OT- How to clean up DC power to improve motor brush life

ORAGRAG · Jan 18, 2018

I am experiencing poor brush life in a application that has a small 120 VDC motor. Currently just using a bridge rectifier to convert 120 AC to 120 DC. Would adding a capacitor across the DC side help clean the power up. If so any recommendation as to the size. Also open to other simple ideas.

wrangler · Jan 18, 2018

Full wave rectifier across 120 volts gives you from what I remember is 1. 41 times ac voltage. That high voltage may be causing your problem.

JST · Jan 18, 2018

That normally is not an issue. Done all the time in KB DC drives.

Wear can be due to "high mica", wrong brush type, excessive current, etc. Even very dry conditions, as with cold weather and heating.

Common caps used for RFI prevention are ,001 uF, ,0047 uF, etc.

If connected from brush to ground, they will want to be ones rated for "Y" usage, .Generally rated Y1 or Y2. Otherwise, if connected across the brushes from one to the next, use ones rated "X".... "X2" etc. Not sure capacitors will improve brush life, but......

Monarchist · Jan 18, 2018

wrangler said:
Full wave rectifier across 120 volts gives you from what I remember is 1. 41 times ac voltage. That high voltage may be causing your problem.

Got your math backwards. 90 VDC is typical off 120 VAC once "integrated". By the motor's inductance. And in the current realm, pragmatically, rather than Voltage realm.

Most folks, however, don't "instrument" appropriately for the environment, are reading the "AC Component" rather than the integrated DC one.

Brush wear here just might be as simple as a series-wound "small 120 VDC" motor and no easy means of "timing" the brushes.

Their brush life is nothing to brag about anyway.

"Shunt" wound OTOH are typically engineered for 2,000 power-on-hour brush life.

Brazillions of different needs that is to be made to work for, so Helwig Carbon & Sputniks carry brazillions of different brushes.

Size and shape are obvious.

Less-obvious is that thermal conductivity, electrical conductivity, and "lubricity" also differ. Same again with spring or "tamper" pressures - also a major player in brush life.

2CW

ORAGRAG · Jan 18, 2018

wrangler said:
Full wave rectifier across 120 volts gives you from what I remember is 1. 41 times ac voltage. That high voltage may be causing your problem.

I will double check with manufacture of the motor on this. On our previous motors brush wear was not a problem. When that manufacture ceased production we were forced to find a new supplier. Since then we have had brush issues. Upon comparison of the old motor to new we noticed that the brushes and commutator were both smaller. They came back to us with it being a power supply issue.

ORAGRAG · Jan 18, 2018

Monarchist said:
Got your math backwards. 90 VDC is typical off 120 VAC once "integrated". By the motor's inductance. And in the current realm, pragmatically, rather than Voltage realm.

Most folks, however, don't "instrument" appropriately for the environment, are reading the "AC Component" rather than the integrated DC one.

Brush wear here just might be as simple as a series-wound "small 120 VDC" motor and no easy means of "timing" the brushes.

Their brush life is nothing to brag about anyway.

"Shunt" wound OTOH are typically engineered for 2,000 power-on-hour brush life.

Brazillions of different needs that is to be made to work for, so Helwig Carbon & Sputniks carry brazillions of different brushes.

2CW

So not a voltage issue then correct?

Monarchist · Jan 18, 2018

ORAGRAG said:
So not a voltage issue then correct?

Havent a klew. Near-zero information furnished so far.

We'd need full sight of the motor nameplate, ability to take that info to go and find its maker's published data sheet, and 'scope traces off your power rig, loaded and not.

If you want a guess?

Mine is "six black helicopters".

JST will now do the math on that, but we still won't KNOW squat until you give us useful measurements and specific motor and power source data.

DDoug · Jan 18, 2018

ORAGRAG said:
I will double check with manufacture of the motor on this. On our previous motors brush wear was not a problem. When that manufacture ceased production we were forced to find a new supplier. Since then we have had brush issues. Upon comparison of the old motor to new we noticed that the brushes and commutator were both smaller. They came back to us with it being a power supply issue.

Me thinks the smaller Comm & brushes are the problem.

That's why some industrial motors have Comms almost as big in Dia. as the armature, and almost as wide as the magnetic section.

ORAGRAG · Jan 18, 2018

digger doug said:
Me thinks the smaller Comm & brushes are the problem.

That's why some industrial motors have Comms almost as big in Dia. as the armature, and almost as wide as the magnetic section.

I am glad that you confirmed this. We gave the new supplier the old motor to reverse engineer. For what ever reason (probably cost) they made their design smaller on both brushes and commutator. Intuitively I thought this was at least part of the issue. However I still thought it prudent to explore the quality of the power issue.

Monarchist · Jan 18, 2018

digger doug said:
Me thinks the smaller Comm & brushes are the problem.

That's why some industrial motors have Comms almost as big in Dia. as the armature, and almost as wide as the magnetic section.

"Symptom" in this case. Problem is ignorance.

Lack of simple research or a smidgen of Engineering input.

No fear. All that is correctible

sym

Monarchist · Jan 18, 2018

ORAGRAG said:
I am glad that you confirmed this. We gave the new supplier the old motor to reverse engineer. For what ever reason (probably cost) they made their design smaller on both brushes and commutator. Intuitively I thought this was at least part of the issue. However I still thought it prudent to explore the quality of the power issue.

