Dear members,

I need a help from anyone. I have a DC motor of rating 5KW. The distance between the motor and the dc source is 150 Mts. How to select the cable for this motor.

Expecting some guidelines

P.S.Pandi

 

Hello,

Firstly, you need to select the cable type based on the installation (ie low smoke, halogen free, to be direct buried, nylon sheath for possible termite attack etc). A reputable cable manufacturer will help you with this. Then you need to size the cable. This is based on three factors:

1) Current carrying capacity - base rating can be obtained from the cable manufacturer. This often needs to be derated based on grouping of conductors, actual ambient temperature and a variety of other factors. There are various standards around the world which can help you with these deratings. Your local cable manufacturer should be able to guide you also.

2) Voltage drop - Related to what the motor can withstand. In many cases 10% (ie 2.4 volts lost in the cable for a 24VDC nominal system) is used in DC applications but it is worth checking. I know of some offshore standards where more onourous DC voltage drop requirements exist. The formula for voltage drop is quite simple: Volt_Drop(volts) = I x R where I is the current in the circuit, and R is the resistance (obtained from cable data). Make sure when you calculate your R value that you allow for the distance from source to destination and destination to source. Most cable manufacturers only quote cable resistance of one conductor in a multi-conductor cable.

3) Short circuit withstand - the cable needs to be able to survive a short circuit. Short circuit fault current is generally derived from the batteries (refer battery manufacturer) as the DC charger is often current limiting type.

The size required is the worst case of the three conditions above. Note that for most low voltage DC applications, the voltage drop requirements are the most onerous.

Regards,
James

 

Any motor cables should be selected based on heat loss/build-up and resistance.

You need a enough-large cable to overcome voltage drop. It would be a safety hazzard, if the cable would be located in/on a cable tray and not considered a proper heat loss.
I do not know the what DC voltage required. You can easily calculate the size and type of conductors.

Newark Electronics and other vendors have all types of cables.

 

In cable selection the following variables are needed:
*power in kva or kw
*permissuble voltage drop (usuall 5%)
*distance to load
*fault current
*mechanical conditions e.g amouring requirements,temperature etc.
Generally
P=sqrt(3)xIxVxP.FxEfficiency for 3-phase motor
OR
P=IxVxP.FxEfficiency for 1-phase motor

-Determine I from the formula.

Because of the high starting current,the current used in selecting cable is
2*I for star delta starting
4*I for direct on line starting

-choose cable that can withstand this current from cable catalogues from manufacturer
-Determine if the voltage drop from the formula
Vd=(mVxDxI)/1000 (V)
for the cable size and distance falls within 5% of supply voltage
If not choose the next cable size from catalogue.

P=motor rating I=calculated current
V=supply voltage P.f=power factor
Vd=voltage drop mV=constant
(voltage drop/ampere/metre) in millivolt
D=distance from source
mV is a constant obtainable from cable catalogue, for a particular cable size and current rating.
This gives a rough method of obtaining cable size for AC motors which i am more conservant with and i want to believe this applies to DC motors also.
For detailed sizing other factors above will be considered.

 

Addressing paulshades reply,

Although the original query concerned DC, your reply covering AC contains an error that too many engineers make:

For the Star-Delta case the nominal current for each of the 6-conductors between the starter and the motor equals (0.577) x (1.0/0.8), or 0.72 times the motor's nameplate rating.

The first multiplier, 0.577 or (sqrt3) / 3, is due to the fact that the connection is that of three 1-phase loads, and not one 3-phase load. The second multiplier, 1.0 / 0.8 or 1.25, results from the fact that 6 conductors are in the same wireway or conduit, not three. Thus, the derating factor is 0.8.

Regards,
Phil Corso, PE
Boca Raton, FL
[[email protected]] ([email protected]) {[email protected]}