Disconnect motor phases at high speeds

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EdTesla   1 µW

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Disconnect motor phases at high speeds

Post by EdTesla » Jan 12 2021 10:49am

Hello people,

I was talking to a colleague last time about why you don't use two motors for very heavy cargo bikes. One motor would be for extremely low speeds (0-8km/h) and another for high speeds (8-25km/h and more).

Now for my idea, which I haven't found here in the forum yet: Couldn't 3 SSR relays be used to disconnect the phases of the slow motor from the controller at a specific speed? Of course, the slow motor should no longer produce torque at this point and the fast motor already should be active.

Of course you could also operate the slower motor with field weakening. But most controllers can't do that.

What do you think?

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spinningmagnets   100 GW

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Re: Disconnect motor phases at high speeds

Post by spinningmagnets » Jan 12 2021 10:59am

There have been several 2WD builds featured. They have a benefit in snow and sand, along with any other slippery condition.

However, if you never ride in slippery conditions, it's cheaper and easier to simply use a more powerful motor, controller, and battery...

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spinningmagnets   100 GW

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Re: Disconnect motor phases at high speeds

Post by spinningmagnets » Jan 12 2021 11:01am

There have been several 2WD builds featured. They have a benefit in snow and sand, along with any other slippery condition.

However, if you never ride in slippery conditions, it's cheaper and easier to simply use a more powerful motor, controller, and battery...

One 2WD build definitely had a cutoff switch for the front motor, when it was not needed...

EdTesla   1 µW

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Re: Disconnect motor phases at high speeds

Post by EdTesla » Jan 12 2021 11:11am

Yes that's right. I have found concepts with multiple motors. But I have not yet been able to find how to disconnect the slower motor. On the DC side is not a good idea because of the extremely high voltages that could occur and possibly destroy the controller. Or?

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amberwolf   100 GW

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Re: Disconnect motor phases at high speeds

Post by amberwolf » Jan 13 2021 2:29am

Why do you need to disconnect the motor? Just stop providing throttle to it if necessary (it isn't really, since if it's not providing torque it's not using significant power).


While the mismatch on my heavy-cargo-hauler 2WDs is nowhere near that you propose, I've never had any issues caused by it, even when I use a single throttle to control both motors at the same time (mostly I've used separate throttles; presently the SBC uses just one so that the CAv3's multiple control/modulation inputs can create a single throttle out to both)

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Re: Disconnect motor phases at high speeds

Post by EdTesla » Jan 13 2021 6:45am

We had a problem with our build at high speeds. A person who had never driven the vehicle before became very fast downhill. The top speed was three times as much the speed that the vehicle reaches on level ground with the slow motor.

When the person stopped, the display of the slow motor was destroyed. Since we have very good mechanical brakes, we do not use recuperation. The current would be too much for our battery. The only problem is that then the voltage increases too much with speed. Due to the body diodes of the mosfets reducing throttle to zero has no effect.

Is that a known problem? I have unfortunately no model name of our controller and our display. If I open the case, I might be able to tell what kind of controller it is.

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dogman dan   100 GW

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Re: Disconnect motor phases at high speeds

Post by dogman dan » Jan 13 2021 7:57am

I'm an idiot when it comes to electronics, but my solution would not be something fancy with the wheels. One good strong motor, one good strong controller, and then use a Cycle analyst for the display.

I just suspect something cheesy with that display/controller. No e bike should blow its display just because you coasted fast down a hill. If this IS something common, ditch that type display.

Re the motors, just one motor strong enough works best. If you desire fast, pick a fast motor. If its very big, it can still run very slow and very efficient on the flat. On the hill, if it has the power, it can still climb a mountain efficiently, just by keeping the speed up enough up that hill. Pick a slow motor ONLY if you want to make it very slow all the time, such as for safety with a delta trike, or tricky tow rig, etc.

