Choosing a motor

Choosing a motor is the heart of an electric car conversion, and it has far-reaching consequences, but the first thing to say is that you can’t choose a motor in isolation from the rest of the system design.  The amount of power delivered by the motor depends not only on the motor but also the battery and its ability to deliver the power. This in conjunction with the vehicle’s weight will impact the performance and range between charges. Change one and it affects the other. They all will affect the cost of the conversion. There is also the matter of where everything can be fitted, as Retro-EVs, it is safe to say, were never designed for a mass of batteries.

Start thinking torque

So how do you select a motor for your conversion?  The first thing to decide is what level of output you want from it.  It is tempting to simply compare the kW or bhp motor rating with that of the original internal combustion engine (ICE), but that is not good starting point. To spring away from the lights, what you need is torque.  Electric motors develop maximum torque at 0 rpm and maintain it up the rpm curve until the motor reaches maximum power.  The Netgain Hyper9 power curve provides a nice illustration.

This is impossible with an ICE engine, and to increase the amount of torque available at low revs, most ICE car makers just up the power.  R-EVs are so much fun because the amount of torque that is available at lower rpm is so large.  So unless you are fixated on top-end speed (where power is what you need) a good starting point is to select a motor with the same amount of torque as the original engine (or a little more!).

Volts determine amps

The next point is the motor voltage. This will determine the size of the battery pack and the current that it needs to provide to enable the motor to deliver the torque. Commercial EVs generally run at about 350-400V, but some cars are running as high as 800V. As the power of the motor is voltage x current, the higher the voltage the less current that is needed to provide a certain power.

This all makes sense, and so you want a higher voltage motor – right?  Yes, but high voltage systems are more expensive to fit and while it might be tempting to fit a Tesla Ludricous motor into a Morris Minor, the rest of the car may not be able to cope or have room for the batteries.  And to deliver the power, higher voltage motors often have higher rpm, so then you need to consider the impact on the transmission – more gearing or use the motor over only part of its range?  As a result, many classic electric car conversions run on a lower voltage such as 100 to 150v, and there are some interesting motors available at these voltage levels.

For example

A good example is Netgain’s Hyper9 – a fabulous sealed-for-life automotive motor that comes in two versions: 110V or 144V.  The two different voltage options give you more flexibility with battery combinations.  It is our first choice for the conversion of mid-range vehicles, and is very adaptable.  The Hyper9 produces 235Nm peak torque (at zero rpm – it is an electric motor!) and 80kW of power between about 5,000rpm and its maximum 8,000 rpm.  Happily for bigger cars, the Hyper9 can also be run as twin motors – so if you do this, you can double the performance figures.

The Tesla Model S 85D (it has 85kWh of battery storage and the D means “dual motor”)  main motor can spin at up to 18,000rpm and runs at 350V.  It officially delivers 660Nm @ 0rpm (though short-term Dyno readings are much higher) and 375kW @ 6,150rpm.  However it also needs a battery pack that match and deliver over 1,000A at 350V. This is why the smaller battery pack Teslas are not as quick: the motor is battery-limited.  Alternatively,  the Yasa 750 R produces 790Nm of peak torque, 200kW of peak power, and a speed range of 0-3,250rpm all within an axial length of just 98mm. To do this, it needs a 750v battery pack however.

Broadly speaking, lower voltage is cheaper:  all the bits and pieces like connectors, contactors, charging systems, etc. cost less for lower voltage ratings and so on.  Don’t forget that some bigger motors will also need a cooling system as well.

So which motor is best for your project?

It’s a complex question, and one that is best answered on the basis of experience, as much as calculation.  Choosing a motor is an iterative process. Start with the desired torque, check voltage options against the available battery storage space, check the motor speed against the transmission options, assess pricing implications for the whole car, not just the motor, and then go back around the loop again. We’ll often work backwards and forwards on a number of combinations with our customers before finding the right balance.

Of course, if budget and space in the vehicle are not an issue, then the world is your oyster.

In summary

Ultimately, what’s important in a R-EV is how it feels when you drive it, so broadly speaking:

  • The properties of the vehicle such as size, weight and aerodynamics are crucial characteristics that will determine speed, torque and power requirements of the electric motor.
  • We usually aim to at least match the peak torque of the original motor.  And then remember that your new Retro-EV will have all this torque available at low (zero) revs, so it will put a bigger smile on your face
  • Considering the continuous power levels will determine if you can stay at a particular high speed.  If you want to cruise on an autobahn at 140 mph, the continuous power needed might be 160kW. Cruise at 70 mph and 60kW could be fine.
  • What is your budget? Higher performance, higher voltage motors can become very expensive to install.
  • And the rest is down to physical space. And rpm limits and the battery pack and the current delivery and the gearing…

There is a lot to consider!