- Location
- Placerville, CA, USA
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- Retired PV System Designer
I, for one, would appreciate it. I heard an explanation once but was not convinced.
Motor guy question
- Thread starter ptonsparky
- Start date
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I, for one, would appreciate it. I heard an explanation once but was not convinced.
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
GoldDdigger...
CSIR Motor Theory
Typically, a split-phase capacitor-start motor, has two parallel windings… S and R, respectively! They are physically displaced by 90-deg around the periphery of the motor’s stator. A capacitor, connected in series with the S-winding, insures that current and time-displacement between the two windings is almost 90-degrees in phase and 8.33 milli-seconds, in time (for a 60 Hz motor).
How Motor is Started
The Starting Relay’s coil is connected in series with the R-winding (no, not a misprint, I said R-winding! It is not voltage-operated, but instead, current-operated. The normally-open contact closes when inrush-current reaches, say, 5-6 times the motor's NP rating, thereby causing current to flow in the series-connected S-winding and capacitor circuit!
How Starting-Circuit is De-energized
As the motor-rotor accelerates, inrush-current flow in the Starting-Relay’s coil decreases. When near full-speed its contact opens, de-energizing the S-winding and capacitor circuit!
Phil
CSIR Motor Theory
Typically, a split-phase capacitor-start motor, has two parallel windings… S and R, respectively! They are physically displaced by 90-deg around the periphery of the motor’s stator. A capacitor, connected in series with the S-winding, insures that current and time-displacement between the two windings is almost 90-degrees in phase and 8.33 milli-seconds, in time (for a 60 Hz motor).
How Motor is Started
The Starting Relay’s coil is connected in series with the R-winding (no, not a misprint, I said R-winding! It is not voltage-operated, but instead, current-operated. The normally-open contact closes when inrush-current reaches, say, 5-6 times the motor's NP rating, thereby causing current to flow in the series-connected S-winding and capacitor circuit!
How Starting-Circuit is De-energized
As the motor-rotor accelerates, inrush-current flow in the Starting-Relay’s coil decreases. When near full-speed its contact opens, de-energizing the S-winding and capacitor circuit!
Phil
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
GoldDigger...
CSIR Motor Theory
Typically, a split-phase capacitor-start motor, has two parallel windings… S and R, respectively! They are physically displaced by 90-deg around the periphery of the motor’s stator. A capacitor, connected in series with the S-winding, insures that current and time-displacement between the two windings is almost 90-degrees in phase (8.33 milli-seconds, in time,for a 60 Hz motor). (if capacitor was not installed maximum phase-displacement is about 25 degrees!)
How Motor is Started
The Starting Relay’s coil is connected in series with the R-winding (no, not a misprint, I said R-winding! It is not voltage-operated, but instead, current-operated. The normally-open contact closes when inrush-current reaches, say, 5-6 times the motor's NP rating, thereby causing current to flow in the series-connected S-winding and capacitor circuit!
How Starting-Circuit is De-energized
As the motor-rotor accelerates, inrush-current flow in the Starting-Relay’s coil decreases. When near full-speed its contact opens, de-energizing the S-winding and capacitor circuit!
Phil
CSIR Motor Theory
Typically, a split-phase capacitor-start motor, has two parallel windings… S and R, respectively! They are physically displaced by 90-deg around the periphery of the motor’s stator. A capacitor, connected in series with the S-winding, insures that current and time-displacement between the two windings is almost 90-degrees in phase (8.33 milli-seconds, in time,for a 60 Hz motor). (if capacitor was not installed maximum phase-displacement is about 25 degrees!)
How Motor is Started
The Starting Relay’s coil is connected in series with the R-winding (no, not a misprint, I said R-winding! It is not voltage-operated, but instead, current-operated. The normally-open contact closes when inrush-current reaches, say, 5-6 times the motor's NP rating, thereby causing current to flow in the series-connected S-winding and capacitor circuit!
How Starting-Circuit is De-energized
As the motor-rotor accelerates, inrush-current flow in the Starting-Relay’s coil decreases. When near full-speed its contact opens, de-energizing the S-winding and capacitor circuit!
Phil
Last edited:
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Phil, I have also seen a circuit with what is labelled as a "potential relay" that, as I recall, was not the primary control for the start winding. Can you clarify that?
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Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
GoldDigger...
No... I can't help you with the circuit you saw. My interpretation of a potential-operated coil is one with a relatively high impedance. Hence, connecting it in series with a motor-winding would severely limit inrush-current!
Two years ago, I designed a braking system for a 1-ph, 240V, 5-Hp DeWalt Radial-Arm-Saw. The Struthers-Dun Starting-Relay Coil had 3-4 turns of #8-#6, solid-conductor! And, its two contacts were about 5/16”-3 /8” in diameter… rated 60-90A! Extremely robust construction!
BTW, it was a dual-cap motor, one for start (electrolytic) the other to improve Pf (oil-immersed)!
Phil
No... I can't help you with the circuit you saw. My interpretation of a potential-operated coil is one with a relatively high impedance. Hence, connecting it in series with a motor-winding would severely limit inrush-current!
Two years ago, I designed a braking system for a 1-ph, 240V, 5-Hp DeWalt Radial-Arm-Saw. The Struthers-Dun Starting-Relay Coil had 3-4 turns of #8-#6, solid-conductor! And, its two contacts were about 5/16”-3 /8” in diameter… rated 60-90A! Extremely robust construction!
