How motor efficiencies affect system designs

A plant's new project requirements: "Make it 25% faster, with 50% fewer rejects," Another requirement could be, "lower energy consumption by 30%." Plants are far more concerned with "parts per minute" than "watts per part." In systems such as packaging, finishing, or a new production line, the largest energy consumers are perhaps electric motors and servos.

06/15/2002


Key concepts

Analyzing motor loads in the design or retrofit stage can significantly reduce operating costs.

Consider ways to turn motors and controls off when idle.

Adjustable speed drives and soft starters can help reduce peak currents.

Sections:
Air power
Hydraulics
Watch out
Using drives on auxiliary equipment
Servo and drive systems
Power quality
Specifying motors
Performance issues
Summary


A plant's new project requirements: "Make it 25% faster, with 50% fewer rejects," Another requirement could be, "lower energy consumption by 30%."

Plants are far more concerned with "parts per minute" than "watts per part." In systems such as packaging, finishing, or a new production line, the largest energy consumers are perhaps electric motors and servos. Looking at the motor loads in the design or retrofit stage can significantly reduce operating costs for years to come. Every system uses different components and technologies, so each offers different savings opportunities.

Air power

If a design or retrofit project includes a small or medium air compressor, you should be aware of some important energy-saving considerations. In most energy audits, the biggest power waster is plant air. The single biggest problem is leaks. A new system should not leak initially. However, it should be designed with maintenance in mind. Look to the compressor and the electric motor for energy savings.

Generally, the compressor either runs continuously, bypassing as the need for air is reduced, or it runs until tank pressure is reached, and then shuts off. Air compressor manufacturers have multiple methods of variable flow for energy savings. When specifying air compressors, these methods should be evaluated to reduce energy cost.

Small motors in a bypass or no-load condition use 50-90% of the full load amperage (FLA). This fact should be considered when reducing energy cost (See Table 1 at bottom).

Make sure the motor is sized to operate without using the motor service factor. When a motor operates in its service factor, it runs hotter. This could shorten its life. In addition, the efficiency of the motor tends to decrease as the load of the motor operates above its nameplate horsepower.

Compare different compressor packages by the cost of energy for expected air requirements. Most suppliers can supply estimated energy cost with the bid or during the initial compressor sizing.

Hydraulics

As they age, hydraulic systems can resemble pneumatic systems because of leaks. Good system design, which will reduce shock and surging, prevents most leaks. However, if leaks occur, they should be repaired as soon as they are observed. This is common-sense maintenance; it saves energy over the long run.

Another important consideration is the efficiency of electric motors running the hydraulic pumps. Many electric motors driving pumps do not come under the new federal mandatory minimum energy efficiency requirements (EPAct) for T frame motors. The overall system cost in energy is related to pump and motor efficiency. Payback depends on operating time. Therefore, three shifts a day or 24/7 operation means a high-efficiency motor has a quicker payback. Motors that run only a few hours a day don't cause as much concern because of the long payback and the low energy consumption (See Table 2 at bottom). Currently, 1, 1.5, and 2-hp motors can be supplied in either in 56 frame or T frame.

Hydraulic pumps usually run continuously. If the dwell time for the hydraulic system is relatively long compared to the operating time, the energy cost considerations become enough to justify the use of another technology. Ask the vendor for methods to reduce idle time energy costs. Also consider using a higher efficiency pump motor than mandated by EPAct efficiency levels. The payback could be measured in just a few months.

Watch out

Many companies design one system to be used in more than one plant location, including plants located in other countries. This practice can create a problem in Canada, for example, which has fewer exemptions in its version of EPAct regulations. Unlike in the U.S., 200-V and 575-V motors, as well as most custom motors are included in Canada's regulations. Mexico's regulations are very similar to those of the U.S. Currently, single-phase motors are not regulated, but premium-efficiency single-phase motors are available. As a general rule, three-phase motors are more efficient than single-phase types.

Another potential problem for European equipment that includes electric motors is that the motors may not comply with EPAct. The standard IEC motors probably will not meet EPAct standards. Specifying EPAct efficiency as a minimum on all T frame motors may save energy — and possibly a large headache at the border.


Fig. 1. The payback on variable torque loads can be very short depending on the load profile. Software to calculate savings is available on web sites from most motor and drive vendors.

Using drives on auxiliary equipment

On production lines, there are usually some types of fans, blowers, or pumps that get the product started or ready for the next production process. Even with a continuous process, the auxiliary devices may not always have to run at full load. Adjustable speed drives (ASDs) can provide lower energy consumption compared to running a constant-speed motor at idle or using a throttling valve (Fig. 1).

