Coil Current Ripple
The peak to peak oscillation created by the current rising and falling in each period in a coil driven by PWM. Its amplitude is inversely proportional to inductance and directly proportional to the PWM period; kept under control it acts as dither, taken to excess it produces heating and visible vibration.
While the switch conducts the current rises, and while it is off the current falls as it flows through the freewheeling path. The average value represents the command; the oscillation is the band around that average.
- If the PWM frequency is lowered the period lengthens and the ripple grows; the spool starts to vibrate noticeably.
- If the frequency is raised the ripple shrinks and motion becomes smoother, but stiction can return and switching losses increase.
- Because the RMS current is greater than the average, excessive ripple creates extra heating in the coil.
For this reason some drives do not need to generate a separate dither; the ripple of the PWM carrier itself does the same job. In the field, if the valve buzzes audibly and the cylinder vibrates slightly, one of the first places to look is the drive's switching frequency and dither setting. On units where these settings are fixed by the manufacturer, the source of the problem is usually oil cleanliness or spool stiction.
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