My motor hums or whistles, how do I stop the noise?
If the pitch does not change with speed the source is electrical and tied to the switching frequency; if it does, it is mechanical. Classify first, then act.
Classify the sound first: a sound whose pitch does not change as the speed changes is electrical and is related to the switching frequency; a sound whose pitch changes with the speed is mechanical. The most widespread cause of a constant-pitch hum is a switching frequency that falls inside the hearing band, and on a driver switching above 20 kHz that sound is typically inaudible. Settle this distinction at the first step so that you do not go looking on the wrong side.
Classifying the sound
- A sound at constant pitch that does not change as the speed changes: It is electrical in origin and related to the switching frequency. This kind of hum is typically inaudible on a driver running above the audible band.
- A sound whose pitch changes with speed: It is mechanical. It comes from a bearing, a coupling, a gear or an unbalanced load.
- A rattle that appears only under load: Gear backlash or a loosened fastener.
- A buzz coming from the driver: It may originate from a capacitor or an inductor; in these components mechanical vibration produces sound.
The quickest way to make the distinction is to uncouple the motor from the load and run it free. If the sound keeps the same character with no load, you are on the motor and driver side; if it only appears under load, look at the transmission.
Hum of electrical origin
The ripple in the current through the winding creates a ripple in the magnetic force; the laminated core vibrates at that frequency and produces sound. If the frequency is inside the hearing band you hear a clear hum, if it is above it you hear silence. But sound is not the only criterion when you choose the PWM frequency: the higher the frequency, the hotter the driver runs. If you are using an adjustable board, the target is the lowest frequency that makes the sound acceptable. On a board with a fixed frequency that route is closed and the work stays on the mechanical side.
What can be done on the mechanical side
- Mount the motor and the gearbox on elastic pads; if a sheet-metal panel is in direct contact, the panel amplifies the sound like a loudspeaker.
- Lengthen the ramp time (soft start/stop on the KS250, the RMP pot on the PT500) to reduce the jolt at starting and stopping, and with it the mechanical noise.
- Check the coupling alignment and the torque of the fixing bolts; misalignment produces both noise and bearing wear.
- Avoid running the motor continuously at very low speed; at a low duty ratio the current ripple and the vibration become pronounced.
- Check the supply cable cross-section; excessive voltage drop makes the motor run at a higher current and more noisily.
- Clip the cables to the panel. A loose bundle vibrates against the sheet with the motor and produces a sound of its own, and that source is hard to find.
The cheap way to find where the sound comes from is to hold the handle of a long screwdriver against your ear and touch the tip in turn to the motor body, the gearbox cover and the bearing housings. Wherever the sound is loudest is the source. You can do the same job by recording with a phone and comparing before and after the work.
Where the sound is not stopped altogether
Not every sound can be removed. The commutator sound of a brushed motor, the gear sound of a reduction box and the air sound of a board with a fan are part of normal operation. What is critical is not the absolute level but the change: a sound that was not there yesterday at the same load is a warning of a fault. If a temperature rise or a rise in current comes with the sound, find the cause before you try to suppress the sound.
What to do
Keep to the order: listen with no load and under load to separate the sound into electrical and mechanical; if it is electrical go to the frequency and ramp settings, if it is mechanical go to the pads, the alignment and the fasteners. Measure the current and the body temperature before and after every change, and move one variable at a time.
The full topic: DC Motor Driver Selection Guide