# Current transformers



## bigcat61 (Feb 4, 2010)

I want to use an analog meter to monitor motor amperage. It will be rated at 150% of the full load amperage. If the motor pulls 64 amps at full load, what ratio current transformer do I need? I'm a bit confused at how to alter the windings to achieve the desired input / output.


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## MDShunk (Jan 7, 2007)

The ratio of CT you use depends on the meter you propose to you. For instance, a 100:1 CT will show one amp on the CT terminals when you have 100 amps flowing on the conductor the CT is around. 

Let's talk about instrumentation.... If you're not using a Simpson Hawk meter, you're screwing up. Okay, that's my personal opinion, but I find the Simpson Hawk meters to be affordable, reliable, easy to program, featureful, and compact. 

Depending on the application, I mostly use a current shunt, rather than a CT, to read current. I just find they're more reliable, and allows you to change the meter without having to remember to short the shunt. You just read the current shunt with a millivolt meter.


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## John Valdes (May 17, 2007)

I did not know you needed to program an analog amp meter? We are discussing analog? right?


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## MDShunk (Jan 7, 2007)

John Valdes said:


> I did not know you needed to program an analog amp meter? We are discussing analog? right?


The OP is. I started talking about the Simpson Hawk meters. They're digital. I love 'em.


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## micromind (Aug 11, 2007)

In a nutshell, almost all CTs are rated XXX:5. The first number is the primary, that's the hole in the middle. The second number is the secondary. that connects to the meter. 

For example,the output of a 100:5 CT will be 5 amps with 100 amps on the primary. It'll be 2.5 amps if there's 50 amps on the primary.

Analog meters designed to be used with CTs will read full-scale at 5 amps. If you want to read percentage of load, get a meter with a 0-150 amp scale. Amps = %. If the CT is 50:5, the meter will read 150 amps (150%) while 50 amps is actually flowing. 

CTs come in 50:5, 75:5, 100:5, and so on. They don't come in 64:5. So you have to trick the CT into thinking there's a different current flowing than what actually is. 

You do this by running the secondary through the hole in the middle, along with the primary. This takes some thought, and polarity matters here. 

The other tricky part here is that 150% load = 5 amps on the secondary, then 100% load will be 3.33 amps. 

Using your example of 64 amps, I would recommend a 75:5 CT. The problem here is that it'd show 100% load with 50 amps. So we need to trick it into thinking there's more current flowing than there actually is. Since the secondary is flowing 3.33 amps at 50 amps, if we run the secondary wire through the hole 4 times, 50 amps will read as 63.2 amps. 5 times will be 66.5 amps. This isn't exact, but it's close enough for an analog meter. 

Brian or Zog (maybe a few others as well) could tell you the proper polarity by using the dot on the primary and black and white on the secondary, I'd have to draw it out, or more likely guess and see what happens. If the polarity is wrong, the meter will reading will be WAY off. 

One thing to remember about CTs, NEVER open the secondary circuit while current is flowing in the primary. Not even for an instant. The voltage across the black and white secondary wires will be VERY high, possibly thousands of volts, and the CT will very likely be destroyed. Not to mention the person who took the wires loose could very easily be destroyed too!

Rob

P.S. My math could easily be off a bit, if so, hopefully someone will correct it. It's been a while since I've done one of these.


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