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Re: Disconnect motor phases at high speeds

Post by fatty » Jan 13 2021 4:32pm

EdTesla wrote:
Jan 12 2021 10:49am
I was talking to a colleague last time about why you don't use two motors for very heavy cargo bikes. One motor would be for extremely low speeds (0-8km/h) and another for high speeds (8-25km/h and more).
You don't use two motors this way because it doubles motor and controller weight and complexity, and then you have the control problem you identified, and dead weight.

Better to size a single motor kV for desired speed and stator volume for desired torque/power.

There are reasons to use two motors for redundancy or 2WD, but generally they would be the same kV.
Don't take advice from:
there is no difference between a mean well CC/CV power supply and a device sold as a charger. they operate in EXACTLY the same way
Testing has demonstrated that ordinary rim brakes thermally outperformed all but the best disc brakes...You'll always add weight and cost, while not equalling the capabilities of comparable rim brakes, if you use discs

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Re: Disconnect motor phases at high speeds

Post by markz » Jan 13 2021 5:20pm

Some question using 2 motors for 2wd for different kV. Having one motor for cruising speed, another for hill climbing. Having one motor dd for regen another for various other reasons. Having one motor is better. Simplicity is better. Reliability is better.
fatty wrote:
Jan 13 2021 4:32pm
You don't use two motors this way because it doubles motor and controller weight and complexity, and then you have the control problem you identified, and dead weight.

Better to size a single motor kV for desired speed and stator volume for desired torque/power.

There are reasons to use two motors for redundancy or 2WD, but generally they would be the same kV.

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Re: Disconnect motor phases at high speeds

Post by amberwolf » Jan 13 2021 5:22pm

EdTesla, see below the quote of your post for possible solutions.

For those reading that don't know about this problem, it is that the faster a motor is spun, the higher the voltage it generates. A motor wound for a very slow top speed under power makes a higher voltage per RPM (kV) than one wound for a higher top speed. If the slow motor is forced to spin fast enough to generate a voltage higher than that which the controller's FETs or other battery-side components can handle, those components will fail (usually dramatically). :(

The motor itself can generally easily handle this, but the controller (and the battery's BMS, if it has one) cannot. And sometimes, when the controller FETs fail, they fail in a way that shorts motor phases together, and at that point the motor becomes a brake, generating significant amounts of heat. If the conditions persist, the motor can be overheated and destroyed. :(

EdTesla wrote:
Jan 13 2021 6:45am
We had a problem with our build at high speeds. A person who had never driven the vehicle before became very fast downhill. The top speed was three times as much the speed that the vehicle reaches on level ground with the slow motor.

When the person stopped, the display of the slow motor was destroyed. Since we have very good mechanical brakes, we do not use recuperation. The current would be too much for our battery. The only problem is that then the voltage increases too much with speed. Due to the body diodes of the mosfets reducing throttle to zero has no effect.

Is that a known problem? I have unfortunately no model name of our controller and our display. If I open the case, I might be able to tell what kind of controller it is.
With this specific problem, then you would indeed need a way to either limit the speed of the system to the max "tolerable" to the slower motor's controller, or to disconnect it's phase wires from the controller, or ensure you use a controller/display system for it that can handle the highest voltage it would output.

The last method is the simplest, if the voltage is low enough for an affordable controller. To find out this highest voltage, you can measure it, with a voltmeter between two phases, set on AC Volts, while riding the bike at the highest speed it will ever reach. If the meter has a Peak Hold feature, use that to make it easier, as you won't need to keep an eye on the display while riding. Then you can use the simple math (that I don't remember) to convert three-phase AC Volts to DC Volts, and then get a controller/display system that can handle *at least* that much, *but* that has either an adjustable LVC, or an LVC low enough to accomodate the lowest voltage you want to be able to run at when that controller's battery is empty.

The battery will also need a BMS that can handle that voltage, if it remains connected to the controller when in this high-voltage state. If both controllers share the same battery, then the other controller *also* has to be able to handle this same high voltage.



Now, if you do want to disconnect the phase wires, you can use regular relays, but there is a small chance that contact chattering can occur under severe vibration. If it happens while current is flowing (using the motor), then the contacts can be damaged over time, or even weld together (so that they don't disconnect when commanded).

