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Matching Motors to Applications

Key Points
  • Motor sizing, efficiency, and design can have a big impact on lifetime operating costs.
  • Premium efficiency motors are 3% to 8% more efficient than standard models.
  • A variety of designs are available to fit the demands that motors must meet.

More than half of the electrical load for most manufacturers is attributable to motors. Because of the significant investment in capital and operating costs, motors deserve attention. In making motor purchasing decisions, companies too often overlook issues such as proper sizing, efficiency, and motor type. Mistakes in any of these areas can have a substantial impact on the bottom line.

Points to Consider

The cost of electricity to run a motor can easily exceed several times the purchase price of a motor. Therefore, even small improvements in motor efficiency can yield significant savings over the life of the motor.

  • The efficiency of an induction motor varies with motor size (horsepower) and motor loading. High horsepower motors are inherently more efficient than smaller-sized motors, but the motor needs to be properly sized to take advantage of this efficiency gain.
  • Motors that are designated energy efficient or NEMA Premium are typically 3% to 8% more efficient than their standard motor counterpart. Energy efficient motors produce the same horsepower output using less electrical input power than a standard motor.

Example

Suppose a manufacturer is considering a three-phase 20 hp motor that will be operating 5,000 hours in a given year. If a standard motor is 86% efficient and a premium motor is 92% efficient, what is the potential difference in energy costs? At 10 cents per kilowatt hour, the standard efficiency motor would cost an estimated $8,675 to operate (0.746 x 20 / 0.86 x 5,000 x 0.10 = $8,675), while the premium efficiency motors cost $8,110 (0.746 x 20 / 0.92 x 5,000 x 0.10 = $8,110) for an annual savings of $565. If the motor runs for more hours or is a higher horsepower rating, then the savings will be greater. If the manufacturer is operating several of these motors, then the potential savings from switching to higher efficiency motors can add up.

You can calculate these and other efficiency comparisons quickly using the Motor Cost Savings Calculator.

The differences between NEMA Premium and Energy Efficient motors are illustrated in the figure above, with specific values for motor efficiency at various horsepower ratings shown in the table below:

Table 1: Motor Efficiencies

Hp

NEMA Premium 

Energy Efficient

1

85.5

82.5

1.5

86.5

84.0

2

86.5

84.0

3

89.5

86.5

5

89.5

87.5

7.5

91.7

88.5

10 

91.7

89.5

15

92.4

91.0

20

93.0

91.0

25

93.6

91.7

30

93.6

92.4

40

94.1

93.0

50

94.5

93.6

60

95.0

94.1

75

95.4

94.1

100

95.4

94.5

125

95.4

95.0

150

95.8

95.0

200

96.2

95.4

250

96.2

95.4

300

96.2

95.4

350

96.2

95.4

400

96.2

95.4

450

96.2

95.8

500

96.2

95.8

If the above motors operate at less than their rated capacity, then there is some efficiency loss, per the figure below:

Motor Design Ratings

In order to further understand the differences in motor design, the terms torque, synchronous speed, and starting current must be explained.

  • Torque—This is the motor's twist, or turning ability, as applied to a shaft. "Locked rotor torque" is defined as the torque produced at initial start. The maximum torque that a motor will produce while running is called "breakdown torque."
  • Synchronous speed—This is the speed at which a motor's magnetic field rotates. It is also called the no-load speed, which is the theoretical speed of the motor if there was no load on the shaft and friction in the bearings. The rotor of an induction motor does not rotate at synchronous speed, because it lags or "slips" with respect to the rotating magnetic field. "Rated speed" is thus defined as the actual speed at the motor's rated power output, and appears on the motor's nameplate. "High slip" motors will have a rated speed significantly lower than that of a "low slip" motor.
  • Starting current—At start up, the motor requires a current that is 6 to 10 times greater than the normal operating current. The current can be reduced with the use of "soft start" devices.

The motor industry has developed different designs to fit the variety of demands that motors must meet. Certain applications favor one design over another, and the user must be sure to select the proper fit. The information below provides a basic description of the most common designs.

  • Design A motors have a higher start up torque than Design B motors. Also, the in-rush current of a Design A motor is not limited by NEMA MG1. These motors can be used for general purpose or for specific applications with high startup torque.
  • Design B motors account for most of the induction motors sold. Often referred to as general purpose motors, slip is 5% or less. As seen from the figure at right, their torque peaks at approximately 80% of synchronous speed.
  • Design C motors have high starting torque with normal starting current and low slip. This design is normally used where breakaway loads are high at starting, but normally run at rated full load, and are not subject to high overload demands after running speed has been reached. Slip is 5% or less.
  • Design D motors exhibit high slip (5% to 13%), very high starting torque, low starting current, and low full load speed. Because of high slip, speed can drop when fluctuating loads are encountered. This design is subdivided into several groups that vary according to slip or the shape of the speed-torque curve. These motors are usually available only on a special order basis.
  • Design E motors are no more. They were originally described as high efficiency motors that could draw a locked rotor current as much as 55% higher than that of a Design B motor with the same horsepower. This could have forced the user to select a higher-horsepower, higher-priced Design E motor to start the existing load. The 2005 National Electric Code formally stated that the specific Design E motor as previously described will not materialize.

Properly matching motors to applications is an important function that can save on capital costs, energy, and operation and maintenance headaches.

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