Recent content by Guineafowl

Got it. Trace 1 to show CH1+CH2, invert 2 to get CH1-CH2.

Yes, it’s been two house moves since I last saw the Hameg manual. As I remember it, the ADD/CHOP button selects dualling options: ADD: alternate sweeps for 1 and 2 CHOP: switches between 1 and 2 during each sweep. Will update when I dig out the manual. I’ve never had one for the Fluke.

I’m not sure either of these have the subtract function, but willing to be corrected. My earthing is TN-S, ie N and E linked at the transformer and run as separate conductors to me. No N/E link at my end. I may have to look for an isolation transformer. I have one mounted in the test bench...

One final problem: the VFD output is mains-referenced, so connecting the probe across phases will trip the RCD. Unless the motor earth will suffice for probe ground, I’ll need an isolation transformer capable of at least starting and running the mill motor at no load. As I understand it...

Cheap and cheerful, I’m afraid: https://www.aliexpress.com/item/1005006427487406.html I have a Hameg HM203-5 20 MHz CRO, and a Fluke/Phillips PM3384A 100 MHz combiscope, but the digital part doesn’t work. Also, some ordinary 100 MHz probes. I assume I’ll need some better kit. I’ve been looking...

Thanks - I see the difference between an RC LPF and the snubber. I’ve ordered a 20-pack of ferrite rings at those dimensions, and have found my LCR meter. What I could do with, is a cheap way of measuring THD before and after installing the filter, is that right? I found this...

No circuit diagram, even in the manual for the decent VFD. At some point, I’ll get around to dismantling one of them as you suggest, but I have to assume the filtering in these devices isn’t adequate to protect older motor insulation, given the literature on the damage they cause. I have a...

Something like this?: I’m not sure if the star point of the snubber should be earthed; all circuits in my shop are RCD-protected to 30mA so I’d like to avoid earth leakage. I’m also not sure of values, but assume that they’re not so critical, because of the wide gap between desired (<=100 Hz)...

No, because the sound isn’t a problem - it’s not annoying - and happens on even the newer motors. On some battery drills, when you just depress the trigger so you can hear the carrier frequency ‘sing’, before the mechanism starts moving and drowns out the noise. It’s that sort of sound. It’s...

There are several motors that might need this modification, one being a Newman ‘Elf’ from 1961, for which I can’t find a datasheet. It’s a very heavy cast-iron motor. Its plate says 1.8 A/phase, permanent star, 415V, but I’ve extracted the star point to enable delta 240V, so I’d estimate 3.1...

I can hear the 3 kHz switching frequency, yes. https://industrialmonitordirect.com/blogs/knowledgebase/eaton-reflected-wave-trap-for-vfd-motor-protection This link suggests the reflected waves (would it be a kind of SWR?) are significant only when the cable length exceeds 15m, so likely not a...

I have a couple of old CROs, but not the right probes for connecting up. The VFD/old motor problem is described here: https://www.mtecorp.com/wp-content/uploads/SP-006-E_Web.pdf Also, when buying new motors, they are often specified as ‘inverter-ready’ as mentioned above, implying that older...

So this would be a dv/dt, or perhaps LC filter, rather than a snubber? What sort of values would work? Elsewhere it’s been suggested 5mH and 0.47uF. I’ll need a meter that measures inductance, unless I can just buy suitable inductors. I don’t mind spending a bit, since they’d be protecting...

The ‘spiky’ output of variable frequency drives (VFDs) can damage the insulation of older motors. They’re often used to run three phase motors on single-phase supplies in hobby workshops. (Modern motors have upgraded insulation and are designated ‘inverter-duty’.) I wonder what cheap and...

It’s better, thanks. You can’t lift halfway, then start again to go higher; the car must be lowered before raising again, to allow the motor a more gradual start. But we did manage to lift it high enough to swap the transfer box, walking underneath comfortably. In fact, this is as high as it...