capacitor
- Thread starter domnic
- Start date
- Status
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
In a roughly tuned series LC circuit the terminal to terminal voltage can be several times the voltage applied to the combination.
In order to most efficiently get a useful phase shift in the current of the start winding the capacitor needs to be sized close to the tuned circuit value at 60Hz. Note that actually tuning the LC to resonance would not be a good thing unless you want the current to be limited only by the R of the winding.
In any case the result is that the capacitor must withstand a voltage higher than 240V.
- Occupation
- Licensed Electrician
We gotta work on this some more. I don't know the why but I know the what.
370V capacitors are running caps and limit the current in the starting winding in a permanent split or cap start/cap run motor. They are usually between 5 to 70 mfd.
Starting caps are 120 or 240V and are usually higher mfd, 80 to 1000.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
The details vary from what I discussed, but the two key reasons for the 370V specification are:
1. Constant use carrying full AC current/voltage is a high stress application for a capacitor, so overspecifiying the voltage may result in longer life.
2. It is still possible that the voltage across the capacitor terminals will be greater than the line voltage, especially when the motor is not loaded.
- Occupation
- Licensed Electrician
Makes sense. Thank you.
junkhound
Senior Member
- Location
- Renton, WA
- Occupation
- EE, power electronics specialty
370V capacitors are running caps (OK) and limit the current (??) in the starting winding (?) in a permanent split or cap start/cap run (OK) motor
No personal criticism intended, but one of the reasons to look at this site is to see how much misunderstanding exists about how 'electricity actually works', e.g. the (?) even from very experienced folks.
Have taught class sections for PE exam for many years, statements as above helps in understanding some misconceptions students often have.
Glad to see that Dave graciously accepted Gold Digger's clarifications.
PS: the physics of power thread has a lot of good examples also - and clarifications, etc.
No personal criticism intended, but one of the reasons to look at this site is to see how much misunderstanding exists about how 'electricity actually works', e.g. the (?) even from very experienced folks.
Have taught class sections for PE exam for many years, statements as above helps in understanding some misconceptions students often have.
Glad to see that Dave graciously accepted Gold Digger's clarifications.
PS: the physics of power thread has a lot of good examples also - and clarifications, etc.
- Occupation
- Licensed Electrician
If I remove the running cap of a cap start/cap run motor or psc motor and wire nut the leads together the amps are going to shoot way up and the starting winding is going to smell really bad and smoke is going to pour out. This I know is true. What is the misconception?
junkhound
Senior Member
- Location
- Renton, WA
- Occupation
- EE, power electronics specialty
The misconception is the current limit 'purpose'. A resistor would be lots cheaper than a cap if the reason was to limit current.
The cap is to provide a phase shift to the start (or cap run) winding so that there is starting (or added running) torque, current limit is incidental with the LC values, and can be HIGHER if the wrong LC values in a resonant or phase shift circuit. For instance, in a split phase motor with no cap, the start winding is wound with lot higher reactance (much different L/R than the main winding) to provide a phase shift without a cap. If the cent sw or the current relay does not open, the smoke gets let out of those also even though designed for no cap.
- Occupation
- Licensed Electrician
Starting cap.
Running cap.
Seems this has more to do with the size of the running cap.
So the current in a starting winding has to be limited either duration or amount, am I correct in saying that?
How is a resistor a legitimate substitute for a running cap?
junkhound
Senior Member
- Location
- Renton, WA
- Occupation
- EE, power electronics specialty
How is a resistor a legitimate substitute for a running cap?
That was a discussion point made to illustrate the cap is not to limit current, a resistor is NOT a legitimate substitute*, but would be IF the reason were just to limit current.
The current does not have to be limited if motor wound differently, but economics of the motor winding say to sell motors competitively the start/caprun winding is lower cost if made of lower gauge wire to take less current.
*in most cases -- could go off an a tangent about lighter wire (higher resistance) on a split phase motor help shift phase on the start winding, but wont complicate things.
That was a discussion point made to illustrate the cap is not to limit current, a resistor is NOT a legitimate substitute*, but would be IF the reason were just to limit current.
The current does not have to be limited if motor wound differently, but economics of the motor winding say to sell motors competitively the start/caprun winding is lower cost if made of lower gauge wire to take less current.
*in most cases -- could go off an a tangent about lighter wire (higher resistance) on a split phase motor help shift phase on the start winding, but wont complicate things.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
If it is just for current limiting the cap still has the advantage over the resistor that it does not waste heat. Less power, less need for cooling.
- Occupation
- Licensed Electrician
If the running cap is not controlling current then what is it doing? It's not providing phase shift, that's what a starting cap does.
I have worked on a lot of single phase motors. The only ones I have seen where the starting winding stayed in have running caps.
