Solving Motor Braking Circuit Problem in Bowling Industry

[H012]

The image is not readable and it doesn't seem to be the Allen-Bradley that was used earlier. Can you give a link to the manual or tell us the make and model No. of the relay?

It is a WEG CWC016 The Allen Bradley's are CF 700 series. The Allen Bradley is the one that went out

 

[H012]

Tom/Jim Let me recap since it's been awhile since I posted that arc was gone. I took a chassis and put in cube relays so I could see if there was arcing. After I wrote that arcing was gone I pulled the cube relays out and put in the Allen Bradley CF 700 series relay. The other chassis has the WEG cwco16 contactor. Both these machines have the resistor and MOV in place. The AB relay is the one that went bad.

 

[jim hardy]

just to be sure which contact welded..

 

[H012]

just to be sure which contact welded..

https://www.physicsforums.com/attachments/106315

Positive on the one I stated

 

[Tom.G]

Tom/Jim Let me recap since it's been awhile since I posted that arc was gone. I took a chassis and put in cube relays so I could see if there was arcing. After I wrote that arcing was gone I pulled the cube relays out and put in the Allen Bradley CF 700 series relay. The other chassis has the WEG cwco16 contactor. Both these machines have the resistor and MOV in place. The AB relay is the one that went bad.

First, please check the wiring of the 8 Ohm resistor. Is it really in series with the contacts that burned?

Next would be if that was a new or used relay. If used, it may have already been damaged and not in good enough condition to last.

You state that the MOV is OK. How was this determined? If it is open there may not be any visual indication. Could also be a bad connection to it. Try putting a cube relay in and check for contact arcing. If arcing is present, replace the MOV.

@jim hardy The link to attachment in post #213 is dead.

 

[jim hardy]

@jim hardy The link to attachment in post #213 is dead.

fixed. Sorry about that..

Positive on the one I stated

The AB relay is the one that went bad.

at first glance i don't see a path through that contact that doesn't go through the resistor.
30 amps 8 ohms is 240 volts
we could possibly get to 240 volts immediately after NO contact opens, while motor current is decaying through MOV.

i'm back to a relay contact race.

 

[H012]

First, please check the wiring of the 8 Ohm resistor. Is it really in series with the contacts that burned?

Next would be if that was a new or used relay. If used, it may have already been damaged and not in good enough condition to last.

You state that the MOV is OK. How was this determined? If it is open there may not be any visual indication. Could also be a bad connection to it. Try putting a cube relay in and check for contact arcing. If arcing is present, replace the MOV.

@jim hardy The link to attachment in post #213 is dead.

I checked the MOV with a meter by unhooking 1 leg. The resistor is in series at TS 27 and center tap of factory resistor. I talked to a guy at direct automation he says to check how long the start windings stay in. There is a time spec. For how long a contact can handle the full start up amps. I would need a scope to see how long the start windings stay in. I'm just sharing what I've learned.

fixed. Sorry about that..

at first glance i don't see a path through that contact that doesn't go through the resistor.
30 amps 8 ohms is 240 volts
we could possibly get to 240 volts immediately after NO contact opens, while motor current is decaying through MOV.

i'm back to a relay contact race.

My thinking also I wanted to share the contact size difference both are AB 700 series just to show how quality has changed and that today's motor circuits have become more efficient so loads aren't as hard I suppose on the contacts

 

[H012]

Here is a better pic

 

[jim hardy]

he says to check how long the start windings stay in. There is a time spec. For how long a contact can handle the full start up amps. I would need a scope to see how long the start windings stay in. I'm just sharing what I've learned.

you can usually hear the "click" when centrifugal switch throws.

Did he say what was that time allowance ?
How long between starts on that sweep motor? Does it reverse electrically midsweep or is the cyclic motion of the sweep arm all done by gears?

 

[H012]

I checked the MOV with a meter by unhooking 1 leg. The resistor is in series at TS 27 and center tap of factory resistor. I talked to a guy at direct automation he says to check how long the start windings stay in. There is a time spec. For how long a contact can handle the full start up amps. I would need a scope to see how long the start windings stay in. I'm just sharing what I've learned.

