Modern houshold meters calculate real power as V x I. They use a constant for the V and for I they calculate the integral – the area under – the current waveform. Power factor correction simply shuffles the area under the current waveform around a bit to make it look more sinusoidal. Like someone mentioned earler, the power company would love you for this. Most household electronics – like TV’s and computers – only draw current as the voltage waveform is peaking and is greater than what it’s storage capacitors current value is. This leads to a houshold current waveform that is a round topped and bottomed spike. The power distribution network has to tolerate extreme high peak current values in relatively short duty cycle. The power distribution would carry a much much higher load if everyone drew sinusiods of power.
The current probe that the guy in the video is using is called an Amp Clamp. It measures the current that runs through the hole when the clam is shut. This is the same mechanism that is used in the power meter for probing the current usage. If you put this clamp around the power cord running to an appliance the meter will always read zero. This is because one of the wires in the clamp has the exact opposite current than the other. You have to open up the wires and separate just one wire to clamp onto in order to get any meaningfull reading. I’ve always wondered if it would be possible to take a couple of extra feet of wire that just went through the current measuring circuit and make a couple of turns lay on top of the amp clamp in the meter to cancel the fields out. You would have to do this without opening the meter or tampering it in any way … of course.
>Modern houshold meters calculate real power as V x I. They use a constant for the V and for I they calculate the integral – the area under – the current waveform.
What? Since when? I still have the spinning disk type on my house. I’m almost certain I read this multiplies V∙I in the instant and integrates the product. Can you send me to some reference that explains this?
A couple more notes about motors:
Induction motors without “run capacitors” and running without mechanical load are almost entirely inductive, drawing little power but lots of out-of-phase current. Induction motors such as “PSC” (permanent split capacitor) motors having a properly sized run capacitor and turning a full mechanical load draw most of their current as true power consumption and don’t have as much of an out-of-phase effect. PSC motors are often used on fans around 1/4 hp to 1 hp, including some HVAC equipment. But HVAC equipment might also use split phase motors, especially in the smaller sizes. Motors that have to start a hard load are the ones that will have start capacitors. Some motors have both.
Most watt-hour meters are still the rotating disc type. They count the number of revolutions of a motor and display the total count on dials marked in watt-hours. Newer meters use solid state multipliers and integrators to get the same result. Either way, they measure and record only the real power and ignore the reactive power which integrates to zero over time.
It’s a sort of nitpick, but induction motors don’t really require a “run capacitor.” Once started they run just fine without any capacitor at all. The capacitor is only needed for starting. When the motor size gets up to 3/4 hp or so, a centrifugal switch is ususally incorporated to disconnect the starting circuit, capacitor and winding, when the motor gets up to speed. For smaller motors the starting circuit is left connected to avoid the expense of adding the centriifugal switch.
Small induction motors have a poor power factor. For example, that 1 hp motor in my air conditioner draws a total of 12 A line current. If it’s 90% efficient the in-phase current is 7.2 A and the quadrature phase current is √(12[sup]2[/sup] - 7.2[sup]2[/sup]) = 9.6 A and the power factor is 0.6.
>It’s a sort of nitpick, but induction motors don’t really require a “run capacitor.”
>Small induction motors have a poor power factor.
Whether induction motors require a run capacitor depends on how much you dislike the poor power factor. They’ll start without one, certainly. That’s why it’s not called a “start capacitor”. But the run capacitor makes the motor run more efficiently and improves the power factor. I think the run capacitor can make the power factor 1.0 at some particular chosen mechanical load. Induction motors don’t strictly require the run capacitor or even the start capacitor to go, though they may need a little spin to get them started. The smallest induction motors, shaded pole motors, don’t have any capacitors or centrifugal switches - they just have a region off to one side of the pole where there’s a heavy copper shorting wire to make that region lag the main pole and create a phase difference to make the motor start turning on one direction.
I think the rule of thumb for fractional hp motor capacitors is to include a starting capacitor (or a split phase winding) if the starting torque will be high (like on an air compressor) and don’t if it will be low (like on a fan) and to include a run capacitor if you want to keep the power factor high and make the motor run cool and efficiently.
What is really required to start a single phase induction motor is a magnetic field component that is out of phase with the main magnetic field. Either a shaded pole or a separate field winding physically located at (usually) 90[sup]o[/sup] to the main field winding and connected to the line by a phase shifting capacitor provides this out of phase field.
This starting capacitor will not do anything to improve the power factor of the main field. If the starting capacitor is left connected the motor has a more continuous, rotating magnetic field that provides smoother torque as opposed to the pusating field of the motor without it.