Using an Electric Motor to Assist Another Motor

[modern_messiah]

Hi all,

This topic is an extension of a question I asked elsewhere. I’m trying not to tread a beaten track and seen as this question is much more specific I don’t think this could be considered a double post…

Anyway, to my question!

How would you incorporate an electric motor to assist with another motor (lets call it the main motor) where the main motor dictates the RPM of the second motor? Lets say the 2 motors are connected via a belt/chain and there is no clutch between them – basically if the main motor was spinning at 2000rpm the assist motor is also spinning at 2000rpm.

Obviously when the assistant motor is not being powered it’s being spun by the main motor. However what are the potential issues when you want to apply power to the assistant motor to not only overcome any resistance it places on the main motor, but also assist the main motor? What’s getting me hung up is that with electric motors you increase the voltage to the motor to increase RPM, and the motor controller pulls more current as needed to reach this desired voltage. But if your RPM is not only uncontrollable (from the assistant motors perspective) but also basically already reached (as it is being spun by the main motor) how can you actually 'assist' the main motor?

Can you just force more current into the motor regardless to increase power? What affect would this have other than potentially damaging your motor? Or, as I suspect, would you actually need to tell the assistant motor to try and spin slightly faster than the main motor? Because the motors are directly connected the assistant motor can’t reach this higher RPM and consumes more juice, and hence more power, to try and reach the higher RPM. By limiting the current available to the motor from the controller (and setting the controller maximum output current to lower than it is capable of) you can prevent burn out but have the assistant motor actively working to increase output power of the main motor.

Does ANY of that make sense? Sorry – it’s 2pm on a Friday afternoon, I’ve been a good boy and have effectively wrapped up for the week but can’t head home yet so I’m brainstorming at work.

I'm trying to think if this is basically the same thing as an e-bike? Think of the electric motor as being the main motor and the pedals as being the assistant motor or vice versa. The difference being that the pedals can freewheel until they reach the same RPM as the main motor in which case they then 'assist' with turning the wheel.

Suggestions or redicule in a sealed self addressed envelope.

Cheers,

- Matt

 

[amberwolf]

Perhaps you should read the various 2 wheel drive threads around here, as that is the same thing as what you want to do, albeit in a mechanically different fashion. Same results, though.

 

[modern_messiah]

Perhaps you should read the various 2 wheel drive threads around here, as that is the same thing as what you want to do, albeit in a mechanically different fashion. Same results, though.

I figured you'd pop to say hi! Thanks....as usual it's about getting a push in the right direction and then the rest (hopefully) falls in place

I'll see what I can dig up.

- Matt

 

[modern_messiah]

I've read KingFish's threadand as far as I can see it's pretty simple in theory, but I'm just a little los as to how it would actually work.

I need to keep the two motor's power and control systems separate (easy done) so that when I want the assistant motor to do it's thing I just open up it's throttle to whatever output I want from it. I just need to gear the assist motor so that when the main motor reaches the desired RPM that I want assistance with, the assist motor is at it's peak (be it torque or power). It should just pull even though it can't exceed the RPM it's 'tied' to.

The thing I'm getting hung up on however is proving tricky to actually explain...If the assistant motor is already spinning at say 3000rpm (as it's tied to the main motor) and I want it to provide more power to the overall drive system, I would open the throttle. What actually happens at this point?! How does the throttle work in conjunction with the assist controller and motor? Is it simply setting a desired speed (RPM) or does it actually just pump in power corresponding to the throttle position (20% throttle for 20% power and so on).

If it's as simple as just applying more power (full open throttle means the motor is outputting maximum power) then it's simple and there's absolutely nothing tricky about this set up.

I'm probably wrong though.

 

[Hillhater]

..If it's as simple as just applying more power (full open throttle means the motor is outputting maximum power) then it's simple and there's absolutely nothing tricky about this set up.
.

you got it right there.
Just imagine it as a hub motor rolling down a hill with no throttle on ( being "powered" along by the bike weight)...then you apply throttle until the motor starts to accelerate... or if the bike has started to roll up a hill by then so it cannot accelerate.... the motor simply settles at a speed (rpm) that matches the power available ( volts & Amps)
Of course, like any Emotor, if you overload it at the wrong speed, it will run inefficiently and start to heat up.

 

[amberwolf]

What the throttle does depends on how your controller works.

Typically ebike controllers are simply speed controllers, so increasing throttle just essentially increases average motor voltage which increases speed.

Some controllers have "power" throttles, so that they control how much power the motor gets, and thus how much torque is delivered, rather than simply how much speed it has. Those are usually higher end units like Sevcon and whatnot.

Such throttle usage can be emulated by external electronics, like the Cycle Analyst, in it's "RC" version for example it is designed to do this.

