When following the new DOE guidelines, don’t forget these considerations.
by Terry W. Bell

For motors used in the U.S., the selection of an energy-efficient instead of premium-efficient motor is no longer an option. The U.S. Department of Energy (DOE) recently made changes to the efficiency requirements for the majority of definite-purpose motors, including pump motors. These new requirements went into effect June 1, 2016.

Active Material Stack LengthFigure 1. The active material stack length for a standard-efficient motor. (Courtesy of Baldor Electric Company
Active Material Stack Length PremiumFigure 2. The active material stack length for a premium-efficient motor.

The new rule, known as the Integral Horsepower Motor Rule, covers 1- to 500-horsepower (hp) (0.75 to 370 kilowatts [kW]), three-phase, alternating-current (AC) induction electric motors and requires the motors to meet premium-efficiency levels as defined similar to National Electrical Manufacturers Association (NEMA) Standard Document MG 1-2014, Table 12-12. Only fire pump motors are allowed to remain at the energy-efficient level.

When replacing a standard motor with a premium-efficient motor, several questions can arise pertaining to form, fit and function.

1. Will the premium-efficient motor be dimensionally larger?

It depends. Going from a standard-efficient to a premium-efficient design requires additional active materials such as copper and high-grade electrical steel. In some cases, the motor size will not change, because it depends on the horsepower, speed and room inside the motor frame. In other cases, the motor could become longer, the frame diameter could be larger or a combination of both. However, the motor size is still bound by NEMA definition to have the standard mounting dimensions.

2. Will the motor weigh more?

Yes. The required additional active materials (copper and electrical steel) could increase the motor weight by as much as 20 percent, so the effect on mounting and handling must be taken into consideration.

3. Will it operate at the same speed?

No. A premium-efficient motor has less slip than a standard-efficient motor, so it will run at higher speeds. For example, a two-pole standard-efficient motor would have a full-load speed of 3,419 revolutions per minute (rpm). The two-pole premium-efficient motor will have a full-load speed of 3,452 rpm. The premium-efficient motor would run 33 rpm faster, and this increase in shaft speed will affect affinity law calculations. This results in an increase in the pump’s flow, pressure and power and causes increased load to the motor.

You may need to contact the pump manufacturer’s representatives to see what they recommend on trimming the impeller to reduce motor load from the higher speed.

4. Will it draw less current?

No. The premium-efficient motor is running at a higher speed, so you will see the same or possibly even higher current draw because the motor is producing more work. Another item that affects current draw is the NEMA design code. NEMA design code B is the most prevalent design and is found in the majority of installed pump applications.

In some cases, a motor must be designed to NEMA design code A rather than B to achieve the required premium efficiency. Design A motors have higher inrush current, which is the instantaneous input current required when the motor is first turned on.

When energized for a few cycles of the input waveform, electric motors may draw several times their normal full-load current.

The selection of overcurrent-protection devices such as fuses and circuit breakers are more complicated when high inrush currents are taken into consideration. The overcurrent protection must react quickly to overload or short-circuit faults but must not interrupt the circuit when the inrush current flows.

5. Will it cost more?

Yes. The increase in active materials (copper and high-grade electrical steel) required to meet the premium-efficiency level adds cost, but the higher efficiency translates into more energy savings and reduced total cost of ownership. The purchase price of an electric motor is only about 2 percent of its life cost. More than 97 percent is electricity.

6. What are the benefits of a premium-efficient motor?

Premium-efficient motors have lower temperature rise, which extends motor life. There is also the added benefit of total cost of ownership, which is made up of a combination of purchase price, cost of running the motor in terms of energy consumption over its lifetime and the cost to a business if that motor fails to run when needed.

7. Will a premium-efficient motor optimize my pump system?

Incorporating a premium-efficient motor in the pump system is a step in the right direction, but one must be mindful of the overall efficiency of the system. The combined efficiencies of each component in the system must be taken into consideration. Optimizing each individual component may improve overall efficiency or contribute to a reduction of the overall efficiency because of the cause and effect of each component, perhaps causing a particular component to operate in a less efficient manner.

Users must consider how each effort to reduce energy consumption and cost affects the entire pump system. System optimization must include looking at how each component functions along with the other parts and pieces of the system, ensuring that optimum efficiency is achieved as a whole.

The benefits of an optimized pump system include reduction in energy waste, maintenance cost and downtime. At minimum, a premium-efficient motor is a must to achieve these benefits.

Form, fit and function for your replacement premium-efficient motor may not be a direct drop-in replacement. Consult with your selected motor manufacturer. The manufacturer can answer these questions and ensure that the replacement motor will work for your pump system.