filter for full wave bridge 1

[BrentW]

I am working on a 1.5 Hp PMDC motor that is run off 115 FWR. If I look at the current waveform on the scope it is pulling a peak current of 28.5 Amps every 120 Hz and the current going to zero in between the peaks. My RMS current reads 14.5 volts. I need to make some sort of filter for the output so I can reduce my peaks and allow for the time current flows to be longer. I have tried a couple things so far but haven't been extremely happy with the results. I put a 7400 microfarad cap in parallel with motor and that sort of did what I wanted. The voltage trace looked like DC and the motor was always pulling current. However my AC amps went way up. I am not sure if I need an inductor to help control the current or what size cap would be the best. Any ideas would be appreciated.

Brent

 

[BrentW]

Thanks for everyones replies as this still is kicking my butt.

The motor is a 4 pole PMDC that has a 5" OD. Currently it is running off of 115 VAC right through a 400V 30Amp bridge rectifier. With the 7400 mF cap (3"x6") it smooths the DC very well and the motor is always drawing current as I would like it. However as I have said my AC current goes up quite a bit.

You need to reduce the commutation time on the bridge SCR's so the filtered DC voltage goes back down to 127VDC.
What is the best way to accomplish this?

Other notes that I have from this post--
What constitutes a low inductance motor? Value?
I just measured it at the two motor leads-- 3mH and 5 Ohms.
The low resistance I think explains why I need such a large cap to control the current. I did also try a 1000 & 2000 uF yesterday afternoon that did not seem to make any change.

Try a PWM drive-
I hooked on up yesterday that is rated for 15ADC. I could run it up to 12ADC @ 20AAC and it would go into current limit.
The drive manufacturer gave me the same story about the low inductance motor but couldn't tell me what was too low.

From what I have read putting an inductor in series with the + lead will help stabalize the current as the field will build when the motor is drawing current and the inductor will have some residual current when the field collapses. Is this correct?

Brent

 

[LionelHutz]

A properly sized inductor will cause a continous current to flow. The AC ripple component will be mostly dropped across the inductor. The motor will then receive DC component which will be the average voltage applied. Hope that makes sense.

Try reading this for the overview;

http://www.tpub.com/neets/book7/27f.htm

I would suggest you look at switched mode power supply design (a buck converter) for ideas on designing a LC output filter. You use the same ideas just at a lower different frequency.

 

[Skogsgurra]

Seen some supercaps. And also some mis-use of m, M and u prefixes. So not surprised. But 74... ? When everybody knows that that is not in the E12 series

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rtronics]

7.4 FARAD at 127 volts in my world is wheelbarrow-sized!!!
regards, Ray.

 

[Marke]

Hello BrentW

If you place the capacitor directly across the output of the rectifier, you will get the peak voltage out of the rectifier as your continuous voltage on the capacitor.
If you fit an inductor (must be DC rated) between the rectifier and the capacitor, the voltage on the capacitor will drop. Provided the inductor is correctly sized, you will get the average of the unsmoothed rectified voltage across the capacitor and that sounds like what you need.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[DickDV]

It sounds like the rectifier in this case is a fixed diode bridge so my suggestion to change the commutation angle of the SCR's wouldn't apply.

Instead, you could accomplish the same thing by using a transformer to reduce the 120VAC down in 5 or 10% steps until the motor's DC voltage came down to spec.

By the time you've done that, you might as well buy a little variable speed DC drive and let it do all this for you.

KB and Dart both make nice inexpensive drives just for this purpose