizing effect of the rotor current. But the rotor current has a lower frequency i.e. slip*stator frequency. Still the reflected current in the stator is of 50Hz frequency. What's the physics behind this?
 
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  • #3
I understand the operation of the motor. In transformers, when a load is connected to secondary, current flows in the secondary. This current tends to demagnetize the core and hence, primary current is increased to nullify the demagnetizing secondary ampere-turns, which gives the expression NpIp=NsIs. But all the currents in transformer have same frequency, say 50Hz. In IM, if rotor current has lower frequency than the magnetizing stator current, how does the stator current due to loading has the same 50Hz frequency?
 
  • #4
cnh1995 said:
if rotor current has lower frequency than the magnetizing stator current, how does the stator current due to loading has the same 50Hz frequency?

Reread that paragraph in the wiki article again, carefully parsing each word. It is like a transformer yes, but a rotating transformer. Focus first on what happens when slip is zero and rotor currents are zero. A regular transformer with shorted secondary does not have zero current.

If you google more, you can find a time domain analysis or a detailed animation that shows one rotation.
 
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  • #5
The stator current frequency will always be the frequency of the source. Why would you expect any different?
 
  • #6
Averagesupernova said:
The stator current frequency will always be the frequency of the source. Why would you expect any different?
Say the motor is operating on 50Hz. The rotor current demagnetizes the air gap and its frequency is less than the stator current frequency. So, demagnetizing mmf has less frequency than the magnetizing mmf. To cancel out the de-magnetizing mmf, the additional stator current drawn has also a frequency of 50Hz. That's why I was wondering how stator current of 50Hz cancels out the demagnetizing mmf of a lower frequency.
 
  • #7
Let's say it is about 3 phases induction motor and for easy explanation let's take a wound rotor. The stator windings dispersion along the air gap and the symmetrical delay between supply voltages [and current] produce a virtual ring of moving magnetic poles.
The velocity of moving poles will be then rpmsynchron=frequency*60/p.
This "ring" actually does not move.
Let's take now the wound rotor.The magnetic process is identical. The virtual velocity is rpmvirtual=slip*frequency*60/p [ since the rotor current frequency is slip*frequency, and the rotor
no. of pole pairs is the same] .But there is also an actual velocity of the rotor rpmactual=
(1-slip)*rpmsynchron
The total velocity of the virtual rotor ring of poles is then:
rpmvirtual+rpmactual=rpmsynchron.
That means both magnetic fields rotate synchronously.
 
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  • #8
Babadag said:
Let's say it is about 3 phases induction motor and for easy explanation let's take a wound rotor. The stator windings dispersion along the air gap and the symmetrical delay between supply voltages [and current] produce a virtual ring of moving magnetic poles.
The velocity of moving poles will be then rpmsynchron=frequency*60/p.
This "ring" actually does not move.
Let's take now the wound rotor.The magnetic process is identical. The virtual velocity is rpmvirtual=slip*frequency*60/p [ since the rotor current frequency is slip*frequency, and the rotor
no. of pole pairs is the same] .But there is also an actual velocity of the rotor rpmactual=
(1-slip)*rpmsynchron
The total velocity of the virtual rotor ring of poles is then:
rpmvirtual+rpmactual=rpmsynchron.
That means both magnetic fields rotate synchronously.
Classic!:smile: This is the explanation I was looking for! The expressions were right in front of my eyes but I wasn't able to see how both the fields rotate synchronously! Thank you very much @Babadag!
 

1. What is an induction motor?

An induction motor is a type of electric motor that works on the principle of electromagnetic induction. It converts electrical energy into mechanical energy to power various machines and devices.

2. How does an induction motor work?

An induction motor has a stator (stationary part) and a rotor (rotating part). When an alternating current (AC) is passed through the stator windings, it creates a rotating magnetic field. This magnetic field induces currents in the rotor, which in turn creates a torque that causes the rotor to rotate.

3. What is the difference between single-phase and three-phase induction motors?

A single-phase induction motor has a single stator winding and is used for smaller applications such as household appliances. A three-phase induction motor has three stator windings and is used for larger applications such as industrial machinery and equipment.

4. How do you calculate the currents in an induction motor?

The currents in an induction motor can be calculated using the formula I = (P * 1000) / (sqrt(3) * V * pf * eff), where I is the current in amps, P is the power in kilowatts, V is the voltage, pf is the power factor, and eff is the efficiency.

5. What factors affect the currents in an induction motor?

The currents in an induction motor are affected by various factors such as the load on the motor, the speed of the motor, the voltage and frequency of the power supply, the design of the motor, and the operating conditions. Changes in any of these factors can result in changes in the motor currents.

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