Advantages of Mechanical vs. Electronic-Field Weakening – White Paper

Dura – Trac Motors, Inc.

DURATRAC MOTORS, INC. 4007 ENGLETON DRIVE, FORT WAYNE, IN 46804 www.duratracmotors.com

L. P. Zepp, CTO 2018:Q4

Executive Summary: Want to learn a dirty little secret about electric motors? Electronic-field weakening to achieve higher

rpm’s uses a significant amount of energy to overcome the effects of back EMF. And using that extra

reduces operating efficiency. In a battery-powered application, using extra energy means reduces range

or operating time. This paper provides more detail about the magnitude of the energy losses using elec-

tronic-field weakening and compares how the DuraTrac mechanical-field weakening can increase effi-

ciency dramatically, which is especially important in battery-powered applications – vehicles and aircraft.

Based on actual dynamometer data, using DuraTrac mechanical-field weakening rather than electronic-

field weakening can increase system efficiency more than 90% at higher rpm’s.

Engagement Motor Max RPM’s1

Dura-Trac

Efficiency

Electronic

Efficiency2

Dura-Trac

Advantage

100.0% 1,200 92.0% 92.0% Same

75.0% 1,600 92.0% 63.9% 43.9%

50.0% 2,400 82.0% 42.6% 92.5%

The balance of the paper addresses:

Efficiency characteristics of permanent magnet motors

Examples of drive-system efficiency in 100% battery/hybrid applications

Formulas to calculate efficiency

EFFICIENCY FALL-OFF AT SPEED IN PERMANENT MAGNET MOTORS. In PM motors, the strong magnets create

an induced voltage (back EMF) in the coils that is proportional to speed. When the induced voltage is

equal to the supply voltage, then the motor is at the maximum rpm. If voltage is electronically elevated

(as in the Toyota Prius drive system), then the motor will run faster at an increased voltage for highway

driving. The Prius system can boost the 200V battery voltage up to 500V for higher-speed operation.

If the voltage cannot be elevated, or if Dura-Trac Motors technology is not used, then electronic-

field weakening must be used by the motor controller to create a field weakening effect and higher rpm’s.

This is achieved by pulsing a reverse current through each stator coil as the rotor magnet passes. For

example, for a 33% increase in rpm’s then 25% of motor current must be applied to buck the North pole

magnet traveling past that stator tooth and coil. This is equivalent to a 75% rotor engagement. Likewise,

for a 50% increase in rpm’s, 50% of additional motor current must be applied by the motor controller.

While the capability of high-speed motor controllers provides electronic-field weakening (bucking) as a

control option, only a small group of technical users seem to understand the extreme efficiency loss that

accompanies this increased reverse current requirement.

PM MOTORS IN BATTERY/HYBRID APPLICATIONS. Many applications using electric traction motors require

very high-motor torque to launch – e.g., cars, trucks, eVTOL (electric vertical take-off and landing aircraft).

Without this high-torque performance, in many cases, the vehicle is not deemed to be capable. The PM

1 For comparisons of efficiency between electronic and mechanical field weakening we used maximum motor rpm’s somewhat below the theoretical maximum. 2 Estimate based on actual dyno data through base speed adjusted for known increased energy required to achieve higher rpm’s electronically. We believe the efficiency is overstated and the DuraTrac gains are likely greater.

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DURATRAC MOTORS, INC. 4007 ENGLETON DRIVE, FORT WAYNE, IN 46804 www.duratracmotors.com

brushless motor characteristic of full torque at zero speed enables this high-torque requirement. The

physics of a PM motor, however, cause severe limitations on top-motor speed. Speed of a PM motor is

inversely proportional to maximum torque. Top-speed limits can be solved by using a transmission to

provide multiple drive-ratios to the wheels. However, the transmission adds cost, complexity and weight.

An all-electric or hybrid-electric vehicle requiring a maximum launch torque of say 400 lb.-ft.

would be limited to a maximum speed of about 10 mph (1,310 maximum rpm) with a conventional fully

engaged rotor. Such limited speed is not a practical for most applications.

