Problems when using two different drivetrains for 4wd?

1

Hi there,

I’m planning to do a 4wd drive hub motor board with my spare parts. I have a Mellow hub drive and a Landwheel and want to use both, Mellow in the front and LW in the rear. (Only the motors on the trucks, without the according docking bay and with vescs)
What I wonder is, what happens if one drive has a higher max speed/rpm compared to the other? Plan is to benefit from both drives, great torque of the LW combined with higher speed Mellow-wise :slight_smile:
Do you know of anyone who maybe already did this?

2

I know only about people doing that with different kV motors on dual drive. There it works without issues.

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3

What happens when the VESC can’t keep up with the forward-pushing of the motor? Does it just stop pushing? In that case it would save power since then the rear drive would just free-roll, wouldn’t it? :smiley:

4

The only problem is if the Kv is really different, once you pass the max speed for a given voltage the higher Kv will generate a higher and higher voltage, you can either fry the electronics if the current is too big or simply the transistors will start to conduct even when off and stat to break

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5

since its Hubmotors, you should be well off any ERPM limits. lower kv motors will have more torque at lower speed and higher kv more torque at higher speed. But as far as the ESC limitation goes you wont face any problems. (unless they have ridiculsly high kv for some reason, which i doubt)

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6

So the lower KV motors will just go higher rpm than they could on their own and the ESC drives them faster too?

7

I guess the lower KV motors will be dragged by the higher kv motors to some extent. Meaning the higher kv motors probably wont reach thier max rpm.

How big is the difference between the two?

8

I’m not sure right now but it is something around 90KV with the landwheels (They ca do 55kmh@10s) and 120 KV Mellow (in a mellow drive with 7s battery their proclaimed 45kmh).

9

A BLDC motor may be essentially identical in construction to a PMSM. But when it is driven as a BLDC, then how it responds to being over-driven is really a matter of how it is controlled. It can be controlled in such a way that it goes into regeneration, but not all controllers are designed to do this.
found on voltage - What happens when a motor is spun faster than its rated RPM from an external force? - Electrical Engineering Stack Exchange

Thats why I ask, what does a VESC do in that case? Can I set it so it goes into freespin or charge? @JohnnyMeduse do you have an idea?

10

The VESC will let the motor free roll unless the voltage it generates is higher than the current battery voltage plus two times the voltage drop of the MOSFETs

For example, let’s say you are pushing both motors to 3000 rpm, the Landwheel will have a back EMF of 33.3 V and the Mellow 25 V

If you battery voltage is lower than 26.4 V the Mellow will start to brake if you push the rpm higher, pretty strong since the VESC has no control over it

As long as you keep an eye on it and don’t push the speed too high you should be good

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11

My suggestions here is to try to have 2 completely separate 2wd systems if possible.
By this I mean 2 separate battery packs with the absolute shortest length of wire between each ESC and its dedicated Battery pack. Also, if you have a way to use 2 identical receivers and pair them both to one Transceiver, this will mean that you essentially have 2 independent 2wd systems under the control of the 1 hand held remote.
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I have found that 4 wheel drive systems have more reiability issues for some reason. So in making 2 seperate systems, you will have similar reliability to a 2wd system.

Now, in the case that you do choose to try using 1 battery for both ESC’s, keep the battery cables to each ESC the exact equal in length and make sure that you divide your 2wd regenerative braking and max brake current settings by 2 since you will now have 2 pairs of motors sending that amount of current to that 1 battery.

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12

when the lower kv motor is pushed above its no load speed it will generate higher voltage than the battery, potentially frying electronics

13

That’s sure for the two battery case, but with one battery, the voltage in the system should stay normal, else you would have a perpetuum mobile, essentially. Or am I missing something?

14

at no load speed, the back emf produced by the motor is equal to the battery voltage. so when the faster motor forces the slower motor to turn faster than its no load speed, the slower motor will produce more voltage than the battery it’s attached to. my understanding is with some vescs, bad things can happen as you approach 60v from the motor.

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15

Yeah I think I see it now, the issue is the ESC, as that is exposed to the higher voltage coming from the motor that is pushed, before it is balanced by the rest of the powersystem.