Can I power a motor with a higher voltage?

I have a set of 56V Lithium Battery packs from an electric lawnmower and a 24V 600W scooter motor.  I don't want to hack the packs apart because I'm hoping to be able to use off-the-shelf packs and chargers when the larger capacities come out.

Can I run these together with just a speed control and not needing an intermediary DC-DC converter?  If not and I want to use the battery, should I run a 48V Kit or a 60V kit (over or under voltage)

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FrankH851 year ago

any luck with this?

Ebay sells 10-60v 20A pwm motor controller have you tried?

I've also got EGO lawnmower battery and charger I'd like to make use of when my son grows up (go cart anyone?)

Thanks

What is this part you call "just a speed control"? Is that not a DC-to-DC converter of some kind? Perhaps the answer depends on the characteristics of your "justaspeedcontrol", e.g. if it can tolerate 56 volts as its input voltage.

jongscx (author)  Jack A Lopez2 years ago

The scooter came with a 24V 'speed control' box. Typical, cheap, chinese 'speed control'. It's a brushed DC motor (which I forgot to mention), so I'm assuming the speed control just PWMs the input voltage. It's voltage tolerant to 36V with proper cooling, so I'll need to get a new one anyway.

My question is, if I have a 60V input voltage, do they make 'speed control' units that I can limit to run the 24V motor? I know PWM dumps the full input voltage then shuts it off really quickly, so it averages to a DC voltage, how do I find out if my motor can handle those 60V pulses on its coils... without just trying it?

For a dc-dc converter to drop that much voltage and with that much current it may coast an arm and a leg. If you could find a 48V motor and controller I would be comfortable running it on 60V. Do you still have the lawnmower motor? you could allways use a pully or gearbox for more tourqe!

It might be convenient if you could build some electrical network, like a resistor and capacitor in series,essentially an RC filter,
https://en.wikipedia.org/wiki/Low-pass_filter#RC_f...

to approximate the dynamic load of your electric motor. I mean something you could plug into the output of your PWM, as a load, to test the output of the PWM, without putting the actual motor there.

I'm not quite picturing the PWM device, but I guess neither are you if you haven't bought it yet. But supposing it had a "knob" on it, like with a dial ranging from 0 to 10, then just from the simple math presented so far, turning the knob to "4" (i.e 40 percent) is as high as you can go without burning out the motor, since 60V*0.40 = 24V, the maximum voltage for your motor.

It would be good to have this test load, for to figure out where "4" is on whatever it is, the knob or whatever, that is controlling the PWM signal, so testing could be done without the actual motor being in danger.

A 1 ohm resistor is about the right size. I mean a 1 ohm resistor would dissipate approximately the same amount of real power as your motor, from the numbers given, since (24V)/(1ohm) = 24 A, and 24V*24A = 576 W ~= 600 W .

An actual resistor 1 ohm resistor with a power rating this high might be hard to find. However, you might be able to fake it with 1 ohm resistor rated for 10 W, but submerged in a small bowl of water, which I imagine to be boiling water, at this amount of power dissipation.

Regarding the size of the capacitor, my guess would be to just pick one big enough so the roll off frequency, f= (1/2*pi)/(R*C), is much lower than the frequency of your PWM. That way the voltage measured across the capacitor looks like smooth DC.

BTW, a DC motor with angular momentum, or connected to something with angular momentum, kind of "looks like" a big capacitor anyway. That is to say spinning mass stores energy proportional to square of angular speed, much like capacitor stores energy in proportion to square of voltage, and for motor I am expecting angular speed and voltage to be coupled, proportional to each other, because of Faraday's Law.

That's sort of a handwaving way of saying, "Well, your motor just 'looks like' (feels like?) a low-pass filter to the PWM... if the PWM is fast enough."

Although, there's no reason why couldn't just put that big capacitor in parallel with your motor, to help smooth out the voltage it sees (feels), if you have fears about the motor not liking the high peaks (60V peak to peak?) of the PWM signal.

You can run some motors on a higher voltage.

Slowly increase the voltage and watch the current.

If the current goes down as the voltage goes up you are OK.

If the current goes down and then back up as the voltage goes up, you need a DC to DC converter.

If the current go up as the voltage goes up you need a DC to DC converter.

Many speed controllers are PWM or pulse width modulators so they may not stop the motor from burning out since the motor will still be getting full power.

jongscx (author)  Josehf Murchison2 years ago

Sorry, should I be measuring the Battery current or the Motor line current? Yes, it is a brushed DC motor currently and I currently do not have a speed controller. I was asking this question so that I know what to buy for what I currently have.

Measure the motor line current and be sure it only goes down as you increase the voltage. For your speed controller you will want a adjustable power supply not a PWM.

Use a 60V kit, but make sure you have over-current control. Current kills motors

jongscx (author)  steveastrouk2 years ago
Why a 60V instead of a 48V? I'm just trying to understand the reasoning?

I remember reading about how under-voltage conditions can cause speed controllers to shut down, but they can be tolerant of higher-voltages since battery Voltage drops off as it's used.
Wouldn't I be starting at a 'low voltage' state already if I used a system meant for 60V?
iceng2 years ago

I would be nervous PWM a motor with double the design voltage.

A lot depends on the motor inductance and if it has brushes it may loose them fast.