This document discusses motor constant (Km) and its use in evaluating motor performance and efficiency. Km is a ratio that indicates a motor's ability to convert power to torque rather than heat. A higher Km value means a motor is more efficient. The document also evaluates several motor and gearbox design concepts for an electric vehicle. It finds that while geared systems can improve efficiency by allowing lower-torque operation, direct drive using large motors may be preferable due to simpler design and packaging compared to adding complexity of a gearbox.

4. 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:

5. 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:

6. 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.

7. 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.

9. 0
500
1000
1500
2000
2500
3000
3500
4000
0 20 40 60 80 100 120 140 160 180 200
HeatLoss(W)
Motor Output Torque (Nm)
Heat Loss vs. Torque of various Motors
06210 at TP
03014 at TP
04512 at TP
06211 at TP
06212 at TP

10. 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

11. 0
500
1000
1500
2000
2500
3000
3500
4000
0 50 100 150 200 250
HeatLoss(W)
Torque generated at Wheel (Nm)
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

13. 0
500
1000
1500
2000
2500
60 70 80 90 100 110 120 130 140
HeatLoss(W)
Torque (Nm)
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

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

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

42. 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.

43. 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.

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

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

64. 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.

65. 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.

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

93. 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 .

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

95. 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.

97. The Truth

98. 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.

99. 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.

100. 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.

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

103. 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.

110. 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 .

111. Disadvantages
• HEAVY HEAVY HEAVY