Solving Motor Braking Circuit Problem in Bowling Industry

[jim hardy]

The relays are turned on an off by means of micro switches riding along a cam faces.

Great - that's something you can see!

The signal to the relay 24vac coils for S & T is done via a PCB.

?? Just a passive pcb that conducts current to the coils, or an active one with circuitry ?

If you remember earlier in this thread I had talked about the capacitance of this circuit

Yes i do remember .
I sure wonder how the design guys arrived at their capacitance values.

That's a fun part of maintenance, figuring out why the designers did things the way they did.
Is the gear mechanism some sort of "geneva wheel" gear mechanism where modest motor coastdown shouldn't matter very much?

 

[Tom.G]

are these the contacts burning up? i only count half enough on the relay picture to agree with the drawing...

i see four NC contacts on relay T. which ones burn up ?

Well?

 

[H012]

Great - that's something you can see!?? Just a passive pcb that conducts current to the coils, or an active one with circuitry ?
Yes i do remember .
I sure wonder how the design guys arrived at their capacitance values.

That's a fun part of maintenance, figuring out why the designers did things the way they did.
Is the gear mechanism some sort of "geneva wheel" gear mechanism where modest motor coastdown shouldn't matter very much?
View attachment 103380

It's a active board The main function of the board is scoring. The other part of the circuitry is for the start timing for when the [T] table and [S} sweep start. As I stated the two components work in unison. That's the problem of the NC burning up If the Sweep motor coasts to far then it out of time with the Table motor. When the PCB sends it signal to start Sweep and Table the machine goes into a what's called interlock The motor just mounts to a right angle gear head.

 

[H012]

Well?

both NC's burn up but never at the same time. The NO's when I take the relay apart always have plenty of contact surface left. This is always the case.

 

[Tom.G]

I'll add my vote to the approaches by @Svein and @jim hardy , it sure seems the simplest and most likely to work. Although I have my reservations about an incadescent lamp in that vibration environment.

I solved it by inserting an 8Ω 50W resistor in series with the relay contacts.

Inserting an 8Ω 50W resistor in series with the relay contacts dropped the current spike to acceptable levels

Maybe a 300 watt incandescent lamp here ?

@jim hardy In your post #29 calculating motor impedance, would it be more appropriate to use Line Voltage and LRA (Locked Rotor Amps)?
Hmm... Maybe not. I just measured the inductance of a couple motors here w/ similar FLA rating and got 10mH and 8.5mH for an induction motor and one with brushes (Skil Saw, compound wound?)

 

[jim hardy]

@jim hardy In your post #29 calculating motor impedance, would it be more appropriate to use Line Voltage and LRA (Locked Rotor Amps)?

I think it would
just sort of working with what's available here
i used 130V which isn't far from line voltage, on premise rotor current is probably high so voltage might increase a bit when it disconnects from line
and i divided by his reported 29 amps because it's around 5X FLA, and 5X sounds reasonable for a small motor
120 volts X 29 amps is 3.48KVA

The starting KVA required by a motor starting at full voltage is determined from the nameplate on the motor or from the manufacturer.
In general it is accepted that small motors requires higher starting KVA than larger motors. Standard 3 phase motors often have these locked rotor codes:
...
less than 1 hp: Locked Rotor Code L, 9.0-9.99 KVA [per hp- jh]
www.engineeringtoolbox.com/locked-rotor-code-d_917.html

1/2 hp then should be in vicinity of 4.5KVA = 37.5 amps, he measured 29

Hopefully he'll read this and say if there's a LRA or KVA code on motor nameplate

i have adjusted from this being a "something is broken" to a "something in design" scenario
and am heartened by your thoughts along same line(if i read your post right)

@Svein When you said you "had that same problem once" - how literal can we take that ? Did you fix this on AMF bowling machines ? Would you share your experience with us ?

reservations about an incadescent lamp in that vibration environment.

The 300 watt lamp was just tossing out an idea. Lamps have that nice nonlinear characteristic , as the motor slows and its voltage decays the lamp cools lowering its resistance which might be helpful
AND it's a high watt resistor you can buy at any decent hardware store.

