Electric motor efficiency is the measure of the ability of an electric motor to convert electrical energy to mechanical energy; i.e., kilowatts of electric power are supplied to the motor at its electrical terminals, and the horsepower of mechanical energy is taken out of the motor at the rotating shaft. Therefore, the only power absorbed by the electric motor is the losses incurred in making the conversion from electrical to mechanical energy. Thus, the motor efficiency can be expressed as
Therefore, to reduce the electric power consumption for a given mechanical energy out, the motor losses must be reduced and the electric motor efficiency increased.
To accomplish this, it is necessary to understand the types of losses that occur in an electric motor. These losses consist
of the following
The power losses (PR in the motor windings) consist of two losses: the stator power losses PR and the rotor power losses PR. The stator power loss is a function of the current flowing in the stator winding and the stator winding resistance—hence the term 72R loss:
When improving the motor performance, it is important to recognize the interdependent relationship of the efficiency and the power factor. Rewrite the preceding equation and solve for the power factor:
Therefore, if the efficiency is increased, the power factor will tend to decrease. For the power factor to remain constant, the stator current I1 must decrease in proportion to the increase in efficiency. To increase the power factor, the stator current must be decreased more than the efficiency is increased. From a design standpoint, this is difficult to accomplish and still maintain other performance requirements such as breakdown torque. However,
Therefore, the stator losses are inversely proportional to the square of the efficiency and the power factor. In addition, the stator loss is a function of the stator winding resistance. For a given configuration, the winding resistance R is inversely proportional to the pounds of
magnet wire or conductors in the stator winding. The more conductor material in the stator winding, the lower the losses.
The rotor power loss is generally expressed as the slip loss:
N = output speed, rpm
Ns = synchronous speed, rpm
FW = friction and windage loss
The rotor slip can be reduced by increasing the amount of conductor material in the rotor or increasing the total flux across the air gap into the rotor. The extent of these changes is limited by the minimum starting (or locked-rotor) torque required, the maximum locked-rotor current, and the minimum power factor required.
Magnetic Core Losses
Magnetic core losses consist of the eddy current and hysteresis losses, including the surface losses, in the magnetic structure of the motor. A number of factors influence these losses:
1. The flux density in the magnetic structure is a major factor in determining these magnetic losses. The core loss can be decreased by increasing the length of the magnetic structure and, as a consequence, decreasing the flux density in the core. This will decrease the magnetic loss per unit of weight but, since the total weight will increase, the improvement in losses will not be proportional to the unit loss reduction. The decrease in magnetic loading in the motor also decreases the magnetizing current and thus influences the power factor.
2. The magnetic core loss can also be reduced by using thinner laminations in the magnetic structure. Typically, many
standard motors use 24-gauge (0.025-in. thick) laminations. By using thinner laminations, such as 26-gauge (0.0185-in. thick) or 29-gauge (0.014-in. thick), the magnetic core loss can be reduced. The reduction in the magnetic core loss by the use of thinner laminations ranges from 10 to 25%, depending on the method of processing the lamination steel and the method of assembling the magnetic core.
3. There has been considerable progress made by the steel companies to obtain lower magnetic losses in both silicon and cold-rolled (low-silicon) grades of electrical steel. The magnetic core loss (Epstein loss) can be reduced by using silicon grades of electrical steel or the improved grades of cold-rolled electrical steel. The type of steel used by the motor manufacturer depends on his process capability. The cold-rolled electrical steel requires a proper anneal after punching to develop its electrical properties, whereas the silicon grades of electrical steel are available as fully processed material. Tables 2.2a and 2.2b illustrate some of the silicon and cold-rolled electrical steels available and the influence of grade and thickness on the Epstein loss and permeability.
However, because of variables in the processing of the lamination steel into finished motor cores, the reduction in core loss in watts per pound equivalent to the Epstein data on flat strips of the lamination steel is seldom achieved. Magnetic core loss reductions on the order of 15-40% can be achieved by the use of thinner-gauge silicon-grade electrical steels. A disadvantage of the higher-silicon lamination steel is that, at high inductions, the permeability may be lower, thus increasing the magnetizing current required. This will tend to decrease the motor power factor.
