sizing of a power cable between a motor and inverter drive

[Lc85]

We have a motor that will be running a fan. It will be driven from an inverter drive. The motor info is
400V,50Hz
355kW
621A nomial current

The inverter drive info is
Output current light duty 701A
Output Power light duty 355kW
Maximum current 1000A
400V,50Hz

My question is, whgat method do you use to size the cable between the inverter drive and the motor, to get the mm^2 value? I know it has to be a screened cable as this is what ABB recommend. The clients specification wants 1.1kV grade PVC Insulation PVC Sheathed and aluminium conductors.

Thanks very much

 

[MikiBg]

See IEC 60364-5-523. First of all you have to specify designed ambient temperature and installation (laying) method of cables. Furthermore, for more accurate calculation, you'll need to specify thermal resistance of surrounding soil, if cables are laid into ground. In that conditions, cables need to bear app. 1.05*621=652A (assumed inverter thermal protection setting). Than you will choose most appropriate type of cable, i.e. conductor and insulation material. For such high load, proposed cable with PVC insulation and Al conductors can result in more than 2 multi-core cables required, which is hard to install, and in this case I would for sure try to avoid single-core cables. My first choice will be PVC insulated, Cu conductor, multi-core, screened cable, like NYCWY for instance. Most European cables are 0.6/1kV rated.
Resulting mm^2 will give you current carrying capacity requirement, voltage drop and EMC conditions should be checked too.

 

[jraef]

The voltage rating request is valid, the output voltage of the DC pulses comprising the PWM going to the motor can cause temporatry voltage impulses of twice the peak voltage of the incoming AC value (not RMS, but the actual peaks). So using 600V rated cable is often leaving you susceptible to cable damage.

As to the cable diameter, that is typically determined by local codes pertaining to sizing any motor conductors, there is no special need to over size if that is what you are asking. So that will usually be dependednt upon the FLC rating of the motor, or the kW rating. Here in the US, we must use a table in the National Electric Code that is based upon motor HP, not the FLC (unless the FLC exceeds the values in the table), then the cables must be sized at 125% of those values. I would hazard to guess that codes in other countries are equally conservative.

"Will work for (the memory of) salami"

 

[squeeky]

A highly complex question. Assumption has been that the cable will go in the ground. Cable ladder will require a different approach. Other cables near by in the same trench or rack will require rating. If it is in the sun or rack at altitude, it requires further rating.

If you run single cores they must be run in Trefoil. If you have more than one set of single conductors, cross phasing is required. If you run a screened /armoured conductor, it must not be steel wire armoured, SWA. As the individual conductors from the starter or into the motor, it must go through an aluminium gland plate. All of this is to prevent or minimize inductive heating and cable failure.

If the motor is a long way from the VSD, then you must consider chokes as the capacitance in the cable may cause problems.

I recommend getting some professional assistance or perhaps supplysome very specific information. Sorry to be negative.

 

[ScottyUK]

MikiBg makes a good point: I also think copper conductors rather than aluminium would a be sensible starting point because it would keep the cable sizes to a sensible / manageable size. You didn't state distance from drive to motor but at that current volt-drop will quickly become more dominant than current-carrying ability of the cable. Copper multicore is available in at least 300mm² and 400mm² sizes, although I caution that 400mm² multicore cable is awkward to terminate unless you have a very generous cable box, something which is almost unheard of on an LV motor.

 

[ozmosis]

A key question is the distance the cable will run. Can you advise?

 

[Lc85]

Thanks for the responses. The cable run will be within 100m

 

[Lc85]

Hi,
Regarding my question above. the run is less than 100m, the cables will be installed on a tray with no more thewn 6 other power cables.

The client wishes to use:
Power cables to be rated 0.6/1kV. Cable conductor (Aluminium) shall be insulated with XLPE. Cable shall also be armoured. Cable shall also contain a screen.

My question is that the terminal box has 2x M75 entries, so with the current of 623A not 621A (Sorry) what will be the maximum size of conductor that will fit the box and be able to handle the current?

Would the client be better with copper conductors?

Thanks

 

[MikiBg]

You forgot to tell us maximum expected ambient temperature.

 

[jraef]

Asking anonymous engineers from around the globe to do the work of a qualified LOCAL engineer familiar with your LOCAL installation codes and who can determine other mitigating circumstances is not the mission of this forum. Someone here might give you an answer that is correct in THEIR location but is in incorrect in yours. Find and hire a qualified local engineer please.

"Will work for (the memory of) salami"

 

[MikiBg]

jraef, I totally agree. However, I don't see anything wrong if someone wants to help, providing note about what standards/codes given solution is based on. On the other side, in this particular case, some basic knowledge on the subject seems to be missing, so helping with it will be a little bit more complex task, I would say

 

[squeeky]

A little knowledge is dangerous. Being fully warned is advantage. There is insufficient information to give a technically correct answer. An assumption or guess could have a high cost. If the result was a fire, destruction, possible loss of jobs and possible death. That is why, in my opinion, people request that the person seek on site qualified advice. Failing this, we should support the colleague as best we can with all the associated warnings.