 

[jim hardy]

@jim hardy In your post #29 calculating motor impedance, would it be more appropriate to use Line Voltage and LRA (Locked Rotor Amps)?

You know, my answer above isn't quite honest

to be truthful i am scared of needing to estimate that motor's subtransient reactance because i have not much idea how.
In fact I've no idea how to approach transient calculations for an induction machine.
And in all of English language, what are the three words does male vanity fear most ? .. "I don't know" ...

That said,
Here's a synchronous machine, LRA would be the steady state region on far right
but to capture the actual waveform on his machine turns this into a research project .

http://www.nuclearelectricalengineer.com/explanation-origin-generators-subtransient-reactance/
At instant of contact closure , what limits current?
Contact bounce and subtransient reactance share a timeframe and it's that first part cycle on far left which his DMM won't see.
meaning that poor little NC contact might suffer terribly in the first milliseconds from current several times greater than LRA.
Is that something designers would have thought about when sizing the relay?
How to protect against that ? Something akin to di/dt for semiconductors? Or a SSR that doesn't bounce?
...just trying to think of "what if's" because Mother Nature so enjoys a tease.

A teeny bit of inductance where we've drawn resistors and lightbulbs might be the technically sound fix
What do you think? Know of any writeups on induction generator fault current ?
That one i linked (the image ) says this about synchronous machines:

Though a damper bar benefits the machine by increasing its efficiency and stability, it also introduces a mutual inductance between it and the various other windings. The complex magnetic flux interactions created by the small resistances and inductances (self and mutual) between the various elements (damper windings, field windings, and rotor body) during a transient condition ultimately act to dramatically and suddenly reduce the machine’s reactance.

and an induction machine's rotor is all damper bars.old jim

 

[H012]

You know, my answer above isn't quite honest

to be truthful i am scared of needing to estimate that motor's subtransient reactance because i have not much idea how.
In fact I've no idea how to approach transient calculations for an induction machine.
And in all of English language, what are the three words a vain man fears most ? ? "I don't know" ...

That said,
Here's a synchronous machine, LRA would be the steady state region on far right
but to capture the actual waveform on his machine turns this into a research project .
View attachment 103414
http://www.nuclearelectricalengineer.com/explanation-origin-generators-subtransient-reactance/
At instant of contact closure , what limits current?
Contact bounce and subtransient reactance share a timeframe and it's that first part cycle on far left which his DMM won't see.
meaning that poor little NC contact might suffer terribly in the first milliseconds from current several times greater than LRA.
Is that something designers would have thought about when sizing the relay?
How to protect against that ? Something akin to di/dt for semiconductors? Or a SSR that doesn't bounce?
...just trying to think of "what if's" because Mother Nature so enjoys a tease.

A teeny bit of inductance where we've drawn resistors and lightbulbs might be the technically sound fix
What do you think? Know of any writeups on induction generator fault current ?
That one i linked (the image ) says this about synchronous machines:and an induction machine's rotor is all damper bars.old jim

Only think that's given on this 1/3 motor is Volts 115, FLA 6.0, Rpm 1725, Service factor 1.25., and Duty cycle Continuous

 

[H012]

I tell you guys that I'm very grateful to you for helping me. I can tell and except the fact that you are far above me in electrical properties.
The only fact I know is that the NC arcing occurs when they open, only because I can see it.

 

[H012]

I tell you guys that I'm very grateful to you for helping me. I can tell and except the fact that you are far above me in electrical properties.
The only fact I know is that the NC arcing occurs when they open, only because I can see it.

Here is some info on the relay. I don't see the spec. for Max switching current for the contacts.

 

[Svein]

@Svein When you said you "had that same problem once" - how literal can we take that ? Did you fix this on AMF bowling machines ? Would you share your experience with us ?

