Electric Motors

Motors run most efficiently near their designed power rating. It is therefore good practice to operate between 75% and 100% of full load rating.

Older installations that have been re-vamped or de-bottlenecked can often be found to have significant line loss. If the voltage at motor terminals is less than design then output power is reduced. Current must then be increased to reach the desired output and the combined I2R loss for motor and supply cables can be significant.

Unbalanced three-phase voltage affects a motor's current, speed, torque and temperature rise. Equal loads on all three phases of electrical distribution helps assure voltage balance while minimizing voltage losses.

The value of power saved by purchasing high efficiency motors can pay out in under 12 months in some cases. They can improve efficiency from 3% for typical process pump applications to eight percent for small units. Efficiency is improved by:

• high grade steel for armature and stator laminations;

• thinner laminations;

• heavier gage copper windings;

• better bearings;

• improved cooling fan design.

The odd shape of the curve in Figure 8.13 reflects typical current industry products. This is a guideline only since there a 2-3% scatter between different manufacturers.

Synchronous motors are more efficient (and more expensive) than induction motors and are mainly used as drivers for ratings in excess of ~3 MW. However, when buying new, it is worth checking whether energy saving outweighs the extra

Figure 8.13 Typical advantage of high efficiency motors.

Motor rating/kW

Figure 8.13 Typical advantage of high efficiency motors.

cost. Multi-pole low speed synchronous motors are a good choice for driving reciprocating compressors.

Synchronous motors can also be used for power factor correction. When fitted with suitable controls, they can maintain system power factor at a pre-set level. This is better than fixed capacitor banks since induction motor power factor changes according to load, with power factor worsening as the load reduces. By generating VARs into the system, power factor is improved and supply current (with its associated I2R loss) is reduced.

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