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Bi-directional meters

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IamSmooth

Tue Oct 16 2007, 06:26PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

When I connected my solar panels I watched my old-style meter spin backwards. I am guessing that there is a small motor that contolled the flywheel. I was wondering how the flywheel "knows" to spin backwards. When I connect an AC motor to an AC current the rotor only spins one way. I can understand that if I am supplying current on the "load" side I can decrease the current on the supply side to zero if they are equal; however, when the current I am supply exceeds the utility-side current I would think the motor would resume spinning in its normal direction. That being said, does anyone know how a bi-directional meter keeps track of supply and production current?

...

Tue Oct 16 2007, 11:10PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The meter isn't an induction motor, at least not like one you have used. IIRC, they work by having 2 coils, on that has a lot of turns that creates a magnetic field proportional to voltage, and one that has a few turns which will create a field proportional to current. An aluminum disk is placed in the field, and an eddy current is induced in the disk, which will oppose the field of the coils, and then it rotates.

IamSmooth

Wed Oct 17 2007, 01:18AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I am familiar with the concept that the meter is an induction meter. What I don't understand is how it spins backwards. With a regular motor the hot wire is an alternating signal. It doesn't matter if I switch the ground and the hot wire - the motor still spins one way. So, why would a meter spin backwards if I supply current on the load side? The current is moving forward and backwards on the supply side anyway. It isn't like this is a DC signal with the current moving only one direction.

...

Wed Oct 17 2007, 02:14AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You need to think of it on a much smaller time frame. One the + cycle, when you are drawing current, you have a + voltage times a + current = a magnetic field in one direction. If you are supplying current there will be a + voltage and a - current, which will give a magnetic field in the opposite direction.

I can't really think of a better way to explain it, it is basically a magnetic multiplier.

Dr. Slack

Wed Oct 17 2007, 07:40AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

And not just a multiplier, but a vector multiplier. It is a 2 phase induction motor.

As you know, a single phase induction motor at stall generates no torque. You generally have an additional start winding, or some copper shorting rings to one side of each pole, to make a second phase to generate a net torque. Once rotating, the start phase can be removed or left in place (note for AC induction generator - shading rings will reduce efficiency).

The meter has two coils, one of a few turns in series with the current, acting next to one of a zillion turns across the supply voltage. The field in the current coil is proportional to current. The field in the voltage coil is not protortional to voltage due to the large inductance, but lags it by 90 degrees. Together, the two coils acting adjacently on the disc create a torque equal to the dot product of the I and V waveforms. This torque is turned into a speed by using a permanent magnet to create an eddy current braking torque proportional to speed.

This type of two coil meter is used for 'normal' import only, because it reads the real power coming in, and does not record the reactive VA circulating. You should be able to put a large PFC across your terminals and draw several amps without the meter speed changing. It also charges you less when the voltage drops, you pay for power, not current.

Now I've thought about it enough to describe it, I wonder how the not-quite-90 lag of the voltage coil field affects accuracy when going between import and export? It will certainly affect the rejection of reactive VA, and be different for when it's lagging or leading.

This is one of the reasons that utilities place power factor restrictions on their users, they don't pass on the cost of supplying the reactive VA. It's not worth them bothering with domestic users (except fluorescent fittings have had PFCs in for years, and PCs above 300W now need PF correction), but factories need to have adjustable banks of PFCs, or some other method, to keep their power factor near to unity.

Marko

Wed Oct 17 2007, 09:59PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I am familiar with the concept that the meter is an induction meter. What I don't understand is how it spins backwards. With a regular motor the hot wire is an alternating signal. It doesn't matter if I switch the ground and the hot wire - the motor still spins one way. So, why would a meter spin backwards if I supply current on the load side? The current is moving forward and backwards on the supply side anyway. It isn't like this is a DC signal with the current moving only one direction.

When you are feeding power back voltage will still have 90 degree lag, and current will now be out of phase than if counter operated forward.

If it is not blocked motor should spin backwards.

But I was never actually sure how it manages to omit reactive power.

Is it simple because imaginary art of current is *always* 180deg out of phase with voltage and cancels out?

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