The Radial Flux Labs optimized motor design has several advantages for electric vehicles: 1. The dual stator design reduces stator length in half, providing very high energy density. The IPM rotor design is robust with high reluctance torque and excellent flux containment. 2. The design achieves a high saliency of 2.79, enabling it to maintain constant torque during field weakening control for improved performance at higher speeds. 3. It has lower iron losses than conventional designs due to its lower pole number, improving efficiency especially at lower loads and regenerative braking.

1. Radial Flux Labs Optimized Motor Design
The Dual Stator design effectively reduces the stator length by half giving very high energy
density. The IPM rotor design is very robust with very high reluctance torque, no demagnetization
under high current loads and excellent containment of flux and good thermal properties
The electric motor design attributes which significantly influence electric vehicle motorperformance
outcomes include the following;
1. High Electrical Efficiency across the desired rev range which determines the distance
travelled on a given charge.
2. High starting torque ensures the start up acceleration capability and hill start and climbing
capability will give useful real world performance with passengers
3. High Energy Density enables a smaller and lighter motor design influencing weight and EV
design package
4. High Regeneration energy recovery for battery recharge under braking deceleration provides
the boost to battery life for useful travel distance, particularly in daily city commuter traffic
Torque for Surface mount PM Motor using Field
Weakening Control
60
50 Rated Speed
40
Torque
30 Constant
Torque
20
region
10 Field Weakening Region Constant Power
for Surface Magnet Rotor
0
0 2000 4000 6000 8000 10000 12000
Speed

2. Saliency
A high saliency in a motor is the key to enabling high reluctance torque and so take advantage of
field weakening and Phase Advance For the following reasons the RFL design achieves very high
saliency.:
In surface magnet motors the Iq and Id are the same and when these motors are run
under field weakening mode, the torque drops away as the speed is increased, giving
constant power.
In motors with buried magnets and high saliency (such as the RFL) the Iq can be much
higher than Id. In this case by utilizing field weakening the Iq torque (Reluctance Torque)
can be made to add to the Id torque (Permanent Magnet Torque) thus helping to
maintaining torque as the speed is increased. Reluctance torque is from the Id current not
the Iq current and is available only over half of the cycle therefore Saliency with a rating of
higher than 2 is needed for the motor to be able to achieve this reluctance torque effect.
Phase Advance enables a doubling of the starting torque without additional current drain.
The higher the saliency the greater this effect is. At a saliency of greater than 2.5 it is possible to
maintain constant torque and output power from the motor when run in field weakening mode..
The RFL design has a saliency of 2.79 and is able to maintain approximately constant torque
under field weakening mode.
Field weakening is a motor control technique that allows a motor to run faster than its rated
speed. Using this control method should provide at least 1.5 times the rated speed, with
embedded or internal magnet rotor designs. (IPM) Spinning the motor up to its rated speed is
referred to as operating in the “constant-torque region,” where the motor’s available torque is
constant as the speed is varied . See graph 2a below for the RFDS torque profile Higher torque in
the higher speed range from the reluctance torque available from the RFL design gives
significantly improved overtaking torque. This is not possible from surface mount designs
Reluctance Torque and Field Weakening
The RFL design has embedded magnets and an overlapping concentrated winding. This
arrangement has high reluctance torque. It has a high salience factor of 2.79 giving high efficiency
under heavy overloads. The Fractional Slot PM motors have no reluctance torque and suffer from
loss of efficiency under high current loads. This effect is clearly evident in Fig 1 below at torques
over the rated torque of 40 Nm. It can be seen that at torques over 40 Nm that the efficiencies
start to diverge with the RFL design staying up and the Kelly efficiency dropping away. This
reflects the high reluctance torque of the RFL design.
Iron Losses
The RFL design has very high energy density yet has lower pole numbers than conventional PM
motor designs and will fit into the same space as the fractional slot designs without their high iron
losses.
Iron losses are proportional to the square of the frequency. The RFL’s lower running frequency
from the lower pole number results in lower stator core losses. This is especially evident at lower
loads and at no load conditions such as downhill running when there is typically still battery drain.
The RFL design reduces this level of battery drain compared to other designs
Rotor and Magnet Losses
The RFL design has a conventional overlapping- concentrated winding arrangement. As a result
the rotor is not subject to large rotor eddy current losses resulting from the 7th harmonics like the
fractional slot machines. The result is a motor with lower losses at high current loads, such as
starting torque and hill climbing.
US Department of Energy analysis into the best design of motor for EV’s which concluded that a
key design attribute is ( high saliency allowing Controller driven filed weakening to enable
optimization of the motor size (power) ,