Ignorant rectifier as supply? Not a DC Drive with regulation capability and limit settings?

How about current limiting? Thermal cutout? Fusing or circuit breakers?

BTW - at an expectation of 120 VDC off 120 VAC, that 120 VAC needs to be three-phase - rare, that - or there needs to be a DC Drive type that boosts it - not just a naked rectifier.

DC motors are predictable items where powered directly off AC mains.
Single-phase, US voltages, that means 90 VDC off 115/120 VDC, 180 VDC off 230/240 VAC.

Only 3-phase readily provides near-as-dammit 1:1 DC to AC conversion. Shallower "valleys" to fill-in for a given span of time.

DC motors need protection against their own nature. They CANNOT "slip", do not KNOW when to quit. Unless protected by internally or externally limited or interrupted power, they will move a load or die trying whilst at locked-rotor.

BTW.. it looks as if the supplier used an intermittent duty design where a continuous duty one was needed.

Which did you specify?

ORAGRAG · Jan 18, 2018

Monarchist said:
"Symptom" in this case. Problem is ignorance.

Lack of simple research or a smidgen of Engineering input.

No fear. All that is correctible

sym

Not a lot of knowledge in this area, just looking for a little help to get pointed in the right direction. No need to go negative.

johansen · Jan 18, 2018

you may be able to make the brushes wider to compensate for the smaller diameter.

or switch to a different brush material.

what is the diameter, rpm, and current density?

Monarchist · Jan 18, 2018

ORAGRAG said:
Not a lot of knowledge in this area, just looking for a little help to get pointed in the right direction. No need to go negative.

That's POSITIVE, Pilgrim.

If I thought you were unable to learn, grow, and make USE of information? I'd simply ignore you outright.

Let's get better sight of your needs, here. Pretend it is a clean sheet design so we can see if the answers come out close to what has already been done, if not, why not, and if so, HOW close.

Not as if PM community were charging you for that assistance, is it?

Or do you wish to risk your THIRD motor choice being even less appropriate than the SECOND one? Motor MAKER was supposed to be the go-to for expertise. But they f**ked up.

Now you need to check their work. Give them stricter goals. Or replace them.

So...

What specs and "type" are on the old motor and new, how much torque or HP needed, what are you asking it to DO duty-cycle and loading-wise, how much space and heat-rise budget do you have, what severe challenges as to wet/dry, dirty/abrasive, special shape or mounting?.... etc.

Very little is so "new" that some OTHER situations have not already provided experience.

They do not "match up" automagically without looking for those matches, though.

Far too many possible types, sizes, options in general.

ORAGRAG · Jan 18, 2018

johansen said:
you may be able to make the brushes wider to compensate for the smaller diameter.

or switch to a different brush material.

what is the diameter, rpm, and current density?

I have what was originally specified from our old motor and sent to new manufacture. However in hindsight did not get what as requested. When the life issue came up we paid more attention to brushes and commutator design. Over past couple weeks supplier sent us different brushes to try. Did not see any improvement in measured wear under low load testing. It is my intention to push the redesign issue back at the supplier. However I am hoping to have at least a rudimentary knowledge of the issue this time around.

Thanks for your help

johansen · Jan 18, 2018

ORAGRAG said:
Thanks for your help

http://www.argointl.com/wp-content/...Select_Brushes_for_Motors_and_Generators2.pdf

here's a start.

Monarchist · Jan 18, 2018

ORAGRAG said:
hoping to have at least a rudimentary knowledge of the issue this time around.

Testing under "light load" will not be getting any useful data. The reverse, rather. "Masking" that there even IS a problem.

If brushes and commutator are undersized, it is HEAVY load - too much energy demanded to be easily passed - that will make it obvious.

A quickly added "turnkey" test device could be a DC Drive.

Even if it is NOT to be used in the product, it lets one set Voltage, Current, and limits for them, ramp up and down slopes, and provides instrument attach points so an ordinary meter can display percentage loads and more.

DC drives have good sensing built in. Have to have. As a byproduct of powering the motor, it can tell you a great deal ABOUT the motor, thereby saving the cost and set-up time of a good deal of separate lab gear.

DDoug · Jan 18, 2018

If the new motor is rated for intermittent duty only, changing
the brush composition isn't going to do much.

A continuous duty motor has more than just different brushes,
different sizes, different wire, cooling, insulation, etc.

ORAGRAG · Jan 19, 2018

I want to thank you all for your information and help. As I can see that this is above my level of understanding I have decided to contract with an outside professional service.

Monarchist · Jan 19, 2018

ORAGRAG said:
I want to thank you all for your information and help. As I can see that this is above my level of understanding I have decided to contract with an outside professional service.

It can be learned. Good to know enough of it to twig to possibly being mis-informed by a vendor with his OWN agenda.

Even if all you do is be "the one" who raises an in-house doubt and sends the latest info off to that third-party (as seems to now be the case, here), your company is far better served than dealing with the costs and delays of another blind-siding from a vendor.

Howard Hughes was eccentric, if not also just plain weird, but he hit on a very profitable truth when he said to his long-time #2, Noah Dietrich:

"Find the experts, Noah. Find the experts."

OT- How to clean up DC power to improve motor brush life

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