Wheel Motor Design Concepts

by Tobias OverdiekMarch 30, 2010

University of Texas at Arlington, FSAE Hybrid Team

Motor Constant Km• Motor Constant (Km) is the ratio of peak torque to the square root of

power input at 25°C and at stall:

• This ratio is useful during the initial selection of a motor, because it indicates the ability of a motor to convert electrical power into torque, rather than into heat.

• Units for Km are:

Motor Constant Km

• A common use of Km is to determine how much power a motor will dissipate in order to generate a certain amount of torque by using the following equation:

Motor Constant Km

• Therefore the larger Km is the more efficient the motor performs by turning less energy into heat with respect to the output.

Motor Constant Km

• Km is independent from KB (KV) and KT

• Motors often are offered with different winding options which change KB and KT, but Km changes based on the frame size, stator width, and diameter.

0 20 40 60 80 100 120 140 160 180 2000

500

1000

1500

2000

2500

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3500

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Heat Loss vs. Torque of various Motors

06210 at TP 03014 at TP04512 at TP06211 at TP06212 at TP

Motor Output Torque (Nm)

Heat

Loss

(W)

Design B – Geared Hub for use with 3014 Motor ( Orange ) 2.727 : 1 Ratio

Design A – Inboard Geared for use with 4512 Motor ( Green ) 2.727 : 1 Ratio

Design C – Outboard Geared for use with 6210 and 6211 Motor ( Red ) ( Blue ) 1.840 : 1 Ratio

0 50 100 150 200 2500

500

1000

1500

2000

2500

3000

3500

4000

Heat Loss vs. Torque of various Motors

06210 geared 1.840 : 1

03014 geared 2.727 : 1

04512 geared 2.727 : 1

06211 geared 1.840 : 1

06212 DD

Torque generated at Wheel (Nm)

Heat

Loss

(W)

60 70 80 90 100 110 120 130 1400

500

1000

1500

2000

2500

Heat Loss vs. Torque of various Motors

06210 geared 1.840 : 1

03014 geared 2.727 : 1

04512 geared 2.727 : 1

06211 geared 1.840 : 1

06212 DD

Torque (Nm)

Heat

Loss

(W)

Design A – Inboard Geared for use with 4512 Motor 2.727 : 1 Ratio

Advantages

• Good sized motor• Gearbox reduces heat dissipation• Possible Aluminum Machined Upright• Convectional cooling may be sufficient• Suspension Geometry will remain the same

Disadvantages

• Housing has to be structural on both ends to support the sun gear stub shaft. This makes the housing unnecessarily heavy. Other designs only have one structural end plate to reduce weight and simplify mounting

• Top A-arm needs minor modifications to clear the housing.

So ‘n so

• Brake Calipers must be resized. A thinner caliper will help package the motor more outward to help clearance the A-arms. The caliper used in the previous slides has already been potentially proven too small yet clearances should be better.

• The design limits this motor for only front hub/wheel use.

Design B – Geared Hub for use with 3014 Motor 2.727 : 1 Ratio

Advantages

• Relatively compact• Only minor changes in Suspension • High Gear Ratio• Suspension Geometry will remain the same

Disadvantages

• The 3014 series motors are designed to run on very high voltage to reduce the amount of current. It is very unlikely to have any 30xx series motor wound for our voltage range, because KB would have to be ¼ of the original value, which would make the current increase by a factor of 4.

So ‘n so

• The small motor leaves no room for the gear box besides in the hub which overcomplicates the design and could prove unreliable and heavier in area’s where other designs are simpler and lighter.

• This design also limits this motor for only front wheel use.

Design C – Outboard Geared for use with 6210 and 6211 Motor 1.840 : 1 Ratio

Advantages

• Robust Gearbox design • Can be fitted to all four wheels• The larger 6211 (blue) motor would provide

powerful regenerative breaking.• Convectional cooling may be sufficient for the

6211 (blue) motor .

Disadvantages

• Even though very light weight, the 6210 (Red) Motor is severely underpowered and dissipates too much heat to be efficient.

So ‘n so

• Bearings could possibly be very expensive and kill the design

• Scrub would increase by 0.5 – 1.0 inches• Brake Rotor diameter would have to decrease

from 8.8 to 7.5 inches.

The Truth

The Truth

• Any geared hub motor assembly above will weigh in between 35-45 lb. Even though the assemblies use smaller motors than direct drive hubs, the weight will be about the same due to gears, extra bearings, cases built to enclose and house oil seals.

The Truth

• Using either the 6211 (blue) or the 6212 (purple) motor with a direct drive setup would also result in a hub weighing 35-45 lbs. The only advantage a gear drive offers is letting the motors operate at a lower torque which decreases heat losses and increases efficiency. However, the extra cost and risk of running gear drives should be taken into consideration.

The Truth

• The limitations of using geared motors is packaging. Although geared motors show many advantages in the graphs, designing and packaging a reliable gearbox may overcomplicate the concept and can make some most motors obsolete and unpractical.

Design D – Direct Drive for use with 6211 and 6212 Motors 1 : 1 Ratio

Design Concepts for Direct Drive

• Since there are no gearboxes, the housing would not have to completely enclose the motor to house oil seals.

• The motors could be clamped to four or so outriggers from the upright, super light weight.

• Convectional cooling would be sufficient for the 6212 (purple) and may be for the 6211 (blue) with a shroud or small fan.

Advantages

• Simple• Using the 6211 (blue) will decrease weight by

at least 5 lb over the geared design.• Convectional cooling may be sufficient for the

6212 (purple) motor .

Disadvantages

• HEAVY HEAVY HEAVY