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

 

[ScottyUK]

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?


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

 

[BrentW]

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.

 

[LionelHutz]

Can you use 3-phase power?

 

[BrentW]

115 VAC 1 ph is the only thing available.

 

[blacksea]

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.

 

[ScottyUK]

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.


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

 

[Compositepro]

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.

 

[DickDV]

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.

 

[BobM3]

Too much capacitance across the brushes can make commutation worse.

 

[Skogsgurra]

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

http://www.gke.org

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[cbarn24050]

You need an inductor in series with the armature.

 

[Skogsgurra]

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

http://www.gke.org

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

 

[BrentW]

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

 

[itsmoked]

7,400,000uF?? !

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BrentW]

Yep

 

[DickDV]

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.

 

[BobM3]

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.

 

[Skogsgurra]

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

http://www.gke.org

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

 

[itsmoked]

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