I have never seen a split phase motor that did not have a starting switch.
junkhound
Senior Member
- Location
- Renton, WA
- Occupation
- EE, power electronics specialty
Bold comments inserted within quote.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
160910-1950 EDT
ActionDave:
A three phase motor has a magnetic vector in physical space that rotates in physical space, and has close to a constant magnitude. Depending on how the stator is wound the rotation speed is RPM = F*60/N. RPM is revolutions per minute, F is supply frequency in cycles per second (Hz), and N is an integer (1, 2, 3, 4, ....). Put a permanent magnetic in that field and it will lock to the rotating vector and run at synchronous speed. This provides a constant output torque to the mechanical load. Put an induction rotor in that rotating field and with rotor slip a current is induced in the rotor that couples with the rotating magnetic field from the stator to produce output torque.
A single phase motor does not have a rotating magnetic field produced by its stator. Rather its stator produces a magnetic vector that has a single fixed angle in physical space and this vector oscillates in magnitude from a maximum + value to a maximum - value and back again continuously. Put a permanent magnet in this field and it won't rotate. However, if you start the permanent magnet to rotate then magnetic pulses from the stator field will add energy to the rotor to keep it at synchronous speed. But you do not have the uniform constant torque that exists in the 3 phase motor.
A single phase motor works very much like a swing where you synchronously add energy to the swing each cycle.
The torque pulsation from a single phase motor can cause problems. Hoover Ball Bearing, a local manufacturer form the early 1900 in our town, was bought by a Japanese company a number of years ago. For a very long time bearing life tests were run on test stands driven by a DC shunt motor. Someone had the bright idea to change to single phase motors on these stands to reduce maintenance costs. All of a sudden bear life dramatically diminished. Thus, no correlation with all prior data.
A 2 phase motor works the same as a 3 phase motor, it produces a rotating magnetic vector in space from the stator.
To make a single phase motor self start you must at least make it a 2 phase motor for a short time. This is accomplished by the various ways that you can find discussed on the Internet.
An interesting characteristic of a capacitor run single phase motor is that it can have a better power factor than a 3 phase motor.
I won't go into capacitor voltage ratings, but you have to understand that ratings on all products are in some way based on experimental data with respect to life, environment, materials, and some criteria. There are no absolutes here.
.
ActionDave:
A three phase motor has a magnetic vector in physical space that rotates in physical space, and has close to a constant magnitude. Depending on how the stator is wound the rotation speed is RPM = F*60/N. RPM is revolutions per minute, F is supply frequency in cycles per second (Hz), and N is an integer (1, 2, 3, 4, ....). Put a permanent magnetic in that field and it will lock to the rotating vector and run at synchronous speed. This provides a constant output torque to the mechanical load. Put an induction rotor in that rotating field and with rotor slip a current is induced in the rotor that couples with the rotating magnetic field from the stator to produce output torque.
A single phase motor does not have a rotating magnetic field produced by its stator. Rather its stator produces a magnetic vector that has a single fixed angle in physical space and this vector oscillates in magnitude from a maximum + value to a maximum - value and back again continuously. Put a permanent magnet in this field and it won't rotate. However, if you start the permanent magnet to rotate then magnetic pulses from the stator field will add energy to the rotor to keep it at synchronous speed. But you do not have the uniform constant torque that exists in the 3 phase motor.
A single phase motor works very much like a swing where you synchronously add energy to the swing each cycle.
The torque pulsation from a single phase motor can cause problems. Hoover Ball Bearing, a local manufacturer form the early 1900 in our town, was bought by a Japanese company a number of years ago. For a very long time bearing life tests were run on test stands driven by a DC shunt motor. Someone had the bright idea to change to single phase motors on these stands to reduce maintenance costs. All of a sudden bear life dramatically diminished. Thus, no correlation with all prior data.
A 2 phase motor works the same as a 3 phase motor, it produces a rotating magnetic vector in space from the stator.
To make a single phase motor self start you must at least make it a 2 phase motor for a short time. This is accomplished by the various ways that you can find discussed on the Internet.
An interesting characteristic of a capacitor run single phase motor is that it can have a better power factor than a 3 phase motor.
I won't go into capacitor voltage ratings, but you have to understand that ratings on all products are in some way based on experimental data with respect to life, environment, materials, and some criteria. There are no absolutes here.
.
- Occupation
- Licensed Electrician
David Goodman
Senior Member
- Location
- Pahrump, NV, USA
Not all run capacitors for PSC or CSCR single phase motors are 370V. There are also 440V capacitors. My limited understanding of the difference is that the higher rated capacitor has more durable dielectric materials, and therefore can operate at higher voltages. The 440V capacitors, because they are more durable can withstand a bit higher ambient temperatures as well.
