PSC motor design 1

[Confused2]

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:

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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 - "Confused2". Thanks for your response.

 

[jbartos]

Suggestions to Confused2 (Mechanical) May 11, 2004
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.
///Yes, there is. Reference:
A.E. Fitzgerald, Charles Kingsley, Jr., Stephen D. Umans "Electric Machinery," 6th Edition, McGraw-Hill, 2003,
Chapter 9 Single- and Two-Phase Motors beginning by page 452
\\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?
///Capacitor(s) provide a phase shift necessary to create rotating electromagnetic field suitable for the motor rotor to start turning.\\ If so, wouldn't changing to a higher rating, say: 7.5 mfd 440 volts, burn-out the start winding insulation prematurely?
///It depends. Contact the motor manufacturer tech support. Also, post the motor nameplate data.\\

 

[Confused2]

I thank you for your response . I understand the split/shift phase function of the start winding and the capacitor amplification of the field . The motor is of the 370 volt PSC series and operates 208-230 volt (1/2 HP ; 2.9 amp ; 1075 rpm ; service factor 1.0 ; 60 Hertz single phase ; thermal protection - auto ; direction - reversible ; 7.5 uf 370 volt capacitor not supplied) . I can't reference the Electric Machinery book you described , just now . The questions : will the difference in voltage 370 to 440 cause a current surge and burn the start circuit windings prematurely ? Does the capacitor put out at it's named voltage when the polarity collapses sixty times per second ?

Thanks again

 

[sreid]

The voltage rating of the capacitor does not alter the electrical function of the capacitor. The voltage rating determines what maximun voltage the capacitor can witstand without blowing up. A 440 Volt capacitor can be used in place of a 370 volt capacitor but not conversly.

 

[Confused2]

Thanks sreid ! That is what the guy wanting to sell them to me is saying , also . It somehow shreds my understanding of such things ; but , may well be the case . According to jbartos (above) , when he wrote "it depends" , it seems a question still open to conjecture . Curiously , what did you make-out of that capacitor I referenced by http address ?

 

[sreid]

If the physical size works for your application, it should work just fine for replacing a 7.5 MFD start/run capacitor.

 

[Confused2]

I just made a rough calculation based on the voltage surge . I calculated the surge difference due to the change from 370 to 440 volts at 7.5 uF to be like +.5 milliamps/sec ; which , would be a negligible number to a winding . If I did it right , I guess the capacitors are interchangable in my understanding . Any other opinions welcomed .

 

[jbartos]

Comment: The capacitor value at your motor has to have a appropriate value that is determined by the motor engineer or designer, namely, the larger capacitor microfarads will reduce Xc=1/(2pifC). Then the auxiliary winding that is in series with the capacitor will receive the higher voltage across its ends. The higher voltage will cause the higher current flow through the auxiliary winding, which in turn will cause a winding temperature increase. The auxiliary winding might potentially be thermally damaged, if the larger capacitance capacitor is applied.

 

[busbar]

From NEMA MG1: “…Permanent-Split Capacitor which has the same capacitance for both..” There is no switching involved—one capacitor is rated for continuous duty and applied voltage is likely ~99% dependent on motor design. (For PSCs, the terms are often main/auxiliary windings; versus start/run.)

 

[Confused2]

