Choosing the Right Motor Cables

 Cable Sizing for Electric Vehicles

When you think of what makes up an EV, cables and wires probably aren’t the first components that come to mind.

Whe­ther hy­brid, all-elec­tric or fuel cell all the­se ve­hi­cle ty­pes need wires and ca­bles to hand­le what are of­ten ex­tre­me cur­rent loads.

These conduits comprise the electrical cardiovascular system that makes a vehicle function from the battery to the motor and all the stops in between.

It's important to know if you are dealing with DC or AC power in a given wire. AC power has some very complex physics which cause some strange effects. This was one of the reasons why AC power was developed in the 1890s, long after DC power.


In AC power current likes to travel near the surface of a wire (skin effect). AC power in a wire also causes a magnetic field to form around it (inductance). This field effects other nearby wires (such as in a winding) causing proximity effect. All of these properties must be dealt with when designing an AC circuit.


In DC power current travels through the whole of a wire.

 

CHOOSING A CABLE

Cable sizing is determined by a number of factors, including current draw and temperature rise created by the current as it runs through the conductor; cable temperature rating; and as other thermal factors such as air flow, other sources of heat.

When working to determine a cabling solution, we use industry standards such as IEC 60287 (Calculation of the continuous current rating of cables) as a starting point.

THE FORMULA

The current carrying capacity of an insulated conductor or cable is the maximum current that it can continuously carry without exceeding its temperature rating. It is also known as ampacity.

Whilst the cables are in operation they suffer electrical losses which manifest as heat in the conductor, insulation and any other metallic components in the construction. The current rating will depend on how this heat is dissipated through the cable surface and into the surrounding areas. The temperature rating of the cable is a determining factor in the current carrying capacity of the cable. The maximum temperature rating for the cable is essentially determined by the insulation material.

By choosing an ambient temperature as a base for the surroundings, a permissible temperature rise is available from which a maximum cable rating can be calculated for a particular environment. If the thermal resistivity values are known for the layers of materials in the cable construction then the current ratings can be calculated.


I = permissible current rating

∆Φ = Conductor temperature rise in (K)

R= Alternating current resistance per unit length of the conductor at maximum operating temperature (Ω/m)

Wd = dielectric loss per unit length for the insulation surrounding the conductor (W/m)

T1= Thermal resistance per unit length between one conductor and the sheath (K m/W)

T2 = thermal resistance per unit length of the bedding between sheath and the armour (K m/W)

T3 = thermal resistance per unit length of the external Sheath of the cable (K m/W)

T4 = thermal resistance per unit length between the cable surface and the surrounding medium (K m/W)

n = number of load-carrying conductors in the cable (conductors of equal size and carrying the same load)

λ1 = Ratio of losses in the metal sheath to total losses in all conductors in that cable

λ2 = ratio of losses in the armouring to total losses in all conductors in that cable.

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