Cable Sizing Vs Starting Current Of Dol Motor
Posted 04 August 2007 - 03:13 PM
I recently facing difficulty to do sizing of my DOL motor.
The motor is 75KW,415V,
What is the factor that i need to consider beside the max. load current when sizing the cable?
ie. Starting current of the motor,Short circuit current of the motor etc.
Posted 05 August 2007 - 04:00 AM
The distance between your motor starter and the motor is another variable which can effect the cable sizing.
If it is within 25 metres, then i would propose 70 sq mm copper.
I would suggest a soft starter for your application, there are may in the market but i use ATS48 series of Telemecanique.
Posted 05 August 2007 - 05:23 AM
There are two major factors to consider:
- Voltage Drop
The voltage drop is often the major determining factor and in many regions, the maximum acceptable voltage drop is 5%. If you select the cable for a maximum voltage drop during start of 5%, it will probably be much larger than the cable size required for continuous running. At the minimum, you should minimize the voltage drop at run to less than 2 - 3% as this is "power" loss in KW.
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Posted 07 August 2007 - 11:22 AM
For checking the impacity of cable, here is a useful calculator. Down load it.
When you would open the page, you would find the download option at the end of page. After down loading go to Mechanical > Wires
Posted 10 August 2007 - 04:37 AM
But yet i still have a question.
For this motor installation, the starting current for the motor would goes up to 1000Amp due to it is using DOL starter.
And we know most of the time the motor failed on the high surge.
So, do i need to size the cable up to the starting current of the motor?
Posted 10 August 2007 - 04:45 AM
The starting current is of such short duration that the normal overload capacity of the wire selected by using the FLA will be sufficient.
Posted 10 August 2007 - 10:54 AM
Itís dependent upon the starting time of motor. If motor starts under heavy load such as flywheel coupled motor where the starting time is too long, we have to select the proper sized cable which may bear the too high starting current for a long period. If we select the cable according to our normal practice, the cable would heat up during starting the motor. According to my experience, if the starting current of motor is greater than 15sec, the cable should be doubled in size as we select the cable under normal load application.
Now, what is the starting time of your motor?
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