Motor requirements

Posted on January 16, 2008


Since I’m trying to develop an electric motor, it would be nice if we knew the target specs, right? 😉

I’m taking the engineering approach and calculating the motor specs in order to guarantee a transparent transition for the vehicle’s driver after the conversion.

Main goals:

  1. Aim at a reasonable top speed (120 km/h is the max limit in europe);
  2. Keep the maximum available torque (for those hard climbs and/or heavy loads);
  3. Assure an efficiency level greater than 75%.

Number 1 should not be difficult to obtain. The LRK motor can spin quite fast. Why 120 km/h instead of a lower speed? because this will be ultimately a conversion to a serial hybrid, which means the top speed of the car is limited by the top speed of the electric motor(s). I could aim for a lower top speed during Phase 1 (Parallel hybrid), but I’m trying to keep the same motors from Phase 1 into Phase 2, for financial reasons.

So how fast does the motor have to spin?

  • Tire max linear velocity = 120 km/h (2000 m/min).
  • Effective wheel radius (hub + tire) = 273 mm (+- 5mm, depending on conditions?…) (0,273 m)
  • Effective wheel circumference = 2 * pi * radius = 1,715 m.
  • Max rotation = Max velocity / Circumference = 1.166 rpm.

1.200 rpm doesn’t look that challenging, does it?

Anyway, this will translate into a switching frequency in the power controller.

  • Max rotation speed = 1.166 r/m (19,4 r/s)
  • Number of magnetic pole pairs in rotor (MP) = 7 (two magnets make a pole pair)
  • Number of active stator pole teeth (ST) = 6 (only half of the 12 total have copper windings)
  • Number of electric phases (Ph) = 3
  • Divider ratio between current freq and rotation = MP x ST / (2 x Ph) = 7/1       (explanation here)
  • Max switching frequency for stator currents = Max rot. / Div. ratio = 136 Hz.

Now the required torque…

I’m using the maximum possible torque generated by the engine as the target. This means revving the engine to the max torque rpm while using the first gear. The intention is to find out the maximum effective torque presently available at the wheel axles.

  • Maximum torque produced by engine (at 3.750 rpm) = 130 Nm
  • Ratio of first gear (manual gearbox) = 3,27/1
  • Ratio of final drive (differential) = 4,06/1
  • Number of driving wheels = 2
  • Maximum torque at wheel = Engine torque x 1st gear ratio x diff ratio / num wheels = 862,95 Nm

Hmm… 863 Nm may look scary, but electric motors (especially PM ones – permanent magnet) are known for their capacity for high torques. Furthermore, I’m counting on a different effect, which the drivers of electric/hybrid vehicles surely have noticed so far: the PM electric motor has its full maximum torque always available, which gives the driver a new set of behaviour options and eventually changes the way the person drives. So ultimately, it may not even be necessary to have such a high torque available… but for now this is my target.

About the efficiency.

This one is a lot more involved… I don’t even have all the angles yet. I do know that:

  • The magnetic circuit must be as wide, short, and little reluctant as possible to carry the maximum flux;
    • small number of air gaps;
    • as thin as possible air gaps;
    • high magnetic permeability materials;
    • short length of magnetic cores;
    • wide passage for the flux in magnetic cores;
    • flux circuit corners cannot be too tight;
  • The magnetic circuit must not be used in flux saturation regime, but just before it reaches it;
  • The electrical resistance of all magnetic parts must be as high as possible to avoid induced currents;
  • The electric power currants must be kept as low as possible to reduce thermal losses.
    • meaning higher voltages, for the same applied power.

… and a few more I haven’t thought of… but you’re welcome to remind me. 🙂

Because of the difficulty in procuring the specific materials, I may have to compromise on efficiency… Let’s hope not.