DC smoothing cap value?

I want to improve the DC field supply on my Monarch lathe. Currently I use a phase-angle chip coupled with a bridge rectifier but it has no smoothing so makes the contactors hum and sometimes rattle. I did add a smoothing cap originally but it wasn't happy with the way it acted with the capacitor so I removed it.

I have a 6a variac and a rectifier but even from using the online calculators I can't get a definitive value for the ideal smoothing cap. The motor needs 115v DC and 1.5 amps. The panel also draws a current but I have no idea how much so assuming another amp or two.

What would be a good smoothing cap to use for this setup? I've tried using a couple of online calculators but they give varying results and ask for some info I can't give.

Peter. said:

I want to improve the DC field supply on my Monarch lathe. Currently I use a phase-angle chip coupled with a bridge rectifier but it has no smoothing so makes the contactors hum and sometimes rattle. I did add a smoothing cap originally but it wasn't happy with the way it acted with the capacitor so I removed it.

I have a 6a variac and a rectifier but even from using the online calculators I can't get a definitive value for the ideal smoothing cap. The motor needs 115v DC and 1.5 amps. The panel also draws a current but I have no idea how much so assuming another amp or two.

What would be a good smoothing cap to use for this setup? I've tried using a couple of online calculators but they give varying results and ask for some info I can't give.

Click to expand...

Caveat: I don't use ANY contactors, relays, or coils for them on the 4Q SSD rigs. No need. No place to put them into the circuit without screwing up the DC Drive's logic, even if I wanted to do.

The 3 HP Large-Frame Field goes nearly silent with just a small Coilcraft choke in series, no age-sensitive caps required. Mind - the SSD 507 I use for Field Power is pretty smooth to begin with. Short 2 V excursion at each Zero-cross, IIRC.

If you use a cap, best to put a choke ahead of it, anyway.

Classical "choke input filter" with the inductive load of field or contactor coils the other side of the cap.

Look for Butterworth and Chebyshev calculators for low-pass filters if you want to go into multiple stages. Hollow-state Audiophile stuff and AM Broadcast power supplies of days (mostly) long gone.

Four stages made utility line hum NEARLY vanish. "Half" of one - choke only - should do yah fine for the 10EE.



Your Field Supply and "heavy" contactor coil supply can, of course, be separately sourced if they are not already. And should be, for best stability.

Several questions. How well regulated does the supply voltage need to be? How much space is available for the filter? How much ripple is allowed?

This a classical design problem back in the day of vacuum tubes. The simplest is the R-C filter which is a resistor in series with a capacitor. Thin comes the pi filter which is C-R-C. The R maybe replace with an inductor yielding C-L-C. Often a resistor was hung of the output of the filter to help regulate the voltage and to bleed down the capacitors.

Most books and handbooks on electronics will have information on this. I Would try Electrical Engineer's Handbook and/or Radio Amateur's Handbook. The Radio Amateur's Handbook is the more practical of the two.

Tom

edit Do a Google search using "dc ripple filter" will give many different filters and means of calculating the ripple.

T:-

Peter. said:

I want to improve the DC field supply on my Monarch lathe. Currently I use a phase-angle chip coupled with a bridge rectifier but it has no smoothing so makes the contactors hum and sometimes rattle. I did add a smoothing cap originally but it wasn't happy with the way it acted with the capacitor so I removed it.

I have a 6a variac and a rectifier but even from using the online calculators I can't get a definitive value for the ideal smoothing cap. The motor needs 115v DC and 1.5 amps. The panel also draws a current but I have no idea how much so assuming another amp or two.

What would be a good smoothing cap to use for this setup? I've tried using a couple of online calculators but they give varying results and ask for some info I can't give.

Click to expand...

When you say a phase angle chip, I assume you are talking about an SCR, that takes a bite out of each cycle to control speed. That leaves a pretty raggady wave to filter. I am no DC motor control expert but I think they usually use some pulse width modulation scheme. The frequency rate can be high and have a lot smoother result.

Sent from my SM-G900V using Tapatalk

Yeah I guessed that the cap I tried was upsetting the csr chip. I've seen plenty of these machines fitted with variac field supply and I have one here that's suitable so I thought I'd make use of it.

Anyway, I don't know what a choke is or what it does and I don't know what my desired ripple is either. I don't suppose that I need any particularly great smoothing since all I'm powering is a DC motor's field coil and some pull-in coils. In fact the lathe's DC panel has some capacitors on it but I don't know what their purpose is. Space is something I'm not short of.

I was kinda hoping that someone would say "use a xxx uf capacitor and it'll be reet"

Peter. said:

I was kinda hoping that someone would say "use a xxx uf capacitor and it'll be reet"

Click to expand...

Use any choke rated for 2A or better, notice an improvement, and generally WITHOUT upsetting the switcher chip as a capacitor generally will do.

In this application, a Capacitor acts as a "spring". It wants to charge to peaks. It can happily participate in Oscillation, and at higher frequency than line at that.

