Why do motors (DC) make a buzz sound when a low duty cycle PWM is applied to them?
Once I applied a 10% duty cycle PWM to a gear motor and it didn't move. just made a buzz sound. What does a low duty cycle do to a motor that makes it buzz?
Electronic – Why do motors buzz when are applied PWM
motorpwm
Related Solutions
The PWM frequency should be high enough that the motor's inductance has a smoothing effect, so current should not go to max as soon the PWM signal goes high. However the current will ramp up, so you should put plenty of capacitance across the power supply to reduce ripple (good ESC's already have low-ESR caps built in, but it doesn't hurt to add more externally). Most brushless ESC's run a PWM frequency of 8KHz or higher, which is fine for high inductance iron cored motors (coreless motors need 30KHz or higher).
If these two conditions are met then the power supply should 'see' a peak current only slightly higher than the average current draw, even if all PWM signals go high at the same time. Just make sure that the average current doesn't exceed 25A and you should be safe. If the motors experience heavy loading during start up or fast throttle changes then you may need to limit the throttle change rate and/or apply current limiting, to keep below the power supply's current limit.
The power absorbed by a propeller is proportional to rpm squared, so at low speed it will put much less load on the motor, eg. if the full throttle current draw was 7A then at 25% throttle it would drop to about 0.3 Amps. So long as your props are small enough to keep the full throttle current below 6.25A per motor you shouldn't have anything to worry about.
The oscilloscope picture below shows current flowing through the windings of a high power brushless motor at a PWM frequency of 12KHz. You can see the current ramping up to about 32A during the PWM ON time (full throttle current draw was 40A). Current at the power supply would have been much smoother than this, as I had several 220uF low-ESR caps across it.
So I found the solution. There where 2 problems I found:
- The encoders that where in the servo use a PID that is in the firmware. I accidentally changed the P, I and D values in a way that all movement of the motors where very inefficient and trembled internally the motor
- Also the DOF 3 motor burned out... maybe because of the PID changes.
In any case, the PID are programmed correctly now and that one motor has been replaced. It is as good as new now.
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Best Answer
When you drive a motor with a PWM signal, you are basically just turning it off and on very rapidly. If the motor is moving while this is happening, it's inertia (along with the winding inductance) smooth out the PWM signal such that the speed of the motor is proportional to the average time the motor is on.
If you use a very low duty cycle, it means that the motor is turned on for only a very small amount of time each cycle. If the average power supplied to the motor is too small to overcome the friction and any load on the motor shaft, the motor will stall. In the case of PWM it will be turned on, stall, then be turned off, over and over again, which essentially makes it vibrate at the PWM frequency. If this frequency is nicely within the audible range for humans (anywhere from ~20Hz to ~20kHz), you will hear this vibration as a buzzing sound.
The rotor vibrates in the magnetic field which is being turned on and off. You turn it on, it tries to rotate but can only move a small way before you turn the field off again, and which point it will stop moving again. It vibrates back and fourth. This may move any air around it, but also moves the whole motor - Newton's law: every action has an equal and opposite reaction! As the whole motor vibrates, the air around the motor will vibrate with it, making sound!
Try putting the motor on a hollow box, this will amplify the vibrations of the stator and the sound will get louder. Conversely, press it on to something like a piece of rubber and that will damp the vibrations - it gets quieter.