My thinking also I wanted to share the contact size difference both are AB 700 series just to show how quality has changed and that today's motor circuits have become more efficient so loads aren't as hard I suppose on the contacts

Where the blue line is where the resistor is. This is the spec sheet for the Allen Bradley relay and the other is for the CWC016 contactor.

 

[jim hardy]

Well this is how it goes when you can't get at it with test equipment, only way you know it's fixed is the test of time.Do i recall relays looking something like this ?

An accidental intermittent between contacts 22 and 32 (which i think are adjacent?) would bypass the 8 ohms we added...

was this machine a particularly bad actor before ?

during braking we have a tank circuit, motor run winding and the two start capacitors as shown by your blue trace. At resonance a tank circuit has current gain. Our 8 ohm resistor increases the damping but we don't know by how much.
You reported ( in post 23 ) measuring 29 amps someplace during braking. Can you measure current through the 8 ohm resistor on this machine during braking ?
Might give a clue, might not...

Contacts weld on make, just like when electric welding with a home buzzbox the rod sticks when it contacts the work...
When those contacts make motor dumps current into the capacitors . Welding can be quick. Presumably motor-cap combination passes through resonance as it slows down but if it's resonant during contact bounce millisecond , well...that's high current.
Sure need a hint at current through those contacts. Remember the 'thump test' ? Perhaps a coil with longer leads ...

surely the 8 ohms reduced your measured 29.

 

[H012]

you can usually hear the "click" when centrifugal switch throws.

Did he say what was that time allowance ?
How long between starts on that sweep motor? Does it reverse electrically midsweep or is the cyclic motion of the sweep arm all done by gears?

Well this is how it goes when you can't get at it with test equipment, only way you know it's fixed is the test of time.Do i recall relays looking something like this ?
View attachment 106332

An accidental intermittent between contacts 22 and 32 (which i think are adjacent?) would bypass the 8 ohms we added...

View attachment 106333
was this machine a particularly bad actor before ?

during braking we have a tank circuit, motor run winding and the two start capacitors as shown by your blue trace. At resonance a tank circuit has current gain. Our 8 ohm resistor increases the damping but we don't know by how much.
You reported ( in post 23 ) measuring 29 amps someplace during braking. Can you measure current through the 8 ohm resistor on this machine during braking ?
Might give a clue, might not...

Contacts weld on make, just like when electric welding with a home buzzbox the rod sticks when it contacts the work...
When those contacts make motor dumps current into the capacitors . Welding can be quick. Presumably motor-cap combination passes through resonance as it slows down but if it's resonant during contact bounce millisecond , well...that's high current.
Sure need a hint at current through those contacts. Remember the 'thump test' ? Perhaps a coil with longer leads ...
View attachment 106338
surely the 8 ohms reduced your measured 29.

I will test again today to refresh. The 29 amps is motor start current which also goes the NC's via the jumper. The instant the NO close the start up current is at the NC's at the instant they open doesn't this cause a heavy arc?

 

[H012]

Well this is how it goes when you can't get at it with test equipment, only way you know it's fixed is the test of time.Do i recall relays looking something like this ?
View attachment 106332

An accidental intermittent between contacts 22 and 32 (which i think are adjacent?) would bypass the 8 ohms we added...

View attachment 106333
was this machine a particularly bad actor before ?

during braking we have a tank circuit, motor run winding and the two start capacitors as shown by your blue trace. At resonance a tank circuit has current gain. Our 8 ohm resistor increases the damping but we don't know by how much.
You reported ( in post 23 ) measuring 29 amps someplace during braking. Can you measure current through the 8 ohm resistor on this machine during braking ?
Might give a clue, might not...

Contacts weld on make, just like when electric welding with a home buzzbox the rod sticks when it contacts the work...
When those contacts make motor dumps current into the capacitors . Welding can be quick. Presumably motor-cap combination passes through resonance as it slows down but if it's resonant during contact bounce millisecond , well...that's high current.
Sure need a hint at current through those contacts. Remember the 'thump test' ? Perhaps a coil with longer leads ...
View attachment 106338
surely the 8 ohms reduced your measured 29.

No this machine was no different before than any other. Yes I will measure the current during braking This isn't easy as the sweep is a little hard to see as well as looking at the meter but I will get it done.

