QotD for 01/31/11: motor capacitor

[derekleffew]

[Pay no attention to the original post date. Standard QotD rules--students only until 02/07/11.]

Hanging off a fan motor is this little black box measuring 1-7/8" x 1-3/8" x 3/4". What is this and what does it do? What do all those numbers mean?

Motor1.jpg picture by derekleffew - Photobucket

Motor2.jpg picture by derekleffew - Photobucket

 

[derekleffew]

Bump, to force the thread to appear as new.

 

[skienblack]

That would be a capacitor. It allows for a momentary bump in voltage to the motor to help it start from a non-moving position.

 

[sk8rsdad]

That would be a capacitor. It allows for a momentary bump in voltage to the motor to help it start from a non-moving position.

Not quite right. I'd be more specific but Derek considers me a professional, and in this instance, he'd be right.

 

[derekleffew]

And the 10µF±5% part?

 

[shiben]

[Pay no attention to the original post date. Standard QotD rules--students only until 02/07/11.]

Hanging off a fan motor is this little black box measuring 1-7/8" x 1-3/8" x 3/4". What is this and what does it do? What do all those numbers mean?

Motor1.jpg picture by derekleffew - Photobucket

Motor2.jpg picture by derekleffew - Photobucket

Its a capacitor with a capacitance of 10 micro farads, plus or minus 5%. It is rated for 250V AC, at 50 or 60 hertz, And should not be used above 70 celcius. The CBB6-1 means its a celing fan capacitor. The capacitor might be working as some sort of frequency filter type thing, I believe they work that way on an AC circuit rather than an energy storage device like it would on a DC circuit. Otherwise skienblack might be correct. Been a while since we did Physics AP E&M.

 

[headcrab]

That capacitor is wired in series with a second winding. Because AC current flowing through a capacitor is shifted in phase (In a capacitor, the current leads the voltage), this second winding, physically rotated from the primary winding, creates a rotating magnetic field which drives the rotor.

 

[LXPlot]

Could somebody put this in very simple terms for those of us whose knowledge of electrics is self-taught or non-existent?

 

[mjw56]

The capacitor corrects for power factor, without digging out a book, it has to do with the way motors cause inductive loads that arent metered well. The utility requires a power factor of .98 or they charge you a premium for the "phantom" power the meter didn't see.

Ill be really surprised if im right as i wouldn't expect such a small motor to do much in a residence.

 

[Chris15]

The capacitor corrects for power factor, without digging out a book, it has to do with the way motors cause inductive loads that arent metered well. The utility requires a power factor of .98 or they charge you a premium for the "phantom" power the meter didn't see.

Ill be really surprised if im right as i wouldn't expect such a small motor to do much in a residence.

Yes you can't discern it from the photo, but this cap will be wired in series.
A PF correction cap will ALWAYS be wired in parallel to the supply.

So keep thinking...

 

[nd925a]

It looks like a relay to me

 

[mjw56]

The Cap in is series with a starter winding (fans have 2 equal windings), the cap applies a phase shift to help start the motor with the starter winding. once up to speed the 2nd winding takes over and the starter operates as an auxiliary winding. When you reverse the fan direction, the 2nd winding becomes the starter winding and the starter becomes the auxillary. the cap follows the starter though so its always there to help start the fan.

 

[TimMiller]

So would a 3 phase motor have the same type of setup as a single phase motor like the fan motor we are looking at? Why or why not? And how is reversing of a 3 phase motor done differently than a single phase motor even if the single phase motor is 220v (2 120v legs)

 

[mjw56]

No a 3 phase wouldn't have the same set up. Because it uses 3 coils which are 120 degrees apart (one for each phase) the motor needs a motor starter. The Starter tuns on the 3 phases in a cascade so that as the sine curve is starting to drop on Phase one, Phase 2's curve is picking it up, and repeated for phase 3. Once the motor is running the motor starter isn't doing anything. To reverse the 3 phase motor the starter just cascades the phases in reverse order. As for reversing the single phase motor, im not entirely sure how that's done, besides just flipping the wires around, which might not wok cause were talking AC here.

In very large motors (think Cement processing plants) a second motor is connected the the main motor shaft to help get it going then is just turned off when the main motor is up to speed.

 

[sk8rsdad]

No a 3 phase wouldn't have the same set up. Because it uses 3 coils which are 120 degrees apart (one for each phase) the motor needs a motor starter.

A 3-phase AC motor does not require a motor starter, except maybe in situations where a very high starting torque is required. The magnetic field in any polyphase motor is inherently rotating because of the displacement of the current between the 3-phases. This rotating field is usually adequate to allow a polyphase motor to self-start.

Single-phase motors, on the other hand, cannot establish a rotating magnetic field without some assistance, hence the reason there is a starting capacitor in the picture that started this thread.

Animation courtesy Wikipedia:

 

[TimMiller]

Upon reading the way I posted this I can understand how I came across a little confusing. I put the questions out there so that some of the new people who are looking to learn can see some of the differences in single vs 3 phase motors since in theater and most other environments you run into 3 phase a lot more than you do single. I'm sure I will put more power related questions into the question of the day.

Yes 3 phase motors can start on their own. In extremely large loads a capacitor bank will be installed with the motor so you don't load down your panel too bad when starting. Though i have started 100+ hp motors under load without a capacitor bank no problem. Now about the single phase 230v motor no matter which way you swap the two hot wires the motor will still turn the same way. There are some that let you retap for direction but most are fixed.

 

[epimetheus]

Yes 3 phase motors can start on their own. In extremely large loads a capacitor bank will be installed with the motor so you don't load down your panel too bad when starting. Though i have started 100+ hp motors under load without a capacitor bank no problem. Now about the single phase 230v motor no matter which way you swap the two hot wires the motor will still turn the same way. There are some that let you retap for direction but most are fixed.

Capacitor banks installed with large motors are most often used for power factor correction rather than starting assistance. Depending on the motor type and size, larger motors are started with the assistance of reduced voltage starters or variable frequency drives. The robustness of the source has the most to do with whether you need starting assistance to bring a large motor up to speed.