SSRs don't have that problem, but like FETs they do have a voltage limit, and a current limit, and internal resistance, and they heat up during current flow. So you need to get ones that can handle all of the requirements of your system, meaning you still need to know the max voltage they'll see, and in this case it's the actual AC phase voltage, not the DC battery voltage. You'll also have to use SSRs taht can handle the highest phase currents the system could see while using the motor as a motor, and like the speed-generated phase voltages, the phase currents are significantly higher than battery voltages. Then you'll need to be sure the SSRs stay cool (which probably won't be an issue, but should be considered when packaging the disconnect unit).


Alternately, you can use a large 3PDT switch, and manually disconnect or connect it. This switch will *also* have to handle the max voltage and current, and might be a bit on the large side. I don't recommend using three separate switches in this case, as turning them on or off at different times might cause issues with the controller.


If you use SSRs or relays, you can setup a voltage monitor on that motor, and when the voltage exceeds whatever limit you set, it autodisconnects. When the voltage drops below whatever other limit you set, it reconnects. Or you can use speed for this. Or do it manually, if you prefer...but then you have to remember to do it, and if you don't....



However, since you don't use regen anyway, but you really want to keep two different speed motors, then what I would really recommend doing is to either use a geared hubmotor with internal freewheeling clutch for the slow motor, or move the slow motor to a middrive thru the pedal drivetrain (which also lets you shift gears for it to use it for a wider speed and torque range). In either case, it is never backdriven. This is a much simpler solution than any of the above, electrically, with less risk of dramatic failure in operation. ;)

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Re: Disconnect motor phases at high speeds

Post by fatty » Jan 13 2021 5:54pm

markz wrote:
Jan 13 2021 5:20pm
Some question using 2 motors for 2wd for different kV. Having one motor for cruising speed, another for hill climbing. Having one motor dd for regen another for various other reasons. Having one motor is better. Simplicity is better. Reliability is better.
Well, it's great to question, discuss, and learn, but there are no mechanical or electrical advantages to such a build -- only disadvantages.
Don't take advice from:
there is no difference between a mean well CC/CV power supply and a device sold as a charger. they operate in EXACTLY the same way
Testing has demonstrated that ordinary rim brakes thermally outperformed all but the best disc brakes...You'll always add weight and cost, while not equalling the capabilities of comparable rim brakes, if you use discs

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Re: Disconnect motor phases at high speeds

Post by markz » Jan 13 2021 7:50pm

There are some advantages, like having 2 motors of the same kv for 2wd in challenging conditions.
Thats about it. Like fat bikes.

If you care about efficiency then obviously that would decrease for 2 motors. More weight, more wiring, more things to go wrong, its just a quagmire. Who knows about the handling, that would be an interesting ride to say the least.

fatty wrote:
Jan 13 2021 5:54pm
Well, it's great to question, discuss, and learn, but there are no mechanical or electrical advantages to such a build -- only disadvantages.

fatty   100 W

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Re: Disconnect motor phases at high speeds

Post by fatty » Jan 13 2021 9:04pm

markz wrote:
Jan 13 2021 7:50pm
There are some advantages, like having 2 motors of the same kv for 2wd in challenging conditions.
Thats about it. Like fat bikes.

If you care about efficiency then obviously that would decrease for 2 motors. More weight, more wiring, more things to go wrong, its just a quagmire. Who knows about the handling, that would be an interesting ride to say the least.
Oh yeah, was referring to 2 motors with different kV.
I'm very skeptical of 2WD via fork drive in general, but I suppose if it were to be tried, same kV on traction-limited surfaces where increasing rear traction is not possible would be the place for it.
Agree with you across the board.
Don't take advice from:
there is no difference between a mean well CC/CV power supply and a device sold as a charger. they operate in EXACTLY the same way
Testing has demonstrated that ordinary rim brakes thermally outperformed all but the best disc brakes...You'll always add weight and cost, while not equalling the capabilities of comparable rim brakes, if you use discs

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