# Control Transformer Tolerance



## GEORGE D (Apr 2, 2009)

My customer has a machine that's been running for years on 208 volts, long story short it has issues now and was wanting thoughts. The CT primary is rated 220 and has both 24 and 12 volt output. My supply is right at 200 and output is 18v and 10v. The HMI is where the problem seems to derive from and was wondering if the low output is causing problem. I know generally electric motors and equipment have that +/- 10% tolerance but seems too low to me for the lowvoltage end (18 instead of 24). Thoughts?


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## HARRY304E (Sep 15, 2010)

GEORGE D said:


> My customer has a machine that's been running for years on 208 volts, long story short it has issues now and was wanting thoughts. The CT primary is rated 220 and has both 24 and 12 volt output. My supply is right at 200 and output is 18v and 10v. The HMI is where the problem seems to derive from and was wondering if the low output is causing problem. I know generally electric motors and equipment have that +/- 10% tolerance but seems too low to me for the lowvoltage end (18 instead of 24). Thoughts?


George,,,What does HMI mean?


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## SteveBayshore (Apr 7, 2013)

Human-Machine-Interface, fancy term for touch screen or control panel.


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## HARRY304E (Sep 15, 2010)

SteveBayshore said:


> Human-Machine-Interface, fancy term for touch screen or control panel.


Thanks Bro..:thumbsup:


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## Big John (May 23, 2010)

So the voltage has been the same all this time but the HMI only started acting up recently? What is it doing?


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## KDC (Oct 19, 2012)

It depends how the HMI (or more likely the PLC that's in there somewhere) is programmed to deal with a low voltage situation. 

Double check the ratio on the transformer (Probably a PT rather than a CT if you're looking for a constant voltage) and compare it to your measurements. 

If it's bad, replace. If it's good, but the HMI is reporting it differently, then it's time to pull prints.


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## GEORGE D (Apr 2, 2009)

Big John said:


> So the voltage has been the same all this time but the HMI only started acting up recently? What is it doing?


Well that's what I'm assuming. For all I know poco may have changed something and brought down the voltage just beyond threshold, and somehow this is the only machine affected. The HMI will not fire up which is the stopping point/tracing of the Japanese or Chinese schematic. That's when I noticed low voltage on HMI. Tracked down transformer, removed all loads and checked voltage which still was low on both 12 and 24 taps, thinking maybe something draining it down.


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## JRaef (Mar 23, 2009)

The voltage tolerance is likely higher than 10% on stuff like that, but the intent of running into a tolerance is for it to survive short term swings in the line voltage, not really for continuous operation at a low (or high) voltage. So what happens is, the internal SMPS (Switch Mode Power Supply) that converts the 24VAC into probably 15VDC, will now have to pull MORE CURRENT at 18V than it did at 24V. More current = more heat and years later, components start to fail because *heat x time = failure*. Most likely it lasted beyond the warranty so it satisfied the OEMs requirement, but it's the end user who is now suffering. If the HMI is still current, you may be able to get a replacement PSU (Power Supply Unit) if it is not integral to the control board, but most likely it is and your HMI is now toast. You can consider sending it out for repair if you have a spare or don't need the machine for a month or so, or just upgrade the HMI to a current version.

But no matter what, I would get a correct transformer and get a lot closer to the 24V input level. That low voltage condition is likely affecting other equipment in the same way.


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## Zog (Apr 15, 2009)

GEORGE D said:


> My customer has a machine that's been running for years on 208 volts, long story short it has issues now and was wanting thoughts. The CT primary is rated 220 and has both 24 and 12 volt output. My supply is right at 200 and output is 18v and 10v. The HMI is where the problem seems to derive from and was wondering if the low output is causing problem. I know generally electric motors and equipment have that +/- 10% tolerance but seems too low to me for the lowvoltage end (18 instead of 24). Thoughts?


I ssume you mean CPT, not CT. Either way, different classes will have different tollerance bands.


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## wdestar (Jul 19, 2008)

I'm not sure if I read all this right - I briefed it. But if you're monitoring the CT voltage you have to program a time frame. 60 ms would appear a norm.


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## wdestar (Jul 19, 2008)

wdestar said:


> I'm not sure if I read all this right - I briefed it. But if you're monitoring the CT voltage you have to program a time frame. 60 ms would appear a norm.


By the way - why would you want to monitor the voltage?


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## Boostinsteve (Apr 25, 2013)

Most things I deal with have some tolerance. That said, 24 volt circuit, can only drop down to about 19 volts before you start running into problems. If this is transformed and rectified, why not look into a dc-dc converter like a victor to keep the voltage where it needs to be for the component.


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## just the cowboy (Sep 4, 2013)

Try an external 240/120vac - 24dc


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