Runnig a 230volt motor on 4oo volts

[mash98]

Dear guys we have some motors which were originally winded on delta with a rated voltage of 230 vlots 3 phase but our winder has changed the motor's internal connection to star( using the same original winding wire) so we can run the same on 400 volts to meet our system requirements.The motors are running fine on the new voltage but I have some questions which I would like you guys to reply.

1- The motor rated current was 8.5 ampere at 230 volts 3ph 50hz and the rated KW were 2.2 as we have changed the winding to star connection and running it at 400 volts now what would be the normal/safe current for this motor?
The motor is taking 5.5 ampere on load while the no load current is around 4 ampere at new voltage.I assume that the same motor can be run safely at around the same rated current i.e 8 ampere or so because the winding wire remains original.Kindly give your valueable comments.

regards

 

[roydm]

When you say the winder changed the motors internal connection I assume you aren't saying he rewound it, just re-connected the windings.
If it drew 8.5 at 230 in delta I would expect the current in each coil to be exactly the same on 400 Star.
No, the winding did not draw 8.5 amps in delta, that 8 amps went to two coils 180 Degrees apart.
BTW, Is your frequency the same?
Roy

 

[waross]

The ratio to use is root three, 3^0.5, or 1.73.
230 Volts x 3^0.5 = 398 Volts or 400 volts.
8.5 Amps /3^0.5 = 4.9 Amps.
Connected in delta te current had two paths through the motor.
Connected in star, each line sees only a single path through the motor.
Due to phase angle differences the factor is root three rather than two.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

A winding can carry a certain maximum current, regardless of whether the motor windings are connected in star or delta. The connection doesn't change anything about the winding, neither the current nor the voltage rating. The differences in voltage and current rating of the motor in delta or star occur because of the way the three windings interact when in delta or star.

When a motor is delta connected the line current - the current drawn from the supply - is [√]3 x the current in each winding. This relationship is a result of how the currents in the two windings, which are displaced in phase by 120[°], sum together according to Kirchoff's Current Law. Therefore each winding is able to safely carry 8.5 / [√]3 amps, i.e. 4.9A. The winding sees a voltage of 230V across it, and this is the rated voltage for the winding.

In a star connection the line currents and winding currents are equal, so the rated line current is 4.9A. The voltage across each winding is [√]3 / line voltage, again because of the way the phase displaced voltages voltages divide across the windings. The result is that the winding voltage is 400V / [√]3, i.e. 230V.

You'll see from the above that the opening statement is true: winding voltage and current rating is the same regardless of star or delta connection.

Your motor is running slightly overloaded and really you should be looking for the next size up, i.e. 3kW. Whether it will make any notceable difference really comes down to temperature: the motor is designed to produce 2.2kW at a specified ambient temperature; if your operating ambient is lower than the design temperature then you can get away with a slight overload.


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If we learn from our mistakes I'm getting a great education!

 

[mash98]

Thanks for the replies the frequency is same that is 50 hz and the motor has not been rewound only the connections have been changed.

regards

 

[ScottyUK]

You guys type too damned quick, there was no reply when I started writing War & Peace.


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If we learn from our mistakes I'm getting a great education!

 

[mash98]

scotty where i can find war and peace.
Any way thanks alot.

 

[roydm]

OOps, I meant 120° apart
ScottyUK, I vote your explanation as the best.
Roy

 

[7anoter4]

I agree with all above remarks except the explanation why no load current is so high in the new star connection. Usually no load current has to be 30-40% and this one -in the 400 V case- is exaggerate [73%].
First of all, the 3thd harmonic current produced in stator due to magnetic field is not canceled as in delta connection and that will increase the no-load current. But, usually the 3thd harmonic current has to be from 10 to 30%[depends upon coil connection].Here, I think, by doubling the number of coils in series the 3thd harmonic is more elevated than usual. Some time, function of no. of coils in series, number of teeth, turn width and other factors it is impossible to maintain the no-load current in the usual limits.

 

[waross]

The harmonic currents in both cases are supplied by the supply transformer. No canceling in the motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

What is your exact measured voltage? What was the measured voltage on the 230 Volt system? A difference in voltage from nominal may explain the elevated currents.

Bill
--------------------
"Why not the best?"
Jimmy Carter