3 phase motor calcs and fuse sizes

[peterhardacher]

Hi am currently employed as testing and inspection on a industrial site just wondered if you could give me some advise
What it is I got a 110 kw motor 180 amp 100 m run full load now I got a 75 mm cable supplying it . I have checked table 4d4a in the regs as it is 3 core armoured . Ref method E . According to this it says am allowed 207 amps without correction factors .

I have also done a cable calc 110000 w / ( 1.73 * 400v ) = 158 amp without power faction correction is this correct ?

I have been in Internet on cable calc program's for cable manufacturers which is telling me 75 mm

Now what it is the cable is protected by a HRC motor fuse 315m 355a according to Ib < In < Iz the fuse size is greater than the cable max amps

Also what concerns me is local to motor the 75 mm goes to a JB and reduces to 25mm I think because there is a lack of space for the bigger cable as the motor has other things around it . Now a 355 amp defo does not protect that cable in case of overload

Now one final thing this motor goes through a variable speed drive but even if it was set at 80 % I still have the problem of oversized fuse . Now I pointed this out to my boss he says its acceptable but I think he's wrong and as am signing the sheets I would great fully apreciate your professional advise on this thanks

 

[AndyOz]

Peter

Try the Bussman site they have some really good info on fuse sizing. Address is http://www.cooperindustries.com/con...l Line Catalog/IEC_British_Standard_Fuses.pdf

This indicates that a 110 kW motor will pull 185A flc and fuse should be rated at 315A. In your case the inverter will provide motor overload protection, if set up correctly. In a motor circuit the overload protection protects against overcurrent and the fuses against short circuit.

Protection against short circuit will enable much larger fuses to be used. For 70mm a maximum of 560A for K115 (70 degree) and 630A for K143 (90 degree) cable.

The reduction to 25mm would also be a concern to me, although you say it is local. If it is only a few meters it could be argued that it is just extending the winding cables outside the motor.

My advice would be to observe the actual running currents of the process. Check the inverter settings for correct overload protection. Thirdly check for any signs of overheating as this plant has been running.

Hope this helps.

 

[peterhardacher]

This helps a lot thanks good reference point for checking fuse sizes to cheers

 

[Brightspark2]

How long is the run in 25mm?...

Over current protection can be placed within 3m of the change in CSA...

 

[bertt]

Hi,
Please, take a picture with the label from your motor.

Thank you.

bertt.

 

[peterhardacher]

Bright spark the run is bout 5 meters I
Bert unable to take pic as this motor is in a EX area hhhmmm which may open another can of worms when the motor is EX inspected ???

 

[AndyOz]

The plot thickens. The 25mm may indeed be part of the motor, allowing termination away from the motor because os the EX environment.

Just a thought. Without seeing the installation it is hard.

 

[peterhardacher]

Ha I know I think the reduction is down to space. Constraints as it is a very cramped area the termination box will allow for the 70 mm gland as a reducer has been used for the smaller cable gland . Also a ex rated gland has not been used and the motor casing has been painted EX d but that's another story ill get round to failing that when am doing Compex inspections Am going to try and get design spec. For the area for 2391 inspections but as you well know these yards don t really keep
records as the plant am workin on was built in the 80 s

 

[peterhardacher]

To find out if this 25 mm cable has been specified and find out why

 

[AndyOz]

As I said it was just a thought. Sounds like it was badly designed and by the grace of god not caused an incident.