Monophase AC motor types using a run winding and a start winding comprise several types based on splitting the phase. The start windings determine the direction of rotation and the induced magnetic field is often amplified with a capacitor in series with those windings. In Capacitor Start motors those windings are shut-off at some percentage of rated RPM. In Capacitor Start/Capacitor Run motors those windings remain active. I believe the Permanent Split Capacitor (PSC) motors are of that CS/CR type. If there is some subtle difference necessitating the name change, I haven't detected it.

My question has to do with the design of these PSC motors and their required capacitors. What I understand about capacitors is that they store a number of electrons at some electromotive force (voltage) and release them (I guess as the AC polarity reverses). With the required capacitor rating example of: 7.5 mfd 370 volt, does that capacitor power the starting winding to 370 volts regardless of the initial "charging" voltage of the circuit? If so, wouldn't changing to a higher rating, say: 7.5 mfd 440 volts, burn-out the start winding insulation prematurely? What is this "dual" rated example (since it doesn't seem to be for the especially confusing "two value capacitor" type motor:

http://www.newark.com/NewarkWebCommerce/newark/en_US/support/catalog/productDetail.jsp?id=46F1246

I am sure the questions seems incompetent to those knowing the answers; hence my handle - "Confused". Thanks for your response.

 

The short answer: no, The rated voltage is what the capacitor will routinely withstand without failure. It should be higher than the line voltage because under certain conditions it will see a voltage higher than the line voltage. But a 440 V rated cap will perform like a 370 V rated cap if their capacitance value is equal. And your theory of operation is interesting, but needs some research.

Regards

cww

 

Not getting any responses... rough calculation yields a +.5 milliamp/sec surge difference from changing from a 7.5 uF 370 volt up to a 7.5 uF 440 volt capacitor. A negligible number to the winding, if I did the calculation right. Other opinions welcomed!

 

All solved now -

The capacitor named voltage is the voltage limit that will breach the dielectric and ruin the capacitor.

The capacitor charged by 240 volts bleeds off from 240 volts. Circuit voltage remains normal.

It is ok to use higher rated voltage capacitors in circuits as long as the rated uF (MFD) remains the same.

In PSC type (+ others) motors the capacitor functions to stall the electron flow and thereby shift the phase in the auxiliary windings. The shift allows for the induced magnetic fields from the main and then the auxiliary windings to force the rotor to turn. The auxiliary (start) windings kind of give the rotor a push.

 

Responding to Confused's May 10 query & May 13 comments:

If I have interpreted you correctly... you want to know if changing of the start capacitor's rated voltage will impair the motor? To impart some basic knowledge while dispelling some of the misconception advanced thus far, following is a qualitative discussion. Without knowing the
motor's parameters a quantitative analysis can not be presented.

Basic Information.
A split-phase motor has two windings... one called the "main" the other the "auxiliary." Their relative displacement in the stator results in their respective currents to be displaced by some angle, usually less than 90 degrees. A starting torque is produced by the two-phase rotating magnetic field in the stator. However, this method has severe limits regarding the maximum starting torque that can produced. Furthermore efficiency and power factor suffer because of the continuously energized auxiliary winding.

To overcome the negative effects noted above, a capacitor is inserted in the auxiliary winding. It can be permanently connected, or just during the starting mode. The former is called the permanent split-phase motor. The latter is called the split-phase capacitor-start motor and
the auxiliary winding is renamed the "start" winding.

The Start-Winding Circuit.
Consider it an R-L-C circuit where R and L represent the winding, and C the capacitor. Starting current, then, is proportional to the vector addition of Rw, Xw, and Xc. It is obvious, then, that the capacitor's impedance influences the starting current. The question now is... does a change of the capacitor's voltage rating affect starting current? If so, how?

Life Expectancy Improved.
No one will argue that increasing the capacitor's rated voltage will reduce the stress factor, thereby increase its life expectancy. But is there a negative side?

Rating Can Influence Current
Of course the classic equation for determining capacitor impedance is,

Xc = 1 / (2 x pi x f x C), Ohms

But, the above is based on rated nameplate voltage, Vr. If the applied voltage, Va, differs from Vr, then Xc must be multiplied by the factor, F, where,

F = (Va/Vr)^2

Thus, the adjusted impedance, Xc', is,

Xc' = F x Xc

Will the above significantly affect current? Jeopardize winding integrity? Or alter starting torque? Probably not! In fact, Confused, it will be a lot quicker to measure the current change, than it is to calculate it.

Confused, your 0.5mA / sec "surge" dimension is unclear? Is it a result of the starting current comarison measurement? Or the difference obtained when connecting the two capacitors to some test voltage?

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected]) {[email protected]}

 

Corrections to my Mon, May 17, 2:56pm reply:

1) Per "The Start-Winding Circuit." 2nd sentence should be:

The start-winding current is inversely proportional to the vector addition of Rw, Xw, and Xc.

2) Per "Life Expectancy Improved." 1st sentence should be:

No one will argue against increasing the capacitor's voltage rating to reduce its stress factor, thereby increasing its life-expectancy.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected]) {[email protected]}

 

I have a Taco 007 Cartridge Circulator and it has a "Permanent Split Capacitor" motor. My question is how does the capacitor work with the motor? Is the the capacitor connected in series or in parallel with the motor? If I take the capacitor out, will the motor still run (turn)?

 

Responding to R. Louie's questions:

Q1) How does capacitor work in a split-phase motor?

A1) In retrospect, it is obvious to me that my May 12th reply was long-winded. Here is a simpler version:

The motor has two windings called he "main" and "auxiliary!". They are electrically connected in parallel and the supply voltage is simultaneously applied to both. Together they produce a starting torque in the desired direction of rotation.

Q2) Is the capacitor connected in series or parallel with the motor?

A2) It is inserted in series with the auxiliary winding.

Q3) Will the motor run without the capacitor?

A3) No! At least not automatically. However, it can be made to run by simply spinning it, manually, in the desired direction of rotation. For obvious reasons, this procedure mentioned is not recommended.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])