Core loss can be lowered by reducing the magnetic flux density and by changing the steel type. The hysteresis component of core loss is determined by the characteristics of the steel when magnetized in one direction then demagnetized and remagnetized in the other direction. This is largely a function of the steel type and processing.
The eddy current component of the core loss results from the changing magnetic field inducing voltages and currents in the steel core. It’s a function of the steel type, thickness, and processing and can be reduced by upgrading the steel type and using thinner laminations. This alloy is more costly and more laminations are needed for a given core length to produce the same power.
Motor lamination alloys are usually known in the industry by their M Grade number. For instance, a common steel used in motors is 24-ga. M56. This steel has a core loss of around 5 watts per pound. The so-called “better steels,” such as 29 ga. M19, exhibit a core loss of about 2 W per pound under similar operating conditions. Using M19 steel in place of M56 steel requires more steel to produce the same output power. And, the number of laminations for a given core length increases. Both factors increase the motor cost.
Friction and windage loss
These losses have two major components: bearing friction and cooling system losses. Bearing friction is a function of the bearing size, lubricant and the approach used to seal the bearing. Because most bearings are chosen as a function of the load and application, it leaves little room for change, other than using better grease and seals.
Windage loss results from having to move cooling air through the motor. The cooling fan constitutes the largest part of the loss. Motors have cooling fans to maintain temperatures within the limitations of the insulation system. The fan can be removed if it’s possible to control the temperature within safe limits in other ways. In most cases, however, some means of cooling is required. Improving the entire cooling system to reduce air flow losses is a reasonable goal. Improved cooling systems usually require the addition of baffles and venturis, which add cost.
Stray load loss
Stray load losses are those not accounted for in the other loss categories. They’ve been the topic of many papers and much discussion over the years. There’s evidence that some stray load losses are caused by eddy currents in the tooth tips and rotor surface, as well as rotor bars being shorted to the lamination steel. These losses can be reduced somewhat by changing the rotor manufacturing methods. But, these additional operations, too, lead to increased costs.
Reducing motor losses will, by definition, increase efficiency. The losses are easily identified and quantified using IEEE methods. There’s a labor or material cost associated with each loss-reduction measure. Improved motor efficiency is achieved by improved materials, additional materials and improved processing. All these cost money and drive up the cost to consumers. Motor manufacturers balance these costs against the improved efficiency benefits and the cost to the consumer.
William H. Yeadon, P.E., is the owner of Yeadon Energy Systems in Iron River, Mich. (www.yeadoninc.com). Contact him at [email protected] and (906) 265-7937.
