Radial Flux Labs Optimized Motor DesignThe Dual Stator design effectively reduces the stator length by half giving very hi...
SaliencyA high saliency in a motor is the key to enabling high reluctance torque and so take advantage offield weakening a...
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Radial Flux Labs Optimised Ev Motor Design

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Radial Flux Labs Optimized Motor Design, Torque/speed chart

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Radial Flux Labs Optimised Ev Motor Design

  1. 1. Radial Flux Labs Optimized Motor DesignThe Dual Stator design effectively reduces the stator length by half giving very high energydensity. The IPM rotor design is very robust with very high reluctance torque, no demagnetizationunder high current loads and excellent containment of flux and good thermal propertiesThe electric motor design attributes which significantly influence electric vehicle motorperformanceoutcomes include the following;1. High Electrical Efficiency across the desired rev range which determines the distancetravelled on a given charge.2. High starting torque ensures the start up acceleration capability and hill start and climbingcapability will give useful real world performance with passengers3. High Energy Density enables a smaller and lighter motor design influencing weight and EVdesign package4. High Regeneration energy recovery for battery recharge under braking deceleration providesthe 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. 2. SaliencyA high saliency in a motor is the key to enabling high reluctance torque and so take advantage offield weakening and Phase Advance For the following reasons the RFL design achieves very highsaliency.: 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 tomaintain 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 torqueunder field weakening mode.Field weakening is a motor control technique that allows a motor to run faster than its ratedspeed. Using this control method should provide at least 1.5 times the rated speed, withembedded or internal magnet rotor designs. (IPM) Spinning the motor up to its rated speed isreferred to as operating in the “constant-torque region,” where the motor’s available torque isconstant as the speed is varied . See graph 2a below for the RFDS torque profile Higher torque inthe higher speed range from the reluctance torque available from the RFL design givessignificantly improved overtaking torque. This is not possible from surface mount designsReluctance Torque and Field WeakeningThe RFL design has embedded magnets and an overlapping concentrated winding. Thisarrangement has high reluctance torque. It has a high salience factor of 2.79 giving high efficiencyunder heavy overloads. The Fractional Slot PM motors have no reluctance torque and suffer fromloss of efficiency under high current loads. This effect is clearly evident in Fig 1 below at torquesover the rated torque of 40 Nm. It can be seen that at torques over 40 Nm that the efficienciesstart to diverge with the RFL design staying up and the Kelly efficiency dropping away. Thisreflects the high reluctance torque of the RFL design.Iron LossesThe RFL design has very high energy density yet has lower pole numbers than conventional PMmotor designs and will fit into the same space as the fractional slot designs without their high ironlosses. Iron losses are proportional to the square of the frequency. The RFL’s lower running frequencyfrom the lower pole number results in lower stator core losses. This is especially evident at lowerloads 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 designsRotor and Magnet LossesThe RFL design has a conventional overlapping- concentrated winding arrangement. As a resultthe rotor is not subject to large rotor eddy current losses resulting from the 7th harmonics like thefractional slot machines. The result is a motor with lower losses at high current loads, such asstarting torque and hill climbing.US Department of Energy analysis into the best design of motor for EV’s which concluded that akey design attribute is ( high saliency allowing Controller driven filed weakening to enableoptimization of the motor size (power) ,

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