another capacitor question???

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new guy
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another capacitor question???

Post by new guy » Thu Jul 06, 2006 11:49 am

How is a capacitor used to help "start" a motor. Since a capacitor can't hold any more voltage than the line it (and the motor) are connected to, how does the capacitor produce more of a kick than just connecting the motor to the line voltage? Is it something to do with the current rush and not the voltage?? Can a capacitor deliver more current, faster than line voltage??

JPKNHTP
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Post by JPKNHTP » Thu Jul 06, 2006 12:41 pm

-JPKNHTP
-God Bless

bearing01
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Post by bearing01 » Thu Jul 06, 2006 12:51 pm

I don't work with motors often... it's been a while... but here's what I remember:

The rotor inside an AC motor likes to see a rotating magnetic field to get it to start spinning and to keep it spinning. Think of it as a magnetic field that is spinning around the rotor. The magnetic field induces a voltage in the rotor, causing a current to flow in the rotor to generate its own magnetic (rotor) field which is opposite to the rotating magnetic field. This causes the rotor to get pushed and turn. The rotor then tries to spin with the rotating magnetic field. When you have a 3 phase power source the 3 phases can hook up to 3 different windings of the motor to get three pulsating currents in a sequence to generate a pulsating magnetic field that when hooked in a circle will appear as a rotating magnetic field. Like a magnet which is spinning!

When you have a single phase 120V power source you can't get a rotating magnetic field from that. Hook up the single phase to the motor and the magnetic field will be non-rotating and only in one direction. This won't induce a voltage in the rotor that will make it spin. Now, if you spin the rotor by hand the non-rotating magnetic field will cut the windings of the rotor and induce a voltage in it and a current will flow in the rotor. This rotor current will generate a magnetic field that is always 90* off from the main magnetic field and as a result the rotor will appear to be experiencing a rotating magnetic field. It will thus get pulled around by it. The problem is that you had to spin the rotor by hand to create this second field that was 90* off from the main field. To create this extra field (that is 90* off from the main field) you can put an extra winding inside the motor (called a starting winding) and physically place/mount it at 90* to the main winding. Now, feed voltage to this starter winding through a capacitor. A capacitor's current leads its power source's voltage by 90*. Therefore, the current and thus magnetic field in the starter winding will be off 90* from that of the power source. The power source also powers the main winding (without a capacitor) and now the two magnetic fields created with both main & starting windings will be out of phase. Because of this, they will appear to rotate. The rotating field will induce a voltage in the rotor to get it to have a current & generate its own magnetic field to oppose this rotating field. The opposition will push the rotor around and make it spin.

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Post by ecerfoglio » Thu Jul 06, 2006 3:42 pm

you just asked
used to help "start" a motor
but you didn't say what kind of motor.

JPKNHTP asumed it was a DC motor, and yes a capacitor can provide the extra current it takes while starting.

bearing01 answered about (single phase) AC induction motors, which you may find in many appliances arround your home or in industry: Refrigerators, water pumps, fans, air conditioners, etc.

Just remember that when the power increases the single phase motor gets replaced with a three phase one (which, of course, needs a three phase suplly).
E. Cerfoglio
Buenos Aires
Argentina

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Chris Smith
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Post by Chris Smith » Thu Jul 06, 2006 7:58 pm

Capacitors are all about "timing"

When voltage is at one point, current is at the other. [AC]

Caps can bridge this difference with AC.

Caps can change the timing of current and power, and not just in motors.

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Post by cato » Fri Jul 07, 2006 4:20 am

So called "starter caps" do not provide a starting current "kick". They provide a current phase shift between different sets of windings in the motor, which sets up a differential magnetic field, which in turn causes the rotor to turn.

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Post by Gorgon » Fri Jul 07, 2006 5:27 am

In the correct direction. Almost like a 'fake' 3 phase motor.

TOK ;)
Gorgon the Caretaker - Character in a childrens TV-show from 1968. ;)

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Post by k7elp60 » Fri Jul 07, 2006 6:21 am

Going a little bit farther, motor start capacitors are generally high values around 300uF, and are generally switched out of the circuit once the motor is turning.
There are motor run capacitors are also used and are generally about 7uF in value and are always in the circuit. The do the same provide a phase shift for the magnetic field.

