This may be a dumb question, but I was unable to find a conclusive answer on the internet, and was hoping someone here could help.
I recently started controlling a 12v brushed dc motor(that is gear reduced) with a stalling current of 15A but that never draws more than 3A under normal loads. I am using an Arduino Uno and an encoder to measure and target a specific rpm, and I was wondering what limits/dangers are associated with it. Mainly, I was wondering how slow I can run the motor under load before damaging it? Is it a problem to run the motor at say 50% or even 25% speed under load?
I am unable to provide a datasheet link for my motor, because it is just an old windshield wiper motor out of a car. But if there is any other information you need about the motor, let me know and I will add it.
It depends on the motor. There is no simple formula for the minimum speed. Even 2 otherwise identical motors will have slightly different minimum speeds.
You can reduce the speed of a motor by using an H-bridge or MOSFET driver and controlling that with an Arduino PWM pin.
Sorry, I should have been clearer in my original question. I meant to ask if it were likely to cause any damage to my motor to try and find the minimum speed. I know stalling the motor is bad for it, but (roughly) how close to stalling it can I get without risking any damge?
Also, I am currently using an h-bridge driver with pwm from the arduino. (I should have said that in the original post, my bad)
Did you verify that using an oscilloscope to show the actual current at the instant of starting? Since you have a brushed motor, the starting current will vary depending on the position of the brushes against the commutator segments.
Thanks for the suggestion. The motor only needs to run as slow as 50% rpm under load, and it looks like it will have plenty of torque at that speed. I was mainly concerned that it might damage the motor to try to run it as slow as it will go without stalling. Should I be concerned about this?
I'm going to suggest looking at your problem a different way.
The %RPM does not matter, except in the case of a fan cooled motor where the fan is on the motor shaft.
The real issue is heat generation and dissipation. And, as always, the answer is.. it depends.
What is the current that the motor will be drawing at that RPM and that particular load compared to the same but at full RPM?
What continuous current is the motor rated for?
What is the duty cycle will you be operating that motor?
That will give you a better idea.
Though again, its a wiper motor and those things are pretty rugged and designed to run for long periods.
It's a wiper motor so it will likely be ok. They're pretty tough (how many have you ever had to change on a car?)
Consider RC rock crawlers and powered wheelchairs. Good motors with an appropriate driver can operate fine at low speed. I'd be willing to bet a donut that the h-bridge would fail due to heat before the motor does. What one are you using or is it home made?
You said it yourself. Lots of current. They are designed under automotive standards and must perform under vibration, wind, snow etc. They're very popular with Hallowe'en and other prop builders for this reason.
the stall-current occurs if the motor is totally blocked mechanically and if you then apply its maximum allowed voltage permamently
Reducing the rpm can be done by using PWM. This means you switch on/off the voltage at high frequency. This means the averaged current will go down.
example numbers to explain more.
motor voltage 12V rpm with no load lets say 1000 rpm
Now with a PWM of let's say 20% time voltage on 80% time off
motor with no load rpm = 300 rpm
The more load you apply the more the rpm will go down until motor is blocked
The more load you apply the more current will flow
If the current that occurs will be still OK depends on how much current the motor can stand without overheating
This can be very different depending on the applied load
very small load PWM down on 60 rpm due to 5% on-time 95% offtime no problem because current is only 0,5A
high load PWM on 40% ontime 60% offtime motor rotating with 120 rpm might overheat the motor due to the too high current of 10A
Yeah, the real issue is friction. A motor that is designed to run on full voltage might not be designed with low friction bearings. So as you slow it down with a lower voltage, the torque might intersect with the friction and stop it, at a surprisingly high speed.
Then, while stopped, the current is higher because there is no back EMF, but lower because less voltage is applied. So it's not really likely to overheat although the possibility remains depending on the specific motor.
Thank you all for the replies, they were all very helpful. After testing, the motor has plenty of torque and doesn't draw too many amps at the slowest speed I need, so I guess I don't need to be concerned. I will continue to monitor it's temperature, but I'm not too worried because it doesn't seem to be working too hard.
Thanks again for the help, I learned a lot about dc motors.