How to convert DC V + A measurements values to AC +A values?

[jjj]

My multimeter does not have AC measurement, but I need to find out how much AC current at 220VAC a 220VDC motor consumes?
I just connected the 220VDC motor onto the 220VAC we have in Chile via a rectifier bridge and a 2.5uF/400V electrolytic, but how to find out how much AC current the motor consumes?
I then did another test, by connecting the same motor via a small inverter with 12VDC input and 220VAC/75W output and it measured 1.3A/16W (at 12.3VDC inverter input).
There must be a way of calculating the approx.220VAC currant consumption from this data or is it not possible?
If not, I just have to buy a cheap AC clamp meter.
Thank you in advance for the help on that...

[g1]

A DC motor runs on DC, so you can't measure how much AC current it consumes as it is not running on AC.
You rectified and filtered the AC, so it is DC now and you would measure the DC current the motor is using.
However, you shouldn't be rectifying AC direct anyways. It should be isolated by a transformer. And 220VAC rectified won't give you 220VDC, what was the DC voltage you fed the motor?

[eschertron]

How precise do you want to be? If you assume that the rectifier bridges don't consume any power while converting from AC to DC (they do, but that's another can of worms), then AC power prior to the bridge = DC power after the bridge. If you are satisfied that the motor did the same amount of work (16W) regardless of the power supply used - based on your observations of the motor running - then you can use that number to calculate the AC current.

The power formula works for AC line voltage the same as for DC voltage, because we specify the AC line voltage as an RMS number, ie, it is the average voltage applied and is equal in quality to a DC voltage at the same level. 220VAC carries the same potential as 220VDC. So if 12.3vdc * 1.3A = 16W, then reversing the equation gives 16W/220v = 0.073A, or 73ma. Unless you had a load on the motor, that will be the NO LOAD amp reading. Under a rated load, it will be considerably higher.

edit: +1 to G-one for pointing out that rectified 220VAC will not give 220VDC. Also agree that without a transformer to isolate the rectifier from the mains, you have a potentially hazardous situation there.

[jjj]

A DC motor runs on DC, so you can't measure how much AC current it consumes as it is not running on AC.
You rectified and filtered the AC, so it is DC now and you would measure the DC current the motor is using.
However, you shouldn't be rectifying AC direct anyways. It should be isolated by a transformer. And 220VAC rectified won't give you 220VDC, what was the DC voltage you fed the motor?

Thank you for your good advice.
It's a 220VDC motor and so, it runs quite smoothly on this simple PS. I can even regulate it via a simple light dimmer. The DC to the motor is 0.09A
It will be about 75mA or so, because it also ran on the tiny 220v/75W car inverter. I'll get myself that $7 ac-clamp meter and see, because under load the current will raise. Apropos safety: I insulated all connections well so, there's no way I'll be touching something.

[jjj]

Thank you master eschertron,
It doesn't need to be that precise, because on load the motor will draw more current. I only thought to have an idea how much AC current it consumes, for if someone wants to run the motor on 110VAC, he/she will need to know which step up voltage inverter to buy. Since the motor ran happily on this lousy 75W car inverter, it can only be under 100mA or about 22W at 220V
To be more certain, l better get myself that $7.70 (shipped) AC-clamp meter (I saw on eBay) and see, because under load the current will raise. Apropos safety: I insulated all connections well so, there's no way I'll be touching something; otherwise I have to isolate all house lights via transformers.
I know... what you mean: DC is yet another "can of worms"... so, you reckon I am a survivor having managed to revamp my 200W tube amplifier??
My trick is: I always have one hand in the pocket while measuring high DC voltages.

[eschertron]

Apropos safety: I insulated all connections well so, there's no way I'll be touching something.

With the 12vdc-to-AC inverter and back to 220vdc, you should be OK. The inverter has built-in protection.

It is when you hook a rectifier network directly to the mains that things get dicey. What's a circuit breaker rated at for on 220VAC lines? 10A? If a rectifier goes bad while you're testing, the could be shorts to ground or 220vac on the DC bus, which could make for a showy display or serious burns before the mains CB trips. There are other dangers inherent in such a circuit too, which is why you can find several threads on MEF about the subject. Do a search using the keywords 'isolation' or 'death' and see what you find.

edit: simulpost. I am sure that you are taking precautions against the dangers that you are aware. Trust that I am not talking down to you.

[diydidi]

Just measure the DC current after the rectifier. It should be close enough to the AC current anyways. The bridge rectifier wont draw more current than the AC.

[J M Fahey]

My multimeter does not have AC measurement, but I need to find out how much AC current at 220VAC a 220VDC motor consumes?
I just connected the 220VDC motor onto the 220VAC we have in Chile via a rectifier bridge and a 2.5uF/400V electrolytic, but how to find out how much AC current the motor consumes?
I then did another test, by connecting the same motor via a small inverter with 12VDC input and 220VAC/75W output and it measured 1.3A/16W (at 12.3VDC inverter input).
There must be a way of calculating the approx.220VAC currant consumption from this data or is it not possible?
If not, I just have to buy a cheap AC clamp meter.
Thank you in advance for the help on that...

Hola webón culiáu
Just a demo that I can speak "Chilean"

So the motor is DC, you will rectify and power it from AC and you want to know the AC power consumption?
To, say, know what wiring, switches, etc. will you need?

1) use a bridge rectifier but not any capacitor , you want 220V RMS equivalent , not 308V peak value.
The motor rotor inertia, plus its inductance, will smooth and even out the full wave rectified (but not filtered) 220V you will get.

2) measure "DC" (actually pulsating full wave rectified AC) after the bridge, in series with the motor itself. no need for an AC clamp .
Precision is close enough for what you need and 1000 times better than indirect calculations involving inverters, estimating efficiency, etc.