AC Induction Motor Design (7)

9. Energy Efficient Electric Motors

The Energy Policy Act of 1992 (EPACT) directs manufacturers to manufacture only energy efficient motors beyond October 24, 1997 for the following: (All motors which)

– General Purpose
– Design B
– Foot Mounted
– Horizontal Mounted
– T-Frame
– 1 to 200 hp
– 3600, 1800, and 1200 RPM
– Special and definite purpose motor exemption

To meet NEMA MG1-1993 table 12.10 efficiency values. The method for testing for these efficiency values must be traceable back to IEEE Std. 112 Test type B.
Energy efficient motors are really just better motors, when all things are considered. In general, they use about 30% more lamination steel, 20% more copper, and 10% more aluminum. The new lamination steel has about a third of the losses than the steel that is commonly used in standard efficient motors.
As a result of fewer losses in the energy efficient motors, there is less heat generated. On average, the temperature rise is reduced by 10 degrees centigrade, which has the added benefit of increasing insulation life. However, there are several ways in which the higher efficiency is obtained which has some adverse effects:
– Longer rotor and core stacks – narrows the rotor – Reduces air friction, but also decreases power factor of the motor (more core steel to energize – kVAR).
– Smaller fans – reduces air friction – the temperature rise returns to standard efficient values.
– Larger wire – Reduces I2R , stator losses – Increases starting surge (half – cycle spike) from 10 to 14 times, for standard efficient, to 16 to 20 times, for energy efficient. This may cause nuisance tripping.
In general, energy efficient motors can cost as much as 15% more than standard efficient motors. The benefit, however, is that the energy efficient motor can pay for itself when compared to a standard efficient motor.
$ = 0.746 * hp * L * C * T (100/Es -100/Ee)
where hp = motor hp, L = load, C = $/kWh, T= number of hours per year, Es = Standard efficient value, and Ee = Energy efficient value — Eq. 5


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