Not bowling machines, no. What I had was a cobbled-together setup for punching holes in a sheet of steel and feeding it through a roller in order to create the blank for a water heater. One of the components was an AC motor that I controlled by a relay. The current rating for the motor was - as far as I can remember - 0.25A and the relays were rated for 10A. Despite this overcapacity, the relay contacts burned up after one or two start/stops. I then inserted my (analog) multimeter in the circuit in order to find out what was happening. What happened was a current peak of about 30A for about 0.5s (at least that was what the meter could track). Since the nominal current was 0.25A, I figured that inserting an 8Ω resistor (which I had in my odds-and-ends drawer) in the circuit would lower the operating voltage by 2V (which still kept the operating voltage way above the minimum), but would limit the maximum current to an acceptable value. And it worked! The relays were still good after 5 years.

 

[wwoollyyhheeaa]

Hi guys. I don't profess to know much about suppressing the sort of stuff you are discussing but it seems to me that if you were to hire a digital recording 'scope with memory and connect one channel to one side of the relevant relay contact and the second channel to the other and make sure the gain is the same on each channel, using frame Earth (or whatever) as the voltage references and run the motor, then you would be able to switch the CRO to A-B and see the voltage between the contacts at any time in the motor's cycle. Suitably expanded on the time axis it might give you another clue. It would also give a numerical description about how many volts are across the contacts. If you also hired a digital current probe with memory and which can be triggered as required it would show how much current is involved. Just an idea.

 

[jim hardy]

inserting an 8Ω resistor (which I had in my odds-and-ends drawer) in the circuit would lower the operating voltage by 2V (which still kept the operating voltage way above the minimum), but would limit the maximum current to an acceptable value. And it worked! The relays were still good after 5 years.

0.25^ X 8 = 1/2 watt at motor current, what a good fix.

The only fact I know is that the NC arcing occurs when they open, only because I can see it.

?? the NC's open on motor start... I've been concentrating on braking, on motor stop.

Take an old relay in your hand and operate it slowly by pushing on the moving plunger , watching the contacts

It's important that when energizing, the NC contacts open before the NO contacts close, ie "Break before Make"
and in other direction, when de-energizing, the NC contacts close after the NO ones open , again "Break before make"
in other words there's a place in the middle of relay travel where both NO and NC are open
else there's a contact race that connects 2.65 ohms of capacitance right across the supply

is the arcing on motor start or motor stop? or both ?

 

[jim hardy]

..

Here is some info on the relay. I don't see the spec. for Max switching current for the contacts.

did you forget to attach it ? Got a picture with part number ?

 

[H012]

Here is some info on the relay. I don't see the spec. for Max switching current for the contacts.

0.25^ X 8 = 1/2 watt at motor current, what a good fix.
?? the NC's open on motor start... I've been concentrating on braking, on motor stop.

Take an old relay in your hand and operate it slowly by pushing on the moving plunger , watching the contacts

It's important that when energizing, the NC contacts open before the NO contacts close, ie "Break before Make"
and in other direction, when de-energizing, the NC contacts close after the NO ones open , again "Break before make"
in other words there's a place in the middle of relay travel where both NO and NC are open
else there's a contact race that connects 2.65 ohms of capacitance right across the supply
View attachment 103434

is the arcing on motor start or motor stop? or both ?

Motor stopping. Reason I can see the arcing is on seven of my machines I have the old style open contact Allen Bradley relays Jim my be familiar with these. They are now obsolete. I'm trying to find a data sheet on the new style 700-CF220J Allen Bradley but am having a hard time finding it.

 

[H012]

..did you forget to attach it ? Got a picture with part number ?

I tried to attach it but for some reason it doesn't

 

[H012]

I'm having server issues I don't usually use windows but for some sites Windows works better.

 

[H012]

I'm having server issues I don't usually use windows but for some sites Windows works better.

OK found that file is too big tried zipping it still to big to upload. Anyway, I read the contacts are rated at 30 amp on make and 3 amp on break.

 

[Tom.G]

H012 said: ↑
The only fact I know is that the NC arcing occurs when they open, only because I can see it.

The relay has 2N.O and 2N.C. contacts but the pinsetter schematic shows 8 Poles; must use two relays. Perhaps relative timing between the relays is the culprit. Can you mark up the schematic to show which relays and which contacts are used in your present configuration?

