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

Discussion in 'General Electronics Chat' started by jjj, Oct 13, 2014.

1. ### jjj Thread Starter Senior Member

Feb 25, 2007
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0
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 car inverter (of 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...

2. ### WBahn Moderator

Mar 31, 2012
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The first thing to note is that you need to be careful hooking up any motor designed for DC to an AC power source, and vice-versa. It may or may not work and even if it does it may not survive long. If nothing else, consider that you are subjecting the motor to voltages well in excess of 300V.

One way roughly estimate the current draw is to consider the power balance. You had 16W of power being drawn by the inverter. If this was converted to power with 100% efficiency, then you would have had 16W being consumed by the motor. Since P=VI (being sure to use RMS values), then your current is, at most,

I = 16 W / 220 V-rms = 73 mA-rms.

Now, this is assuming that the output of the inverter is reasonably sinusoidal, which it may or may not me. Also, this is assuming that the motor is only drawing real power and you also have reactive power so your actual current draw could be considerably higher, just largely out of phase with the voltage.

3. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
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I think your post is against forum rules.

You are supposed to have transformer between mains (wall outlet) and the device, it acts as isolator. According to safe practices of the forum you are supposed to have: mains->transformer->rectifier->device. Since you don't have transformer, your ciruit is considered unsafe and violates forum safety practices. Expect your post to be locked by moderators.

4. ### WBahn Moderator

Mar 31, 2012
18,087
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I don't know that it does, at least not for that reason. The basic reason, as I understand it, is for relatively low voltage circuits so that they can't get nailed by the full AC voltage if they get shorted to the supply. But in this case the load is running at basically the same voltage as the mains, so the transformer involved would be pretty close to a 1:1 anyway.

Also, I see that I didn't realize that he is rectifying the AC power to run his DC motor, so some of what I said regarding feasibility and safety doesn't apply. But the output of the rectifier is still going to be in the 300+ V range.

Also, I don't think a 2.5uF cap is going to do much of anything. If it is charged to 310V at the peak, then at a 100mA DC current draw it would droop 200V just by the time the input voltage crossed zero (at 50Hz). It would seem that you are going to need at least 20x the capacitance to achieve something on the order of 10% voltage regulation at whatever load you were using in your test. You probably actually need quite a bit more, in practice.

5. ### wayneh Expert

Sep 9, 2010
12,382
3,237
Your meter can measure RMS voltage across a shunt resistor in series with the motor. Done.

Correction - put the shunt resistor in series with the bridge, on the AC side. The motor is on the DC side.

And the warning above are all true. Rectified AC voltage is much higher than the nominal AC voltage, and will shorten the life of the motor or even destroy it suddenly.

You'd need a MUCH larger capacitor to accomplish any useful filtering.

Last edited: Oct 13, 2014
6. ### jjj Thread Starter Senior Member

Feb 25, 2007
111
0
Thank you all for you good advice.
I ran the 220VDC via a bridge rectifier and 2.5uF/400V Elco to the motor and the motor runs smoothly w/o a problem. I even can control its speed via a 500W lamp dimmer. I also insulated all connections well so there's no way I can touch any DC. Of course the current will rise on motor load. Best will be I buy myself that cheap \$7.70 AC clamp meter, I saw on eBay...
Incidentally, I run another 220VDC geared motor from a Chi-machine under load and for years w/o a problem. Here's Pic of it: https://app.box.com/s/nv0zftm0vhbx5dbwfpx9
So, you reckon I should swap the 2.5uF for a 100uF/400V for better filtering?

Last edited: Oct 13, 2014
7. ### WBahn Moderator

Mar 31, 2012
18,087
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If you were to look at the voltage waveform going to the motor you would probably find that it is full-wave rectified AC with full peak voltage at over 300V. The motor may well run what seems to be smoothly since it's getting slammed with power 100 times a second but the mechanical inertia is filtering that out, but it is probably being pretty badly abused and may not last very long.

8. ### jjj Thread Starter Senior Member

Feb 25, 2007
111
0
So, you reckon I should swap the 2.5uF for a 100uF/400V for better filtering?

