In a Capacitor Run motor generally your phase shift is 90 degrees. In theory I know putting in a larger capacitor can harm the motor if not rated. However, my question is in theory could you ever get to 360-degree phase shift with a large enough capacitor? Or would this stall the windings. When motors have a start cap I'm assuming you're generally starting with a larger phase shift as well. Let me know what you think thank you!
Phase Shift 360 degrees?
- Thread starter kjroller
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LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
No, it doesn't work like that. The shift is based on each half of the sine wave.
So max would be 180?
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
No, the max would still be 90 deg.
So in other words a larger capacitor would just release more current on initial discharge vs a smaller one no greater phase shift
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- Licensed Electrician
I don't think there's even a 90° phase shift. The capacitor just sends the current ahead of the voltage enough in the starting winding to a kick the rotor to get it spinning.
Someone with a bigger brain that can do math may have a more detailed analysis.
Someone with a bigger brain that can do math may have a more detailed analysis.
It's 90deg. There's not a lot to say about it.
Joethemechanic
Senior Member
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- Hazleton Pa
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- Electro-Mechanical Technician. Industrial machinery
I remember reading about capacitors for phase shift (motor starting), building induction generators, and remediating low lagging power factor in this book about 30 years ago and if I remember right there were formulas for sizing in there. I used the formulas so much I forgot them.
ISBN:0132515474
ISBN13:9780132515474
Release Date: January 1990
Publisher: Prentice Hall
Length: 752 Pages
All I can tell you is adjust capacitor size until you get the current right or as close to right as you can get it
Electrical Machines, Drives, and Power Systems
By Theodore WildiISBN:0132515474
ISBN13:9780132515474
Release Date: January 1990
Publisher: Prentice Hall
Length: 752 Pages
All I can tell you is adjust capacitor size until you get the current right or as close to right as you can get it
- Occupation
- Licensed Electrician
In a single phase motor without something to kick the rotor in one direction to start the rotation the rotor will sit there frozen in place so a starting winding is added to the coils in the motor. A starting cap is added to the starting winding and sends the amps out of phase with the voltage and that allows the north magnetic pole that is being induced on the rotor from the running winding to move towards the south magnetic pole that is being generated in the starting winding.
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- Lockport, IL
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- Semi-Retired Electrical Engineer
I think not. My understanding is that the (approximately) 90 degree phase shift created by the starting capacitor causes the magnetic field surrounding the rotor to spin, in much the same way as occurs in a 3-phase motor. The rotor will "chase" the spinning field. Once the rotor is moving near to full speed, the starting capacitor is no longer needed.
Note: I see that ActionDave posted a similar response while I was typing.
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- Licensed Electrician
I feel validated by Charlie who is one of the big brains on this forum.
I will add that depending on the motor design the starting winding and cap may stay in the circuit to give some extra HP.
I will add that depending on the motor design the starting winding and cap may stay in the circuit to give some extra HP.
Then why not just use a Run Cap and not a Start Cap? The reason I ask is 90% of fans or compressors in AC units are dual Run Caps and the phase shift is induced through the run cap so why ever use a start cap vs a run cap? The way I understand the difference is the Start cap yes does cause a phase shift however its initial discharge of electrons causes more current (flow of electrons) to kick start the motor then after a bit it will switch to the run cap so it can continue the phase shift but with moderate flow of electrons.
Last edited:
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
I won't claim to have a bigger brain, and the one I have is almost as old as I am. But here is some math:
The voltage induced in a capacitor is equal to the value of its inductance (a constant) multiplied by the rate at which the current passing through the capacitor is changing. The formula is, V = L (di/dt), where "V" is the voltage value at any point in time, "L" is the inductance, "di" is the amount of change in the current value over a very short period of time, and "dt" is the amount of time during which that change in current is taking place. If the current is following a sinusoidal curve, then so too will the voltage, but the two curves will be offset by a 90 degree angle.
Here is a lesser-math way to visualize this 90 degree offset. Draw a sine wave (i.e., smooth, rolling hills), starting at the zero points of both the horizontal and vertical axises. Draw another curve on a separate set of axises immediately below the first one. The value you plot on the lower curve at any point of time is to be the slope of the upper curve at that same moment. At time zero, the upper curve has a positive slope, but that slope gets smaller as time passes. When the upper curve is at its peak, it has a zero slope (i.e., at that brief moment, the upper curve is flat). Then the upper starts sloping downwards (i.e., has a negative slope). When you finish the drawing and look at the two curves, you will see that both are sine waves, and that they are offset from each other by 90 degrees.
