Some motor questions..

Ravenkallen

Hey everybody!! I have a question regarding the use of higher voltage and motors. My new robot platform that i bought has motors that are rated at 7.2 volts. While this is fine and it is still a step up from my previous bots, i would like it to be a little faster. Could i get way with raising the voltage a little(Maybe to 9-10 volts)? Or will i probably cause damage? If i keep the current low, it will still be using the same amount of power right? Or could i use PWM? I know some about motors, but i just wanted to ask first...

Mike Green

Maybe you'll get away with it and maybe you won't. People rate components for particular voltages or particular currents for good reason. On the other hand, sometimes the ratings are at environmental temperatures or with duty cycles different from what you intend to use.

The voltage rating of a motor affects the amount of current through the windings and the product of the two gives you the amount of power used. Some portion of that power gets turned into heat and that causes the temperature of the windings and that of the motor itself to rise. At some point that may damage the motor or the insulation on the wires or the bearings of the motor. You could certainly do some careful experimentation with higher supply voltages vs. operating temperatures. Remember that the current will increase as you put a mechanical load on the motor. Remember also that mechanical stalls do happen and the current through the motor will peak. You could use some kind of current sensing and cut off the motor drive if the current exceeds some amount that would prevent any thermal damage.

LoopyByteloose

Motor specifications are a rather confusing area. It is best to consider what exact is the weak link in the motor's configuration - wire size, brushes, or overall heat accumulation.

Many of the motors we buy in appliances cannot run 24/7. The heat build up is too much. For example, a food blender is meant to run for a few minutes and rest. The manufacturer provides parts that can heat up and cool off during rest periods. Other motors - like fans - have to dissipate enough heat to not fail over years of continous operation. If too much heat builds up, insulation gets fried and failure soon follows.

Small dc motors use copper enamel wire. The insulation is pretty good up to a few hundred volts. So a 7.2 volt motor doesn't have a problem with insulation - other areas are its weakness. The foremost is wire size as the smaller the wire, the more heat under load. And a second problem is the quality of the brushes. Some brushes are intended for rotation in only one direction and yet we build robots with h-bridges that frequently ignore this fact. There isn't much you can do if you already have the wrong brushes, but it is best to be sure that you purchase motors that are intended for rotation in both directions.

I suspect using a 7.2 rated motor at 10 volts for short burst and not under a heavy load will do fine. But if you run it all out for extended periods or with big pay loads, you may suffer failure. Most hobby motor situations have motors that are extremely tolerant to higher voltages if the duty cycle and payload are light. The exception to this is the R/C servo motors as the electronics included cannot tolerate much in the way of over-voltage - even if the motor can.

But remember when you start looking into motors that are expected to produce horsepower, you pretty much have to pay more to get more.

erco

10 volts is 39% more than 7.2 volts, that's a pretty hefty jump. As Mike & Loopy have pointed out, heat is your worst enemy. In light, intermittent use, you may be fine at 10 volts. Much depends on your gearing and total vehicle mass (weight). Personally, I prefer lower gearing with lots of torque, which gives lower speeds with better acceleration and control. In general, higher voltage will increase demands on your mechanical and electrical system :

If you are geared low, increasing motor voltage will cause a small current increase and a big increase in mechanical forces and potential wear.
If you are geared high, increasing motor voltage will cause a larger current increase and a smaller increase in mechanical forces & wear.

ElectricAye

It might also depend on what type of motor it is. Very often stepper motors have ratings that are far below what they can take. It's not unusual to have a stepper motor rated at 3 VDC but you'll need to run it at 24 VDC to get good performance. I have no idea why stepper motor ratings look like that. If somebody can explain it to me, I'd be interested in hearing why.

JasonDorie

I think steppers are rated at a given voltage to not overheat without current limiting. Beyond that voltage, the stepper driver has to limit the current to keep the motor from overheating. People use higher voltages to decrease the time it takes for the magnetic field to reach full strength (rise time) but doing so without limiting current would fry it. They don't have brushes or electronics to burn out, so as long as the current is kept in check they're pretty tolerant.

ElectricAye

JasonDorie wrote: »

I think steppers are rated at a given voltage to not overheat without current limiting. Beyond that voltage, the stepper driver has to limit the current....

That makes sense. Thanks for the insight!

Ravenkallen

Wow, thanks for all the comments guys. So the basic gist is that you have to watch out for heat generated by the excess voltage. My plan would be to not have the higher voltage on all the time, but rather have a "boost" option that could provide a little faster of a ground speed for a few seconds.

kwinn

The main problem with running motors over their rated voltage occurs when the motor is stalled, starting, or under heavy load. If you can control or limit the current you can get higher speeds and faster acceleration. Current sensing used as a control signal for PWM can do this for you.

lardom

I have a motor that got too hot at its rated voltage of 12V. I ended up putting a ceramic resistor in series with it. There was a slight loss of power but it stayed cool.