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Suitable Ammeter question

Bob-J-H

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Joined
Nov 14, 2006
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Camarillo Ca
I need to get a volt/ammeter to monitor the performance of a large electric motor. I see them on ebay for up to 100 amps, which is within the range of the motor, but the inrush is much higher than 100 amps. My question is, what will that do to the meter? This is digital by the way. Anyone with any experience with this? I hate wasting my money. Bob
 
I need to get a volt/ammeter to monitor the performance of a large electric motor. I see them on ebay for up to 100 amps, which is within the range of the motor, but the inrush is much higher than 100 amps. My question is, what will that do to the meter? This is digital by the way. Anyone with any experience with this? I hate wasting my money. Bob
No, don't worry about inrush, unless you want to be able to read it. Inrush happens so fast that even if the current transformer saturates, it recovers quickly.

So that brings up another issue. Most analog ammeters work on 1A or 5A and use what is called a "current transformer" (CT) which you buy separately as a ratio of xxx:1 or xxx:5 respectively. Then you select that ratio so that your expected load falls roughly in the middle. In your case if you want to look at an expected load of 100A, you get a 200:5 CT and a 5A meter input, that way 100A falls right in the middle of the range so you can see either side of it well. If you bought a 100:5 CT ratio, at 100A the meter is pegged all the time, you have no indication of it going higher. But on digital meters you must follow whatever the meter says, sometimes it is pre-set. So just be aware that there may be another component you need here. Download the instruction manual ahead of time to see.
 
More specifically what are you attempting to monitor for performance? Do you just want to see what your run current is during its run? Like said above the meter will just go over limit for that short time during start up but will be fine.

The online motor test rig we use was almost $20k but we're monitoring every aspect imaginable. If you just want to see what your run current is, grab a amp clamp for your digital multi meter or get an amp clamp meter that will do a min Max and average. The start current are so fast that an oscilloscope, power quality analyzer, online motor test rig is what is needed to accurately catch it.

Is there a problem with the motor that your trying to troubleshoot?
 
If you do go that meter/ct route, there is one important characteristic that you should be aware of. In order to protect you and the ct, the secondary is SHORTED unlike a voltage transformer. So you could go the meter/ct route with a shorting switch for startup, then open the switch.

Tom
 
Thanks for the replies guys. What I am trying to do is monitor the load on a machine so that full load amps. are not exceeded. Such as on a wide belt sander. Bob
 
A current shunt would probably be the most cost effective solution if you want to install it permanently. These have 4 terminals, 2 heavy ones for current and 2 smaller ones that connect to a meter (millivolt range).

You would probably want something like a 500A/50mV one. 100 amperes would read as 10 millivolts. The most convenient way to use it if you do not want to dedicate a meter to it full time is to mount it in an enclosure with tip jacks to plug the standard meter probes into. One precaution is that although the meter is reading only millivolts the leads (and meter internals) are at the full voltage above earth.
 
Why go through all this when there commercial products sitting on the shelf designed for just this application? They are called motor starters with Overload Relays.

Tom
 
What he wants is a load meter. I have these on both lathes and the CNC mill I regularly use. They are very useful for finding the maximum stock removal without overloading the motor. He just wants to buy a digital ammeter with matching current transformer. You should be able to get the overload tolerance from the manufacturer.

Shunts do not always give the correct reading on AC. It appears to be a magnetostrictive effect. I have been looking for information for over 30 years and have not been able to find a single reference to it. Sometimes I think I am the only person in the world who knows about it. I ran into it when a manufacturer was trying to get acceptance tests on current transformers for ground fault detectors for the C 130 transport. We finally used a piece of non magnetic stainless steel for the shunt in a bucket of water, constantly feeding ice cubes to keep a constant temperature.
 
never run into an effect I thought was magnetic in any way. My understanding is that only ferromagnetic materials show magnetostrictive effects.

Manganin contains copper, manganese, and a small amount of nickel. Nickel is directly ferromagnetic and magnetostrictive. Some manganese alloys can be ferromagnetic, and presumably magnetostrictive, I don't know if it would be in manganin. The copper isn't.

With a high current, there will surely be a magnetic field, and there will also be some forces on the material due to expansion with warming, that will follow the AC. So it makes sense that there might be an effect. I would have thought it was small, very small, or there would be more comment on it.

I have run into issues with meter response because the shunt is wide-band, and the meter is not. In case of ANY significant harmonic content, particularly in 400Hz aircraft power, many meters will read wrong. Usually, but not exclusively, they read low.
 
Hook a low voltage transformer capable of supplying a few amps through a current transformer and an ohm or two Nichrome resistor. Load the CT with a metal film resistor and look at it with an oscilloscope. At the zero crossing you will see a kink and the leading ramp of the waveform will be concave, lower voltage than it should be because the resistor has a greater than rated resistance during the rise. This is not inductive. Most meter shunts I tested showed the same distortion. The first "resistor" I tested that did not have the problem was a piece of aircraft safety wire. I tried a piece of mild steel welding rod. It had the same effect until I got it glowing red and rediscovered the Curie effect. When I reduced the current and let it cool down, the effect was severe because I had annealed it. I wish you would confirm this. 30 plus years of people smiling patronizingly and laughing when I left is enough.

Bill
 
Why the CT?

I would have thought that a direct look at the voltage would be best, no possible effects of the CT iron. But maybe the gain is not enough?

