Week-6 Challenge: EV Drivetrain

1. Power converters used in electric vehicles are :

• AC to DC converter or Rectifier: charger of an EV
• DC to AC converter or Inverter: Motor drive, where DC from battery is applied to AC motor.
• DC to DC converter or Chopper: converts one DC voltage level to another DC voltage level. Generally used in auxiliary components of Vehicle like music systems.
• AC to AC converter: where voltge and frequency can be varied, used as a fan regulator.

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4. In the article author, wally ripple talks about the differences in DC brushless(BLDC) motor and the Induction motor(IM) and compare them with respect to the vehicle requirements.

• Both BLDC and IM have virtually same stator with distributed winding they differ in the rotor structure where BLDC has permanent magnet (PM) rotor where as for IM rotor is stacked steel laminations that are shorted at thier ends.
• IM are compatible with the conventional utility supply ,which is the main reason for the success whereas BLDC produces no staring torque with fixed frequency, so they inverter in order to run.
• In 1990's, all of the electric vehicles except one were powered by the BLDC drives. Today all the hybrids use BLDC with no exceptions.
• Heat generated in BLDC rotor is less compared with IM as there are no windings in rotor. BLDC can be operated at unity power factor(UPF) where as for IM the high power factor is 85% .
• To vary the torque the fluX density(B) must be varied. BLDC has no provision for varying B as there are no methods to vary B of a PM. This condition is exploited by the IM as B can be varied by varying the V/f ratio by supplying motor through a modulated inverter.
• Coming to the digital control of inverter the control difference is the control code. BLDC require Absolute position sensor, IM drive require only a speed sensor.
• As BLDC size grows the magnetic losses also increase proportionately and part load efficiency drops but with IM as size grows magnetic losses do not necessarily grow.Thus IM are favoured for high performance, peak efficiency is less compared with BLDC but average efficiency may actually be greater than BLDC.
• PM are expensive and also difficult to handle due to very large forces that come into play when anything ferromagnetic comes near to them. IM are easier to protect and less costlier.
• IM are difficult to control, the control strategies are difficult to understand. Incur added development cost but no recurring costs.

The Author concluded by saying that BLDC dominate in hybrid and plg-in hybrid where as for pure EV IM drives are prevalent. As both BLDC AND IM drives hardware is similar we may see them side by side running the vehicle in coming era.