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how to wire an ammeter to 7.5 hp ac motor

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ww

05-02-2008 05:57:26




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ammeter has two termnals do I split one 220 leg and wire it there or what?




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36 coupe

05-03-2008 03:48:53




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to ww, 05-02-2008 05:57:26  
You need a shunt or current trans former to use a high current ammeter.If you look on the meter face you wiil see 50 millivolts on the bottom of the scale.If you try to use with out a shunt the meter is toast.A clamp type meter will cost less than a shunt or current transformer.



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dr sportster

05-02-2008 15:01:45




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to ww, 05-02-2008 05:57:26  
What you need is an ammeter called[brand name]Amprobe.Now Greenlee makes one not called that because not their brand.Ask supply house for the 40 dollar Greenlee brand Amprobe actually just called Greenlee Ammeter.Clamps around wire for readings as the more expensive Amprobe brand does.The one you have may cause burns etc if you not sure what its for.Just so you know present OSHA regs call for an arc protective suit to be worn during this testing[nobody does it but there is a reason why---guys got burned up badly]Be careful good luck.

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ww

05-02-2008 11:33:50




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to ww, 05-02-2008 05:57:26  
it is a meter from a factory they tore down. so what do i need to buy?



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Red Dave

05-02-2008 12:02:16




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to ww, 05-02-2008 11:33:50  
You would need to get a CT that meets the primary and secondary currents you are working with, then you would have to re-scale the meter.

Is it a D'Arsenval movment? If so, when you "pinned the needle" you may have damaged it.

Look for small print close to the bottom of the face. Sometimes the meter movement full scale amps is written there. You will need that to determine the correct CT ratio. If not, you will have determine it experimentally. Then you will have to pick the proper CT for the expected primary amps and the secondary current that the meter movement needs.

McMaster-Carr used to sell CT's.

This whole thing may be more trouble than you wanted to get into. Unless you need continuous monitoring, I suggest a good clamp-on amp probe.

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ww

05-02-2008 15:50:14




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to Red Dave, 05-02-2008 12:02:16  
I need to load my hammer mill. does that help?



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ww

05-03-2008 02:40:29




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to ww, 05-02-2008 15:50:14  
I ment to say how much to load my hammer mill



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buickanddeere

05-02-2008 09:07:14




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to ww, 05-02-2008 05:57:26  
Hold on, you are scaring everybody.There is a reason why professional electricians are hired.

Any standard panel AC meter CT type that reads current is rated for 0 to 5 amps. Only the scale on the face is changed and the proper matching current transformer used. b.t.w. an open circuited current transformer can be a fast and messy way to die. We are not talking ordinary everyday wiring techniques let alone some "home fixes". That 7.5HP single phase 240V motor will have approx 40.0 amps of running current requiring at least wires rated for 50.0amps which is #6/2 copper. Short circuit protection provided by a maximum of either P rated fuses of 125amps, D fuses of 75 amps or a 100amp breaker. Overload protection must be a thermal heater set at 50.0amps. To run your ammeter at mid scale at full load. An 80 amp current transformer of 80:5 or having a ratio of 16 to 1 is required. To get the meter to read 40amps when it's really indicating 400amps. It will require a 120amp CT. All AC panel meters using potential transformers run at 120V full scale. So a panel meter with a full scale of 1000V requires PT of 1000:120 which has a 8.333 to 1 ratio. A shunt type ammeter which you definitely don't have meter an internal resistor and a volt meter marked in amps.

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John T

05-02-2008 08:46:03




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to ww, 05-02-2008 05:57:26  
ww, okay heres the deal and its pretty much what Red is tellin ya. That ammeter IS NOT letting the full 1200 amps pass thru it sooooo oo it has to still accurately measure current by one of a couple ways. 1) The actual current passes through a resistor in paralell (shunt) with the meter or 2) Theres a torroidal coil current transformer CT (sometimes called a donut) that encircles the wire and the ammeter reads the induced voltage or current and converts that into amps as read on the meter. Either way requires the correct sized matching CT or shunt resistor to match the ammeters range n I cant tell ya what that is HOWEVER the main thing to know is you gotta have a shunt resistor or a CT that matches up, buttttt ttt didnt such come with the meter???

John T

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IH2444

05-02-2008 06:12:17




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to ww, 05-02-2008 05:57:26  
Is the ampmeter designed for AC ?



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IH2444

05-02-2008 06:14:23




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to IH2444, 05-02-2008 06:12:17  
If it is a DC ampmeter it will just vibrate itself to death. And Most ampmeters for over 10 amps or so require an extrernal shunt.



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buickanddeere

05-02-2008 06:06:30




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to ww, 05-02-2008 05:57:26  
Put the ammeter in series with either Line 1 or Line 2. If you put the ammeter across Line 1 and Line 2 as if it were a volt meter. You will hear a loud bang and see/smell smoke. May as well get a volt meter and install at the same time. it's really eye opening to see voltages drift with load and time of day. Assuming here you have a ammeter with a built in shunt. And not one made to work with a current transformer?

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ww

05-02-2008 07:06:17




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to buickanddeere, 05-02-2008 06:06:30  
it is an ac 1200amp ammeter. i have it wired into leg 1. it pins the needle. what is a shunt? is that what they call a donnut? is there different values?



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Red Dave

05-02-2008 07:45:28




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to ww, 05-02-2008 07:06:17  
It sounds to me like what you have is an indicating instrument that indicates 0 to 1200 AC amps, but it is not a complete AC ammeter. If it is what it sounds like, you need Current Transformer (CT) to feed it. A CT is sometimes called a donut. The CT ratio would be determined by the actual range of the indicating instrument, which may be only a few amps. It might be 1200 amps primary on the CT, but the secondary that feeds the indicator may only be 1 or 2 amps.

If that is so, it will not do what you want it to without more equipment.

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IH2444

05-02-2008 08:19:05




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 Re: how to wire an ammeter to 7.5 hp ac motor in reply to Red Dave, 05-02-2008 07:45:28  
Yeah what Red said.



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