The application of the Dura-Trac Motors technology provides a family of torque/speed curves that

can extend dynamic speed to meet driving requirements without a transmission. In effect, the motor field

weakening ability acts like a magnetic continuously variable transmission (CVT).

The Dura-Trac Motors field weakening overcomes the limitations by sliding the magnet rotor axi-

ally along a splined motor shaft. After the

maximum torque launch, some torque (and

magnet engagement) can be traded for ex-

tended motor speed.

The increase in efficiency using the

DuraTrac can be dramatic. The line chart in-

dicates system efficiency when the duty cy-

cle demands that the base speed of the mo-

tor must be doubled. Through 1,200-1,300

rpm’s, the efficiency of the motors is the

same – each motor operates in its “base”

condition.

To achieve the higher rpm’s, the identical motors must be field weakened about 50% to overcome

back EMF. One motor is weakened electronically, which is the approach most widely used. Notice how

efficiency declines to just over 40.0% approaching maximum desired motor speed.

Higher speed for the other motor – identical voltage, same number of windings, etc. – is achieved

by mechanical-field weakening – separating

the rotor and stator by 50.0%. Efficiency re-

mains above 80.0%. As rpm’s increase, the

efficiency gain also increases. In a battery-

powered application, the higher efficiency

of the DuraTrac motor would result in sub-

stantially extended range (operating time

for aircraft).

The DuraTrac PM rotor can be dy-

namically varied from 100% to 25% by a

servo controlled actuator. The actuator al-

lows 100% rotor disengagement for near

zero magnetic torque drag when the electric or hybrid system is on standby mode or during higher-speed

driving. The actuator is under closed loop control of the motor controller, as determined by driver inputs.

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DURATRAC MOTORS, INC. 4007 ENGLETON DRIVE, FORT WAYNE, IN 46804 www.duratracmotors.com

CALCULATING MAXIMUM SPEED AND TORQUE. Have time for some math? The following paragraphs explain

the physics behind calculating the efficiency and torque of permanent magnet motors. Take time to read

and understand. We promise there’s no test.

Permanent magnet motors are linear with respect to their torque / speed performance and the ratio

changes created by field weakening.

Motor Maximum Speed

100% rotor engagement, 1,310 rpm’s, at 96 V (Fig. 1 and 5).

75% rotor engagement, 1,746 rpm’s. 75% of the rotor is exposed to the stator coils or the mag-

nets are bucked to reduce their attraction to 75%. Maximum speed at 75% will be speed at 100%

multiplied by 1 the decimal engagement

1 ÷ 0.75 = 1.33 𝑥 1,310 = 1,746 max rpm’s

1,746 rpm maximum agrees well with the dyno data for 75% rotor engagement.

50% rotor engagement, 2,620 rpm’s. 50% of the rotor is exposed to the stator coils or the magnets

are bucked to reduce their attraction to 50%.

1 ÷ 0.50 = 2.00 𝑥 1,310 = 2,620 max rpm’s

2,620 maximum agrees well with the dyno data for 50% rotor engagement.

Motor Maximum Torque

Torque is proportional to amps and % engagement of the magnet rotor.

100% rotor engagement. In actual dyno performance (Figure) 5 shows an input of 450 amps pro-

duces a maximum torque of 425 Nm, or 0.944 Nm / amp. Using dyno recorded torque/ amp as a

constant, a 600 amp input will produce:

600 𝐴 𝑥 0.94 = 564 𝑁𝑚 Max Torque

For further torque comparisons at 600 amps, the 564 Nm max torque will be used.

75% rotor engagement. Because maximum torque is linearly proportional to rotor engagement,

a 75% rotor engagement at 600 A will produce

564 𝑥 0.75 = 423 𝑁𝑚 Max Torque

423 Nm maximum torque agrees well with the dyno data.

50% rotor engagement at 600 A will produce

564 𝑥 0.50 = 282 𝑁𝑚 Max Torque

282 Nm maximum torque agrees well with the dyno data.

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Std PM

Motor