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#72573 - 12/04/06 02:51 PM Multi-speed PSC Motor Windings  
wendel  Offline
Member
Joined: Aug 2004
Posts: 55
On a typical HVAC 3 speed PSC motor are speeds determined by individual windings with only the neutral being common to all or are there taps off one large winding?


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#72574 - 12/05/06 11:06 AM Re: Multi-speed PSC Motor Windings  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA
wendel,

Brief answers, to your question regarding the PSC Motor:

Quote

On a typical HVAC 3 speed PSC motor, are speeds determined by individual windings - with only the neutral being common to all; or are there taps off one large winding?


The PSC (Permanent Split Capacitor) type Single Phase Induction Motor achieves a reduced speed, by reducing the Voltage observed at the Motor's Main and Auxiliary Windings.
The PSC Motor uses an intregal Autotransformer for speed control selections, rather than including multiple Poles for altering the Frequency - as would be done with other types of Squirrel-Cage, "Short-Circuited Rotor" type Induction Motors.

The Autotransformer is wound to the same Stator Assembly, as the Main and Auxiliary Windings are wound to.
The Autotransformer is connected to the Main and Auxiliary Motor Windings' "common" connection point on one side, and brings in one side of the 2 wire circuit through one of the tapped speed selection leads (or directly to the Motor Windings for full speed).

The Autotransformer is tapped at various points along the Winding's "Build", relative to the number of turns required for a certain Voltage.
These taps are connected to the AC input through a selector switch, which allows only one lead to be connected at a time (or no leads connected to turn the Motor off).

Example Speed Selections For A "3-Speed PSC Motor":

<OL TYPE=A>

[*] The "Full / High Speed" input selection tap is made between the "_END_" of the Autotransformer Winding, and the "_BEGINNING_" of the Main Motor Winding.
This selection point bypasses the speed control Autotransformer, and runs the Motor at Designed RPMs - applying full Voltage to the Motor Windings,


[*] The "Medium Speed" input selection tap is made on the Autotransformer's Winding - at apx. 50% of the Winding's length, which reduces the Voltage at the Motor's Windings by apx. 20% (+ or -),


[*] The "Low Speed" input selection tap is made at the "_BEGINNING_" of the Autotransformer's Winding - the opposite end from which the Motor Winding leads are connected. This reduces the Voltage at the Motor's Windings by apx. 40% (+ or -).
</OL>

The remaining Motor Windings' "Ends" are connected directly to the "2nd Conductor" of the 2 wire AC circuit.
Typically, on Circuits using a Grounded Conductor for "L-N" 2-Wire AC Circuitry, this "2nd Conductor" is the System's Grounded Neutral Conductor.
For "L-L" 2-Wire AC Branch Circuitry, no Grounded Conductor is utilized on the Circuit, so that "2nd Conductor" is an Ungrounded Conductor.

Since there is no relative "Polarity" involved with a 2-Wire AC Circuit, it does not matter which Conductor is terminated to any certain Motor Input Lead. Direction of Motor rotation depends on "Which Side Of The Main Winding Gets Reduced By The Auxiliary Winding", not by the 2-Wire AC input Circuitry.
(i.e. The input 2-wire circuit's terminations to the Motor's input may be reversed, with no change in direction for the Motor's rotation).

Attached are a few Schematics, which describe some Single Phase Induction Motors.

Feel free to comment or ask additional questions, regarding this topic &/or the Schematics.

Scott35

Images Below...........................

[Linked Image]

Fig. 1.1: PSC Induction Motor Circuit Diagram-C.C.W Rotation (normal rotation)

"L1" + "L2" = Motor Input Line Leads,
"1" = Main Winding,
"2" = Auxiliary Winding,
"3" = Capacitor (always in circuit),
"4" = Squirrel Cage rotor,
"5" = AC Line Input,
"6" = Intregally Wound AutoTransformer (for speed control).

[Linked Image]

Fig. 1.2: PSC Induction Motor Circuit Diagram-C.W Rotation (reversed rotation)

"L1" + "L2" = Motor Input Line Leads,
"1" = Main Winding,
"2" = Auxiliary Winding,
"3" = Capacitor (always in circuit),
"4" = Squirrel Cage rotor,
"5" = AC Line Input,
"6" = Intregally Wound AutoTransformer (for speed control).

