filter for full wave bridge
filter for full wave bridge
(OP)
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
Brent





RE: filter for full wave bridge
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Sometimes I only open my mouth to swap feet...
RE: filter for full wave bridge
RE: filter for full wave bridge
RE: filter for full wave bridge
RE: filter for full wave bridge
But maybe recommended to check motor inductance - maybe choke in series with motor winding still be need.
RE: filter for full wave bridge
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Sometimes I only open my mouth to swap feet...
RE: filter for full wave bridge
RE: filter for full wave bridge
The reason the measured AC current goes up is twofold: first, the DC voltage goes up when you add the filter as well so the motor will run faster. Reduce the motor to the same load and speed as before and you will find that the AC current isn't that much higher after all, second, your ammeter is probably measuring PEAK current rather than true RMS current. The RMS current will be less than you measured.
RE: filter for full wave bridge
RE: filter for full wave bridge
If you are having problem with commutation when having a 7400 uF (It is 4700, isn't it?) smoothing capacitor and continuous armature current, then it probably isn't current ripple that is the problem. You can always try to increase capacitance (increased capacitance does not make commutation worse).
I think that the increased voltage may be the culprit. Running without capacitor gives one voltage and with (a sufficiently large) capacitor increases voltage about 40 percent. That increases motor speed - maybe above rated speed. And that is not good for the commutator.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: filter for full wave bridge
RE: filter for full wave bridge
It is only with low-inductance motors and some PWM and SCR controllers that extra inductance is needed. I do not think that is the case here.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: filter for full wave bridge
What do I need to know to size an inductor to put in line?
Brent
RE: filter for full wave bridge
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: filter for full wave bridge
RE: filter for full wave bridge
You need to reduce the commutation time on the bridge SCR's so the filtered DC voltage goes back down to 127VDC.
If you do that, your AC current will settle back to normal too---assuming your ammeter can read thru the harmonics and give a true RMS measurement.
RE: filter for full wave bridge
We've used a capacitor across the brushes of our universal motors for quite some time now. Up to a point the sparking at the commutator goes down. When we increase the capacitance above a certain point the sparking at the commutator gets worse.
RE: filter for full wave bridge
If you are using a capacitor across brushes to suppress noise, then your capacitor is probably sub-microfarad. Here, the capacitor is used to produce a smooth voltage out of a rectified sine.
The situation is quite another.
What motor size and voltage are you using? And at what capacitor size does arcing get bad?
Still puzzled by the 74... capacitor size. Must be a very special one. Never seen that before. But plenty of 47... capacitors.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: filter for full wave bridge
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: filter for full wave bridge
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
RE: filter for full wave bridge
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.
RE: filter for full wave bridge
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: filter for full wave bridge
regards, Ray.
RE: filter for full wave bridge
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
RE: filter for full wave bridge
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