Electric DC Motors

Vicinalvictor · Jul 7, 2018

I built an electric cart, 800w 36v gear motor. Problem is it blows fuses at 36 volts upon take-off. 24 volts runs the motor, but tends to overheat. The motor is geared down to a safe speed, and I use a momentary switch as a foot throttle. Any idea what I need to do or install to get this motor to run as intended on 36 volts? I know cutting the voltage increases the amps, and that is probably why the motor tends to get hot. I am running 10 ga. wire, the wire does not get hot, but I do not run the motor very long either.....

MarkB_MI · Jul 7, 2018

A DC motor is essentially a dead short at zero rpm. I suggest you switch to a "slow blow" fuse or circuit breaker that can handle the momentary overload when you start up.

If you have it geared so you're running at the desired ground speed on 24 volts, it will want to run 50 percent faster on 36V. So you need to gear it down quite a bit. Note that current through a dc motor is proportional to torque, so reducing the torque requirement (by re-gearing) will reduce the current requirement and the motor won't run so hot.

Vicinalvictor · Jul 7, 2018

Thank you. I have it geared for the 36v. I thought perhaps the momentary switch was throwing too much at it all at once, and that was causing the fuse to blow. Any suggestion where I buy a "slow blow fuse" or circuit breaker that can handle the momentary overload?

Jon Hagen · Jul 7, 2018

Some good info here, you may just need to increase fuse size slightly.
Load VS fuse size.

guido · Jul 7, 2018

Hello vicinalvictor,

You need to measure the amp draw. Fuse should be 50% more of the amp draw
1)10 amp draw = 15 amp fuse,

Guido.

Vicinalvictor · Jul 7, 2018

Thank you Guido. I know if you take the watts and divide it by the volts 800/36, you get 22.2 amps.

Vicinalvictor · Jul 7, 2018

I keep blowing 40 amp fuses.

David G · Jul 7, 2018

Sure sounds to me like it is geared wrong.

Tramway Guy · Jul 8, 2018

DC typically does not have ?inrush? like an AC motor, at least for shunt wound or permanent magnet motors.

But it if it is series wound, that might be a different story.

fixerupper · Jul 8, 2018

You need bigger than 10 ga wire. 4 gauge is more suited for your application. An undersized wire will blow fuses while the wire does not get hot.

Vicinalvictor · Jul 8, 2018

Thanks, I thought about larger gauge wires, but then I was looking at the two wires coming from the DC motor, and they I think they are 10
gauge wires. Do you think I need to run those wires through a diode or something?

Vicinalvictor · Jul 8, 2018

I do not think this is a shunt wound DC motor, although I am not sure. The motor has more torque than speed. I have it wired in series, 800w, 36v. But for some reason, the flow of current is blowing 40 amp fuses at start up. If I power it at 24v, it runs slower, but tends to overheat, and then blows the fuse. Perhaps a "slow blow fuse" as earlier suggested may remedy the situation, or perhaps I need a diode or something in the line. I am not sure.....

Vicinalvictor · Jul 8, 2018

What you say makes sense to me. Perhaps if I change the driven gear to a much larger one than what I am using now, would slow everything down more, thus reducing the current flow (amps) needed at start-up. Keep the voltage at 36v, am I right?

Vicinalvictor · Jul 8, 2018

Thanks, I did increase the fuse size from a recommended 30 amp, to 40 amp. It makes no difference. I think there has to be diode, or relay resister, something inline to regulate the current flow.

MarkB_MI · Jul 8, 2018

> Perhaps if I change the driven gear to a much larger one than what I am using now, would slow everything down more, thus reducing the current flow (amps) needed at start-up.

Yes, if the motor is turning faster for a given ground speed it will draw less current, because it has to produce less torque. (The torque is gained by the gearing.) As for start-up current, the initial current draw will be about the same regardless of the gearing, but the current drops with rpm, so the quicker acceleration offered by the lower gearing means the current will drop sooner. (The faster a DC motor turns, the less current it draws, and at "no-load" rpm the current draw is almost zero.)

As for "slow-blow" fuses, consider using the "MAXI" series automotive blade fuses. They can handle a 100 percent overload for about ten seconds. However, they are only rated for 32 volts. I'm wondering if you really need fuses in the circuit; fuses normally serve to protect the wiring; if the wiring is heavy enough, fuses really aren't required. For example, automotive starter cables are never fused.

MarkB_MI · Jul 8, 2018

> I was looking at the two wires coming from the DC motor, and they I think they are 10 gauge wires.

It depends on how long your wires are relative to the wires supplied with the motor. If they're a lot longer, you should probably go up a size to 8 gauge. It never hurts to have heavier-than-necessary wires, assuming you can still route them and connect them to your switch.

> Do you think I need to run those wires through a diode or something?

Running a diode in series with the motor won't do anything but drop the voltage available to the motor by about 0.7 volts. And diodes that can handle 30 amps continuous are expensive.

That said, diodes are sometimes placed in parallel with motors to protect solid-state switches from damage. It's not normally done with mechanical switches, although it can reduce switch arcing. The diode is reverse-biased so that no current flows through it when power is applied to the motor, but when the switch opens the current in the motor has a low-resistance path to follow, so there's no inductive "kickback".

Vicinalvictor · Jul 8, 2018

I think I understand, thank you! This is why the 36v motor gets warm if I under volt it at 24v, and try to run it under load. I was also wondering about the fuses too, and if I really needed an inline fuse like you say. But I cannot run it like it is with 36v, no fuse, because it would burn out the momentary switch, would it not? If I install a larger secondary driven gear, the motor would run faster but cooler, right? because the torque would be more in the gearing and less amp draw on the motor to produce the torque.

Vicinalvictor · Jul 8, 2018

Yes, I never thought too much about the gearing, and the effects it can have on the torque of the motor until this post.

shinnery · Jul 8, 2018

Speaking of electric motors, VW just sent an electric race car up Pikes Peak in 7 min. 57 sec. for a new record. When I drove up Pikes Peak the last part was dirt. Mt. Evans w of Denver was the world's highest paved road.
https://www.engadget.com/2018/07/07/watch-vw-ev-pikes-peak-record-setting-run/

MarkB_MI · Jul 9, 2018

> If I install a larger secondary driven gear, the motor would run faster but cooler, right?

The heat put off by a motor is a function of how much current is going through it. And the current through a DC motor is proportional to the required torque.

Another thing to consider is "back EMF", which is the voltage induced in the motor as it turns. This voltage opposes the flow of current through the motor and increases as motor speed increases. At "no load" speed, back EMF equals the applied voltage. That's why DC motors turn faster the more voltage you apply to them, and why they draw less current as their speed increases.

As for burning up your momentary switch, that depends on the switch. As I mentioned earlier, what kills DC switches is not so much the peak current through them, but the arcing that results when you interrupt an inductive load. That's why most AC/DC switches are derated on DC: a switch that's rated for 10 amps AC is often only rated at 2 amps DC. (AC power turns itself off 120 times per second, so you get less arcing on AC.) But if your motor is getting hot at 24 volts, it will get even hotter at 36V if you don't change the gearing.

Vicinalvictor · Jul 9, 2018

I got the motor to run on 36v today, I removed the inline 40a fuse. So now I shall concentrate on the gearing like you say. Makes sense now that you explained it to me. Thank you!!

Electric DC Motors

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