Some Rough Estimations of Power Requirements

I’ve been doing some research into power requirements and to be honest a lot of the math such as calculating torque curves is going over my head. I’m sure I could get a grasp of it if I really applied myself but I think this would require considerable effort to develop the required underpinning knowledge. Fortunately I have been able to find some very useful online tools that help to make these calculations easier.

Before I go into them though, I want to share some of my sanity checks.

A typical “powerful” child’s quad bike uses a single 1000w motor. The motor is connected to the rear axle using a chain. Being able to adjust top speed/torque by changing gear ratios would be advantageous. This 1000w quad bike is faster than our needs so that same motor could be geared to give low speed torque instead.

Keeping in mind that in the video above the rider is on a flat even surface and we don’t see how long it’s taken to accelerate to this speed, it does at least give some idea of what a 1000w motor running off a 48v battery is capable of and gives us a reference point for future calculations.

The bike in the following video uses a single 3000w motor and is clearly capable to carrying the rider up hills at significant speed.

A typical Land Rover winch might have a 3,500w motor.

In all of these examples though, the power is coming from just one motor. If I stick to the rocker bogie design then the wheelchair will have six motors, so six times more power (Although it’s obviously not as simple as this as not all wheels will have equal traction etc). Given this EXTREMELY BASIC research, it would appear that 3000w or similar motor might be up to the job.

I found this very useful calculator which given a few known variables can calculate the required power output:

1. Vehicle Speed. The wheelchair needs to be able to move at a brisk walking speed of 5kph.

2.Vehicle Weight. Impossible to say at the moment. just to get a rough idea, lets go with the combined average weight of 4 UK adults. 77 x 4 = 308kg.

3.Passengers. 1 child.

4.Nominal Battery voltage. The higher the voltage given to the motor, the higher its torque. To keep the weight of the wheelchair down, I want to try and avoid going over 48v if possible and prefer to keep it even lower.

5.Weight of one battery. Difficult to say as I don’t yet know which batteries I’ll be using. For the moment I won’t add any weight here and just count it as already having been included in the overall weight.

6.Motor current on level ground. This is a difficult one as you usually cannot guess or measure it until after you’ve finished making the vehicle. The website recommends that if you’re unsure just use their value of 10 as it’s likely to be a fairly small part of the total.

7.Hill climbing ability. A rocker bogie vehicle will usually topple over on a gradient of more than 45 degrees. Ideally I’d go and measure the gradient of the Llanberis Path but for the moment I’ll use 30 degrees as an example. This is a gradient of about 55%.

8.Length of hill. It’s an 18 mile return journey but it won’t need the peak current for all of the time. If I said 15 miles then I think this is quite generous. 15 miles is 24km.

9.Acceleration. Acceleration time isn’t too important in terms of how long does it take to reach top speed. More importantly though is when it’s climbing over obstacles, how quickly can power be transferred to each wheel. I’ll experiment with this value to see what happens.
The results:

Given the values above, the calculator has suggested a motor of 2,500w. Given the earlier Youtube references as a sanity check, this seems OK.

In addition to this, changing the voltage supply and time to accelerate doesn’t make any difference to the output of the calculator in terms of which motor is needed.

The calculator doesn’t take into consideration things like friction, uneven surfaces, and weather etc, however it does assume that one motor will be carrying the full weight of the vehicle. In light of this, a 2,500w motor will probably be OK.

When I start shopping around I will consider 3000, 4000, and 5000 watt motors and if the cost isn’t too prohibitive will invest in bigger motors. However, for the moment at least I have a rough idea.

I like the simplicity of being able to a buy a motor which is already embedded into the hub of a wheel as it will make building the wheelchair far simpler. Although UK suppliers have been difficult to find, I have found this Chinese manufacturer which has a huge range to choose from.

There is also a 43 page forum thread started by the manufacturer on  I’ve only glanced at the discussion but comments on there seem positive. Both the Endless Sphere forum and the Chinese manufacturer look a good place to obtain some advice.

Having said this though, despite the complications of mounting a motor separately to the wheels, it does give the advantage of being able to change gear ratios and makes it possible to work with local manufacturers.

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