PSC motor design
PSC motor design
(OP)
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 - "Confused2". Thanks for your response.
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 - "Confused2". Thanks for your response.





RE: PSC motor design
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.\\\
RE: PSC motor design
Thanks again
RE: PSC motor design
RE: PSC motor design
RE: PSC motor design
RE: PSC motor design
RE: PSC motor design
RE: PSC motor design
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.)
RE: PSC motor design
RE: PSC motor design
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.
RE: PSC motor design
RE: PSC motor design
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.
RE: PSC motor design
http://www.lmphotonics.com/single_phase_m.htm
for single-phase motor circuits that include capacitors
RE: PSC motor design
RE: PSC motor design
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.
RE: PSC motor design