Friction and Windage Losses
Friction and windage losses are caused by the friction in the bearings of the motor and the windage loss of the ventilation fan and other rotating elements of the motor. The friction losses in the bearings
TABLE 2.2a Typical 50/50 as Sheared Epstein Data for Silicon-Grade Electrical Steel
|steel||Standard||thickness,||at 15 kg,||at 15 kg,|
|grade||gage||in.||60 Hz||60 Hz|
Note: The Epstein core loss is for fully processed steel; lower losses can be attained with semiprocessed steel and a quality anneal. Source: Courtesy Armco Advanced Materials Co., Butler, PA.
are a function of bearing size, speed, type of bearing, load, and lubrication used. This loss is relatively fixed for a given design and, since it is a small percentage of the total motor losses, design changes to reduce this loss do not significantly affect the motor efficiency. Most of the windage losses are associated with the ventilation fans and the amount of ventilation required to remove the heat generated by other losses in the motor, such as the winding power losses PR, magnetic core loss, and stray load loss. As the heat-producing losses are reduced, it is possible to reduce the ventilation required to remove those losses, and thus the windage loss can be reduced. This applies primarily to totally enclosed fan-cooled motors with external ventilation fans. One of the important by-products of decreasing the windage loss is a lower noise level created by the motor.
TABLE 2.2b Typical Epstein Data Inland Steel Nonsilicon Cold-Rolled Electrical Steel
|Inland type||in.||W/lb at 15 kg||at 15 kg (min.)|
Note: The Epstein values are typical for semiprocessed steel annealed after punching.
Stray Load Losses
Stray load losses are residual losses in the motor that are difficult to determine by direct measurement or calculation. These losses are load related and are generally assumed to vary as the square of the output torque. The nature of this loss is very complex. It is a function of many of the elements of the design and the processing of the motor. Some of the elements that influence this loss are the stator winding design, the ratio of air gap length to rotor slot openings, the ratio of the number of rotor slots to stator slots, the air gap flux density, the condition of the stator air gap surface, the condition of the rotor air gap surface, and the bonding or welding of the rotor conductor bars to rotor lamination. By careful design, some of the elements that contribute to the stray loss can be minimized. Those
stray losses that relate to processing, such as surface conditions, can be minimized by careful manufacturing process control. Because of the large number of variables that contribute to the stray loss, it is the most difficult loss in the motor to control.
Summary of Loss Distribution
Within a limited range, the various motor losses discussed are independent of each other. However, in trying to make major improvements in efficiency, one finds that the various losses are very dependent. The final motor design is a balance among several losses to obtain a high efficiency and still meet other performance criteria, including locked-rotor torque, locked-rotor amperes, breakdown torque, and the power factor.
The distribution of electric motor losses at the rated load is shown in Table 2.3 for several horsepower ratings. It is important for the motor designer to understand this loss distribution in order to make design changes to improve motor efficiency. In a very general sense, the average loss distribution for standard NEMA design B motors can be summarized as follows:
|Motor component loss||Total loss, %|
|Stator power loss I2R||37|
|Rotor power loss I^R||IS|
|Magnetic core loss||■10|
|Friction and windage||9|
|Stray load loss||16|
This loss distribution indicates the significance of design changes to increase the electric motor efficiency. However, as the motor efficiency and the horsepower increase, the level of difficulty in
TABLE 2.3 Typical Loss Distribution of Standard NEMA Design B Drip-Proof Motors
Polyphase four-pole motor, 1750 rpm. % loss = percent of total losses. PU loss = loss/(hp x 746).
improving the electric motor efficiency increases. Consider the stator and rotor power losses only. To improve the motor full-load efficiency, one efficiency point requires an increasing reduction in these power losses as the motor efficiency increases:
|Original||Increased||in power losses|
|hp||efficiency, %||efficiency, %||required, %|
These loss reductions can be achieved by increasing the amount of material, i.e., magnet wire in the stator winding and aluminum
|25 hp||50 hp||100 hp||>.m hp|
FIGURE 2.2 Per unit losses for standard design B four-pole motors.
conductors in the rotor or squirrel-cage winding. However, a loss deduction of only 5-15% can be achieved in these power losses without making other design modifications. These modifications can include a new lamination design to increase the amount of magnet wire and aluminum rotor conductors that can be used, combined with the use of lower-loss electrical-grade lamination steel in the magnetic structure and the use of a longer magnetic structure. The level of difficulty and, consequently, the cost of improving the electric motor efficiency increases as the horsepower rating increases. This is illustrated in Fig. 2.2, which shows the decrease in per unit losses as the horsepower rating increases, thus requiring a larger per unit loss reduction at the higher horsepower ratings for the same efficiency improvement.