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Tool Talk Discussion Forum

Re: JohnT?

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John T

12-16-2013 04:57:56
216.249.76.176



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Good morning "Curious George" and I mean that as a compliment, I remain curious about this "lectricity" stuff myself. Its been over 40 yrs since my first EE job with The Century Electric Motor Co but heres my best recollection:

1) "I've seen 1/4 hp shaded pole blower motors on a furnace, which should only use about 200 watts, less than 2 amps, draw 8-10 amps. The reason, the shaded pole motor isn't powerfactored."

I think the reason is the shaded pole winding (unlike if a centrifugal switch cut it out at X RPM) remains in the circuit drawing extra current and producing heat even though its function is long over........

2) "In theory if it were powerfactored, it should draw close to 2 amps, right?"

Not sure what you mean by "powerfactored" Indeed if the start winding were cut out of the circuit via a centrifugal switch, there would be less current draw thereafter and such would relate to the motors single phase induction and HP and load BUT NO LONGER THE EXTRA START WINDING.

3) "Our residental electric meter can only sees the 2 amps, even if motor isn't PF. Am I right again?"

NO the electric meter sees the total amperage which consists of BOTH what the run winding draws PLUS what the paralell shaded pole winding continues to draw (cuz no switch cuts it out at X RPM)

3) "A shaded pole fan motor generates a lot of heat. In theory, the heat produced is calculated by I squared R. So, is the heat produced inside a shaded pole motor based on the 10 amps or the 2 amps? If it's the 10 amps and our energy meter only sees 2 amps, are we getting more heat energy than we are paying for?"

The heat produced is I Squared R AND THATS THE TOTAL I (Run PLUS Start Winding)

NOTES: Many cheap shaded pole motors like say for a ceiling fan application have very low starting torque and can hardly pull a rubber chicken, but thats NOT to say that start winding cant be bigger and have more coils of wire and produce much more starting torque if such is required (Shaded pole is NOT ONLY for small fan motors). The "shaded pole" is another winding in addition to the run winding which is magnetically offset from the run winding so the motor knows which way to start spinning plus actually starts turning that way ITS JUST THAT IT REMAINS IN TEH CIRCUIT unlike a split phase induction motor where a centrifugal switch cuts it out once started.

NOTE Power factor is the cosine of the angle between current and voltage. If the load is pure resistive (Not L or C) they are in phase and the cosine of 0 degrees is one A UNITY POWER FACTOR.

NOTE A motor is mostly an Inductive load and in an inductor the Current lags the Voltage thus they are NOT in phase so theres a less then unity power factor. If a capacitor is placed in the circuit (Voltage lags current) that helps balance out the current and voltage bringing them back closer to sync and improving power factor.

I think youre confusing power factor and all that real versus apparent power stuff and phase angles of voltage and current versus the total current a shaded pole motor draws even after start up......

Nuff for now, I gotta take my dear old 90 yo mother to a Dr Appointment, back this evening.

John T

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George Marsh

12-16-2013 07:05:20
50.121.6.110



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 Re: JohnT? in reply to John T, 12-16-2013 04:57:56  
JohnT,
Hope momma is doing well. Before my mom died at 88, I made many trips to Florida to watch her. It was a blessing she passed on Easter Sunday 3 years. No one should have to live with cancer and all the pain she went through.

Follow up question. I'll try to make it simple.

My 3/4 hp water pump on 110v will use about 12 amps. I put 200 MFD's in parallel with motor and dropped the current to 9 amps. Measured PF, it's 0.99. The motor still draws 12 amps, the line current going to motor is 9 amps.

It's my understanding that with or without the cap bank. My electric meter only sees the 9 amps and not the 12 amps. Is that correct?

There will be a follow up question.

"Curious George"

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John T

12-16-2013 14:22:45
216.249.76.176



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 Re: JohnT? in reply to George Marsh, 12-16-2013 07:05:20  
Hey George, I, as you know, meant no offense with the "curious George" nickname. Heck I'm curious as a cat myself and seems my kids years ago watched a cartoon about some curious George, character, but I'm not sure.

Not having your meter or any specs or anything at all, its still my "best guess" you're correct, it sees the amperage which would be that if you had a unity one power factor, which means it does NOT see the vector sum of the resistive current component flow PLUS the Inductive (out of phase if not corrected) component. The electric company watt hour meter doesn't take power factor into consideration far as I know (44 years ago Duncan Meter Co gave a talk in our EE school but Ive slept since then) HOWEVER at the place I worked they still monitored Power Factor (even if meter didn't show) and charged us a huge penalty if we were off Unity by too much........

Tell you what, my friend the good Buick man (unlike long retired me) is very active in this business and knows this power factor stuff better then me, but its still my guess your meter DOES NOT SEE or take power factor or out of unity power factor current into consideration. See what he and the other more current in the practice sparkies have to say NO WARRANTY on my best guesses about how your particular meter works.

