What's a starter cap do anyways?

[Rod Sheridan]

The starting capacitor does two things for the motor;

- it has the correct capacitance to cancel the inductance of the start winding, which results in the current being limited by the winding resistance, as opposed to inductance. This greatly increases the winding current and motor starting torque.

- the cancellation of the start winding inductance by the capacitve reactance causes a phase shift between the run winding current and the start winding current. (Run winding current is phase shifted due to the run winding inductance).
The phase shift between the two windings creates a rotating magnetic field, so the rotor can spin instead of sitting there vibrating.

Regards, Rod.

P.S. A cap start/cap run motor has a second continuously rated AC capacitor that stays in series with the run winding at all times. It provides a small current, phase shifted in relation to the run winding current. This improves the motor smoothness and efficiency.

[Mitchell Andrus]

A minor clarification:
The starter cap is NOT charged when the motor starts. Charging it up takes some energy away from one set of windings, causing less torque to be applied in one direction, causing the motor to always begin to spin the preferred direction.

Ooooohhhh. So it's like when Grandpa pulled on a rope to start the lawnmower. Kinda like that. Kinda.

I though that fairy story was a bit 'off'.
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[Jim Rimmer]

The starting capacitor does two things for the motor;

- it has the correct capacitance to cancel the inductance of the start winding, which results in the current being limited by the winding resistance, as opposed to inductance. This greatly increases the winding current and motor starting torque.

- the cancellation of the start winding inductance by the capacitve reactance causes a phase shift between the run winding current and the start winding current. (Run winding current is phase shifted due to the run winding inductance).
The phase shift between the two windings creates a rotating magnetic field, so the rotor can spin instead of sitting there vibrating.

Regards, Rod.

P.S. A cap start/cap run motor has a second continuously rated AC capacitor that stays in series with the run winding at all times. It provides a small current, phase shifted in relation to the run winding current. This improves the motor smoothness and efficiency.

So, when do the fairies run out?

[glenn bradley]

Great explanation, but you've got way too much time on your hands Glenn!

What's frightening is that my years in management (before I decided to do go back to doing something useful) has trained me to spew meaningless gibberish like this for what seems like hours while other folks in business suits would sit around and nod in a reverent and knowing way so that all their counterparts would think that they are wise and all-knowing and should be handed large sums of money to keep their worsted wool pressed, their wingtips shiny, their projects behind schedule and over budget but their bosses happy.

[Joe Chritz]

What's frightening is that my years in management (before I decided to do go back to doing something useful) has trained me to spew meaningless gibberish like this for what seems like hours while other folks in business suits would sit around and nod in a reverent and knowing way so that all their counterparts would think that they are wise and all-knowing and should be handed large sums of money to keep their worsted wool pressed, their wingtips shiny, their projects behind schedule and over budget but their bosses happy.

This is likely the most intelligent thing I have read in months.

Joe

p.s. Monty Python still makes me laugh until my stomach hurts.

[Ken Fitzgerald]

What's frightening is that my years in management (before I decided to do go back to doing something useful) has trained me to spew meaningless gibberish like this for what seems like hours while other folks in business suits would sit around and nod in a reverent and knowing way so that all their counterparts would think that they are wise and all-knowing and should be handed large sums of money to keep their worsted wool pressed, their wingtips shiny, their projects behind schedule and over budget but their bosses happy.

So you are the guy that taught them that spew I have had to listen to for the last 34 corporate years?

[Van Huskey]

Glenn...


The key here is 3 phase motors can be started direct on line due to the method of electrical delivery.

Single phase induction motors need a "bump" to start and can spin in either direction. Larger non-shaded pole motors like we deal with have a second stator winding which is sent the oit of phase current from the starter cap to get it moving and moving in the correct direction.

A simple analogy is the battery/starter in a car, it helps the engine overcome the starting torque so the internal combustion portion can start providing the power and starts the engine in the correct direction...

[Rod Sheridan]

So, when do the fairies run out?

Why at dusk of course, that way the wee folk aren't seen by the muggles.......

[Ken Fitzgerald]

Please Rod....don't introduce "Muggles' to this. I am already having trouble understanding the fairy theory of starting caps. Please don't add in any more varaibles.

[Lee Schierer]

Thankfully my tea is still too hot to drink. And to think I almost didn't read this post because, I couldn't remember the exact technical explanation. I too like the fairy theory.

Maybe Glenn can explain how transitors or electron tubes work.....

[Mitchell Andrus]

Maybe Glenn can explain how transistors or electron tubes work.....

That's easy. Scientists shrink Beefeater guards that are raised on farms in Great Brittan. They stand guard at a gate and only allow (or stop) the flow of electricity if the guard is tickled with a little electricity. The jolt of electricity has to be just forceful enough to convince the guard to change his gate and allow or stop the flow. Some guards wear thick armor, some very thin armor so it takes differing amounts of electricity to convince the guard to change the gate.

Because the guards are so tiny, they can open and close the gate thousands of times per second. Some guards open their gates, some close their gates depending what type of gate they're guarding.

In this way, a very tiny amount of electricity can control a much larger amount of electricity allowing us to crank the volume when Apollo 13 blasts off... now on Blu-Ray.

One of the reasons stuff built with transistors are so much smaller than stuff built with red-hot vacuum tubes is that the guards needed air conditioned space suits so they could work in the hot environment and that added a LOT of weight. That's why we don't jog with those console radios of the 40's anymore.

See?

Now ask about diodes.
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[Ken Fitzgerald]

Now ask about diodes.
.

I'll guess. They are little bitty guards that are like me? One-way and bigoted? If they are zener diodes they are biased?

[Ken Fitzgerald]

BTW.....You are missing David DeCristoforo's dissertation based on extensive testing Mitch. Check this thread: http://www.sawmillcreek.org/showthread.php?t=143490

[Mitchell Andrus]

If they are zener diodes they are biased?

Depends on whether they're mounted on the board parallel to an edge or not.
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[Ray Bell]

It is truly amazing what you learn on the site. I am recommending this place to all of the EE's that I know.