 

[jim hardy]

The instant the NO close the start up current is at the NC's at the instant they open doesn't this cause a heavy arc?

that's the relay contact race. I think they are break before make so NC should be open before NO closes. If you can see the contacts you can verify by taking relay in hand and operating the plunger slowly while you watch. Never should both be made at same time. Try it in several different positions, if it's close enough to be sensitive to gravity there's something not right. I can't recall whether those had a specified mounting plane.

I;ve found a lot of intermittents over the years from abraded insulation.
Our old Westinghouse BF relays were susceptible to debris falling down into the contacts.

The question to be answered is "There's enough current flowing to hurt those contacts. Where does it go ? "
It'd be nice to know how much it is. Surely those start capacitors will absorb a lot, and we thought we'd limited that with the 8 ohm resistor. Maybe we need fifteen or twenty ? The subtransient motor current can have fast rise time, rising while contacts are still bouncing which is deadly.

If that current is going where it's supposed to we attack its amplitude. If it's going someplace we don't expect , we work on that.
Looks like the NO contacts constrain it to where we expect, on paper.

old jim

 

[Tom.G]

Well, that's the third model relay data sheet, AB 700 HA. You were using AB 700 CF.

The 700 CF is rated for twice the vibration and three times the shock resistance of the 700 HA, and a bit better than the CWC016. That could easily be the critical difference. Considering the harsh operating environment, I recommend sticking with the AB 700 CF relay, at least for now. That will reduce the contact bounce. Once they are proven to work well (or not), you can try other replacement options.

 

[jim hardy]

okay, more speculation on relay race

we've not examined what drives these relay coils. I'm pretty sure i remember mention of a microcomputer someplace, presumably located in what we plant guys would call "the brain box".Relays depend on a rapid change of flux to move the armature smartly so that it'll break the microwelds formed at "make" time.
Anything that slows rate of change of flux is detrimental to the mechanics of operation.

See this interesting article...
www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN

Relay deenergization or “drop-out” in typical clapper-type relays normally
develops as follows: As the coil supply is interrupted, the magnetic flux
decays to the point where the decreasing magnetic holding force (trying
to keep the armature seated) drops below the spring forces (trying to
unseat it), and armature opening commences. As armature opening
continues, spring forces reduces according to the armature position; the
countering magnetic force, however, reduces both with armature position
and with decay of coil current (both of which reduce coil magnetic flux).As
the electrical current in a relay coil is interrupted, an induced voltage
transient of the order of hundreds or even thousands of volts may be
generated across that coil as its magnetic flux, which is linked by the coil
turns, collapses. This induced voltage, plus the coil supply voltage, as
shown in Fig. 1, appears across the coil interrupting switch in a simple
series switching circuit.
...
A slowly decaying magnetic flux (the slowest is experienced with a simple
diode shunt across the coil) means the least net force integral available
to accelerate the armature open. In fact, rapid loss of the opening forces
supplied by stiff NO contact springs, coupled with slowly decaying
magnetic forces, can actually cause a period of net force reversal where
the armature velocity is slowed, stopped, or even momentarily
reversed until the flux further decays, finally permitting available spring
“return” forces to cause transfer to continue.
It is equally important to realize that when the contacts of a typical power
relay make, connecting very fast rising (e.g., resistive) medium or high
current loads to the voltage source, a minute molten interface occurs
between the mating contacts, giving rise to a microweld or stick condition
that must be separated at the next opening transfer.
The “stick” force is normally well within the ability of the net opening
force, aided by the momentum of the moving armature, to break the
stick and effect contact transfer. However, the loss or even reversal of
armature velocity (under conditions of simple diode shunting as described
above), and accompanying loss of armature momentum needed to help
break the contact stick, can result in failure to break the stick, and a
contact “weld” is experienced.
The more rapidly the coil current decays, the less the magnetic hold
back, and thus the greater the armature momentum and contact stick
“break-ability.”