Q.E.D
I guess I understand the phase shift 100% but the applicable differences between the start and run cap is what I am looking for as they both cause a phase shift. However, one has a faster discharge being the start capacitor so I am under the impression the number of electrons then flowing into the Start winding or Current would be much greater at start up because of this discharge. Because with a run cap you could still get the phase shift with less discharge on the Cap
Joethemechanic
Senior Member
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- Hazleton Pa
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- Electro-Mechanical Technician. Industrial machinery
The current with the rotor sitting still is something about like locked rotor current for a brief instant. So that is why you use the start cap. If you look at the values of the capacitors on a motor with both start and run capacitors you will find that the start cap is many times the microfarad value of the run cap. It needs to be that big to handle that brief instant of high current when starting. The thing about start caps is they are only designed for very short duty cycles. Run caps are designed for continuous duty and can stay in the circuit
So, the Start cap doesn't add current but needs to be rated for the inrush on larger motors? (If I'm understanding right) And reason run caps work on a compressor is because of the low start up current on the motor itself for a bigger AC unit you may need a Start cap. For example, my ac unit uses a 40uf/5uf dual run cap. In other words, it can handle and discharge a large amount of current for the phase shift for a short period of time?
Eddie702
Licensed Electrician
- Location
- Western Massachusetts
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- Electrician
We used to have issues with some roof top units with multiple condenser fans that had 5mfd capacitors. When one compressor started (first stage) it would start 2 condenser fans. When 2d stage started it started the second compressor and 2 more condenser fans. The second stage fans would sometimes start backwards as the fans were all in the same condenser air space plenum. The air from the fans that started first would have the 2d stage fan blades spinning in the wrong direction before they were powered.
The fix (probably not the right fix) was to put 7.5 mfd capacitors on the two 2d stage fans.
We tried starting all 4 fans when first stage fired up but during cooler weather that would cause first stage compressor to run with lo head pressure with all the fans on.
We even put some sheet metal dividers in the condenser air plenum but that didn't work to well.
The fix (probably not the right fix) was to put 7.5 mfd capacitors on the two 2d stage fans.
We tried starting all 4 fans when first stage fired up but during cooler weather that would cause first stage compressor to run with lo head pressure with all the fans on.
We even put some sheet metal dividers in the condenser air plenum but that didn't work to well.
JoeStillman
Senior Member
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- West Chester, PA
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- Electrical PE
My intelligence is strictly artificial, so I asked the AI...
I see a few instances of misinformation so I'll attempt to clarify. You should not think of the capacitor as a device which is somehow charged up and waiting to discharge and provide the energy for motor rotation... this is an AC system and the state of charge of the capacitor at start up is unknown and may in fact be zero. The capacitor is there just to provide a phase difference between the current in the aux winding the main winding. This creates a somewhat imperfect rotating field similar to perfect one in a 3 phase motor. If you don't have a capacitor, the field does not rotate and you can typically start the motor in either direction with a manual spin with your fingers and it will stay running.
In a PSC (permanent split capacitor) motor (typical furnace blower before ECM), the starting torque requirements are low so you can get by with a run capacitor that always stays in circuit.
For higher starting torque loads, think table saw, compressor, a starting capacitor is used for more phase shift and higher starting torque. These capacitors are only rated for short duty cycle and are removed from the circuit typically by centrifugal switch (table saw) or potential relay (A/C compressor). If a start capacitor is used, a smaller running capacitor is likely also left in circuit to improve running vibration and power factor.
In the HVAC situation mentioned above, the 40uF/5uF dual run capacitor is just what it says, the 40uF is the run capacitor for the compressor and the 5uF is the run capacitor for the fan. They are just in one "can" for convenience and cost. There is typically a larger start capacitor somewhere in system that is removed by start relay.
In a PSC (permanent split capacitor) motor (typical furnace blower before ECM), the starting torque requirements are low so you can get by with a run capacitor that always stays in circuit.
For higher starting torque loads, think table saw, compressor, a starting capacitor is used for more phase shift and higher starting torque. These capacitors are only rated for short duty cycle and are removed from the circuit typically by centrifugal switch (table saw) or potential relay (A/C compressor). If a start capacitor is used, a smaller running capacitor is likely also left in circuit to improve running vibration and power factor.
In the HVAC situation mentioned above, the 40uF/5uF dual run capacitor is just what it says, the 40uF is the run capacitor for the compressor and the 5uF is the run capacitor for the fan. They are just in one "can" for convenience and cost. There is typically a larger start capacitor somewhere in system that is removed by start relay.