9. ### WBahn Moderator

Mar 31, 2012
18,087
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Yes, or even 220uF.

10. ### jjj Thread Starter Senior Member

Feb 25, 2007
111
0
Yes, I can imagine what it "might do" to the motor. This motors came from a Chinese leg massager and a Chi-machine were originally connected to a speed regulator boards, which I disconnected to save space. The massager motor's control board has only line filtering, but no 400V electrolytic and the Chi-machine motor has only a Triac (for speed Ctrl) and a 22uF/400V Elco and somehow these motors can take it in strive, without suffering. To my hearing the motors run very quietly and smoothly.
To increase its speed, I even pumped the AC voltage via Variac to 290V, which of course drove the DC voltage to almost 400VDC, but the motors took it without ado and just ran a bit faster. So it seems, these motors are tailor made to simplify its applications?

11. ### WBahn Moderator

Mar 31, 2012
18,087
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You can certainly try it and see how it goes. The particular motors you are using might be able to take it, in which case you are good. It could be argued the doing it "right" by using proper filtering is the proper way to do it and, if the cost, space, and weight aren't prohibitive, that's the way you should do it, but on the other hand, if it's good enough, then it's good enough.

12. ### wayneh Expert

Sep 9, 2010
12,382
3,237
The value needed depends on the current draw of the motor, which we don't know (but might be less than 100mA?). Compare the time constant RC of the capacitance C and the resistance R of the motor, with the peak-peak time for 120Hz. If RC is not roughly equal or greater than 10ms, it won't make much difference in smoothing the ripple and will only contribute to the peak rectifier current.

If we assume the 71mA value, the R of the motor is V/I=R = 220/0.071=3,100Ω
The time constant for a 220µF capacitor is 220 x 10E-6 µF * 3,100Ω = 0.68 seconds Plenty!

For a 2.5µF cap: 2.5 x 10E-6 µF * 3,100Ω = 7.8 ms Not enough!

The NEED for any filtering is a separate discussion.

13. ### jjj Thread Starter Senior Member

Feb 25, 2007
111
0
Yes, I too think a bigger Elco is the right way to go, for it only benefits the motor and so, will last longer than the China engineers like it to last...
So, I thank you all for sharing your wisdom. I will help another person in need and this will make our world a better place than Islamists aim to achieve.

14. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
While I agree with your last sentiment, it's best to keep politics off the boards, especially the non-Off Topic forums. Just omitting the last five words would have been perfect. Good luck.

15. ### jjj Thread Starter Senior Member

Feb 25, 2007
111
0
Pardon me! The problem is, I am so much into philosophical pondering that I too easily and unwittingly equivalate one reality with another.
I have written several books, titled: "A Guide to... Personal Contentment" and "Grandpa's Insight"; that's why! Good luck to us all!
Besides, I also wrote about the "China engineer's obsolescence timer mentality", which might explain their 'weird electronic circuit designs'...s and I hope I did not offend anyone with this (sadly) factual observation.

16. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Pondering about planned obsolescence and it's causes and culprits isn't a problem -- it is closely enough related to relevant topics that there is pretty wide latitude given. It really was just the last few words that could raise the hackles of the mods, whether they agree or not. Don't worry, you're fine.

17. ### jjj Thread Starter Senior Member

Feb 25, 2007
111
0
Disaster...
I inserted a 220uF/400V Elco and three components blew up: the increased power burned the switch, the bridge rectifier and the 500W/220V lamp dimmer !!
I then replaced the three components and connected a 47uF/400V and now I'm back to where I was before the good advice. That must have bene the reason that the Chinese electronic technicians didn't insert a bigger sized Elco for this Chi-machine motor!
From what I learnt is: that the amount of uF stands in relation to the power level. So, now with the 47uF the motor got just the right amount of juice and runs very smoothly and silently, as it did so that last 3 years.
I only wanted to know how to measure its AC current of another 220VDC motor. Yet, since this is pretty complicated, I buy myself this cheap AC current clamp meter. End of the problem!

Last edited: Oct 17, 2014