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How to properly measure solenoid coil current.

PowerHydro

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Feb 7, 2024
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Poland
Hi everyone.
Question as in the topic.

For example, I have a hydraulic pump equipped with a proportional solenoid valve that controls pressure.

The solenoid coil is powered by PWM 24 V with a frequency of 200 Hz.
I set PWM to 50% and measure the solenoid coil current.
When I measure with a true RMS multimeter, I have an indication of 0.86 A on the DC range.
When I measure on the AC+DC range, the reading is 1.38 A.

Which measurement is correct?
 

PowerHydro

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Feb 7, 2024
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2
Location
Poland
This is a 24V DC coil.
Of course, it is measured in series, but the question is whether we measure the average value (I DC range) or the effective value (better multimeters with the AC+DC function)?
How do you measure?
 

wlhequipment

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Sep 3, 2017
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Sheridan, CO
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Mechanic
I'm confused. You're powering a coil (DC) with a PWM source (AC), and you want to know whether to use a peak reading or RMS? I think that would depend on what you're trying to accomplish. Pretty old memory here, but RMS is calculated by taking the p-p reading (AC) and dividing by the square root of 2 iirc, to give you the DC component that creates the same power or heat as the AC component. So RMS gives you a DC value while p-p gives you an AC value. You might have better luck poking around an electrical engineering forum.
 

wlhequipment

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Now you got me wondering about this. Would an AC solenoid valve even work? I think so. The iron in the valve is responding to the magnetic field generated by the coil. It doesn't care about the polarity of that field, it's just moving because the field is there (or not). So, an AC coil could do the job, I think. The problem would come from the inefficiency in the circuit because it's constantly building and collapsing the field. The AC circuit would have to work much harder to generate the same effect as a similar DC circuit, but I think it's possible. I'm getting pretty far out of my comfort zone here, but it was a fun trip :)
 

1693TA

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FAA Radar Engineer, (Retired)
Now you got me wondering about this. Would an AC solenoid valve even work? I think so. The iron in the valve is responding to the magnetic field generated by the coil. It doesn't care about the polarity of that field, it's just moving because the field is there (or not). So, an AC coil could do the job, I think. The problem would come from the inefficiency in the circuit because it's constantly building and collapsing the field. The AC circuit would have to work much harder to generate the same effect as a similar DC circuit, but I think it's possible. I'm getting pretty far out of my comfort zone here, but it was a fun trip :)
You are correct in a simple ac coil will work on dc current and vice versa. The coil itself is not polarity sensitive. He is referencing proportional control of a mechanical operator. Usually this starts off as an AC voltage at relatively high frequency and is either voltage, or frequency shifted to move a slug or valve.

As far as the resistance value he is looking for, that is a design feature and dependent upon amount of force required to operate the mechanism. The 200hz carrier is quite common but the ounce inches in torque required to operate a valve or such varies by design. The coil and magnetic field produced has to be sufficient to act upon the operand.
 

wlhequipment

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You are correct in a simple ac coil will work on dc current and vice versa. The coil itself is not polarity sensitive. He is referencing proportional control of a mechanical operator. Usually this starts off as an AC voltage at relatively high frequency and is either voltage, or frequency shifted to move a slug or valve.

As far as the resistance value he is looking for, that is a design feature and dependent upon amount of force required to operate the mechanism. The 200hz carrier is quite common but the ounce inches in torque required to operate a valve or such varies by design. The coil and magnetic field produced has to be sufficient to act upon the operand.
Oh! I totally glazed over the proportional thing. That used to be called a synchronous setup right?. Not to oversimplify, but a guy would turn a dial 10degrees and a valve or whatever would move 10 degrees. Something like that? Weren't they usually multi-phase input devices, delta wound and the actuator was Y wound? Man I'm really digging back into some dusty memory. You're obviously more qualified to answer the OPs question, if he comes back, so I'll see myself out. Thanks!
 

Graham Hamilton

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Aug 18, 2019
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32
Location
Scotland
Just to say 24V PWM with 200Hz frequency is a DC 24V supply were the voltage is switched On and Off at a rate of 200Hz. 100% PWM duty cycle is 100% On, and 0% duty cycle is 100% Off. So when the coil sees 24V it will be at full current but pulsing On and Off dependent on the PWM duty cycle. An oscilloscope would show this On/Off pattern. (I guess there are magnetic fields building and collapsing effects on this but small)
I've built a few PWM circuits and whenever I've been checking the output I just use a Volt meter which will give the "average" voltage for the PWM. So in this case at 50% PWM duty cycle I would expect to see 12V and 50% of the maximum current draw for the solenoid.

Hope this is of some help.
 

63 caveman

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Nov 11, 2017
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346
Location
western Pa.
My memory is not what it used to be but if I remember correctly!

I was working on a bobcat joystick actuator that was acting flakey. One of my multimeters (kline I think) on the Hz scale toggled to PWM %. With it back probed into the actuator I could see the erratic behavior of the signal.

Not sure if this helps out on the main topic of this thread but may help someone reading it. FWIW the meter was less than $100 usd.
 
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