Pumps and blowers are normally variable-torque devices. A speed reduction reduces loads proportionally. Using an ASD on these loads reduces energy consumption in proportion to load reduction as the speed is reduced. Pumps or fans that do not require full load most of the time are candidates for ASDs. Processes using fans and pumps can use the PID loop built into most drives to maintain a constant pressure or temperature instead of the less efficient dampers or valves. Using an ASD, instead of a bypass valve for pumps or dampers for fans, controls the process better, making it more repeatable. Small-horsepower ASDs cost little more than a starter and can offer great energy savings.

Another advantage is that a drive is inherently a reduced-voltage motor starter. Generally, the largest part of the utility bill is the penalty for peak usage, so reducing peak current should be a goal. For large motors in the project, consider using ASDs. However, if variable speed is not needed, consider using solid-state starters to reduce the peak current. Solid-state starters can even be less expensive than mechanical starters for larger motors. Also, using the PLC to bring one drive or motor on at a time during startup can reduce peak demand.

Servo and drive systems

Servo amplifiers and ASD regeneration resistor banks should be looked at first for energy savings in newly designed or retrofitted systems. Some applications require the repetitive stopping of large inertial loads. The control's regeneration resistor banks must dissipate this stopping energy. The regeneration resistors then transfer the heat to the plant air, adding to the heat load in the building. For motors around 10 hp and larger, consider a line regeneration control that will put the energy back on the line (run the meter backwards) for a direct saving — plus, it eliminates the additional heat load. The extra cost of the regeneration drive is offset by the elimination of the regeneration resistors, the recovery of the electricity, and the reduced plant cooling cost. They are offered in vector, ASD, and ac servo pulse width modulation (PWM) drive configurations.

The extra heat from drive regeneration resistors can present heat loads on the plant cooling systems. One way of reducing this energy cost is to use a multiaxis drive. The energy from one servo will be regenerated into the common bus and used by the other drives first. The regeneration resistor is only used by the control as a last resort. This package is smaller, simpler, and more energy efficient. It also can have a lower installed cost compared to individual drives.

In servo systems, it is also common to see servo motors in the hold mode. The control is commanded to hold or have torque at zero speed, even when it is not necessary. A more energy-conscious method would be to decelerate to a stop then set a mechanical brake to hold motors requiring zero shaft movement. After setting the brake, the drive can be disabled, thereby saving energy. The brake does not wear because it does no braking of the load. Consider 20 servo motors on a machine, all energized, with nothing to do. They are no longer servos, just heaters! Also consider stopping the motor, and then disabling the drives. This stops the heat-producing current flow to the motors.

Power quality

Power quality can be directly related to good system design. It is common to see single-phase power being supplied from one transformer to all the control devices, PLCs, and even to single-phase servos. And as power grids get weaker, voltage imbalances can become an issue on three-phase circuits. Even small voltage imbalances can increase the power consumption of each motor (Fig. 2).


Fig. 2. Voltage balance is important. It is often monitored on incoming power, but not inside the plant distribution system. Regardless of the system design, a 4% voltage imbalance can negate great design work.

There are many nonlinear loads, such as PLCs, ASDs, and PCs, connected to power sources. These nonlinear loads degrade the power quality. Electric motors convert nonlinear power to heat. The bottom line is that bad power requires more power to do the same work. By controlling harmonics with filters, proper wiring techniques can extend the life of the electronics, and reduce power consumption by motors (For more information, read NEMA standard MG1 Part 30.1.2).

Motors and most other components are designed for maximum performance and efficiency when operated at nameplate voltages. At most plants, the operating voltage is not 460 V (as listed on the applicable nameplates), but closer to 500 V. As voltage increases, the efficiency decreases for the motor loads, thus increasing power consumption (Fig. 3).


Fig. 3. This chart from the EASA Electrical Engineering Handbook shows how a motor behaves as voltage changes. Voltage increases do not necessarily decrease operating amperage.

An engineer's calculations and tabulations are based on servo and motor performance charted by manufactures at the nameplate voltages. When a system operates at 495 V, it can reduce the servo's regenerative performance, increase motor speed, decrease motor efficiency, and create other undesirable performance characteristics.

Specifying motors

If a project requires ac electric motors, the engineer should be careful in specifying "premium efficiency" motors. This term had meaning in the past, because it was defined in NEMA guidelines. The established EPAct motor efficiencies are the old NEMA premium efficiency level. Every motor manufacturer now offers a motor line with efficiencies above those specified by EPAct. But NEMA has not published a new "premium efficiency" table. This situation creates confusion as to the specification of premium efficiency motors. A good specification to consider is the one from the Consortium of Energy Efficiency (CEE) used by many electric utilities (Table 3). It is also important to check with the local utilities for energy rebate programs that will cover most or all of the cost for upgrading to premium efficiency motors. Many electric utilities can provide engineering assistance to help reduce power costs.