How you want it to work will depend on what exactly you want it to do.


If I were going to build a multimotor system, I would only do it with separate motors for separate wheels, because it is lighter and mroe efficient to have one larger motor than mutliple smaller ones, in most cases, if the main purpose is simply to have more speed.

Most likely I would also use only one throttle to control both motors, with electronics in place to ensure they both share the load proportionally as needed.

If the purpose for multiple motors is for traction, rather than more power, then I would have separate throttles ganged together but operable separately, so that normally I would just throttle up as usual for normal riding, and in traction control situaitons I could manually throttle separately with a slightly different kind of movement.

Alternately, I would build electronics to do automatic traction control to ensure both wheels maintained the same speed, slowing the motor that's running suddenly faster than previous overall ground speed, so it doesn't keep contribuiting to a spin-out or skid.



FWIW, I have an eventual plan to use chaindrive on my main bike(s), with a small backup freewheeling-geared hubmotor in one wheel simply for emergency use in case of failure of the chain system. It is not likely to be anything like your use for the system, but it also is a two-wheel-drive system, technically, as they *could* be used at the same time.


Anyhow, there's a lot of ways ot use the motors or to control them; which ones you use depend on exactly what your goal is and why you are using multiple motors to achieve it.

 

[modern_messiah]

Thanks for the well explained reply...it helps a lot!

FWIW this is all in relation to my post in the general discussion area and the whole 'replacing an alternator with a BLDC motor' question. I'm just doing some more research on whether or not it technically possible before I do something stupid. So far I see no reason that it wouldn't work, and your comment regarding 'power' throttles appears to basically be exactly what I am after, although I feel a simple speed controller would also work as the motors are connected and if I tell the assist motor to run faster than it is allowed (as it is tied to the main motor) it will draw more current, and hence apply more power, to try and reach this speed. I think that would work.

Meh - early days!

 

[amberwolf]

Just remember that when it is not powered, it is still using up power--taken mechanically from the other motor. Even if it is not set to braking (regen) it still has friction/etc that sap power from the system.

If you do not need any regeneration from it, you can use a freewheeling output on the motor so the motor is never spun by the system until you power it up (like my "emergency backup" motor would be).

That's a part of why it is usually more efficient to use one larger motor instead of multiple smaller ones, for many (most?) cases where two could be used.

Also, you have to ensure that whatever speed the second motor adds to the system does not push the first motor past it's design RPM, or that motor will then simply be adding drag and heat and wasting the power the other motor adds. Yet another reason to only use one motor.


So the real question is: Why would you want ot use more than one? What very specific reason do you have to do so? If you can't answer that in detail, then you don't need more than one.

 

[modern_messiah]

Just remember that when it is not powered, it is still using up power--taken mechanically from the other motor. Even if it is not set to braking (regen) it still has friction/etc that sap power from the system.

If you do not need any regeneration from it, you can use a freewheeling output on the motor so the motor is never spun by the system until you power it up (like my "emergency backup" motor would be).

That's a part of why it is usually more efficient to use one larger motor instead of multiple smaller ones, for many (most?) cases where two could be used.

Also, you have to ensure that whatever speed the second motor adds to the system does not push the first motor past it's design RPM, or that motor will then simply be adding drag and heat and wasting the power the other motor adds. Yet another reason to only use one motor.


So the real question is: Why would you want ot use more than one? What very specific reason do you have to do so? If you can't answer that in detail, then you don't need more than one.

Well in this case the assistant motor won't have the power to pull the main motor past it's design RPM, since we're talking about a 20HP electric motor trying to out do a 100kW ICE...

It's more of a 'just because I can' sort of thing. Replace a stock alternator with a reasonably large BLDC motor to not only act as a replacement alternator (to charge the car's stock battery), but when the time comes, provide the overall system with more torque and a bit more power. You could have it set for the assistant motor to come on low in the rev range to assist with starting acceleration, or have it come on at the ICEs peak power to add more top end acceleration and speed. What kind of figures you'd be looking at depend entirely on what sort of motor you can get in place of the alternator. You could even just have a push button that applies power on tap.

I have no intention of using this as a 'on all the time' system. I'd actually love it to be automatic - it only activates when the vehicle throttle is open 90% or more, or say between 3000 and 5000rpm (my vehicles peak power output is at approx. 4000rpm). Think of it like the Porsche GT3 Hybrid....but not so hardcore.

I think it'd be relatively simple to do - the most complex thing I can think of is making a mounting bracket for the BLDC motor so it can be mounted using the existing alternator mounts, and some custom pulleys to allow for a chain to be used instead of a traditional belt (as I feel the belt might not like the extra power).

- Matt