[kirk_wallace]

Taking AC readings on a DC supply gives you an indication of the AC component of the DC. If a diode in the rectifier section of your power supply is shorted or open, it will show up in your AC reading. Taking AC or DC readings cost nothing and if the readings don't match the expected readings, then it can indicate a problem. Taking a DC reading on an AC signal can indicate grounding or isolation problems.

Motor power supplies are generally unregulated, so it should be expected to have a significant amount of AC on the DC output. If you get a lot of AC or none, then there is a problem somewhere. In this case the AC reading is unstable, which isn't much help. It may help to turn the auto-ranging off and try again, otherwise skip it.

[SCzEngrgGroup]

Taking AC readings on a DC supply gives you an indication of the AC component of the DC. If a diode in the rectifier section of your power supply is shorted or open, it will show up in your AC reading. Taking AC or DC readings cost nothing and if the readings don't match the expected readings, then it can indicate a problem. Taking a DC reading on an AC signal can indicate grounding or isolation problems.

Motor power supplies are generally unregulated, so it should be expected to have a significant amount of AC on the DC output. If you get a lot of AC or none, then there is a problem somewhere. In this case the AC reading is unstable, which isn't much help. It may help to turn the auto-ranging off and try again, otherwise skip it.

No, with a DVM, if you have anything but a stable ~60Hz AC component, you're reading garbage, which will even be significantly different using different DVMs, as the circuitry in the meter is incapable of giving a meaningful reading when the input is not stable for the entire sampling period of it's A/D converter, which is typically no more than a few hundred Hz, if that. An analog meter will give you something of limited value, if you know how to read it, because the analog movement will essentially filter the reading, giving you a good average. A digital meter will just give you garbage, with no correlation to what's really happening.

Regards,
Ray L.