Wow ! I haven't made the deal on the capacitors yet ... so ... you are describing the circumstance as I thought I understood it . If this were an election with me abstaining , however , there would be no clear winner . I can't vote because the reliability of my calculation is poor . The total number of electrons stored in the capacitor will exaust through the circuit sixty times per second . That they would do so faster at the higher named voltage was my original suspicion . You have caused me to reflect that the number of electrons stored is also greater in the higher voltage capacitor and thus the circuit current must also be stronger during exaustion (beyond that due to increasing the voltage) . By definition the 7.5 uF at 370 volts contains only 84% of the electrons the 7.5 uF at 440 volts contains when charged and the voltage also drives them faster through the circuit . I only have a technicians understanding of these things . It seems the reason sreid discounts the effect is also understandable and is why I have come to the experts . I believe that the arguement made is : is there enough electrical current produced during capacitive discharge cycles to matter since at that moment the line power to the circuit is essentially off . That also presumes the electrical capability of the winding insulator will stand the increase of 70 volts (I believe it will) ; eventhough , the motor names the the 370 volt capacitor . The current , then , is the problem and I am trying to get my mind around the electrical discharge again . The calculation from before yielded .5 milliamp/sec difference in discharge current . But , like what the meaning of "is" is , the per second (unresolvable denominator) has me confused since the process occurs at 60 Hz . Sixty times .5 milliamps may be a problem (still only 3/100 of an amp in a second , though) . If I go back and look at the 16% increase in total electrons and regard them as also moving faster at the higher voltage - that is cause for alarm ! Is the capacitive discharge current an issue worthy of worry ? Thanks to all but the quandary continues ... I am hopeful for a definitive though simply answer (not as simple as buy the 370 volt capacitors , stupid ! lol ) .

 

[sreid]

The voltage rating of a capacitor has nothing to do with the number of electrons stored in it. The number of electrons stored is caused by the voltage applied to the capacitor. This voltage is your AC line voltage (120/240 VAC) and is a fixed value.

A mechanical analogy; suppose a bolt is rated to not break if torqued to 10 lb-ft. You can use it at 1, 2, 3, 10 lb-ft but if you torgue it to, say 20 lb-ft it will snap. The voltage rating of a capcitor is the same, you can use the capacitor at any voltage up to its maximum voltage rating but not any higher.

 

[Confused2]

Because that is the voltage that breaches the dielectric . Great analogy - I get it ! Thanks

 

[jbartos]

Suggestion to the previous posting: Please, do not become shocked by the eng-tips replies. We are slowly getting there. It just needs a little bit of patience. The following is good to notice:
1. The AC capacitor function is somewhat different from DC capacitor where electrons and charge associations with the capacitor are helpful. The ac capacitor behaves more like a passive element in the circuit, similarly as an inductor or a resistor. Voltage drops across its terminal, certain amount of current flows through it during the steady state operation, the initial charge at the circuit switched "ON" may be large or may be zero Coulombs, etc. Normally, this charge is not considered. The only things considered are the initial transient currents and capacitor value in Farads that can be used to calculate phase shifts or displacement angles versus other vectors or phasors, such as resistive phasor or inductive phasor. This phase shift of displacement angle is important for the single motor to start turning, develop starting torque, etc. The displaced phase by the capacitor is rotating, e.g. at 60Hz, i.e. 60times per second. This rotating frame keeps the motor rotor turning.

 

[jbartos]

Suggestion: Visit

http://www.lmphotonics.com/single_phase_m.htm

for single-phase motor circuits that include capacitors

 

[Confused2]

Thank You ! You have been helpful . I found the website link immediately above very interesting . In your opinion does the schematic for the capacitor start/run motor seem to be the same configuration as these so-called PSC motors ?

 

[jbartos]

Suggestion: Lets consider the definition of PSC motor from IEEE Standard 100-2000 "Dictionary":
Permanent-Split Capacitor Motor.
A capacitor motor with the same value of effective capacitance for both starting and running operations
Therefore, the closest fit to the IEEE Std 100 definition is on the referenced website:

2. Capacitor Start Capacitor Run
This configuration comprises two windings W1 and W2, a centrifugal switch SW1 and two capacitors C1 and C2.
The two windings are wound with a geometric offset, effectively making a second set of poles phase shifted within the stator. The capacitors provide a phase shift to the current flowing in W1 and we therefore have a "two phase" motor. When the motor is almost up to speed, the switch opens disconnecting the capacitor C1. C2 remains in circuit to provide a continued second phase, reducing torque pulsations and noise. The motor can be reversed by reversing the connections of either W1 or W2 (but not both!)
The start winding (W1) and the capacitors provide for a rotating magnetic field in one direction enabling the motor to start.

The referenced website would have been substantially more attractive if it happened to follow certain industry standards, e.g. IEEE, IEC, NEMA, etc.

 

[Confused2]

Ok , thanks!