You can test the potential of a capacitor while the motor is operating simply by using a volt meter on the capacitor terminals. The A/C condensor on my home came with a 440V 40uf capacitor from the factory. It tested while running as follows:
Name plate RLA - 17.2 Amps
Potential between capacitor terminals - 377 V (440 rated cap.)
Current - Run cap. for compressor - 4.6 amps.
Black lead to compressor - 17.8 Amps (Hmmm, over the RLA)
Red lead to compressor - 16.8
Calculated uf of capacitor was only 32.2 uf. That indicated that the capacitor was failing. So, I replaced it with a new 440V 40uf.
After replacing the capacitor I took new readings while the compressor was running.
Potential between capacitor terminals - 385 V (440 rated cap.)
Current - Run cap. for compressor - 5.3 amps.
Black lead to compressor - 16.8 Amps (now below RLA)
Red lead to compressor - 15.6
New measurement taken: Amp. clamp around both red and black - 5.2 amps.
Calculated uf of capacitor, (5.3 X 2650)/385 = 36.5 mfd (now within cap. spec.'s, but still a little low)
A properly functioning capacitor reduced the amperage draw to within the nameplate RLA. A 370V capacitor would not have been able to handle the voltage required for proper operation of this motor. I can't tell you why the voltage was 385V. Maybe one of you can.
- Occupation
- Licensed Electrician
We have 440V caps in our shop, 370V is more common though. I don't doubt that a 440V cap is made of tougher stuff than 370V caps.
Everything else you said confirms what I have been saying.... Running caps limit current and the mfd of the running cap change how much the current is limited.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
160811-0750 EDT
ActionDave:
To repeat a capacitor run motor is a 2 phase motor. The two stator coils are physically about 90 deg apart. The motor can be wound with the two coils having the same number of turns, but the number of turns may be differnt for cost reasons. This is really a question of how much starting torque is required.
The function of the run capacitor is to control the phase angle of the current in its associated coil. The choice of a run capacitor is different than that of a starting capacitor. A starting capacitor is selected to provide the required starting torque. The run capacitor is selected for power efficiency, power factor, and torque ripple.
The run capacitor in series with the second stator coil is selected in combination with that coil to produce an electrical current phase shift of about 90 deg compared with the other coil that is directly connected to the source voltage. At or near full load the series resonant circuit of the capacitor, coil inductance, and a resistor determines the coil and capacitor current, and the capacitor voltage. The resistance value is a function of the coil resistance and the power being fed to the rotor from this coil.
Power factor can be almost unity, but this may be not the optimum place to operate.
See pages 304 and 305 in "Alternating-Current Machinery", by Bailey and Gault, University of Michigan, 1951, McGraw-Hill. In 1955 the book was $6.00 .
Running performance of an actual motor is discussed on p 310 and following.
.
ActionDave:
To repeat a capacitor run motor is a 2 phase motor. The two stator coils are physically about 90 deg apart. The motor can be wound with the two coils having the same number of turns, but the number of turns may be differnt for cost reasons. This is really a question of how much starting torque is required.
The function of the run capacitor is to control the phase angle of the current in its associated coil. The choice of a run capacitor is different than that of a starting capacitor. A starting capacitor is selected to provide the required starting torque. The run capacitor is selected for power efficiency, power factor, and torque ripple.
The run capacitor in series with the second stator coil is selected in combination with that coil to produce an electrical current phase shift of about 90 deg compared with the other coil that is directly connected to the source voltage. At or near full load the series resonant circuit of the capacitor, coil inductance, and a resistor determines the coil and capacitor current, and the capacitor voltage. The resistance value is a function of the coil resistance and the power being fed to the rotor from this coil.
Power factor can be almost unity, but this may be not the optimum place to operate.
See pages 304 and 305 in "Alternating-Current Machinery", by Bailey and Gault, University of Michigan, 1951, McGraw-Hill. In 1955 the book was $6.00 .
Running performance of an actual motor is discussed on p 310 and following.
.
A higher voltage than line voltage is generated in the start winding. Refrigeration service men have more experience with this than electricians. Most home air conditioners only have a run capacitor this is kind of deceiving because it does start the compressor. Manufacturers do this to save money. The same compressor could be found on a condensing unit for a walk in cooler and have a start capacitor added and a potential relay added to remove the start cap. from the circuit. On an electric motor that has to start under a very heavy load the start winding and start cap. are designed to give the absolute highest torque, after startup the run cap is sized to give the smoothest running torque without burning the winding so in a way everyone is right. Potential relays rely on back emf from the the start winding to pick up or open the circuit containing the start cap. A potential relay can be chosen by measuring this voltage then choosing a relay that picks up at about 80 percent of this.
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
- Status