Performance issues

High-efficiency motors do perform differently. With each increase in efficiency in motor design, tradeoffs are made, and most are transparent to the application. First, the locked rotor amps (LRA), also called starting amps, increase. Typical motor LRAs before EPAct had been approximately six times the FLA. The new EPAct motors have an LRA approximately six and a half to seven times the FLA. And in premium efficiency motors LRA, is approximately eight times FLA.

Also, older motors generally ran at slightly slower speeds. A typical pre-EPAct, 4-pole motor might have run at 1725 rpm, whereas a replacement high-efficiency motor may run at 1750 rpm or higher. This difference is a consideration for pumps and fan applications where increasing the speed increases load. Check performance data for exact speed at full load. The higher the efficiency, the cooler the motor operates. A cooler motor means less heat load.

Summary

When considering energy costs in a design or retrofit project, first look to the power utilities to see if rebates are being offered. The utility's engineering staff can provide assistance. Vendors should supply energy calculations to determine energy costs for different technologies.

Regeneration resistor banks are energy wasters; try to eliminate or reduce their use to lower the plant's air conditioning heat load. The estimated wattage from calculations to size the regeneration resistors can be used to estimate air conditioning costs and pay back. The plant's HVAC supplier can calculate the additional heat load cost or the savings from reduced heat load.

Consider ways to turn motors and controls off when idle. Cycle motors and controls "on" one at a time to reduce peak current. Consider ASDs and soft starters to help reduce peak currents.

After the installation is completed, make sure the voltage is balanced and the harmonic voltage is controlled to reach the project objective.

— Edited by Jack Smith, Senior Editor, 630-288-8783, jsmith@cahners.com

Table 1. Typical motor efficiencies at full load vs. no load

Hp

Efficiency

No load amps (NLA)

full loadamps (FLA)

NLA/FLA%

0.25

64

0.6

0.67

90%

0.33

68

0.7

0.8

88%

0.75

76.2

1.2

1.5

80%

1

78.6

1.2

1.7

71%

1*

82.5

0.7

1.4

50%

5*

87.5

3.6

6.7

54%

10*

89.5

7.9

14.2

56%

Note:*=EPAct efficiency motors


Table 2. Examples of paybacks for premium efficiency motors

10 Hp Open

EFF

1 shift 2000 h

Payback in months

3 shifts 6000 h

Payback in months

24/7 8260 h

Payback in months

NonEPAct

85.5

$1,222

$3,141

$4,586

EPAct

89.5

$1,167

3.6

$3,001

1.2

$4,381

0.8

Premium Efficiency

91.7

$1,139

20

$2,929

6.7

$4,276

4.6

10 Hp TEFC

EFF

1 shift 2000 h

Payback in months

3 shifts 6000 h

Payback in months

24/7 8260 h

Payback in months

Non-EPAct

85.5

$1,047

$3,141

$4,586

EPAct

89.5

$1,000

7.9

$3,001

2.6

$4,381

1.8

Prem Eff

91.7

$976

35.9

$2,929

12

$4,276

8.2

Note for Table 2: The calculations are based on 6otor. NonEPAct motors may not be available, but would be the most expensive choice — even when used just 40 hr per week.


Table 3. EPAct vs. CEE efficiencies for 1800-rpm motors

Open Hp

TEFC EPAct

CEE

Hp

EPAct

CEE

This table compares 4-pole motor efficiency at EPAct and CEE levels. EPAct covers 2-pole and 4-pole motors.

1

82.5

85.5

1

82.5

85.5

1.5

84

86.5

1.5

84

86.5

2

84

86.5

2

84

86.5

3

86.5

88.5

3

87.5

89.5

5

87.5

89.5

5

87.5

89.5

7.5

88.5

90.2

7.5

89.5

91

10

89.5

91

10

89.5

91

15

91

92.4

15

91

92.4

20

91

92.4

20

91

92.4

25

91.7

93

25

92.4

93.6

30

92.4

93.6

30

92.4

93.6

40

93

94.1

40

93

94.1

50

93

94.1

50

93

94.1

60

93.6

94.5

60

93.6

94.5

75

94.1

95

75

94.1

95

100

94.1

95

100

94.5

95.4

125

94.5

95.4

125

94.5

95.4

150

95

95.8

150

95

95.8

200

95

95.8

200

95

95.8





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