×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

filter for full wave bridge

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

RE: filter for full wave bridge

The problems you are seeing are inherent in a simple rectifier-capacitor arrangement. A large inductor will help to calm the current pulses, but will be expensive, bulky and lossy. What problem is the load current causing as it stands?
 

----------------------------------
  Sometimes I only open my mouth to swap feet...

RE: filter for full wave bridge

(OP)
I am trying to improve the commutation.  After about 12 hours of run time the commutator starts pitting and the motor really goes down hill.  I tried running it with an off the shelf SCR drive and it performs the same way.  If I run the motor on straight DC it performs beautifully.  My goal is to get the rectified DC closer to actual DC with out spending a lot of money.

RE: filter for full wave bridge

Can you use 3-phase power?

RE: filter for full wave bridge

(OP)
115 VAC 1 ph is the only thing available.

RE: filter for full wave bridge

The servo amp with PWM power stage (instead SCR one) would be better solution. The switching frequency will be no less than 20kHz in this case.
But maybe recommended to check motor inductance - maybe choke in series with motor winding still be need.

RE: filter for full wave bridge

Define 'a lot of money' - is this something you're building for the military, or something you're knocking up on the kitchen table? The definition of 'cheap' and 'expensive' varies depending on who you talk to.
 

----------------------------------
  Sometimes I only open my mouth to swap feet...

RE: filter for full wave bridge

There are DC motors designed to run on SCR speed controllers. Motors that are designed to run on pure DC do not do well on rectified AC as you have observed.

RE: filter for full wave bridge

The cap across the motor is still likely to be your best choice short of buying a PWM DC drive.

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

Too much capacitance across the brushes can make commutation worse.

RE: filter for full wave bridge

Hi,

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

RE: filter for full wave bridge

You need an inductor in series with the armature.

RE: filter for full wave bridge

That is very rarely needed. If the voltage is smooth enough, it is not necessary.

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

RE: filter for full wave bridge

(OP)
I miss spoke earlier.  The cap I used (rather large) was a 7400 mF.  I happen to have a couple laying around.  Just to clarify, my commutation was greatly improved when I used the cap vs. no cap.  Currently out of my rectifier I am running 127V RMS.  With the cap in there it bumps the voltage to 154V RMS.  The main issue is that with out the cap in parallel I have a current spike every 8.33 msecs causing a large rush of current to the motor and inbetween cycles the motor is not pulling any current. The main problem with the cap is that the AC line current goes up a lot.   

What do I need to know to size an inductor to put in line?

Brent

RE: filter for full wave bridge

(OP)
Yep

RE: filter for full wave bridge

Read Skogsgurra's message again.  Your own data confirms that the bridge output voltage increases from 127VDC (not RMS) to 154VDC which will run the motor overspeed.

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

Skogsgurra -

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

Bob,

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

RE: filter for full wave bridge

Skogs; The 74 bothers you and not the fact that it is a 7 FARAD cap.. er, battery, no I mean cap?!?!

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: filter for full wave bridge

(OP)
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  

RE: filter for full wave bridge

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.

RE: filter for full wave bridge

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 smile

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: filter for full wave bridge

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

RE: filter for full wave bridge

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

RE: filter for full wave bridge

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources