A while ago I got lots of great advice on this forum about how to reduce motor-related electrical noise in the espresso machines I work with. Improved grounding and snubbers made a world of difference for us. Along the way, I tried to do reading to better understand what was going on, and the general consensus seemed to be that motors mostly cause problems when switching off, not switching on. I also heard that the worst of the electrical noise wasn't directly from the back-emf of the motor, but from the arcing across relay contacts that the emf could produce - I was under the impression that the snubber preventing the arcing. None of this ever quite explained our scenario, where we most often saw glitches in our low voltage system when the motor turned on, not off.
Last week I finally got an oscilloscope and tried to "see" the noise and test which of a few noise fighting strategies was most effective. Below are pics of my oscilloscope settings and the resulting traces.
It seems like there are two very different noise signatures here. When the motor starts, there's a large, solid drop in the trace, about 4ms long and down 200mV at its worst. When the motor stops, there's a fuzzy "cloud" of high frequency noise for about 2ms, maybe 100-150mV p-p.
From what I've seen and heard, I'd have expected the snubber to totally wipe out the cloud of noise when the motor stops, but it seems to have little to no effect on that. Instead, it does a remarkable job of eliminating the dip upon motor startup that most people seem to think shouldn't exist to begin with.
I'm pretty confused and want to better understand all this. Can anyone help me make sense of this?
Sorry forgot to describe setup: Oscilloscope is connected to +5VDC power and DC ground wires that power all of the control systems in an espresso machine which also includes carbonator pump motors (240VAC, centrifugal switch start.) Wiring schemes include switching 240V to motor with DPDT relay, switching only hot leg to motor through relay and leaving neutral maintained, switching both legs with relay and adding snubber, and finally switching just the hot leg with an SSR.
One of our clients had noticed that using SSRs eliminated their noise issues, but later decided that the reason it helped was the maintained neutral, not the SSR itself. These tests would seem to contradict that.
As always, sorry for rambling. Thanks in advance to anyone who gets through all this.
[EDITED to reduce file size of pictures]
Last week I finally got an oscilloscope and tried to "see" the noise and test which of a few noise fighting strategies was most effective. Below are pics of my oscilloscope settings and the resulting traces.
It seems like there are two very different noise signatures here. When the motor starts, there's a large, solid drop in the trace, about 4ms long and down 200mV at its worst. When the motor stops, there's a fuzzy "cloud" of high frequency noise for about 2ms, maybe 100-150mV p-p.
From what I've seen and heard, I'd have expected the snubber to totally wipe out the cloud of noise when the motor stops, but it seems to have little to no effect on that. Instead, it does a remarkable job of eliminating the dip upon motor startup that most people seem to think shouldn't exist to begin with.
I'm pretty confused and want to better understand all this. Can anyone help me make sense of this?
Sorry forgot to describe setup: Oscilloscope is connected to +5VDC power and DC ground wires that power all of the control systems in an espresso machine which also includes carbonator pump motors (240VAC, centrifugal switch start.) Wiring schemes include switching 240V to motor with DPDT relay, switching only hot leg to motor through relay and leaving neutral maintained, switching both legs with relay and adding snubber, and finally switching just the hot leg with an SSR.
One of our clients had noticed that using SSRs eliminated their noise issues, but later decided that the reason it helped was the maintained neutral, not the SSR itself. These tests would seem to contradict that.
As always, sorry for rambling. Thanks in advance to anyone who gets through all this.
[EDITED to reduce file size of pictures]
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