MH's Basic Electrical Theory - Lead and Lag ???

TwoBlocked

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Industrial Electrician
So I am perusing MH's Basic Electrical Theory book and am startled by how lead and lag is explained in Unit 10. In the text, you are to consider the relationship of VOLTAGE to current instead of the other way around resulting in inductive circuits having a leading characteristic and a capacitive circuits having a lagging characteristic! I remember it being opposite. Checking other references, I find they agree with my memory. I apologize if this has been covered before in this Forum. I did a bit of a search but didn't turn up anything.

COMMENTS ???
 

jim dungar

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I believe the text states that voltage is always used as their reference point on the waveform plot. Lead or Lag is dependent on if the current peak occurs before or after the voltage zero point.
 
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Mr. Serious

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Capacitive circuit: current leads voltage because the current has to flow into the capacitor first, to charge it up, before you start to get voltage on the capacitor.

Inductive circuit: current lags voltage. Inductors always oppose a change of current through them. When the voltage changes, the magnetic field around the inductor has to change before the current can change.
 

TwoBlocked

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Industrial Electrician
Thing is, when you look at a power meter, such as on a generator, it just says leading or lagging, not which is leading which. (OK, I have NEVER seen a leading power factor on a meter, but that's not the point.) And thanks, Tom, for the mnemonic. I was never into them, but ought to know them to understand what others are talking about. The other week I was explaining to an intern that Watts is the unit of power and is found by multiplying Volts times Amps. Another electrician exclaimed "PIE!" and I had to ask if he meant 3.14...
 

LarryFine

Master Electrician Electric Contractor Richmond VA
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I also think the voltage should be the reference, because voltage is the cause, and current is the result.
 

Mr. Serious

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Larry, you're trying to say that it makes more sense to say current leads/lags voltage; rather than voltage leads/lags current?
 

TwoBlocked

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Location
Bradford County, PA
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Industrial Electrician
I agree with Larry, but the bothersome thing for me is the way the book describes it I believe is not conventional. It is the book I have been having apprentices read and just now started really looking at it. I'd hate for one of them to look at the power factor on a meter and say "What? Why is the PF lagging when there are so many motor loads? It should be leading!" I gotta go back through the magnetism part, too. Something seemed odd there... But for the most part, I very much like the book.
 

jim dungar

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...Why is the PF lagging when there are so many motor loads...
ELI the ICE man
Inductive loads cause the change in current to lag behind the change in voltage. Our industry calls this a lagging power factor.

Capacitive loads cause the change in voltage to lag behind the change in current. Our industry calls this a leading power factor.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Capacitive loads cause the change in voltage to lag behind the change in current.
What we're discussing here is that you should be saying, "Capacitive loads cause the change in current to lead ahead of the change in voltage."

That's because the voltage's timing is not what is varying, the current's timing is.
 

jim dungar

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What we're discussing here is that you should be saying, "Capacitive loads cause the change in current to lead ahead of the change in voltage."

That's because the voltage's timing is not what is varying, the current's timing is.
Correct.
The voltage is used as the reference when our industry talks about Power Factor.
 

jim dungar

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I am sure I read it in there, as a general statement. But heck that was last month.
 
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