This is very interesting. I simmed the circuit with a 5uF cap, and the asymmetry in the voltage wave almost completely disappeared. Then I performed the same sim with a 10uF cap, and the symmetry improved even further, but the motor sub-circuit drew a little less current. It went down from 1.1A to 1.0A. Question, the measurement of which component would better represent the current flowing through the motor? Should it be the current flowing through R2 (or L1), or the current flowing through R

Without capacitor, motor dissipates energy (heating) every half-cycle for acceleration and deceleration and consumes extra current.
With bigger value of capacitor, dissipated energy (heating) decreases and current becomes lower.

 

Would such a thing make a difference here (and I mean in respect to EMI)? That is, would it make a difference if I were to use a 200 nF polyester cap in parallel with the 10 uF cap at the diode bridge output?

Sure.
Electrolytic capacitor + film or ceramic capacitor.

 

And a last, but extremely important question regarding capacitors. You said the larger, the better... fine...

Which cap of the following would be best?

A 68 uF @ 200V, electrolytic capacitor:

​

A 30 uF @ 370V motor "working" capacitor:

​

Or a 108-130 uF @ 220VAC motor "startup" capacitor:?

​

 

And a last, but extremely important question regarding capacitors. You said the larger, the better... fine...

The (bottom) motor start version will be bi-polar, but Chinese origin are noted for high failure rates!
Max.

 

The (bottom) motor start version will be bi-polar, but Chinese origin are noted for high failure rates!
Max.

Thanks, Max. I don't need bi-polar since the cap will be used on the DC side of the circuit. My main concern is if an ordinary motor start capacitor can be used for a continuous application such as this one ... as for its being Chinese, welllll... I guess there's only one way to find out if the quality is good enough.

 

Do not use start capacitor it may crushes when working time will more than few seconds.
Look inside computer power supply - usually 470uF relatively small capacitors.

 

The bi-polar start cap only gets warm when used on continuous AC and you only have to check out the number of premature failures of Chinese origin versions on the DIY or CNC machinery sites.
Max.

 

Do not use start capacitor it may crushes when working time will more than few seconds.
Look inside computer power supply - usually 470uF relatively small capacitors.

Thanks. I've located a "work" capacitor of 80 uF @ 440VAC:

​

Would that work?

 

"Start capacitors give a large capacitance value necessary for motor starting for a very short (seconds long) period of time. They are only intermittent duty and will fail if energized too long. Run capacitors are used for continuous voltage and current control to a motors windings and are therefore continuous duty. They are generally of a much lower capacitance value."
http://www.capacitorformotor.com/motor_capacitor.html

 

"Start capacitors give a large capacitance value necessary for motor starting for a very short (seconds long) period of time. They are only intermittent duty and will fail if energized too long. Run capacitors are used for continuous voltage and current control to a motors windings and are therefore continuous duty. They are generally of a much lower capacitance value."
http://www.capacitorformotor.com/motor_capacitor.html

Finally, a clear and reliable source of info regarding motor capacitors! ... thanks for posting, I'll give it a through read.

 

470uF, 400V.
https://eu.mouser.com/ProductDetail...iMZZMtZ1n0r9vR22d%2blFeYd%2bH4Ezb3hjX2aK%2bo=

 

470uF, 400V.
https://eu.mouser.com/ProductDetail/Nichicon/LLG2G471MELB35?qs=sGAEpiMZZMtZ1n0r9vR22d%2blFeYd%2bH4Ezb3hjX2aK%2bo=

... this one beats them all, alright ... and the best part is Mouser delivers directly in Mexico. I guess it's settled now. Thanks!

 

I guess the Aerovox 55μf start cap I am using in a DC supply is an exception to the rule?
It is marked 350VAC.
Max.

 

I guess the Aerovox 55μf start cap I am using in a DC supply is an exception to the rule?
It is marked 350VAC.
Max.

Maybe it's still working because you're using it with a much lower voltage than it's rated for?

 

In a AC start situation they conduct a very high current both polarities for a short period, if left in circuit, overheat.
If not subjecting them to this, such as a DC power supply where they are just maintaining the DC level they don't seem to have a problem.
They are after all just two electrolytic s back to back.
Max.

 

I was just reading this thread, and the problem with ageing electrolytic caps came to mind. Is this an issue too with motor work caps?

 

I guess the Aerovox 55μf start cap I am using in a DC supply is an exception to the rule?
It is marked 350VAC.
Max.

It is not start capacitor. It is run capacitor. Simple film capacitor.
"AC Film Capacitors for Motor Run, UPS and General Purpose Applications
Aerovox’s line of SuperMet™ motor run and general purpose capacitors are constructed with metalized polypropylene film technology using the latest design techniques."
http://www.aerovox.com/Products/MotorRunCapacitors.aspx

 

I was just reading this thread, and the problem with ageing electrolytic caps came to mind. Is this an issue too with motor work caps?

How long these capacitors work in your computer PS at extremely high temperature condition?

 

I think I'll go for this beauty of a cap: 560 uF @ 400V. It's a little more expensive than the other one, but its performance on the sim is noticeably better.

I intend to connect a 10K @ 5W wire wound resistor in parallel to it for safety reasons. Question: should I be concerned about the inrush current going into the cap when it's first connected to the power supply?

 

Question: should I be concerned about the inrush current going into the cap when it's first connected to the power supply?

You should connect in series with power line special thermistor, as in tube monitor PS.
http://www.littelfuse.com/products/temperature-sensors/power-thermistors.aspx