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dacflyer
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Post by dacflyer » Fri Jul 07, 2006 9:51 pm

heres a wrench in the works.. what about run caps.. i seen in ceiling fans and such,
the run caps do something a bit differen't
i had one that had a cap that was bad,, couldn't find exact size. i replaced it with one that was a bit bigger...from a 3Uf to a 5Uf.. it worked, but cause the fan to spin a bit faster. and hum a little bit more..
critical ? i do not know,,but it worked for years still...

some motors also have a start / run switch.. i guess the start cap does just about the same thing,,except its not mechanical.

from what i rember. some how the start winding was sort of like a R/C circuit in a way..the start coil was like a resistor. and had a heavy current. thus giving the motor the kick start it needed, untill the cap was charged, then the current to start winding was disapated.
after the motor was turned off, the cap would be discharged.

sounds logical to me anyway.. i asked a buddy here that has been working at a armarture motor shop here.

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Chris Smith
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Post by Chris Smith » Fri Jul 07, 2006 10:16 pm

Timing is everything?

Everybody considers the three or four quadrants of ohms law? [wow?]

So why is time always left out?

Mentality? [small ones]

Some 3d genius said this is all there is to electronics,...mean while Einstien said wake up?

Were still snoozing ,....duhhh

Move on dummies....

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Post by dyarker » Sat Jul 08, 2006 5:18 am

Memory aid for inductors and capacitors in AC circuits: ELI the ICE man.
In an inductor (L) the phase of the voltage (E) leads the current (I).
In a capacitor (C) the phase of the current (I) leads the voltage (E).

In an AC motor with brushes, at turn-on the phases of the current in the stator and rotor are in phase. The may go clockwise, counter-clockwise, or just sit there and burn up. So a start winding is added to stator with a capacitor in series with it. The phase shift of capacitor guaranties that the motor always starts in the same direction and gives torque boost. A centrifical switch turns off the start winding. The phase shift won't be 90° because the capacitor is in series with the winding which has inductance and resistance.

From the description of the ceiling fan, the winding with the capacitor is intended to stay in the circuit all the time ( the fan "... worked, but cause the fan to spin a bit faster ..."). The larger capacitor has less impedance to the AC, so more current flows.

The bit about the cap "charging" and "disapating" would only make sense
for a motor running on DC. The fan is not running on DC, or it would not continue to "spin faster" after the cap charged.

A capacitor across the terminals of a DC motor absorbs noise spikes from the brushes. It does not boost start up. It slows start up a little because current is going into charging the cap instead of through the motor. If there is resistance in series with motor, then a cap in parallel with the resistance would boost start up.

Cheers,
Dale Y

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dacflyer
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Post by dacflyer » Sat Jul 08, 2006 7:37 am

my comments were refering to A/C motors...just to clear up any confusion :)
have a good weekend all !

bearing01
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Post by bearing01 » Sat Jul 08, 2006 2:17 pm

Chris Smith wrote:Timing is everything?

Everybody considers the three or four quadrants of ohms law? [wow?]

So why is time always left out?

Mentality? [small ones]

Some 3d genius said this is all there is to electronics,...mean while Einstien said wake up?

Were still snoozing ,....duhhh

Move on dummies....
You're making capacitors seem like a big deal. It's not. If you want to do hand calculations instead of run spice simulations then yea you have to deal with a first or second order differential equation. However, if you look at it with a Leplace transform then it's very easy.

In an AC system it's not necessarily about timing.

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Chris Smith
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Post by Chris Smith » Sat Jul 08, 2006 5:51 pm

Im sure Tesla rolls over in his grave when he hears, "caps arent every thing"?

The inventor of the poly phase, etc.

Physics and electronics are very specific.

Dont kid your self?

bearing01
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Post by bearing01 » Sun Jul 09, 2006 8:18 am

Chris Smith wrote:Im sure Tesla rolls over in his grave when he hears, "caps arent every thing"?

The inventor of the poly phase, etc.

Physics and electronics are very specific.

Dont kid your self?
Sorry man, you're approaching this way too general (for me) and apparently from a philosophical point of view. Are you a physicist?

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