Specifying energy efficient motor, cabling and local power transformer

Why energy efficient? Let’s see.

Many energy saving actions can encourage specifiers and engineers to look closely at the savings which can be made by specifying an energy efficient version of a particular component.

This technical article presents the case for taking the important step and looking at the overall benefit of specifying energy efficient options throughout the system, looking at the motor and cabling together, and if appropriate, the local power transformer.

1. The motor
2. The cable
3. The transformer

1. The Motor

As an example, consider a ventilation fan motor, working full time, at 100 metres from the distribution panel.

Typical Motor parameters:

The input power is given by the following formula:

At a unit cost of £0.0443 per unit, the annual saving is £97.02, while the incremental cost of the energy-efficient motor would be about £95.

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The Cable

A conductor cross-section of 1.5 mm² would be considered adequate – from a thermal rating point of view – to carry this current (< 10A). Such a cable would have a resistance of 25 milliohms per metre.

The power loss in the cable is:

p_L = I² × R

The reduction in power loss in the cable due to the use of an energy efficient motor is:

p_D = (I₁² – I₂²) × R
p_D = (9.842 – 9.512) × 25 × 10^-3 × 100
p_D = 5.796 × 2.5 = 14.49 W

Energy saved per year = 127 kWh

For a 1.5 mm² cable supplying an energy efficient motor the power loss is:

= 9.51² × 25 × 10^-3 x 100 = 226.1 W per phase, or 678.3 W total for all three phases.

Using, say, 10 mm² cable would reduce the total loss to 103.1 W, giving an annual saving of

575.2 × 8,760 = 5,037 kWh

At a unit price of £0.0443 per unit, the annual saving would be £223.14. Since the cost of installation will not be significantly higher than for the smaller cable, the only additional cost is the purchase cost of the cable.

This table shows the power saving achieved so far:

The overall annual power saving is 7,227 kWh, or £320.16.

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3. The Transformer

There are dozen of factors which should be considered when selecting local transformers (core losses, load losses, winding material, etc.). By careful selection of the motor and cabling we have already reduced the input power by 10%, and consequently the attributable transformer load loss by 21%.

Using the standard and low loss transformers from Table 1 and assuming that it is running at 80% load, the incremental loss associated with a 1 kW load for each transformer can be estimated at 24.4 W and 21.25 W respectively.

TDR – Test discount rate
REC – Regional Electricity Companies

For the incremental load of the motor, the additional losses are 195.2 W for the standard and 170 W for the low loss transformer, so that the low loss transformer saves a further 221 kWh per annum. The additional cost of selecting a low loss transformer is equivalent to £5.37 per kW of rating (based on a 315 kW unit), so that attributable to the motor load (remembering that the overall loading is 80%) is £53.60.

The overall saving is 7,446 kWh per year, equivalent to £329.85 per year (at £0.0443 per unit).

It must be remembered that the selection of more efficient downstream components, including cabling, reduces losses in other components upstream.

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Reference // Electrical Energy Efficiency by Copper Development Association