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Why PSC PERMANENT SPLIT PHASE (SINGLE PHASE) MOTORS HAVING LOW SPEED COMPARED TO OTHER SINGLE PHASE MOTORS .

PRAVIN_KHARDIKAR

Welcome to ECN!

I am unsure exactly what your Question is directed towards, so could you please add some more information?

Basically, the PSC Motor is an excellent candidate for Fan use - especially Axial Blowers ("Propeller" Fans).
These Blowers are normally used for Applications requiring larger volumes of Air at lower pressures (as compared to Centrifugal Blowers / Fans), so high speeds are not necessary.

Additionally, a direct connection may be done to an Axial Fan Blower with the PSC Motor, instead of connecting through belt an pulleys to achieve a low top speed.

Maximum RPMs (highest speed) depends on the number of Poles on the Stator.
I have not seen a 2 Pole PSC Motor (would have Max. high speed of 3600 at no load, slipping down to around 3450 RPMs).
Seen 4 Pole (1800 RPM), 6 Pole (1200 RPM) and 8 Pole (900 RPM) PSC Motors.

I think the typical Ceiling Fan would be a 16 Pole Motor, with a Maximum high speed of 450 RPMs.
These have speed controls via an Auto transformer wound into the Stator assembly.

Looking forward to your reply.

Thank you.

Scott

They're used because they're cheap.

I understand it as mostly having to do with the way the armature is wound and mechanical forces; salient-pole motors, with the coils simply just wrapped in loops around the rotor like it's a rotating bar magnet, are not practical for high speeds, but are cheap to manufacture. The magnetic field offered by a cheap permanant magnet inherently limits the power and speed possible, too, but is cheaper to manufacture for small motors than other types of cores.

RPMs? or RPM (Revolutions Per Minute)

(minor pet peeve)

Larry C

Hi ,

Question is like ....why PSC Motors having lower RPMs compared to other motors ?

More poles reduces the speed.

2 poles 3600 rpm slipping to 3450 typically
4 poles 1800 rpm slipping to 1725 typically
6 poles 1200 rpm slipping to 1150 typically

and so forth....

LarryC,



Feel the same way, just can't stop myself with the RPM pluralisation!
It comes out naturally, and I do not even think about it until later

Some of my Pet Peeves:

* Calling AA Dry Cells "Batteries"
(get some wierd looks when requesting "Double A Dry Cells")

* Calling Electric Lamps "Light Bulbs"
(plant the "Bulb" and it emits Light???)

* Calling a single Locomotive or a single Railcar a "Train"
(there are a lot of Trains in that Classification Yard!)

* Someone mistaking the name "AutoCAD" as if the CAD Package is completely "Automatic", expecting instant results
(I sit down at my Work station, and exclaim: "Computer; Draw Project 'X'".
I sit back, doing nothing, and wait while "The Computer" completely Engineers & Drafts the entire Project + Prints all Sheets out perfectly - all in less than 5 minutes.
I submit Bill for $10,000.00)

* Referring to Applications / Routines / Functions / Network Services as "The Computer"
(Does Your Computer have the Internet?)

* Taking the topic of a Thread wayyyy off topic ... OOPS, I just did that!

With all this said, I wonder if the OP's question had been successfully answered?

Looks like we went to as many extremes possible, without too much jargon.

Most concise answer is: They run slow because they just do!

Scott

I think you are thinking of a Shaded Pole motor. PSC stands for Permanent Split Capacitor. They are NOT necessarily cheaper than Cap-Start motors and have less starting torque, but they have the advantage of no moving parts (other than the rotor of course).

Permanently Split motors are quite prevalent.
Where they fall down however, is with constant stopping and starting.
They can't be used on a variable frequency drive, because, by design, it will burn the bearings out over time, with circulating currents.
This is why the Induction motor (Cage-type) has become the work-horse of industry.
With a VFD, you can have any speed you like, you can ramp up to speed and ramp down again.

Actually, of all of the different types of 1 phase motors, PSCs are one of only two that CAN be used with 1 phase VFDs. The other is Shaded Pole type, although those can also be controlled with simpler traic type voltage control, aka Dimmers.

The circulating current issue is potentially true for all AC motors run by VFDs; cage type, wound rotor, synchronous, 3 phase or 1 phase. But it can also be easily mitigated.