All 4QDs controllers have a switching frequency of (around) 20 thousand times per second – 20kHz. This is high enough to give no whistle or whine from the motor but, more importantly, it means that as far as the motor is concerned the voltage to it is pure d.c. so the current it draws is continuous – not pulsed as it used to be in older thyristor or similar controllers. This reduces motor heating and gives better control.
There is a more detailed description in our circuits archive.
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