A stout filter-choke of the sort used for power circuits is closer to a flywheel in its effect. It resists excursions, Dee Cee and upward.

Transformer windings are also "chokes". So are motor windings.

Primaries - the 120 or 240 Volt side - of common "doorbell" and control transformers do not ordinarily pass enough current 1.5 to 2A here. "Heavier" ones - or grounds-lighting or buck/boost transformers may or may not, but you can test with such if you have one handy.

A 'two lead' choke is all you need, and for this purpose, not all that much of one.

Knock down the switching artifacts - sharp spikes - and the contactor coils are reasonably comfortable at doing their own "integrating" of twice-line Hz pulsed DC and operating in a stable manner, even if they "hum".

Or use that Eurotherm SSD 507 I sent you instead of the chip?

There are a few "discontinuities" in this.........

A field supply would not normally NEED any smoothing. It is a current activated deal, and the inductance vs the pulsing DC is enough to work fine, either from rectified AC, OR as per monarch 10EE, via a phase angle setup using thyratrons or conceivably thyristors (SCR).

BUT, you are trying to use the same supply for relays as well as the field? Is that a stock setup in yours? I have not looked at the 10EE schematic for a while, but I seem to recall they had a side chain for the relay power.

JST said:

There are a few "discontinuities" in this.........

A field supply would not normally NEED any smoothing. It is a current activated deal, and the inductance vs the pulsing DC is enough to work fine, either from rectified AC, OR as per monarch 10EE, via a phase angle setup using thyratrons or conceivably thyristors (SCR).

BUT, you are trying to use the same supply for relays as well as the field? Is that a stock setup in yours? I have not looked at the 10EE schematic for a while, but I seem to recall they had a side chain for the relay power.

Click to expand...

Peter worked-through his DC panel with Cal "a while" ago. AFAIK, it is OEM or near-as-dammit. His Armature drive is a 1Q Eurotherm/Parker SSD 512C.

Those have more "balls" than a Beel/BICL but still need external contactor-reversing and resistive help with braking that a 514C "4Q" SSD does not need.

AFAIK, it is only his field supply that is "DIY".

Field OR Armature, the MG was "rotary" derived DC power, had no significant ripple, let alone AC-line derived pulsations. Some other Pilgrim will have to put a 'scope on WiaD or Module drive - I don't have either.

For my one - former MG, or Mark's - former Modular, each with single-phase-derived Eurotherm/Parker SSD SCR-class DC Drives on both Armature and Field?

Using chokes for ripple-filters gives us cleaner revs, prevents "watermarking" on finer cuts as well as reducing annoying / distracting acoustic noise. Goal was MG smoothness without MG noise and losses nor DC panel moving, wearing, arcing, sparking, spiking parts.

IF .. I use TWO chokes, and about 300-400 MFD in between on the Armature supply?

- Motor windings go nearly silent. Brush sliding noise is contained by the CI "hull" of the 10EE, covers on. The larger CHOKE - in the belly where the MG once lived, WILL hum a bit on hard acceleration, fast braking, or on-the-fly reversing. Otherwise, not so much.

No tool in the cut, all one hears at the operator position is rolling bearing elements, and surfacing feed input subsection whir...

Until the rest of the feed or threading geartrain is engaged, anyway.

That's "normal mode" for most work, and at that point DC supply - regardless of type - is no longer of much concern acoustically.

On a "1Q" system, however, the stability of contactor and relay hold-in and predictability of response becomes MORE important once in the cut and threading or surfacing under power.

Use of a 4Q drive simplified life to where my entire DC panel left Virginia to go hide-out as spares somewhere in New Jersey. Can't say as I miss the bugger atall.

Peter, I would strongly suggest using switched mode power supply. Bridge+cap will cause voltage fluctuations and also, problems with startup surge.
I think that you could find 115V and 2A supply for about 50..100 dollars, ask from Farnell or RS.

We use the term "choke" as if everyone knows what it is. In simple terms it is an inductor, an example would be a transformer type construction without a secondary winding. An inductor opposes a change in current, so it is placed in series with the supply. A capacitor acts to maintain constant voltage, so they are placed in parallel.

Tom

TDegenhart said:

We use the term "choke" as if everyone knows what it is. In simple terms it is an inductor, an example would be a transformer type construction without a secondary winding. An inductor opposes a change in current, so it is placed in series with the supply. A capacitor acts to maintain constant voltage, so they are placed in parallel.

Tom

Click to expand...

Well, no, not exactly. Left to its natural preferences, a capacitor helps sustain a waveform that resembles a sawtooth. It's just their nature.

Where useful, they need either a resistor - or better yet, an inductor - in a tuned - or deliberately DE-tuned circuit. "LOW pass" filter in this case where "low" is wanted to be pure DC but won't ever get all that close to it.

In general, "utility mains derived" Hz is in a band where that can be cleaned up easily with a stand-alone inductor, no lumped capacitor present atall.

