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VFD
- Thread starter kd4gij
- Start date
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- Sep 11, 2012
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- 329
Gentlemen;
Had a long chat last night with my brother-in-laws Dad. Thirty three years a meterman at PP&L, (Pacific Power and Light). I will be the first to admit that metering is a very weak subject of mine but after a quick refresher this may help. First, there is no direct connection in the electrical component and the metering component of a meter. Old mechanical style with the wheel, or new digital type. Hence the meter does not care about voltage. The only thing that drives the meter is the magnetic coupling between the two sections of the meter. The voltage labeling on a meter is so you do not plug the meter into too high of a voltage. Like a lot of things there is a maximum voltage but no minimum voltage to operate the meter, or of what it will measure.
If you peer into the side of a visible meter 240/120v 1 ph, you will see the secret of how it works. There are two coils that wrap around a "C" shaped laminated armature that has the disk centered in the air gap of the armature. The secret is INDUCTANCE, magnetism, the strength of the field is measured in Henries. The magnetism or Henries are created/induced by current flow through the two coils. This field is created regardless of voltage level but due to current. All the little Henries (magnetism) reach down and grab the spinning metal plate, each side adding its own little addition to the strength of the field spinning it along at changing rates due to power usage. This field value is also affected by capacitance and power factor but that is also another whole discussion for the future. Again, this bit of wonder happens due to current flow creating magnetism regardless of voltage. Due to the current flow through each field of the meter.
Gene, my meter buddy, also told me of a situation that may help explain why, and how a Rotary Phase Converter operates and why people claim the large horsepower idler consumes no power. (Yet to be proven to me but I digress) Gene said he was sent out in the field one day to check a faulty meter. It was a 10 hp irrigation pump installation out in a hop yard. Motor is running but the meter is not. He inserts the circuit closers, pulls the meter, installs a brand new Saginaw Meter, 240/120v 3 ph Delta. Removes the circuit closers and observed the same result. No perceptible meter movement. Back to the Van to radio the Supervisor. While in conversation the Ranch hand shows up and opens a valve so the pump now has a load and is now pumping water. Before the motor was just running along trying to pump against a closed valve, hence not doing any work. Gene goes back to the meter to answer a question from his boss and lo and behold, the meter was spinning like crazy. Goes through the whole procedure and replaces the old meter and it reacts the same...... What is going on?
Looking over the whole site, one pole away was a rather large bank of power company installed power factor correction capacitors. Apparently the bank supplied enough capacitance to totally cancel the inductive reactance from the motor so it it would overcome its own losses and continue to run while drawing no power from the transformer bank. Opening the valve made the motor go back to work and start to tally up the charges again. Gene said even the EE's at the office could not explain all this for sure.
Been a good chat. Made me dig deep and really think again and I thoroughly enjoyed it.
Mr. Walltoddj, have you considered that your savings in your shop may be the fact that you're retired now, like myself, so you do not work your machines as long and hard as you used to, so that is part of the reason for the savings? I would like to chat with your son, he being a Journeyman Electrician to see where his ideas come from. I myself am a Master Electrician. Thus the reason for my enjoying these discussions, so I don't forget everything!
Have a nice day,
Had a long chat last night with my brother-in-laws Dad. Thirty three years a meterman at PP&L, (Pacific Power and Light). I will be the first to admit that metering is a very weak subject of mine but after a quick refresher this may help. First, there is no direct connection in the electrical component and the metering component of a meter. Old mechanical style with the wheel, or new digital type. Hence the meter does not care about voltage. The only thing that drives the meter is the magnetic coupling between the two sections of the meter. The voltage labeling on a meter is so you do not plug the meter into too high of a voltage. Like a lot of things there is a maximum voltage but no minimum voltage to operate the meter, or of what it will measure.
If you peer into the side of a visible meter 240/120v 1 ph, you will see the secret of how it works. There are two coils that wrap around a "C" shaped laminated armature that has the disk centered in the air gap of the armature. The secret is INDUCTANCE, magnetism, the strength of the field is measured in Henries. The magnetism or Henries are created/induced by current flow through the two coils. This field is created regardless of voltage level but due to current. All the little Henries (magnetism) reach down and grab the spinning metal plate, each side adding its own little addition to the strength of the field spinning it along at changing rates due to power usage. This field value is also affected by capacitance and power factor but that is also another whole discussion for the future. Again, this bit of wonder happens due to current flow creating magnetism regardless of voltage. Due to the current flow through each field of the meter.
Gene, my meter buddy, also told me of a situation that may help explain why, and how a Rotary Phase Converter operates and why people claim the large horsepower idler consumes no power. (Yet to be proven to me but I digress) Gene said he was sent out in the field one day to check a faulty meter. It was a 10 hp irrigation pump installation out in a hop yard. Motor is running but the meter is not. He inserts the circuit closers, pulls the meter, installs a brand new Saginaw Meter, 240/120v 3 ph Delta. Removes the circuit closers and observed the same result. No perceptible meter movement. Back to the Van to radio the Supervisor. While in conversation the Ranch hand shows up and opens a valve so the pump now has a load and is now pumping water. Before the motor was just running along trying to pump against a closed valve, hence not doing any work. Gene goes back to the meter to answer a question from his boss and lo and behold, the meter was spinning like crazy. Goes through the whole procedure and replaces the old meter and it reacts the same...... What is going on?