I can check easily up to about 30A AC.
 
Why the CT?

I would have thought that a direct look at the voltage would be best, no possible effects of the CT iron. But maybe the gain is not enough?

I can check easily up to about 30A AC.

You have to look at the actual current. the voltage is what it is and follows the standard sine wave. The current is reduced by the magnetostriction push back, which is only a few volts at best. I don't think it shows as much on a higher voltage circuit. You can see a similar effect with a magnetostrictive transducer. I have some sonic dental scaler transducers around. I may try them. You might be able to see the effect on a 304 stainless shunt in series with the Nichrome resistor. It is easy to set up a situation where the effects cancel and don't seem to be there.

The original problem was that the distortion was being read as phase shift, which failed the CT under test. The apparent phase shift was in the load resistor, not the CT.

If you want, I will set this up for you and demonstrate it.

Bill
 
I meant the voltage on the resistor under test..... probably with another resistance of a different type in series....If the current changes, the effective resistance would have to change in an exactly complementary way in order to keep a nice sine of voltage coming from the resistor. if the current changes BECAUSE OF the resistance change, that's different, and I think that's what you are saying above.... driving the shunt with a voltage source.

With a different type resistor in series, it is no longer a voltage source.... the effect should show up. And if the other resistor does not have the same effect in it, the chances of cancellation are reduced.

From what you described originally, you seemed to be saying that the shunt voltage did not follow the current correctly... That would seem to indicate a change in effective resistance.

There can also be funny effects from transformers at low currents...... as at "crossover" of the sine wave.

If you can demo it, I'd for sure look at it.... Things like that interest me.
 
The effect is that there is some sort of inertial quality during the rise in voltage that means it draws less than the calculated current for the instantaneous voltage. It is not inductive. If it was, the inductance would be inhibiting the current as it rose from the negative peak. It only starts at zero crossing, is maximum at the beginning and fades as the voltage reaches the peak. A high hysteresis magnetic material might do it. It is definitely magnetic, as witnessed by the disappearance when the resistor reaches the Curie point. Hysteresis would imply that it would be less after the iron is annealed, but the result is the opposite.

"From what you described originally, you seemed to be saying that the shunt voltage did not follow the current correctly... That would seem to indicate a change in effective resistance."

Actually it implies that the current does not follow the voltage. If you look at the voltage across the resistor, it is a sine wave. That is why you need a CT, which shows the current without these effects.

Bill
 
We have a meter outside from yardcsale that has inputs for boltage snd current transformers for doing what you need in 3 phase.

It looks expensive snd is a later day project.

It uses simple coils that tge power wire to be measured passes through.

Consider a clamp on ammeter that is not a clamp on but just a coil of wire.

Non touching of the wire as it just picks up the mag field.

Voltage is easy.

There should be plenty of devices on the market for doing this already...just need to figure out how you are going to interface to them and use the data then budget.
 
Actually it implies that the current does not follow the voltage. If you look at the voltage across the resistor, it is a sine wave. That is why you need a CT, which shows the current without these effects.

Bill

If you have a 'current shunt" and a current that is set by some outside conditions, which is the normal use of a current shunt (measuring current without significantly affecting the circuit), then the actual measurement is of the voltage on the shunt.

The voltage is a result of the current, and "by definition" the shunt resistance should be chosen to be so small as to not affect the current. So if the voltage across the resistor does not follow the current, the resistance must be changing, (or it's not the only path for the current). You would not expect the current to be changed unless the shunt were a poor choice resistance-wise, OR the shunt had a really gross effect occurring.

it seems that the errors in the test you mention must have been due to something about the shunt changing, presumably the effective resistance.

The case of a test with a voltage source applied to the shunt resistor seems to be different.... the resistor could change quite a bit and the voltage would not change, but of course the current would. So the true test would be a constant current source, with the voltage across the shunt read out as usual.

Not all the effects of a CT are "classically inductive".... there are a bunch of non-linear semi-resistive loading effects as well, especially at currents well below the expected load current range for the CT. What happens as the core is magnetized by the measured current can be somewhat anomalous vs the actual current. Only once the core is magnetized does the CT begin to read out correctly.

Not saying there is no effect.... just thinking about how any OTHER effects can be eliminated so just the effect under study can be isolated.

I need to do some experimentation.
 
The resistor in the original test was in effect both the load and current shunt. The circuit was just a transformer that was a fairly stiff source, the CT under test, and a load resistor that was only an ohm or so. The test was to read the phase angle between the voltage across the resistor vs the output of the CT with a specified load across its winding. An independent test lab recommended by McDonnell Aircraft kept rejecting sample transformers for phase shift. The CT was in its proper operating range, around 3000- 4000 Gauss core flux. The voltage across the resistor looked normal but the output of the CT was distorted as I described. The distortion read as phase shift on a phase angle meter. We eliminated all the effects like the initial magnetization current and after s lot of frustration realized that the resistor was non-linear. We tested a lot of resistors, mostly the edge wound ribbon types because we were working with a low voltage high current system.

As you say, the resistance is slightly higher from zero crossing on the leading edge of a half cycle until it is near the peak. I don't know that it is magnetostrictive, but that is the best explanation so far. If a magnet will stick to the resistor, you will have the distortion.

I know about the distortion in a CT's output from operating outside of the optimum flux density. That isn't the problem.

Bill
 








 
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