Can anybody exactly tell me why a three phase induction motor takes a very high starting current?

Question

I do not wnat off-the-textbook answers, auch as "no back emf" et. all. Please explain as you would to a layman.

Answer 1

It appears that you have asked about the synchronous motors that are used to run flourmills.

These motors can be initiated at a particular RPM only. For that, a small induction motor runs the synchronous motor and brings it to a particular RPM. As soon as it happens, the main synchronous motor is switched on.

You might have seen a lever switch for this setup… the operator operates the lever to the upper position for starting induction motor… then on assessing that the motor has reached the minimum required RPM he brings down the lever to the downward position for switching on the main synchronous motor.

Answer 2

This explanation is hopefully in laymans terms.

At the moment an induction motor is started, the only impedance to the circuit is resistance.... copper wire resistance R which is relatively low --------
It takes a few milli secs for iron in the motor (or transformer) to build up the magnetizm. As soon as the magnetic flux is built up, the back EMF, causes a high impedance (Z) which is additive to the copper wire resistance. The resultant is that the high current at the instant of starting into a low resistance is immediately reduced as soon as the high impedance is seen in the circuit.

This means that at the instant of switch on, there is an instantaneous high inrush (restistance only) and several milli seconds later the inrush peaks and rapidly decays as the impedance part of the circuit (magnetic element) builds up.

Typically, a cage motor will draw 4 to 8 times the full load current at start. The actual draw is dependent on how the motor is designed.


Source: Electrical Engineer (ret.) with 45+ yrs experience.

Answer 3

It can be shown mathematically that "The total instantaneous power for a balanced 3-phase system is constant and is equal to 3 times the average power per phase.” That means that the rotating stator and rotor magnetic fields in a three phase motor are not pulsating at all as they are in a single phase motor. Edit 1 I would agree that size, fan etc. probably have more to do with the differences in sound than 1ph vs 3ph, but I suspect this is an academic question that should have been preceded by "all other things being equal."

Answer 4

Any induction motor is rated for a certian rpm(rotation per minute) and always tries to run at that rpm, whatever the circumstances.
But a running motor runs slow when there is more load than usual, which in turn makes it draw more amps(current) to get back to its rated rpm.
So, the slower it goes, under load the more current it draws to get back to its rated rpm.
When the load is too heavy, the motor might even come to a standstill which causes it to draw the maximum current to get back to its rated rpm.
I think it is now clear to you that the closer the motor runs to its rated rpm it draws less amps.And the greater the decrease in speed from the rated rpm, the more amps it draws.
So, to go to the rated rpm from zero rpm, the amps drawn will be the maximum.
This is why the starting current for any induction motor is very high.
"Star/ Delta" starters are fitted for large induction motors.

In the Star position this starter helps to reduce the ampere load on the motor when it is starting by "misleading" the motor to run as one rated for a much lower rpm than it is actually rated for.

Once the motor has reached this "falsely" rated rpm, the starter assumes its Delta position, which causes the motor to realise it is running slower than its real rated rpm, and it speeds up without too much pull of amperes.
I think that explains the reasons in the simplest layman's terms.

Answer 5

Windings are made up of Copper, which have very low resistance (in Micro / Milli Ohms). Due to high number of turns around iron core, evry phase of winding becomes inductive in nature.

When given a supply, magnetism is induced in windings & core. In three phase Induction Motors the magnetism is in form of rotating field. This makes rotor move by force of attraction.

Its a property of any inductor coil that when ever its energised it produces the magnetism & this magnetism (due to interaction with current carrying conductors) oposses its own cause i.e. the current through the coil. Actually emf is induced due to relative movement between magnetism & current carrying conductors. This emf acts oppossing to the one supplied to coil hence called as back emf.

Due to this & the force required to move the rotor along with coupled load, any motor draws higher current while starting if not used with special type of starters.

Hope your doubt is cleared.

Thus

Answer 6

In the most basic of terms, the large starting current need to get a three phase motor going is due to the higher magnetic force that has to be overcome in order for the motor to begin spinning. Once the motor is running the higher curent is no longer needed.

Answer 7

hi
first of all think of starting current.why r spacially specifying the starting current ?
means that it is important for any motor to start.
Actual need--

induction motor -means the electrical power transmitted to rotor is through induction process.once the power is induced then currnet flows through each conductor on the armature at a time the magnetic field exists & couples to the rotating power in the stator.as the rotor at starting is rest,no rotation,the power required to make the rotor to rotate is more than running.thus the induced power is not sufficient to couple with rotating power of stator.so it draws more power(not voltage),current,to establish more magnetic strength to revolve along with stator's rotating power.