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Motor Problem Help Needed...


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hello all



I have a motor i need help with its attached to a fan and the motor has no plate so i don't know the details, Its installed

in a steel duct and has six leads running to a connection box and seems to be connected in delta. Could i connect this to a

star delta starter and how would i find the correct rated contactors for this application...

much help would really be apprieciated as im new to all this, thanks..




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Hello Simon.


Supposing you have an asynchronous motor,

first you need to make sure motor's shaft with the fan is turning freely - nothing is jammed anywhere.


Then, using a multimeter (I am sure you have one) measure the resistances of the windings - with the bridges set as they are now on the motor (for delta as you have said). Let's say A-B reads 10 Ohm, A-C 10,3 0hm, B-C 9,8 Ohm. Shaft (fan) by that should stay still, otherwise you would not get any measurements. Difference between values read should not be more than some +/- 10 % (I may be incorrect here however, but for the range +/- 5% I am pretty sure).


Then measure whether there is no ground fault to earth - best to use a megaohmeter, but if you do not have one, then use the same multimeter set to megaohms range. Windings-to-earth resistance should not be less than 1megaOhm at least.


If you have measured and named here windings resistance values (now in delta) it can be possible to surmise approximate ratings of the motor's power and the curcuit breaker needed to protect your installation in case something went wrong (dont start the motor without the protection).


And do not try the motor in delta, try it in star - as the nominal voltage (and frequency) are unknown. Your supply voltage may just turn to require the motor windings to be connected in star.


And finaly, you will need a clamp meter - you can do without a megaohmeter, but without the former - in no way. You will start the motor for few seconds, controlling the current drawn. If currents not too big and do not differ very much across the all three leads, motor runs without much noise, then you can run it for few minutes and then see if it does not heat too much.


Hope this can help you.

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Hello Sims


If the motor is connected in delta and in this connection it performs correctly on your supply, then yes it can be star delta started provided that the windings can be separated.


If you can run the fan under maximum loading, you can measure the current into the motor from the supply.

As you do not know the rating of the motor, I would assume that the rated current is 20% higher than your measured current for the purposes of selecting the contactors.


The main and the delta contactors should be sized at 57% of the assumed current above. The star contactor should be sized at one third of the assumed current.


Best regards,


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