Whilst batteries are a key component of any Retro-Electric build, at the heart of all electric vehicles is the motor. The motor is the equivalent of the engine in an ICE vehicle, so making sure your conversion has the right motor when designing your build is paramount.
Over the years we have all read about, or indeed driven, great looking cars that have been spoilt by a poor engine. So making sure your selection is the right one is key, get it wrong and it will be an expensive mistake that you will be stuck with.
Selecting the right motor for your Retro-EV isn’t straightforward, but Steve Drummond, co-founder of Electrogenic, explains some of the variables and what you need to consider when choosing the right motor.
The first thing about choosing a motor for your conversion is that there isn’t a wide range of choice available. However, the choice that you do make has far-reaching consequences.
The amount of power you will get from your motor depends not only on the motor you choose, but also on the batteries and their ability to deliver the power. This, in conjunction with the vehicles weight will ultimately determine the performance and range of your car.
So how do you select the right motor for the job?
Powerrrrrr is not everything, Jeremy!
The first thing to decide is what level of power do you want from your motor? It’s tempting to simply compare the kW or bhp motor rating with that of the original ICE engine, then match this, or indeed double it for a street racer, however this the wrong way to start.
Performance is delivered in a different way with an electric motor, BHP is not as important as with an ICE. What you need, to fly away from the lights, is torque! This is where an electric motor excels.
Electric motors develop max torque from 0 rpm and maintain it through the rev range. This is impossible with an ICE, so to increase the amount of torque available at low revs, most car makers just up the power. This is why Retro-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 producing a Porsche-beater on top-end speed (where kWh or BHP 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!).
Volting out the trap.
The voltage of the motor is important, this will determine the size of the battery pack and the current that it requires to enable the motor to deliver its torque. Most commercial EVs generally run at about 350-400V, however some run as high as 800V. The power rating of the motor is calculated as “voltage x current”. So, the higher the motor voltage the less current it needs to provide its given power.
This all makes sense, so you want a high voltage system, right? Yes, but high voltage systems are more expensive to fit and while it might be tempting to fit a Tesla Ludicrous motor into your Morris Minor, the rest of the car will have to be seriously upgraded to cope and you may struggle to find room for the batteries. And 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 Retro-EVs run a lower voltage motor, somewhere between 100 and 150v. The good news is that there are some interesting motors available at these voltage levels.
Lower voltage motors are not only cheaper, but all the bits and pieces like connectors, contactors, charging systems, etc. cost less for lower voltage systems too. Plus, some bigger motors will also need a cooling system, which increases cost and complexity.
One of the best motors out there is the Netgain 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 the first choice for a conversion for us here at Electrogenic and is very adaptable.
The Hyper9 produces 235Nm peak torque (from zero rpm – it is an electric motor!) and 80kW of power between about 5,000rpm and its maximum 8,000 rpm. For larger conversions, the Hyper9 can also be ran as twins motors, and If you do this, you can double the performance figures.
Alternatively, the Yasa 750 R motor 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. This is a very powerful and compact motor, perfect for many Retro-EV conversions however it needs a 750v battery pack, which limits the vehicles or the budgets its available to.
To compare these motors with a new EV, 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 of torque and 375kW @ 6,150rpm. However, it also needs a battery pack that delivers over 1,000A at 350V. This is why the smaller battery pack Teslas are not as quick: the motor is limited by the battery power.
So which motor is best for your project?
It’s a complex question, and one that is best answered based on experience rather than a simple generic calculation.
At Electrogenic, when we are specifying a motor it is often an iterative process. Starting with the desired torque, checking voltage options against the available battery storage space, checking the motor speed against the transmission options, assessing pricing implications for the whole car, not just the motor, and then often going 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 space and budget is not an issue with your vehicle, then the world is your oyster and it shouldn’t be hard to create a vehicle that can take a Tesla on at the lights.
Ultimately, what’s important in a R-EV is how it feels when you drive it, so broadly speaking:
1 – 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.
2 – You should aim to at least match the peak torque of the original ICE. But remember, your new Retro-EV will have all its torque available from zero revs, so it will put a bigger smile on your face.
3 – Consider the continuous power levels you are likely to need. If you want to cruise on an autobahn at 140 mph, the continuous power needed is around 160kw. Cruising at 70 mph and 60kW could be fine.
4 – What is your budget? Larger motors can become very expensive to install if they need cooling systems and complicated battery packs.
5 – The rest is down to physical space, the battery pack and the current