BTW, it was a dual-cap motor, one for start (electrolytic) the other to improve Pf (oil-immersed)!
Phil
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
GoldDigger...
Further to my earlier comment...
Paralleling the potential-operated relay to either the S or R winding, precludes its ability to discern duration of the Inrush-current flow! That is, how would it know when to de-energize the start-winding circuit?
Phil
Further to my earlier comment...
Paralleling the potential-operated relay to either the S or R winding, precludes its ability to discern duration of the Inrush-current flow! That is, how would it know when to de-energize the start-winding circuit?
Phil
kwired
Electron manager
- Location
- NE Nebraska
The schematic in post 5 which is for the OP's motor in question is wired in parallel to a portion of the motor winding.
Not saying it is never done, but I can't recall ever seeing a starting relay that is in series with a motor winding.
Not saying it is never done, but I can't recall ever seeing a starting relay that is in series with a motor winding.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
171118-1116 EST
In post #22 8.33 milliseconds is indicated as being 90 deg at 60 Hz. Not so , use half that value. 1/60 second is about 16.666666667 milliseconds.
A true single phase motor will not self start. You can mechanically start it with a rope and pulley, or you can temporarily or permanently make it a two phase motor and it will self start because some degree of a rotating magnetic field is produced.
A 90 degree shift is not required to get some form of rotating field.
.
In post #22 8.33 milliseconds is indicated as being 90 deg at 60 Hz. Not so , use half that value. 1/60 second is about 16.666666667 milliseconds.
A true single phase motor will not self start. You can mechanically start it with a rope and pulley, or you can temporarily or permanently make it a two phase motor and it will self start because some degree of a rotating magnetic field is produced.
A 90 degree shift is not required to get some form of rotating field.
.
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
Gar...
1) You are correct! Mea culpa... my value 8.33 milliseconds is in error. But so is your 16.66 value, although closer! The actual lag or displacement angle, hence time, is determined by the capacitor in series with impedance of the start-winding. It alters the phase angle between Run- and Starting-winding impedances!, resulting in a significant increase in starting-torque than could be obtained without the cap!
2) No mention was made of a alternative starting methods!
Phil
1) You are correct! Mea culpa... my value 8.33 milliseconds is in error. But so is your 16.66 value, although closer! The actual lag or displacement angle, hence time, is determined by the capacitor in series with impedance of the start-winding. It alters the phase angle between Run- and Starting-winding impedances!, resulting in a significant increase in starting-torque than could be obtained without the cap!
2) No mention was made of a alternative starting methods!
Phil
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
171118-2204 EDT
My 16.6..... was correct because the sentence was giving the approximate value for the period of 1 cycle of a 60 Hz signal. An exact decimal value can not be provided for 1/60 second.
One full cycle equals 360 degrees, and 90 degrees for 60 Hz in time is approximately 16.6..../4 or about 4.166.... milliseconds. The exact value is 1000/240 mS.
.
My 16.6..... was correct because the sentence was giving the approximate value for the period of 1 cycle of a 60 Hz signal. An exact decimal value can not be provided for 1/60 second.
One full cycle equals 360 degrees, and 90 degrees for 60 Hz in time is approximately 16.6..../4 or about 4.166.... milliseconds. The exact value is 1000/240 mS.
.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
Wherever you decide to stop the repeating decimal, you need to round the last digit up to 7.
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gar
Senior Member
- Location
- Ann Arbor, Michigan
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- EE
171118-2412 EST
GoldDigger:
True if I were to round it, but I was trying to imply an infinite number of digits. But this does bring up the question of whether that infinite digit should be a 7. Possibly it should.
.
GoldDigger:
True if I were to round it, but I was trying to imply an infinite number of digits. But this does bring up the question of whether that infinite digit should be a 7. Possibly it should.
.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
171119-0808 EST
GokdDigger:
I did not mean to imply that the repeating digit would would be 7, but that possibly the very last digit out at infinity might be considered to be 7, but there really is no last digit there is always one further out.
.
GokdDigger:
I did not mean to imply that the repeating digit would would be 7, but that possibly the very last digit out at infinity might be considered to be 7, but there really is no last digit there is always one further out.
.
ptonsparky
Tom
- Occupation
- EC - retired
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
171119-0915 EST
If there are an infinite number of significant places (digits), then is there ever a least significant digit? I don't need a response. This is for philosophy students to consider.
.
If there are an infinite number of significant places (digits), then is there ever a least significant digit? I don't need a response. This is for philosophy students to consider.
.
kwired
Electron manager
- Location
- NE Nebraska
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
My son, who is a computer/network tech has a t-shirt that reads: "My password is the last 8 digits of pi."
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
171119-1319 EST
GoldDigger:
What are those digits? Does anyone give him an answer? Another one for the philosophers.
Sorry for the misspelling of your name. K and L are next door, and I did not catch it. I try to proofread, but I am not good at it, and too many things slip thru. It gets worse when trying to figure out whether I said what I thought I was saying.
.
GoldDigger:
What are those digits? Does anyone give him an answer? Another one for the philosophers.
Sorry for the misspelling of your name. K and L are next door, and I did not catch it. I try to proofread, but I am not good at it, and too many things slip thru. It gets worse when trying to figure out whether I said what I thought I was saying.
.
ptonsparky
Tom
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- EC - retired
- Status