See, Im as curious as you are and always willing to learn.......

A rusty OLD John T

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MarkB_MI

12-16-2013 17:06:38
75.219.42.89



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 Re: JohnT? in reply to John T, 12-16-2013 14:22:45  
John, I know power engineering was your speciality, and I've never worked in the field. But I'm fairly sure kwh meters measure actual power, not volt-amps. The traditional mechanical meters have both voltage and current windings; if current and voltage are out of phase, that will be reflected in the number of watt-hours recorded by the meter. I think the newer "smart" meters can record not just kwh, but average current, voltage, power factor and a bunch of other stuff.

Of course, utilities can and do penalize customers with low power factors. Look at it this way: if the power company could charge based on volt-amps rather than watts, they wouldn't be too concerned about customers with poor power factors.

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John T

12-16-2013 18:04:48
216.249.76.176



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 Re: JohnT? in reply to MarkB_MI, 12-16-2013 17:06:38  
Hey there Mark, Actually AC Power Distribution was more my specialty and while I worked a lot with Power Factor and Power Factor Correction, Im very rusty in that area as well as the types of modern or "smart" KWH Meters etc. Like I used to tell my kids "Old dad knows some things but NOT everything" HOWEVER the things I DID SAY about what PF is and its definition etc is I believe accurate.........I may have to break down and refresh myself in this area but Im too lazy and old for that. The "stuff" I do know I'm glad to share and help people with as you well know.

John T

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George Marsh

12-16-2013 14:42:16
50.121.6.110



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 Re: JohnT? in reply to John T, 12-16-2013 14:22:45  
JohnT,

No offense taken.

So, if I got things right in my head, with or without caps, the motor uses the same current and generates the same heat.

The only thing that benefits is the circuit breaker in the load center is seeing less amps and there is less I squared loss in the wire going to the motor.

What about a generator? Is it better for a genny to PF?

George



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John T

12-16-2013 15:27:35
216.249.76.176



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 Re: JohnT? in reply to George Marsh, 12-16-2013 14:42:16  
"So, if I got things right in my head, with or without caps, the motor uses the same current and generates the same heat."

I cant say that's exactly right, but its hard to explain and I'm way too lazy to dust off my books lol. Here's what I will say, however. If the PF is unity meaning, the current isn't leading or lagging the voltage, its in phase, the vector sum of the resistive current component and reactive (L/C) current component is the same as the resistive ONLY i.e. C canceled out the L. However, if the PF is less then unity, the current is the vector sum of the 2 out of phase components (R + L/C) even though a meter doesn't take that into account, its watts NOT Volt Amps.

"What about a generator? Is it better for a genny to PF?"

The utility or a genset generates a sine wave and it has to "work" less if the load is unity PF. If the PF is lousy the genset or utility is actually having to produce the in plus out of phase current components (vector sum) and that's why we were charged a penalty for a lousy PF even though our watt hour meters registered the same

HEY GEORGE IM RUNNIN OUTA SMARTS ON THIS PF STUFF this is my best recollection but certainly not warranted correct. When I believe an answer is correct I just state it, but Im NOT sure on this PF stuff, its been too long, dont take any drastic steps based on the above, consult someone more current in the trade whose not been retired over 20 years ........

Sorry, but that's all I got

John T

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George Marsh

12-16-2013 19:16:24
50.121.6.110



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 Re: JohnT? in reply to John T, 12-16-2013 15:27:35  
thanks JohnT,
I am very familar with vectors, series and parallel tank circuits, XL, Xc, R, Z, all the equations in electronics. Never really applied them to motors.

What I'm not 100% sure of, but going to take a guess an educated guess on what you call unity(powerfactor).

When the phase angle between the vector sum and R is zero, the cos of zero is one and the Power Factor is 1, unity.

So, if you were to find the phase angle then the cosine of that angle transulates to the power factor. example. The phase angle is 45 degrees, the cosine of 45 is .707. Therefore you would say the power factor is .707. As the phase angle approaches 90 degrees, the power factor approaches 0. As phase angle approaches zero, power factor approaches 1.

If you can follow this chatter, good luck.

Yes. I have too much time on my hands.

So, is the cos of phase angle = power factor.

Inverse cosine of power factor = phase angle.

I'm kinda of rusty too. Only been retired 10 years. I pitched all my books.

George

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John T

12-17-2013 03:45:52
216.249.76.176



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 Re: JohnT? in reply to George Marsh, 12-16-2013 19:16:24  
"So, is the cos of phase angle = power factor. "

YOU GOT IT.........And if they are in phase the angle is zero and the cosine of zero is one...

Fun huh

John T



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George Marsh

12-17-2013 06:11:34
50.121.6.110



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 Re: JohnT? in reply to John T, 12-17-2013 03:45:52  
JohnT,
Don't think I ever read that any place. Why do some try to make it so complicated?
George



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