This article is the second part on the discussion about voltage drop calculation. We shall be providing real world example for you to be able to appreciate it better as it may have already been a part of what you have already done or currently doing in your design.

We shall be presenting an IEC standard equipment in our sample calculation.

A 120 mm^{2} 3core XLPE insulated cable 120m in length supplies a 110 kW induction motor that has a starting current of 6 times the full-load current of 180 amps. The starting power factor is 0.25 lagging.

If the sending end line-to-line voltage is 400 volts, the specific resistance ** r** and reactance

**for the cable are**

*x***and**

*0.197***ohm/km respectively at 90**

*0.072*^{O}C and 50 Hz.

Find the percentage volt-drop on starting the motor.

### Solution

**For the 120 mm ^{2} cable**

the series impedance is:

R = rl = 0.214 x 120 / 1000 = 0.02568 ohms/phase

X = xl = 0.0808 x 120 / 1000 = 0.009696 ohms/phase

**For the 110kW motor**

the starting current is:

I = 6 Ã— 180.0 = 1080.0 A

The power factor is:

cosØ = 0.2500, therefore sinØ = 0.9682

Let us assume the sending voltage remains constant at 400 volts,

** AB = 1080.0 x 0.02568 x 0.2500 = 6.9336 volts/phase BE = 1080.0 x 0.02568 x 0.9682 = 26.6805 volts/phase EF = 1080.0 x 0.009696 x 0.9682 = 10.1386 volts/phase DF = 1080.0 x 0.009696 x 0.2500 = 2.6179 volts/phase**

** %V _{d} = ( 1.732 / 400 ) x 1080.0 x ( 0.02568 x 0.25 + 0.009696 x 0.9682 ) x 100% %V_{d} = 4.6764 x ( 0.00642 + 0.006738 ) x 100% %V_{d} = 6.15%**

Therefore,** V _{r} = 400 x ( 1 - 0.0615 ) / 1.732 V_{r} = 216.74 volts/phase**

Typical minimum starting voltage for motors is 80% x rated voltage. Since the calculated volt-drop in our example is less than 20%, the motor will accelerate to full speed easily.

In practice, since the actual motor parameters are not known during the design stage, the minimum starting voltage is raised to 85% to provide a factor for safety.

For Part 3, we shall be providing more examples for voltage drop calculations.