Dazed and Confused - About Capacitators

When I was in school learning about electricity and motors, there was a saying: "ELI the ICE man".
What this referred to was the difference in Inductive (L) circuits and Capacitive (C) circuits in that Voltage (E) leads Current (I) in an inductive circuit and Current (I) leads Voltage (E) in a capacitive circuit.
In a motor, voltage is basically the force that results in the "PUSH/PULL" capability of the motor. So in an Alternating Current (AC) circuit, voltage builds to a maximum, returns to zero, then builds to a minimum, and returns to zero.
By adding a "Starting Winding" with a capacitor in the circuit, the voltage downstream of the capacitor builds slower than in the "Run" winding circuit. This, in effect, turns your power supply into 2 phases instead of 1.
To picture this, think of riding a tricycle or a bicycle where the pedals are connected directly to the wheel. Think of your legs as the voltage. Now think of trying to start peddling starting on an uphill slope (representing starting load). Now imagine trying to peddle using only one leg.
You can see that if you started on an even slope (no load) or downhill slope (already spinning), this could be possible depending where the peddle was positioned when you started to push. If the peddle started in the down position, you can push all you want, but won't go anywhere.
Now we add the second leg (starting winding), and now we can get going. Once in motion, we can use just the one leg to keep moving.
Yes, on a bicycle, using both legs all the time is easier, but think about it if the peddles were just a few degrees apart instead of 180 degrees apart. That's why the starting winding cuts out once the motor is close to full speed.
This is why a 3 phase motor doesn't require a starting capacitor. In the example, think of 3 peddles 60 degrees apart and three legs doing the peddling. Regardless of where the peddles start out, at least 2 of the legs are able to get it going.
 
JPigg55 said:
When I was in school learning about electricity and motors, there was a saying: "ELI the ICE man".
What this referred to was the difference in Inductive (L) circuits and Capacitive (C) circuits in that Voltage (E) leads Current (I) in an inductive circuit and Current (I) leads Voltage (E) in a capacitive circuit.
In a motor, voltage is basically the force that results in the "PUSH/PULL" capability of the motor. So in an Alternating Current (AC) circuit, voltage builds to a maximum, returns to zero, then builds to a minimum, and returns to zero.
By adding a "Starting Winding" with a capacitor in the circuit, the voltage downstream of the capacitor builds slower than in the "Run" winding circuit. This, in effect, turns your power supply into 2 phases instead of 1.
To picture this, think of riding a tricycle or a bicycle where the pedals are connected directly to the wheel. Think of your legs as the voltage. Now think of trying to start peddling starting on an uphill slope (representing starting load). Now imagine trying to peddle using only one leg.
You can see that if you started on an even slope (no load) or downhill slope (already spinning), this could be possible depending where the peddle was positioned when you started to push. If the peddle started in the down position, you can push all you want, but won't go anywhere.
Now we add the second leg (starting winding), and now we can get going. Once in motion, we can use just the one leg to keep moving.
Yes, on a bicycle, using both legs all the time is easier, but think about it if the peddles were just a few degrees apart instead of 180 degrees apart. That's why the starting winding cuts out once the motor is close to full speed.
This is why a 3 phase motor doesn't require a starting capacitor. In the example, think of 3 peddles 60 degrees apart and three legs doing the peddling. Regardless of where the peddles start out, at least 2 of the legs are able to get it going.
Click to expand...

Hey Jpigg,
lol I remember ELI the Ice man... And " Brown Beer Rots Our Young Guts " for resisters, we had a lots of sayings In school I cant remember anymore cause it was 36 yrs ago, but ELI the ICE man I do... we had an Italian kid that made us all laugh when he would say something like, Don't Make Me Call The ICE MAN..

Thanks Jpigg, that brought back some fond memories of school days gone by.
 
Been dazed and confused for so long it's not true
Wanted a capacitor, never bargained for you
Lots of people talk and few of them know
Soul of a capacitor was created below....
 
[QUOTE="Susan_in_SF, post: 529126, member: 47131". With all the AC motors in my garage, none of them, except for this one and for my little surface grinder, has capacitors. I tried reading up on it via google, but didn't find answers to my questions. So, I hope someone here can educate me.
.[/QUOTE]

Oh... lecture mode ON:
AC motors are like a bicycle crank; you move the pedals up and down, and the shaft goes around.
Unlike bicycles, though, the clockwise motion that you want is NOT guaranteed by the relatively
well-trained human who knows better than to pedal backward. In order to ensure motors run in
the direction of choice, it's necessary to fiddle the timing of two distinct windings (like, two pedals)
so that the DOWN phase of each only occurs when that direction of force is appropriate for the
clockwise motion we want. You need a time delay.

For three-phase power, three different times (three different phases) are available, and you can
have a perfectly good motor without any other time-adjusted power signals. Three-phase motors
work fine with no capacitors. But single-phase motors DO require at least one
time-shifted powered motor winding, and this is done by
(1) adding inductors
(2) adding capacitors
(3) adding switches ( this is the closest to how a human controller keeps pedals going clockwise).