We have here 'safety ' relays which means a single welded contact won't prevent armature movement..Conversely that means the armature can't apply a whole lot of force to unstick it...
Might we have a complicated sequence of events here ?
IF a NC contact fails to "unstick",
then as you pointed out, the instant the NO closes a LOT of current flows through your stuck NC which melts the microwelds and that frees the NC contact, it gets to open and interrupt a LOT of current as it opens. And it wipes out evidence of the stuck contact. But it suffered a big arc.

Now, since adding the 8 ohm resistor, the current that flows at instant of NC closure is a whole lot less than it was before. Maybe enough less that it didn't unstick the contact ?
Is this the very first one you've found stuck instead of just melted ?
Maybe Mother Nature is playing Cat&Mouse with us,. ( She loves to do that and seems to particularly enjoy making me work overtime)
There's a mechanism that increases likelihod of relay contact race right there in that PDF. It's the protection circuit of relay drivers in whatever drives those relays, if there is protection. ( Probably there is. ) Simple diodes are the most common approach and are also the deadliest as explained in that article. They'll increase likelihood of a stuck contact.

Do you have any information about the relay drivers in your "Brain Box" ?

If they use simple diodes across the coil for protection then we have a possible design flaw, relay drivers that make the relays slow to release inviting relay sticking that immediately wipes out its own evidence.
Your 8 ohm resistor might have preserved the evidence.

It is with trepidation i post such a bizarre hypothesis . But I've seen stranger things.
It's possible that relays thirty years ago were more tolerant of flywheel diodes than are modern ones. That's something the engineer picking replacements might well overlook.
Spring tension is one of those little details with big consequences. Armature spring is probably different for each combination of NO and NC contacts, for it has to accelerate mass of relay's moving parts and its force gets summed with force of contact springs. Moral- Don't go around swapping relay parts...

Please check my logic ? Any ideas how to test for sticks ? Probably upping resistor to fifteen or twenty ohms would reduce size of microwelds.

We need to know what's just inside that brainbox. Relay drivers, inductive kick protection...
If they use zener diodes for flywheel , well, the design guys did a good job and we just need to verify they still break down at nominal Vz not just one diode drop..



old jim

 

[H012]

Well, that's the third model relay data sheet, AB 700 HA. You were using AB 700 CF.

The 700 CF is rated for twice the vibration and three times the shock resistance of the 700 HA, and a bit better than the CWC016. That could easily be the critical difference. Considering the harsh operating environment, I recommend sticking with the AB 700 CF relay, at least for now. That will reduce the contact bounce. Once they are proven to work well (or not), you can try other replacement options.

Thanks Tom How did you come across data sheet for the cf model when I went to AB site the HA is what kept coming up. I was going to do some test on the start switch to see if I could tell how long it stays in the reason here is that these motors had centrifugal switches originally which worked by rpm the new switches are load detecting Solid State.
I couldn't do any testing yesterday as my help decided he didn't have a ride to work.
I'm going to take a picture of motor so I can show you guys something. I'm glad you guys can understand the data sheets because a lot of it I don't. The vendor for the relays have been jumping all over offering different relays maybe a supply/price problem but it's frustrating. I own my own have been trying to find a contactor more suited for the 120 vac and current but the draw back is the 4PST with 2no& 2 NC don't offer very high current limits in the area that is needed.If I was using 208 vac and only needed NO these are limitless.

 

[H012]

that's the relay contact race. I think they are break before make so NC should be open before NO closes. If you can see the contacts you can verify by taking relay in hand and operating the plunger slowly while you watch. Never should both be made at same time. Try it in several different positions, if it's close enough to be sensitive to gravity there's something not right. I can't recall whether those had a specified mounting plane.

I;ve found a lot of intermittents over the years from abraded insulation.
Our old Westinghouse BF relays were susceptible to debris falling down into the contacts.

The question to be answered is "There's enough current flowing to hurt those contacts. Where does it go ? "
It'd be nice to know how much it is. Surely those start capacitors will absorb a lot, and we thought we'd limited that with the 8 ohm resistor. Maybe we need fifteen or twenty ? The subtransient motor current can have fast rise time, rising while contacts are still bouncing which is deadly.

If that current is going where it's supposed to we attack its amplitude. If it's going someplace we don't expect , we work on that.
Looks like the NO contacts constrain it to where we expect, on paper.

old jim

Hey Jim I can try and put a meter on the NO and another meter on NC to see if my eyes are quick enough to tell when they make/break. Also read my reply to Tom's post.