[Linked Image]

Fig. 1.3: Speed Control Switch Detail

*Note: The AutoTransformer shown in Fig. 1.1 and 1.2 is wound onto the same "core"
as the Motor's Stator windings. Additional taps may be inserted for increased levels of speed control.
The "100%" tap is both the end of the AutoTransformer and the Motor's Lead.
Connecting directly to this lead will result in full Motor speed.

[Linked Image]

Fig. 1.1: Capacitor Start Induction Motor Circuit Diagram-C.C.W Rotation (normal rotation)

"L1" + "L2" = Motor Input Line Leads,
"1" = Run (Main) Winding,
"2" = Start (Auxiliary) Winding,
"3" = Centrifugal Start Switch,
"4" = Squirrel Cage rotor,
"5" = Start Capacitor,
"6" = AC Line Input.

[Linked Image]

Fig. 1.1: Capacitor Start/Run Induction Motor Circuit Diagram-C.C.W Rotation (normal rotation)

"L1" + "L2" = Motor Input Line Leads,
"1" = Run (Main) Winding,
"2" = Start (Auxiliary) Winding,
"3" = Centrifugal Start Switch (shown in the "Run" position, connecting the "Run" Capacitor),
"4" = Squirrel Cage rotor,
"5" = "Run" Capacitor,
"6" = "Start" Capacitor,
"7" = AC Line Input.

[Linked Image]

Fig. 3.1: Shaded Pole Induction Motor Circuit Diagram

"L1" + "L2" = Motor Input Line Leads,
"1" = Run (Main) Winding,
"2" = Shading Coil (Auxiliary Winding),
"3" = Short Circuiting Connection,
"4" = Squirrel Cage rotor,
"5" = AC Line Input.

[Linked Image]

Fig. 1.1: Resistance Start Induction Motor Circuit Diagram-C.C.W Rotation (normal rotation)

"L1" + "L2" = Motor Input Line Leads,
"1" = Run (Main) Winding,
"2" = Start (Auxiliary) Winding,
"3" = Centrifugal Start Switch,
"4" = Squirrel Cage rotor,
"5" = AC Line Input.

---end


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#72575 - 12/05/06 03:37 PM Re: Multi-speed PSC Motor Windings  
wendel  Offline
Member
Joined: Aug 2004
Posts: 55
Many, many thanks, Scott. That's a great explanation. Always nice to learn and understand new things. The bottom line for my purposes is if any speed tap demonstrates a short to ground then all speeds are condemed to the same fate.


#72576 - 12/08/06 07:51 PM Re: Multi-speed PSC Motor Windings  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA
Glad to help out here Wendel!

As to the follow-up question:

Quote

The bottom line for my purposes is if any speed tap demonstrates a short to ground then all speeds are condemned to the same fate.


A:
_ If the Motor is energized, and the Tap which has the Ground Fault _"IS NOT"_:

- The Selected Speed Tap,
or
- The "High Speed" Tap;

Then the Autotransformer Windings will have circulating Currents flowing from the Motor Lead, to "Ground" - via an EGC.
This will not have an affect on the Motor's performance - UNLESS - the Autotransformer becomes excessively hot - at which point will cause heat issues throughout the Motor.

B:
_ If the Ground Fault occurs on the Winding - AHEAD OF - the Speed Selection Tap point, this would dramatically affect the Motor's performance, as the Motor Windings would see an extremely reduced Voltage.

C:
_ If the Ground Fault occurs on "The Selected Speed Tap", then this would be a Short Circuit, which "Should" trip the OCPD.

D:
_ If the Ground Fault occurs on the "High Speed Tap", and the Speed Selection is made to one of the Tap points "Ahead Of" the High Speed Tap, then the Motor would see very low Voltage (if any).

E:
_ If the Ground Fault occurs on the "High Speed Tap", and the Speed Selection is made to the "High Speed" point, this would be a Short Circuit Ground Fault.

Hope this makes sense!

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!


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