Looking over the whole site, one pole away was a rather large bank of power company installed power factor correction capacitors. Apparently the bank supplied enough capacitance to totally cancel the inductive reactance from the motor so it it would overcome its own losses and continue to run while drawing no power from the transformer bank. Opening the valve made the motor go back to work and start to tally up the charges again. Gene said even the EE's at the office could not explain all this for sure.
Been a good chat. Made me dig deep and really think again and I thoroughly enjoyed it.
Mr. Walltoddj, have you considered that your savings in your shop may be the fact that you're retired now, like myself, so you do not work your machines as long and hard as you used to, so that is part of the reason for the savings? I would like to chat with your son, he being a Journeyman Electrician to see where his ideas come from. I myself am a Master Electrician. Thus the reason for my enjoying these discussions, so I don't forget everything!
Have a nice day,
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The Ohm's law formulas work well for DC. AC is a whole 'nother animal including things like form factor correction for best efficient operation. Not really a simplification but a relative comparison.
Uh oh, you went there - power factor. It was drawing power all right, just not "real" power. Power companies hate this, they have to pay for the line losses which occur due to the reactive I squared R loss when they only measure and charge for "real" power.
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The fancy meters used for large industrial and commercial customers record the power factor and the customers are charged extra if it's high.
Or they or the customer pay for capacitor banks to correct it. Capacitors are a one time expense, paying constantly for PF charges is forever.
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- Nov 16, 2012
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In a nutshell, the meter works the same as a clamp-on Ammeter. When an Alternating current flows through a wire, it creates a magnetic field. That magnetic field has a relationship to the flowing current and the higher the current, the stronger the magnetic field.
The clamp-on ammeter has a built-in coil of known resistance and when you clamp that coil around a conductor it induces a voltage (from the magnetic field in the wire you're measuring) in the internal open coil. The ammeter then measures that secondary voltage and since it knows the resistance of the internal coil, it uses Ohms law and calculates the theoretical current generated in the internal coil (if that coil were to be shorted). The ammeter now knows the voltage and theoretical current in it's internal coil. It then deduces the current in the wire you're measuring by using transformer ratio laws such as
N1/N2 = A1/A2. N1 is the number of turns in the internal coil, A1 is the theoretical Amperage in the internal coil. N2 is the number of turns in the wire you're measuring and since the wire only goes through the clamp coil once, N2 = 1. Everything is known and all you do is solve for A2 which is the amperage flowing in the wire. -Done.
I presume the meter box knows the voltage on the wires so, knowing the voltage and current, it easily knows the power used.
(If you don't believe me, take your clamp-on ammeter and loop the same wire through it twice. It will read twice the current because you effectively changed N2 to equal 2).
Also, if you take the two hot lines of a 220 circuit under load and clamp the meter on both of them, it will read zero because, when one line is high, the other is low (they're 180 degrees out of phase) and they cancel each other out. If you measure the neutral/ground, it should read zero or very close to zero. If it doesn't you got a serious problem with that motor or possibly a ground fault in your system.
If you measure each hot line of a 220 motor and see that one shows a very slightly higher reading than the other, it's because one of the windings is slightly different than the other. The difference should be very small and this is a diagnostic test of motors. Each motor should have specs about line-to-line variance. Maybe one rotor blade got wrapped with a few extra turns or, the slight variance in the diameter of the long wires was enough to change the current in that segment.
This by the way is a theoretical description of how to read the current flowing in a wire. There are other sensors that read flux density and perform a mathematical integral over time based on flux density. Various meters probably use different types of sensors as dictated by economics etc.
Ray
Tsk, tsk, and you were doing so good too!
It's acting as a current transformer, a single winding primary and a multi-turn secondary (yes it steps up the voltage while it's at it, and to surprising levels). You terminate the circuit into a known load and measure the current through that load, you care only about the ratio.
Current transformers are all over the the place, power plants, substations, just everywhere, they've been used for decades. We even have machines here that use them only for indicator lamps. If that light's lit you know there's current through that heater.
Here's a baby one from China on eBay, there are over 4,000 listed.
http://www.ebay.com/itm/0-5A-Input-...uit_Breakers_Transformers&hash=item484fbf7f57
You don't want to let a CT go open circuit, imagine one on a 120KV transmission line open circuit, not only magic smoke but magic arcs.
Last edited:
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LOL... Caught on a technicality I suppose.... But don't be afraid to read the first sentence of the last paragraph...
"This by the way is a theoretical description of how to read the current flowing in a wire."
In reality, if I had to design a meter, I'd go look for a single part to measure the current that does the whole shebang in one shot -and it probably would cost 30 cents in quantity lots...
Ray