alternating current & alternating voltage

[peetey]

I was wondering if anyone can explain alternating current and alternating voltage polarity as it relates to time. If frequency is 60 Hz then I have 60 cycles a second. If a cycle is half positive and half negative then every 120th of a second I have current reverse and polarity reverse. Can anyone explain this and maybe include a diagram that illustrates it? I am a meterman and I am wondering why the meter wouldn't reverse direction as the polarity and current reverse. Help

peetey

 

[PMACP]

"I am a meterman and I am wondering why the meter wouldn't reverse direction as the polarity and current reverse."

This is because the multimeter measures the effective value of the magnitude and not the instantaneous value.

 

[dpc]

The same reason an ac induction motor doesn't reverse direction 120 times a second. An alternating magnetic field is created by the stator. This induces a current in the rotor. The magnetic field exerts a force on the rotor and "drags" it around as the field alternates. In a single-phase motor, the only trick is getting the rotor started. In a three-phase motor, there is a rotating magnetic field that pulls the rotor around.

I'd check out the Meterman's Handbook.

 

[sibeen]

peetey, remember a simple bit of algerbra that a negative multiplied by a negative gives a positive result.

When both the current and voltage are both in the positive part of the cycle then V x I = positive.

When both the current and voltage are in the negativee part of the cycle then -V x -I = positive.

So the power flow is positive in both sections of the cycle.

It does get way more complex than this, but it gives a general idea.