 

[jim hardy]

Do you have a clamp on ammeter ? Or a donut toroid ?

I'd like to rule out my crazy hypothesis.
Trying to think of a way to detect the current pulse that would result from my "stuck NC" .
Its only distinguishing features are : it would appear when relay energizes closing the NO , it should produce a spark as the NC opens, and shouldn't appear very often only when the relay contact sticks.

A small donut toroid around the wire with a meter on secondary should give a brief voltage pulse when current surges. Maybe a LED there would flash . Problem is discerning normal current from stuck contact current, we don't know if they're very different.

Say - what do you think of this ? a volt meter across the 8 ohm resistior should give a higher reading when NC sticks than when it doesn't stick. That should be easy, write down the reading for say ten operations and note any unusual sparking.

Try your two meter check. I don't think i'd be able to see that fast.

Have you a description of the relay drivers in "brain box" ?

old jim

 

[Tom.G]

How did you come across data sheet for the cf model

See my PM to you. There was a link in my post but it got deleted.
Also, which model AB relay is it that just burned up? the HA or the CF or...?

If I was using 208 vac and only needed NO these are limitless.

Could you add Auxilliary NC contacts to any of those higher-rated relays?

Say - what do you think of this ? a volt meter across the 8 ohm resistior should give a higher reading when NC sticks than when it doesn't stick. That should be easy, write down the reading for say ten operations and note any unusual sparking.

Sounds like a good idea. There should be a voltage across that resistor ONLY during breaking.

 

[jim hardy]

..this is the HA ? from AB bulletin 700, page 2

hang on trying to uninstalll recent wimndows update so i can find things

 

[H012]

See my PM to you. There was a link in my post but it got deleted.
Also, which model AB relay is it that just burned up? the HA or the CF or...?Could you add Auxilliary NC contacts to any of those higher-rated relays?Sounds like a good idea. There should be a voltage across that resistor ONLY during breaking.

The AB are 700 CF I will do some looking into axillary NO's I saw some that have one axillary NO but will look more. My new inductive amp meter I bought isn't working. Off to Home depot to get my money back.

 

[H012]

okay, more speculation on relay race

we've not examined what drives these relay coils. I'm pretty sure i remember mention of a microcomputer someplace, presumably located in what we plant guys would call "the brain box".Relays depend on a rapid change of flux to move the armature smartly so that it'll break the microwelds formed at "make" time.
Anything that slows rate of change of flux is detrimental to the mechanics of operation.

See this interesting article...
www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN

We have here 'safety ' relays which means a single welded contact won't prevent armature movement..Conversely that means the armature can't apply a whole lot of force to unstick it...
Might we have a complicated sequence of events here ?
IF a NC contact fails to "unstick",
then as you pointed out, the instant the NO closes a LOT of current flows through your stuck NC which melts the microwelds and that frees the NC contact, it gets to open and interrupt a LOT of current as it opens. And it wipes out evidence of the stuck contact. But it suffered a big arc.

Now, since adding the 8 ohm resistor, the current that flows at instant of NC closure is a whole lot less than it was before. Maybe enough less that it didn't unstick the contact ?
Is this the very first one you've found stuck instead of just melted ?
Maybe Mother Nature is playing Cat&Mouse with us,. ( She loves to do that and seems to particularly enjoy making me work overtime)
There's a mechanism that increases likelihod of relay contact race right there in that PDF. It's the protection circuit of relay drivers in whatever drives those relays, if there is protection. ( Probably there is. ) Simple diodes are the most common approach and are also the deadliest as explained in that article. They'll increase likelihood of a stuck contact.

Do you have any information about the relay drivers in your "Brain Box" ?

If they use simple diodes across the coil for protection then we have a possible design flaw, relay drivers that make the relays slow to release inviting relay sticking that immediately wipes out its own evidence.
Your 8 ohm resistor might have preserved the evidence.

