Measuring 'true' AC current through two parralleled servo valve motors. A mystery....

Thread Starter

Dynaman

Joined Jan 17, 2008
60
Greetings AAC'ers. I have a mystery on my hands. I have a 24VAC 3A transformer with a high quality 2AMP circuit breaker on the secondary. I also have two 24VAC Jandy/intermatic type servo motors (3 way valves used in swimming pools). When the valves are transitioning about half way, the circuit breaker trips. They are rated at 750ma ea.

So I placed an ammeter in between the transformer secondary and the two paralleled valves, and measured 1.26AMPS using A/C TRUE RMS ammeter when the breaker tripped. Huh??? Perhaps a bad breaker you say???

So then I removed the the two servos and substituted a 14 ohm resistive load. Low and behold the current held forever at 1.8amps. It never tripped.

My question is, why would the servo motors trip the breaker at a much less current? My hunch is the phase of the motors might be interacting with each other perhaps driving up the real current? Any ideas? If so, best way to measure the actual current draw?

Any help appreciated.

Aram
 

panic mode

Joined Oct 10, 2011
1,782
TrueRMS is ... well ... 'relative'. What meter you have there? There is limit to everything and some meters will show reasonable value up to 1kHz, some up to 4kHz etc. Most servos use carrier frequency that is 8-16kHz so this can be a problem.
I am guessing that you are using fast acting over-current protection? Inductive loads are normally protected by devices with time-dalay.
 
Inductive loads draw their restive counterpart briefly. The coils have a DC resistance. That amount of load is drawn momentarily, so fuses and circuit breakers for such loads need to have high trip times or be called "time delay" like time delay fuses.

There is a little EE law that says, "The current in an inductor cannot change instantaneously" and that's what's hitting you.
 

Thread Starter

Dynaman

Joined Jan 17, 2008
60
Gentleman, thank you.. @ Panic mode-- The meter is a fluke 289. Since we are talking 24VAC @ 60 hz. I don't see much issue. I am not clear on the servo carrier frequencies you are speaking of. These are not stepper motors. Just standard A/C motors. Please clarify.

@kiss-- I would say the trip time is high on the breaker, because it runs at least 20 seconds before tripping. A full cycle is about 35 seconds.

Best way to measure true current draw?
 

BobaMosfet

Joined Jul 1, 2009
788
Gentleman, thank you.. @ Panic mode-- The meter is a fluke 289. Since we are talking 24VAC @ 60 hz. I don't see much issue. I am not clear on the servo carrier frequencies you are speaking of. These are not stepper motors. Just standard A/C motors. Please clarify.

@kiss-- I would say the trip time is high on the breaker, because it runs at least 20 seconds before tripping. A full cycle is about 35 seconds.

Best way to measure true current draw?
Just something to consider. Why don't you learn a little bit about your meter to see just how much effort Fluke put into getting it right for you?

https://www.fluke.com/en-us/learn/blog/electrical/what-is-true-rms

As with all things in AC, sometimes you have to take 'time' out of the equation and that's what RMS is all about for voltage and current.
 
A 2 A breaker and a 1.5A inductive load isn;t going to cut it.

Remember too that DVM ammeters add a series resistor to the circuit, The voltage burden is usually < than 0.6 V.

You need to do a 4-terminal resistance measurement of the motor coils. You measure the voltage across and the current in the circuit. Say use a 1.5 V battery and some resistor. Guess assume the 24 V current is about 5x the 750 mA to get the approximate R. Size the measuring resistor accordingly. Breaker should be a little larger than this calculated I with the measured R.
 
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