The 'inductors' solution only gives a time delay if you also rely on electrical resistance to
set a time constant, which means it creates waste heat: this is OK for a clock motor,
but not seen in large motors. Shaded-pole clock motors have two metal-wrapped-around-iron
things that don't LOOK like they're electrical parts at all... but they are.

The 'switches' solution is usually implemented with a commutator and brushes which is complex.
but allows high speed: this is the efficient way to get a fast-turning Dremel tool.

The 'capacitors' solution can either use wire resistance, OR inductance, to get a time delay. If you
use wire resistance, the delay is power-wasting, so those motors use a START capacitor that
is switched OFF after a brief turnon period. Flywheel effect is relied on to keep the motor turning.
But, if you use inductance and a capacitor, a RUN capacitor, the torque is smooth, the motor can
run quietly and efficiently. Until, that is, the capacitor expires (they're inexpensive, and compact,
if you just allow for only-a-few-years lifetime).

Lecture mode OFF

START capacitor motors usually have a centrifugal switch, that can get dirty, and many woodworking
tools get sold cheap when a bit of sawdust gets inbetween the switch contacts. Switch
cleaning and capacitor replacement are routine maintenance. START capacitors,
if they fail, result in a motor that hums but literally doesn't know which way to turn. And
RUN capacitors handle a high current, if they fail the motor hums or stalls under load.
Sometimes, there's both a START and a RUN capacitor.

So, mainly, people prefer three-phase motors, if they can get 'em.
 
Wow. I am so touched (in a non-sexual harassing way) by all the lovely responses :) . I'm still dazed in terms of when do I need to install a capacitor to my motor. In my garage, I have tons on old motors laying around. None of them have capacitors hanging anywhere.
I have 2 vertical bandsaws (1 for wood resawing, 1 for metal). I would like to upgrade the motor in my wood bandsaw to a motor with a little more hp.

Do I need a capacitor for the upgrade motor I put into the wood bandsaw? The old motor in the bandsaw now has no capacitor, juse like my tablesaw.

Oh, and then there's the issue with treadmill motors. I have a few KB Electronics pwm drives I was planning to install with treadmill motors to get variable speed out of my vintage machines (SB lathe, Craftsman metal bandsaw). I just assumed all I needed was the drive and the motor.

Am I assuming wrong?

Life was so much easier before I thought about capacitors. Ignorance is bliss - fo' realz. :)

Susan
 
Hi Susan,

You shouldn't need much other than the KB drives to get the treadmill (DC) motors running, capacitors are only needed in a subset of AC single-phase motors (and usually come with them, in a little "dog kennel" attached to the motor casing).

What is a Good Idea with the KB drives is fuses - one on the incoming power (or if it's a 220v in drive, one on each leg) and one between the drive's output and the motor - this could save either the motor or the drive if/when the other fails.

I'm assuming you know you're going to want a speed control potentiometer (usually 5 K ohms, 2 watt rating, MUST be a linear taper or all the speed adjustment will be in the last few degrees of rotation, Strongly Recommend a plastic shaft as the potentiometer sits at AC line voltage!) and a 2-pole on-off-on switch that can take the full motor current if you want to be able to run the motor in reverse. Insulate the potentiometer and switch very well, 220v DC can give nasty surprises...

Dave H. (the other one)
 
OK, if they don't have capacitors fitted, and no sign of them ever having been fitted, they probably don't need them. Some (but not all)capacitor type motors will give details of the capacitor size needed on the motor rating plate, this will be a number followed by (usually) mFd (microFarads) sometimes also this symbol -| |- which is the ISO symbol for a capacitor . Capacitor start increases the starting torque, so if the motor is started on load, like a compressor, or any other machine that a substantial part of the machine starts turning with the motor, you will need a good starting torque motor, which probably means one, or sometimes two capacitors. Machines that start with no load, ie where the load is switched in after the motor is running at full speed, just have a start winding which is switched out of circuit when the motor is up to speed. Some of these also use a capacitor. In your case, the thing to do is try the motor on the bench, and see if it starts and runs up to speed without any problem (make sure it is properly grounded) leave it running for say 30 minutes and check with a clip on ammeter that it is running below the current on the rating plate, which is marked as FLA, or Full Load Amps., If it starts, runs, remains cool (not cold, there will be some heat) and no other problems occur, try it on the machine. If it bogs down on starting and fails to accelerate, or just sits and hums, turn off, you need more starting torque. The above description of single phase motor types and starting systems is by no means complete, but I feel it covers the most common types. Experiment! Always make sure that the polarity of the equipment is correct, and most importantly that the motor AND the machine are properly grounded. From my college days, we used "Beer bottles rolling over your garden because people got wet" to remember resistor colour code.
Phil, UK
 
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