Cable selection for AC induction motor starting

[Brant]

Hi All.

Can any one help is there any articles written on cable selection to reduce voltage drop in supply cables to a large 3 phase motor i.e. 250 Kw 4 pole 415 volt 50/60 Hz i.e. to help reduce the voltage during a fully loaded start
(basic rules & guide lines)

 

[DaiWei]

If in the U.S., start with the NEC section 430.

Ken Culverson

 

[waross]

Basics;
1> Cable ampacity 125% of motor ampacity.
2> Overall voltage drop (feeders plus branch circuits) not to exceed 5% at running conditions.
3> With a large motor on a relatively small system, check and evaluate system voltage drop under starting conditions. Basically the regulation of the supply transformer.
4> What DaiWei said.
5> In Canada start with the Canadian Electrical code, Section 28, Motors and Generators.
yours

 

[jraef]

Well with 415V you are obviously not in the US or Canada, but certainly there is a similar guiline for cable sizing per your local codes. Cable size is not the only factor in determining voltage drop however, in fact it isn't even a very big contributor unless the distance is extreme or the cable was seriously undersized to start with. Are you using reduced voltage starting or DOL? That might be a better first step.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Hello jraef
I agree with you on the 415 V, the 250 Kw is another tip-off.
However I have seen one exception to the 415 Volts not in the US or Canada.
We use North American standard Voltages (sort of).
I had a lightning strike take out two 277 volt distribution transformers at a resort. There were no 277 volt transformers available in the country. Plan "B" go 240 volt transformers, crank the taps down to the bottom and set the taps on the dry types to the top. This produces about 114.6 volts in place of 120 volts and 198.7 volts in place of 208 volts. This is 1.7 volts above the minimum required by many refrigeration compressors. Local knowlege, Not much 208 at the resort, mostly 120/240, and the primary line was cronically high.
The next step was to verify that the dry type transformers had 5% taps. Imagine my delight to find that as well as the standard taps, the dry types also had 416V taps. We ended up with a 416V hotel distribution system with all North American equipment And 120/240 and 120/208 secondaries..
This gives rise to another question. Are there any utilities in the US that are changing old Delta banks to Wye connections or are starting to use Wye connections with a lot of 120/240 distribution transformers in stock? It was the first and only time that I saw North American transformers with a 416V tap. (208 x 2 = 416)
respectfully

 

[ScottyUK]

Brant,

Once you have a cable with adequate thermal and fault rating for the motor circuit, you need to consider the maximum volt drop which your drive can deal with and still provide sufficient accelerating torque.

You could consider the cable as a mild form of reduced voltage starting from the point of the view of the motor. For example, a fan will probably accelerate even with a fairly severe volt drop, whereas a laden conveyor equally probably won't. The load plays a big part in this calculation.

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