Star or Delta 2

[AlwaysConfused]

Hi, for a squirrel cage induction motor (d.o.l) it is generally considered that anything over 4kw should be connected in delta. Why is this? because of the higher torque? I know someone that connected a 15kw in star accidentally (it was originally in delta) and the motor burnt out, but I couldn't think why it would burn out, other than that there would not be enough torque to turn the device it was connected to, and i take it with a 2 third drop in current the overload would be pretty ineffective too. I this correct?

Thanks

Richard

 

[itsmoked]

I U R correct.

 

[AlwaysConfused]

Is there any reason why a bigger motor can't be designed to be run in star?

 

[davidbeach]

No reason why it couldn't be in wye. Large synchronous machines are often high impedance grounded wye connected.

 

[Marke]

Hello AlwaysConfused

Three phase induction motors can be designed to run in star or in delta at their rated voltage.
The performance is essentially the same provided that it is connected in it's design format at the design frequency and voltage.

In the 50Hz world, the most common form of reduced voltage starter is the star / delta starter and for this to operate, the motor must be designed to operate in delta at line voltage and frequency. Typically, this would apply to motors greater than around 4KW.
These motors can be operated in delta at 400 volts, or star connected at 690 volts.
Motors below 4KW are often designed to be star connected at 400 volts, this enables them to be run from 230V three phase by connecting them in delta. This is useful when using a single phase input three phase output inverter type speed controller.

The major issue is to ensure that the motor is connected according to it's design conection at rated votage and frequency. It will then give full performance.
Connecting a motor that is designed for delta connection, in star, at rated voltage and frequency will reduce the voltage across each winding by rt three and the rating is effectively reduced to one third.
Connecting a motor that is designed to operate in star, in delta at it's rated voltage and frequency will probably let the smoke out, and it is very hard to put back again. You will overflux the iron resulting in much heat. The current will be well above the rated current at rated load.

In effect, the motor has two voltage ratings, one for star connection and one for delta connection. Both connections at the appropriate voltages, will give similar performance.

Best regards,

Mark Empson

http://www.lmphotonics.com