Why overmotoring an application is not such a good idea

Bad practice

In many instances, the practice has been to overmotor an application, i.e., to select a higher-horsepower motor than necessary.

The disadvantages of this practice are:

1. Lower efficiency
2. Lower power factor
3. Higher motor cost
4. Higher controller cost
5. Higher installation costs

Let’s see one example of overmotoring in the case of the varying duty applications. Consider the comparisons of the 40-hp motor that could have been selected based on the peak load versus the 30-hp motor that can be selected on the basis of the duty cycle:

1. Motor cost // List price of standard open 1800-rpm drip-proof motor:

• 30 hp = $1160
• 40 hp = $1446

2. Control Cost // NEMA-1 general-purpose motor, 240-V starter:

• 30 hp, size 3 = $600
• 40 hp, size 4 = $1350

Figure 1 shows the difference in the input watts and Figure 2 the difference in the input kilovolt-amperes for 30- and 40-hp motors operating at the same output.

At loads above 36 hp, the input is more favorable for the 40-hp motor. However, at loads below 36 hp, the kilowatt and kilovolt-ampere inputs are lower with the 30- hp motor.

IMPORTANT! In general, the larger the difference between the actual load and the motor rating, the higher the input requirements for the same load.

The common practice of motor oversizing results in less efficient motor operation, higher motor current, lower power factor and higher energy loss in the power distribution system.

Although some situations may require oversizing for peak loads, you should otherwise always select a motor that will operate efficiently in the 75 to 100 percent load range. The efficient load range varies for some motor types, designs or applications. In some cases, downsizing the motor may yield energy demand savings.

Should you replace an oversized motor?

Motors rarely operate at their full-load point. Field tests of 60 motors at four industrial plants indicate that, on average, they operate at 60% of their rated load. Motors that drive supply or return air fans in heating, ventilation and air conditioning (HVAC) systems generally operate at 70% to 75% of rated load.

They are the load on the motor, the operating efficiency of the motor at that load point, the full-load speed (in revolutions per minute [rpm]) of the motor to be replaced, and the full-load speed of the downsized replacement motor.

Using software to conduct analyses of oversized motor replacement opportunities

MotorMaster

MotorMaster software contains a speed/correction algorithm such that when the nameplate full-load speed of the motor to be replaced is entered, the increase or decrease in load for the replacement motor is automatically calculated.

Speed change effects are thus used to determine annual energy and dollar savings and the simple payback from investing in a new, energy-efficient motor.

MotorMaster also contains full, 75%, 50%, and 25% load and power factor data for most motors. An oversized motor replacement analysis can readily be made, with MotorMaster interpolating to determine the efficiency at the appropriate internally computed load point for the new downsized motor.

Equipment and installation cost data for the replacement motor are automatically entered into the analysis and speed correction is again automatically accounted for.

References //

• Energy efficient electric motors by Marlin O. Thurston; Department of Electrical Engineering The Ohio State University Columbus, Ohio
• Motor Efficiency, Selection and Management – A Guidebook for Industrial Efficiency Programs by Consortium for Energy Efficiency
• Replacing an oversized and underloaded electric motor by U.S. Department of Energy