Permanent Magnet Motors for Commercial and Industrial Applications
Motor: Permanent Magnet AC vs. Constant Speed Induction with Variable Frequency Drive
Permanent Magnet AC (PMAC) motors with powerful ferrite, ceramic or rare earth magnets attached to the rotor to establish a permanent magnetic field feature to reduce rotor resistance losses, exceed "Super Premium" efficiency levels, and provide variable speed operation.
Item ID: 431
Commercial, Industrial, Agricultural
Motors & Drives--Motors
Brushless permanent magnet (PM) motors operate with an AC power supply so are often referred to as PMAC motors. Use of permanent magnets eliminates the need for conductors (rotor bars) so rotor I2R losses are eliminated. This design makes it possible to combine high efficiency, low speed, and high torque in a single package. For small motor sizes, the efficiency of the PM motor may be 10% to 15% greater than older, standard-efficiency motors at the same load point. These efficiency gains hold over the entire range of typical motor loads.
One motor manufacturer offers 1800 RPM, totally enclosed, blower-cooled PMAC motors and drives in the 10 hp to 150 hp size range as standard product offerings. Compact, low-speed “pancake” designs are available for specialty applications, such as cooling tower fan-drive motors. PM motors are available in both “drop-in” NEMA and IEC frame sizes, and can be specified when new process equipment is being ordered or retrofitted onto existing equipment. Frame-size compatibility is important so mounting bolt holes will line up and the motor shaft height and diameter will align with the driven equipment shaft. PMAC motors are available in general purpose as well as explosion-proof and wash-down-duty enclosures.
Due to the need for a drive or controller, variable-speed PMAC motors cost much more than constant-speed Premium Efficiency induction motors. PM motors have variable-speed capability, however, so are equivalent replacements for an electronic pulse-width modulated variable frequency drive (VFD) controlling a new Premium Efficiency inverter-duty motor. When replacing constant-speed motors in variable flow applications, energy savings due to the variable-speed capability of the PMAC motor will greatly exceed the savings due to the increased efficiency of the motor itself. Permanent magnet motors provide improved efficiency over their entire operating range, and meet or exceed the International Electrotechnical Commission (IEC) IE4 efficiency standards. The IEC 60 Hz motor standards exceed the U.S. NEMA Premium full-load efficiency standard levels.
Baseline Description: 20 hp Premium Efficiency Motor with VFD Flow Control
Baseline Energy Use: 1278 kWh per year per hp
Assume a 20 hp Premium Efficiency motor that operates for 3,400 hours per year with VFD flow control. The VFD produces 40% energy savings compared to constant speed operation with an average load of 75%. Efficiency of this motor is 93%. Annual energy use is thus:
3,400 hours/year x 60% x 20 hp x 0.746 kW/hp x 75% load / [93% x 96% (VFD Efficiency)]
= 25,569 kWh/year or 1,278 kWh/year per hp
Note: this is a conservative choice of baseline for comparison. If the baseline was an Energy Efficient or Standard Efficiency motor with a VFD or constant-speed motor serving an application that would benefit from variable flow, the energy savings could be considerably greater.
PMAC motors have found a favorable energy savings retrofit niche as cooling tower fan drive motors. The most common method for driving the fan in modern cooling towers is a right-angle gear reducer drive shaft, along with a standard foot-mounted, constant speed AC motor. Vertical shaft, compact PMAC motors can be designed to directly couple to the fan shaft while operating at a synchronous speed in the range of 90 to 230 RPM. Mechanical losses associated with the gearbox are eliminated and the PM motor has inherent variable speed capability. Replacing a 50 hp conventional constant-speed motor with a PM motor at an Intel facility resulted in 47% energy savings, equivalent to 89,607 kWh per year. The original two-speed motor operated for 5,110 hours per year (Nichols, 2013). Note that current codes in Washington state require variable speed drives for cooling tower motors above 10 hp, so PMAC motors are mainly a retrofit technology or an alternative to installing conventional technology, including gearbox, in new installations.
Manufacturer's Energy Savings Claims:
Savings Range: From 2% to 20%
NovaTorque literature cites a 30% to 50% reduction in wasted energy along with a 4% to 20% reduction in energy use. Marathon’s Sy-Max literature states that their ultra-high efficiency PM motors will have 20% to 25% fewer losses when compared to NEMA Premium motors. Electrical energy is projected to account for 95% to 97% of the total life cycle costs for motors, so even small savings can be significant (Munz, 2011). Note that for a 94% efficient motor, wasted energy is equal to only 6% of input energy. Thus, a 30% reduction in wasted energy is equivalent to a 2 percentage point efficiency gain.
Best Estimate of Energy Savings:
"Typical" Savings: 2%
Energy Savings Reliability: 3 - Limited Assessment
PMAC motors are available in NEMA and IEC frame sizes (from Marathon Electric, Leeson, and WEG), and can be specified when new process equipment is being ordered or retrofitted onto existing equipment. Motor performance is tested and verified in accordance with IEEE and IEC testing protocols. Their efficiency advantages are well known, at both full and part-loads. PMAC motors, however, have variable speed capability and are often specified in lieu of a Premium Efficiency induction motor with a variable speed drive. That being the case, energy savings can vary widely due to variations in annual operating hours, baseline motor performance (single versus two speed, operating hours, load profile, flow modulation technique employed, and base motor efficiency).