It is with trepidation i post such a bizarre hypothesis . But I've seen stranger things.
It's possible that relays thirty years ago were more tolerant of flywheel diodes than are modern ones. That's something the engineer picking replacements might well overlook.
Spring tension is one of those little details with big consequences. Armature spring is probably different for each combination of NO and NC contacts, for it has to accelerate mass of relay's moving parts and its force gets summed with force of contact springs. Moral- Don't go around swapping relay parts...

Please check my logic ? Any ideas how to test for sticks ? Probably upping resistor to fifteen or twenty ohms would reduce size of microwelds.

We need to know what's just inside that brainbox. Relay drivers, inductive kick protection...
If they use zener diodes for flywheel , well, the design guys did a good job and we just need to verify they still break down at nominal Vz not just one diode drop..



old jim

You would think there would be a protection circuit for the relays but there isn't. The coil is 24 vac. The way the coil works is: The A1 is hot at all times the A2 is connected through the circuitry of the microprocessor board.as needed

 

[Tom.G]

Oops! I have a typo in my post #230:

Referring to @jim hardy post: #229 "a voltmeter across the 8 Ohm resistor..."

Sounds like a good idea. There should be a voltage across that resistor ONLY during breaking.

SHOULD BE:
Sounds like a good idea. There should be a voltage across that resistor ONLY during motor braking.

 

[H012]

Oops! I have a typo in my post #230:

Referring to @jim hardy post: #229 "a voltmeter across the 8 Ohm resistor..."

SHOULD BE:
Sounds like a good idea. There should be a voltage across that resistor ONLY during motor braking.

Checking it today. I tried the two meter approch
To see if there is a delay of the the NC opening when NO close. If there is it is to fast for me to tell looking at both meters it looks like it's simultaneous but that's probably not the case as I don't believe human eyes and the brain to process such rapid change.

 

[H012]

Checking it today. I tried the two meter approch
To see if there is a delay of the the NC opening when NO close. If there is it is to fast for me to tell looking at both meters it looks like it's simultaneous but that's probably not the case as I don't believe human eyes and the brain to process such rapid change.

Ok been awhile. What I did was add a timer in between jumper wire from 43 to 31NC used the NO of timer at .05 secs. It closes after start windings have deenergized.Before if you remember There was 25 amps going through NC also as they were opening. Now by delaying the timer closes after the 25 amp draw has stopped. Now I only have 6 amps at any time going to NC's

 

[jim hardy]

Refresh my memory

where's 43 and 41 ?

 

[jim hardy]

If so, you seem to have fixed a contact race.

 

[jim hardy]

If they use simple diodes across the coil for protection then we have a possible design flaw, relay drivers that make the relays slow to release inviting relay sticking that immediately wipes out its own evidence.
Your 8 ohm resistor might have preserved the evidence.

Now I'm really curious about that microprocessor's relay driver circuitry.

The way the coil works is: The A1 is hot at all times the A2 is connected through the circuitry of the microprocessor board.as needed

What kind of microprocessor is it? Do you have a manual or link ?

 

[H012]

If so, you seem to have fixed a contact race.

To put it simply, I fixed the contact race and took the 25 amps out of the braking NC'd I put to new cube relay in sweep we'll see if it changes anything. Does my idea sound OK to you?

 

[H012]

If so, you seem to have fixed a contact race.

It's the jumper from 43 NO to the 31 NC of relay

 

[jim hardy]

o put it simply, I fixed the contact race and took the 25 amps out of the braking NC'd I put to new cube relay in sweep we'll see if it changes anything. Does my idea sound OK to you?

If i understand where your new contact is, where i guessed it, that sounds fine. Great thinking !
What powers the timer's coil ?

My curiosity always wants to understand what was going on,
that's why I'm curious about what circuit drives the relay coil and just how sharp is its turn-off , ie how fast does it drive relay coil current to zero.

 

[H012]

Refresh my memory

where's 43 and 41 ?

View attachment 107051

 

[H012]

Trying to upload a picture. Having hard time

If i understand where your new contact is, where i guessed it, that sounds fine. Great thinking !
What powers the timer's coil ?

My curiosity always wants to understand what was going on,
that's why I'm curious about what circuit drives the relay coil and just how sharp is its turn-off , ie how fast does it drive relay coil current to zero.

I just jumped from relay contact to timer contact.

 

[H012]

yes that looks right were you're indicating on attachment.