Coreless pager motor PWM frequency and smooth acceleration

Other Hardware Motors, Mechanics, Power and CNC
1

I am building a model train controller for a small tram layout, and I am trying to figure out what PWM frequency to use with my tram's tiny coreless motors. I have found through various sources that coreless motors prefer a high frequency PWM signal, but when I try to run them at a high frequency (32kHz), they stay stalled until around 50% duty cycle. Problem is, as soon as they start, they fly, and only then can I then reduce the speed to a nice slow crawl. When I use the standard PWM freq (488 Hz), the tram starts at about 15% duty cycle and accelerates nice and slow, like a real one would. So my question is, would using a low PWM frequency be problematic, and if so, how could I make the motor start sooner at the higher frequency?

I am using the Arduino Uno R3, and the motors are 3 volts each and draw about 9mA unloaded. I am using a 1N4007 as my flyback diode in a basic single-transistor (4N3904) driver.

2

I don't know whether a low PWM frequency wold be problematic for those motors, but in general, smaller motors have lower inductance and need higher PWM frequencies to run efficiently.

The problem with using a high PWM frequency may be the 1N4007 flyback diode. Rectifier diodes like that have slow recovery times, so they absorb a lot of power at high switching frequencies. As the motor current is so low, a small signal diode such as 1N4148 would be better. If that still doesn't work well, perhaps you should try an intermediate PWM frequency, such as 3kHz.

3

Looks like the 1N4007 diode was the culprit, the tram starts moving now at about 20% with a nice slow start.

4

On the other hand, maybe not. It is better, but it still seems inconsistent; sometimes it accelerates beautifully, other times it stays stuck.