By contrast, a stand-alone capacitor with no inductor doesn't help a naked switcher - SCR or other - until one is well up into the audio band, if not RF. And higher.

The legacy approach has been RC or LC filters, and in as many as nine stages.

Dynamic, or "active" filtering has become lighter and smaller than passive. Hence the sophistication and value-add of packaged power supplies, many races and tribes.

Money WAS put where my mou.. 'er "keyboard" is, BTW. Shipped Peter an SSD 507 for use as Field Supply back when he first started to use the SSD 512C on his 10EE.

I'm ahead, actually, as more recently he gifted ME with a superb scraping tool he made himself.

But I DO wish he would try that SSD 507 for the field. Intended as Armature supply for low/medium HP DC motors, they are pretty damned stable, even with neither add-on L or C.

We don't actually use their "Field" section. That part is just an ignorant "naked" FWB rectifier, neither filtered nor regulated.

It is the intended-as "Armature" output as makes a bitchin' adjustable, regulated, "rampable", Field supply with adjustable limit-points - even sensors and alarms that are compatible with the larger 512C/514C used for the 10EE's Armature supply.

ANY decent regulated DC supply will do for the Field.

Those of us who use SSD Drives simply find a bit of extra value in that control and signal compatibility with the heavier Armature drive.

Anyhow......

OK, the field has an SCR system. What's this about the relays? They mormally are AC relays, and have no issues, OR they are DC relays, which have a rectified supply with "some" smoothing" applied. Relay inductance is not always (or often) enough to keep down the buzzing.

But, usually, a simple unregulated supply works. Relays usually close doen to 70% of nominal volts, and may "hold in" down to 20%. Might not be good for full current unless pulled in at a decent force, but do stay closed...

If you had someone go thru the unit back before, then something changed now? Find that.

Of call your pal, who must be around that area, and see what he makes of it.

I dunno what that last crack is meant to mean but these are DC contactors and a DC motor and everything works fine. All I want to do is smooth the DC to stop, or at least reduce, the buzzing in the contactors. I tried putting a capacitor on the output of the bridge rectifier but it didn't like it (this was a few years ago now and I can't remember why) but now that I have a variac to use instead of the semiconductor voltage regulator I want to use that to dial down the input voltage to the bridge rectifier and use a capacitor to reduce the ripple.

I was hoping that the info offered above would be enough for someone to help me with a ball-park value. I'm gonna source some caps and go with trial and error I think. Once it shuts up the buzzing I'll set the voltage and forget about it.

I assume it worked OK at one time, so what changed that makes it buzz now? You, or the fellow that helped out on it can find that, and if fixed it would be back to its "original working self".

If you need an amp at 115V DC, at reasonably low ripple, then you need around 85-95 VAC input, rectified (assuming a fairly smooth DC with reasonable ripple voltage but nowhere near no ripple). Somewhere around 100 to 330 uF should give a smooth enough voltage to avoid buzzing. I expect the relays do not take as much as an amp, so it may need less than that.

Smart thing to do is to have the relay supply separate, then it is not an issue, and you can do whatever, without problems. But if the thing has a scheme that runs the relays on part of the field supply, watch out that the circuit is not DEPENDING on some relay current to do its job correctly. If it is, then you will have to either leave them in where they are, or provide an equivalent load if you move them to another supply.

I can only guess why you weren't successful years ago. The reason you are having buzzing with DC relays is that there is not enough stored energy to carry the magnetic field during the time when the rectified current goes through zero. That is what a shading coil is for on an AC magnet.

In order to determine the values of a filter, measure the amount of DC current is required to operate the relays. Lets assume it is 2 amps at 120 VDC. Further, assume that during the time that the rectifier is not charging the capacitor that the voltage not dip below 100 volts during half of a half cycle or .005 seconds.

The equation to solve is E = Eo( e(-t/RC)) where E is 100 volts, Eo is 120 volts, t is .005 seconds, R is 60 ohms. Solving, C = 342 microfarads.

This is highly simplified to yield a ballpark value. The non-conducting time will be less than 5 msec, the voltage can dip more than 20 volts, and the charging rate is a sine wave, not just on/off.

To limit the peak amps when energizing the power supply, I would put a 10 ohm 50 watt resistor in series with the bridge and the capacitor. Adjust the variac to give 120 VDC no load. Under full load the voltage drop should be less than 20 volts.

Tom

Just to follow up on this I have a 400uf cap on the way but in the meantime I have installed the variac along with a boost transformer into a tidy metal enclosure and I couldn't help giving it a try. Turns out all the humming and contactor noise was due to the voltage regulator chip because with that removed and replaced with a variac it has all disappeared. I will still fit the capacitor when it arrives mainly because it will give me a truer reading on my DC volt meter.

Out of curiosity, what "chip" was it as far as maker and type number?

Some are not happy unless they have other parts with them, including capacitors.

This one

(PDF) CSR14 Datasheet PDF Download - CSR/64/14/154
POWER REGULATOR CHIP
INTRODUCTIO