Motors and Motor Controllers

Owned by Ria Baijal
Last updated: Mar 15, 2022
3 min read

 

https://avt.inl.gov/sites/default/files/pdf/fsev/power.pdf

Power system of electric vehicle consists of just two components:

Motor: Provides power – converts electrical energy into mechanical energy, DC and AC motor used in EVs

Controller: Controllers application of this power

  • In contrast the power system of a gas-powered car has lots of components including the engine, carburetor, oil pump, exhaust system etc

DC motor components:

  • Coils that create magnetic forces which provide torque

  • A rotor or armature mounted on bearings that turns inside the field

  • Commutating device that reverses magnetic forces and makes the rotor turn(this provides power)

AC motor components:

  • Same components but no commutating device because there is continuous current reversal

AC motor:

Advantages:

  • Lighter weight

  • Reliable

  • Only one moving part(the shaft) and therefore should last a long time

Disadvantages:

  • While the motor itself is cheaper than a DC motor, the cost of the electronics needed to invert the battery’s direct current to alternating current makes it overall more expensive

DC

Advantages:

  • Cheaper overall

  • Simpler controller

 

IN the past EVs used DC motors/controllers because they operate with the battery’s current without lots of complicated electronics. Now that these electronics are getting better and better lost of EVs use AC motors/controllers due to lighter weight and better efficiency

 

Controllers:

  • The controller is the electronics that operate between the batteries and the motor to control the speed/acceleration

  • Regulates energy flow from the battery

  • For ac motors it transforms the battery’s direct current into alternating

  • Controller converts the motor to generator(regenerative braking)

  • Reverses motor rotation so car can reverse

  • Modern controllers change speed and acceleration by a process called pulse width modulation

o   Silicone-controlled rectifiers interrupt the electricity flow to the motor rapidly

o   High power when intervals are short(when current is turned off)

o   Low power when long intervals

o   Older controllers for EVs with dc motors a variable resistor controller was used

  •   Full power always draw and high resistance was used to reduce current to motor when it wasn’t necessary

 

https://electricbikereview.com/forums/threads/whats-the-criteria-for-selecting-a-battery-based-on-motor-and-controller.41035/

Criteria for selecting battery based on motor and controller

  • Battery voltage should match motor rating(you can overvolt a motor usually?)

  • Battery AH rating = motor power rating/motor voltage rating x 1 hr

  • Controller current rating should be quite a bit higher than motors max current draw – eg min 20A controller for 10.4A that a hypothetical motor draws

 

https://www.motioncontroltips.com/selecting-the-right-batteries-and-motors-for-battery-powered-commercial-equipment/

  • Rated power of motor is calculated from the combination of speed, torque and duty cycle; this establishes critical voltage, current, and capacity requirements of battery

  • Battery discharge current rating should meet or exceed motor current input requirements

  • Battery discharge curve is important because output voltage directly affects output speed of motor

 

https://auto.howstuffworks.com/electric-car3.htm

General info:

Typical motor: 20 000 watt-30 000 watt range

Typical controller: 40 000-60 000 watt range – eg a 96 volt controller will deliver a max of 400-600 amps

 

https://www.danatm4.com/blog/10-things-consider-choosing-electric-motor/

Things to consider when choosing a motor:

  1. Vehicle Characteristics:

  • Size

  • Weight

  • Aerodynamic

  • These determine speed, torque and power requirements of electric motor

 2. Driving cycles

  • Lots of stops/ vs long distances with only a few stops

  • Use to determine battery pack size, series/parallel

 3. Maximum Speed:

  • Targeted max speed, how long does it need to be sustained

  • Rolling radius of wheel

 4. Maximal torque:

  •  need to know if car will be ascending very high angles

  • Can calculate highest torque required by ev considering gearbox(do electric vehicle’s even use a gearbox?)

 5. Max power:

  • need to take into account drag, friction coefficients

  • peak power of electric motor cannot be sustained continuously, need to take time constraints into consideration(the car won’t need to climb the worst hill for the entire drive)

 6. Battery Capacity

7. Battery Voltage:

  • Normally two ranges of voltages: 300-450 Vdc and 500-750V dc

  • Voltage limitations of IGBT(Insulated Gate Bipolar Transistors) used in motor controller and there are two main standard voltages available for them: 600V dc and 1200V dc