Delta Star Connection Problem

[MasudDU]

hello,
gentlemen please help me with this motor problem described below...

an electrical motor (400V, 133.5Amp, 50Hz, 75KW, 2960 RPM, COSθ=0.89) was running on delta connection.
after rewinding its getting hot when running on delta connection. it ran hot at under full load,
taking more currents than the rated full load value,
rated voltage supplied to the motor, and cooling was good.
the re-winder suggested to run the motor in STAR connection
but my senior engineer said it can't be run on STAR connection..
but when the re-winder asked him the cause, he didn't answer.
now the question is- why we can't run this motor with STAR connection?
please help...

now if i run this motor on star connection, will there be any problem ?

 

[zlatkodo]

Hi, Masud,
Need more information: motor application, FLA before and after rewinding...
I can say from practice the cause was probably a bad rewinding:
- Mess with winding data, internal connection, parallel paths etc. because, rewinders often change the original winding trying to make a new one that is "easy to do", or
- Core damage due to excessive heating during extraction of the old winding.
The exact cause will be known only when all the information about the winding and core are available. See here.
BTW, in general, the motor that runs in delta connection could be reconnected to the star only if the load is highly reduced.

 

[jraef]

If, when it was originally made, it was a SINGLE voltage and could be started as Star-Delta, or started DOL, then the full voltage connection would have been in Delta.

If, when they re-wound the motor, they did it as a DUAL VOLTAGE design, as in 230/400V, then the LOW voltage (230V) would be the Delta connection and the HIGH (400V) connection would be Star.

So what it sounds like to me is that your re-winder paid no attention to the original configuration design of the motor as it was when you gave it to them and just wound it they way they wanted to...

"Will work for (the memory of) salami"

 

[waross]

How many leads?
Integral HP motors in Europe generally have 6 leads and may be connected in star or delta for starting or dual voltages. The voltage ratios will be 1.73:1
Integral HP motors in North America generally have 9 leads and may be connected in parallel or series for dual voltages. The voltage ratios will be 2:1
There are lots of exceptions.

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

 

[LionelHutz]

Motor operate on a V/Hz (voltage per frequency) curve. So, there is only a single appropriate voltage to use for the Y and delta connections at 50Hz. Hopefully, the re-winder either built the motor to be 230V/400V, 50hz or 400V/690V, 50hz when connected Y and delta. So, does the re-winder have the winding data which tells you the two voltages? If he's given you the data, then whichever connection is made for 400V, 50Hz is the correct one to use.

 

[Marke]

Try running the motor delta connected with nothing connected to the motor shaft.
The current drawn will be essentially iron loss, which on a motor around this size and speed, should be in the order of 20% - 25% of the rated current of the motor.
If the winding is wrong for the voltage and frequency, IN would expect that the open shaft current will be too high or too low.

Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd

 

[petronila]

Hello masudDU,

I will suggest that you a take a look of name plate and note the rated voltage, also verify how manny leads has the motor.

If you found that is 230/400 and six leads, the motor is wrong connected.

If your motor is 400/690 and six leads then the motor most be connected for 400 V in delta connection and can´t be connected for star like your workshop sayds (will smokes up).

Taking account you don´t sayd nothing about overcurrent or instantaneus breaker trip, I would sayd that first to do is verify the load, alignment and mechanical issues in order to know where comes from the overload condition , if you don´t found nothing abnormal the motor could be drawning more current due to manny things one of this could be wrong alignment between rotor and stator, squirrell cage open or by magnetic stator core saturation, so could be good that you perform an stator core test and also a rotor single phase test or with growler.

Regards

Carlos

 

[electricpete]

a parallel thread by op has a few more details...
thread404-371930

It is apparently drawing more current than full load. A minor detail apparently. How much more we're left to guess. Like Mark I had suggested uncoupled run among other questions.

=====================================
(2B)+(2B)' ?

 

[Marke]

Hello MasudDU

Your questions :
"why we can't run this motor with STAR connection?" Under open shaft conditions, you can run the motor in star or delta connection provided that it is would with sufficient turns to be connected in delta at the voltage and frequency that you are using. If the motor is designed to run in star connection at that voltage, then running in delta will over flux the iron and the motor will get very hot and fail. Most motors of this size at 400V 50Hz, are designed to be 400 volt delta connected and 690V star connected. Connecting such a motor in star at 400V 50 Hz, will reduce the flux in the iron and reduce the maximum torque capacity of the motor. Effectively, the rating of the motor is reduced to 1/3 of the delta connection rating at 400V.

If the motor was designed to be star connected at 400V, then delta connection at 400V will saturate the iron causing a very high temperature rise, high current and motor failure.

"now if i run this motor on star connection, will there be any problem ?"
If the motor is designed for delta connection of 400V and you run it in star connection, the affective rating of the motor is reduced to one third. While the motor may be able to run the load at full speed, the slip will be three times full load slip (or more) and will result in excessive slip losses in the rotor which will overheat and fail.

First, run the motor in an open shaft condition and measure the current. If the current is significantly high under open shaft conditions, then you either have a severe mechanical overload such as bearing problems, or you are operating on a high voltage or low frequency for the connection being used. If the open shaft current is in the range of 20% - 30% Rated Full load current for this motor, then the connection is correct.
If the open shaft current is significantly lower than 20% of rated full load current, then the voltage is low or the frequency is high for that connection. The flux in the iron is proportional to V/F. Increase the voltage and you must increase the frequency for a given connection in order to keep the V/F constant.

Separating the motor from the driven load, isolated the driven load from being part of the problem.

Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd

 

[squeeky]

Dear MasudDU,

I totally agree with Mark about the torque issue if it was wound to be a 400v Delta and you wish to run it in star. I would ask the motor rewind company for their rewind data sheets. They may not mean much to you but if they don’t supply them quickly then there may be some altering going on.

I have not really thought of running a motor on no load to check the connection to make a comparison, but it makes sense. If it is running with reduced voltage the torque will be reduced by the square root. My data sheets have the motor (IE2) at 128.3A for Full load and 42.0A at no load. The slip at no load should almost be zero so your running speed should be very close to 3000 rpm. At full load it should be 2965rpm. When your machine runs, can you check the speed accurately? If so do this as Mark’s comment on running with a high slip is valid and it will heat up. If the speed is not very close to rated you could have a problem.

I have added some WEG data sheets if you want them.