VSD MOTOR SUPPLY CABLES GETTING HOT

[salarkdr]

We have six VSD's mounted adjacent to each other on a switchroom wall supplying 30 kw motors via 3c 16mm² XLPE cables (approx 145m). The cables leave the base of each unit and converge onto a common cable tray.

The problem is the cable is heating in the crotch where the cables come together.

Has anyone any experience of this related to earthing or harmonics?

 

[oceanamber]

Although you give the sizes of the motors and cables you have not given us a line voltage. It is usual to include a de-rate on your cables when bunching them together like you seem to have done. The U.K. I.E.E. regs would apply a 0.74% derate onto the cables installed like this. Certainly where the cables bunch you can localised heat.

 

[Dopler]

Look at AB site:

http://files.awdm.com/e-files/ra/drives/drives-in001a-en-p.pdf

chapter 4 page 10, "Cable trays"

 

[advidana]

yes to both, It is a poorly designed system.

 

[ICEMAN]

Suggest you measure the harmonic current distortion at the input the drives.

If they have no AC line or DC bus reactors you will be looking at around 65-70% (maybe higher) total current distortion per drive at rated load (more if the load is less due to the discontinous current).

If they have AC line reactor or dc bus reactors then you will be looking at ~38/40% Ithd (if 3%) or ~ 34-37% Ithd (again if 3%) at rated load.

All these harmonic currents add up and are disspated as heat (as well as having other effects like skin effect on cables).

Harmonic currents increase the total current drawn from the mains as well as reduce the true power factor not the displacement pf (which is the fundamental pf only).

In your measurements check also for resonance between any local power factor correction capacitors if not fitted with detuning reactors and drive harmonics. It'll be obvious to you if you have resonance.

ICEMAN