# Electric motor power increase question

#### eKretz

Apr 8, 2013
251
OK, I have a project I'm working on, where I need to increase the available stall torque of some existing motors, it's important not to change the motor size if at all possible. The motors are already having some issues with heating up and losing power after about 30 minutes of continuous use. They are DC brushed motors, (ball-bearinged) of the 550 variety and run at 24V, about 4000 RPM no-load, and about 4 amps at stall. Most of the time they are running near stall, and work in pairs. They are already running a pretty minimal air-gap. Will switching to neo magnets increase torque without too much current increase? What about switching to a larger motor (If I can convince them) and running a limiting resistor? Would I get more torque than the smaller motors? I'm sure this would help combat the heat/power loss issue since the wire gauge would be larger but with less current running through it, wouldn't it? I'm hoping someone has some insight so I don't have to go through a whole bunch of experimentation/prototyping.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Gearing the motor will give you more torque.

Anything with a series resistor will give you far less torque.

#### eKretz

Apr 8, 2013
251
OK, so if I had a non-stock motor of larger diameter that was trying to draw 30 amps but was limited to 5 it would have less torque than the stock 5-amp motors? Is that because of the motor being optimized to run in the amperage range it was designed to use? So the magnetic fields are optimal at the design amperage and drop off exponentially or something at lower amperage? Gear reduction is no good because it will drop the RPM too far.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
I assume you're referring to my statement about the resistor.

I you place a resistor in series with a motor, it causes the voltage the motor sees to drop as load on the motor increases. This results in either a reduced increase in torque, or a reduction in torque. Both of these problems are exactly what you do not require.

#### eKretz

Apr 8, 2013
251
Right, but what my question was meant to ask was is it possible for a higher amperage draw/torque motor than the one in current service to be current limited and produce higher torque and less heat than the stocker? I know that for a given motor a series resistor will obviously reduce torque from what it would be without. I'm not sure if you understood the original question was regarding two different motors or if you are saying that the series resistor would be bad even on the larger motor.

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#### duke37

Jan 9, 2011
5,364
You are likely to burn out a DC motor if you stall it and put a large current through.

The armature windings will not share the current and the commutator will get heated assymetricaly. Normally a motor takes a lage current to start but the heat is distributed as the motor rotates, then as the motor speeds up the current reduces and windage cools the armature.

#### eKretz

Apr 8, 2013
251
Yep, I know that. But these FFB wheel makers have been doing it for the last 10+ years with most of them having minimal problems. I'm just trying to figure out if there's something I can do (since I don't have to worry about the company bean counters insisting on cheap parts like they do) to make them work better.

#### duke37

Jan 9, 2011
5,364
"Yep, I know that. But these FFB wheel makers have been doing it for the last 10+ years "

How do they achieve the force feedback?

#### eKretz

Apr 8, 2013
251
I'm not sure I understand the gist of your question, as it seems very general in nature, but they are taking the FFB signals in and outputting power to the motors through an H-bridge controlled by a Microchip PIC-16? controller. Not sure if PIC-16 is correct, going by memory. There is a photosensor and optical grid wheel for location reporting, as well as a hall sensor and steel pin for "zero" calibration.

#### duke37

Jan 9, 2011
5,364
So the computer could know that the motor is stalled and could reduce the current to a safe level. A short burst now and then could be generated to see if the motor would be able to overcome the resistance.

#### eKretz

Apr 8, 2013
251
Nope that isn't happening as far as I can tell. When the wheel pulls to the left for instance while you are making a right hand turn, the force is constant until you are through the turn. It only applies as much power as is necessary to simulate the force needed, and it scales the force to the wheel's ability to produce torque, so it's not constantly stalled at full voltage and amperage. I have an ammeter on the power supply, so I think I would see it if it was fluctuating much. I am now very tempted to hook up my oscilloscope and get a snapshot of exactly what is happening though. I have 4 channels but only 2 probes at the moment or I'd probe the motor leads and see exactly how the voltage and current (with a sensing resistor) is adjusting.

My biggest hurdle is really the cooling issue I guess. I have a setup that is working but it's complex and I'd really like to find a simpler solution. I was hopeful that using a bigger motor but taxing it less (by using a series resistor maybe to reduce the larger motor's current draw) might do the trick, but it sounds like Steve is saying that wouldn't work.

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