For this example, assume a 20 hp PMAC motor has a 95% efficiency at full load with a flat efficiency curve versus a NEMA Premium Efficiency motor with a full-load efficiency of 93% (and also with a flat efficiency curve). Both motors are controlled by drives with an efficiency of 96%. The energy savings of the PMAC motor when meeting the same load profile is: 100% x (1 – 93/95) = 2.1% Note that energy savings would increase if the new PMAC motor was a replacement for a Standard Efficiency motor or an Energy Efficient motor. (Data on PMAC motor performance was extracted from Marathon SyMAX Brochure, SB385.)
Energy Use of Emerging Technology:
1,252.4 kWh per hp per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
Potential number of units replaced by this technology:
The motors most likely to be a good application for permanent magnet motors are those with a fluctuating load such that they either already have or would benefit from a variable speed drive (VSD). According to the U.S. DOE's United States Industrial Electric Motor Systems Market Opportunities Assessment in 1998, the percentage of industrial motors with a fluctuating load is 19.3% of all industrial motors (Xenergy, 2002, App B). Marathon Electric indicates that virtually any application suitable for induction motors is suitable for replacement (except for Premium Efficiency motors that operate constantly at full load). Centrifugally loaded variable-speed pumps and fans are ideal, as are loads driven by belts, chains, or gearboxes. Eliminating power transmission devices results in increased energy savings due to eliminating the power transmission device plus dollar savings in the form of reduced maintenance requirements.
Estimating the total number of horsepower installed in such applications is challenging. What we can more easily estimate, which is ultimately the more important number, is the total energy used by motors with fluctuating loads. In this analysis, we only consider industrial motors up to 500 hp. Drop-in replacements are available for many NEMA-frame motors. Using national numbers from the Market Assessment, we get a total energy use for fluctuating loads in those sizes of about 89,000 GWh/yr. According to the EIA, approximately 8.2% less energy is used by industrial motors in 2011 compared to 1998 when the original survey was completed. Adjusting for that, and taking 4% of the total (since the Northwest has a population of 4% of the U.S.), we get approximately 3,200 GWh/yr being used by induction motors with fluctuating loads in the Northwest. Dividing that annual energy use by the baseline example energy use of 1,278 kWh/yr/hp, we get an effective installed base of industrial motors in the Northwest with a fluctuating load on the order of 2,503,912 hp.
Regional Technical Potential:
0.06 TWh per year
Regional Technical Potential of an Emerging Technology is calculated as follows:
Baseline Energy Use * Estimate of Energy Savings (either Typical savings OR the high range of savings) * Technical Potential (potential number of units replaced by the Emerging Technology)
Installed first cost per: hp
Emerging Technology Unit Cost (Equipment Only): $103.80
Emerging Technology Installation Cost (Labor, Disposal, Etc.): $0.00
Baseline Technology Unit Cost (Equipment Only): $69.90
The installation cost is not included as a new motor must be installed. In the event of a motor failure the old motor must be removed and a new motor installed independent of whether the new motor is a PMAC motor or a conventional motor.
Simple payback, new construction (years): 14.7
Simple payback, retrofit (years): 45.1
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.
SyMax provides list prices for their PMAC motors in standard NEMA frame sizes (SyMax brochure). Prices for 5 hp, 10 hp, and 20 hp 1800 RPM PMAC and Premium Efficiency 1800 RPM TEFC motors are given below:
| || |
Motor Horsepower Rating—1800 RPM
| || |
SyMax PMAC Motor
Premium Efficiency Motor (from Baldor Electronic Catalog)
Price Premium, %
Purchasers typically obtain a discount off of motor list price. For large industrial customers, the list price can vary from 40% to 50%. Assuming a 50% price premium, a 20 hp PMAC motor would have a purchase price of 50% x $4,152 = $2,076 (from Marathon SyMAX Brochure SB385). This is equivalent to $103.80/hp. Cost for the conventional Premium Efficiency motor is 50% x $2,796 = $1,398, or $69.90/hp. Both motors would be driven by variable speed drives of comparable costs. Note that PMAC motors can be controlled by the same variable frequency drives used for conventional AC induction motors.
Reference and Citations:
McElveen, et. al.,
Recent Developments in Motor Technology Allow Direct Drive of Low Speed Cooling Tower Fans
Cooling Technology Institute
"Super Premium" Efficiency Motors are Now Available
Washington State University Extension Energy Program
Melfi, et. al.,
Permanent-Magnet Motors for Energy Savings in Industrial Applications
IEEE Industrial Applications
Advanced Electric Machines Technology
Workshop on Future Large CO2 Compression Systems
Baldor is Redefining the Future of Cooling Tower Performance
Baldor Solutions Magazine
Cooling Tower Motor Solutions With Direct Drive Technology
ABB Technology Show
United States Industrial Electric Motor Systems Market Opportunities Assessment
Frequently Asked Questions—Permanent Magnet AC Motors