Here is the link to the AB relay. The "J" part No. suffix is for 24v 60Hz coil.

http://search.rockwellautomation.co..._fe&output=xml_no_dtd&proxystylesheet=main_fe

Interestingly, the Repair Parts list shows only covers, labels and coils, but NOT contacts.

 

[H012]

H012 said: ↑
The only fact I know is that the NC arcing occurs when they open, only because I can see it.

The relay has 2N.O and 2N.C. contacts but the pinsetter schematic shows 8 Poles; must use two relays. Perhaps relative timing between the relays is the culprit. Can you mark up the schematic to show which relays and which contacts are used in your present configuration?

Here is the link to the AB relay. The "J" part No. suffix is for 24v 60Hz coil.

http://search.rockwellautomation.co..._fe&output=xml_no_dtd&proxystylesheet=main_fe

Interestingly, the Repair Parts list shows only covers, labels and coils, but NOT contacts.

That's correct. [1] 4PST for the [T] table and [1] 4PST for the sweep

 

[Tom.G]

OK. Double-Break contacts with NO wired in parallel. Mis-read the schematic. No need for a marked up one.

With just one relay in the pinsetter, relative timing between relays isn't an issue.

The remaining possibilities are:
1) There is a large charge retained on the Start capacitors.
2) The relay being used has overlapped contact timimg.
3) The motor is still coasting.
[5) Ghosts or Gremlins]
There just aren't any other power sources.

Can you hang a DC voltmeter on the capacitors to check for a retained charge during operation?
When a relay starts degrading, is there arcing on every operating cycle or just occassionally?

 

[H012]

Hey Tom, There are two 4PST relays. One for each assembly of the pinsetter. Table is one assembly and the sweep is the other assembly.
The circuitry for each assembly is the same. Breaks down like this: The Table has two caps, one 625 ohm resistor and a 4PST relay.
The sweep has it's own two caps., one 625 ohm resistor and a 4PST relay. The arcing is every cycle. I will check the with a DC meter tomorrow when I go to work at 3 PM Thanks for the feed back.

 

[jim hardy]

Can you hang a DC voltmeter on the capacitors to check for a retained charge during operation?

that'll be worth doing. Watch for several operating cycles because it's random where in the line cycle relay contacts open...

Here's another low-tech test
make yourself a coil about 4" diamter, 100 turns of #18 or so, whatever you have, just remember it has to carry braking current
Connect it here so braking current has to flow through it

now you have done two things
connected a tiny inductance in series
and given yourself a "poor man's surge detector"
six amps won't affect your coil
but if you are getting hundred+ amp surges from that motor, you'll feel your coil 'jump' as the wires push against one another from the sudden current rise
and that'll be a clue
maybe we can figure out how to put a better number on it.
I noticed the "jump" when applying short circuits to an inverter , but i had meters too and learned guesstimate my surge current from how hard the coil jumped.

In troubleshooting we have to use all our senses
and if your current surge is that strong we might be able to use inductance against it.
First step is investigate it further.

Thanks to TomG and Svein for diving in here - I'm over my head , but the whole is greater than the sum of its parts !

old jim
I really don't know, just one more bit of data gathering.

 

[H012]

that'll be worth doing. Watch for several operating cycles because it's random where in the line cycle relay contacts open...

Here's another low-tech test
make yourself a coil about 4" diamter, 100 turns of #18 or so, whatever you have, just remember it has to carry braking current
Connect it here so braking current has to flow through it

now you have done two things
connected a tiny inductance in series
and given yourself a "poor man's surge detector"
six amps won't affect your coil
but if you are getting hundred+ amp surges from that motor, you'll feel your coil 'jump' as the wires push against one another from the sudden current rise
and that'll be a clue
maybe we can figure out how to put a better number on it.
I noticed the "jump" when applying short circuits to an inverter , but i had meters too and learned guesstimate my surge current from how hard the coil jumped.

In troubleshooting we have to use all our senses
and if your current surge is that strong we might be able to use inductance against it.
First step is investigate it further.

Thanks to TomG and Svein for diving in here - I'm over my head , but the whole is greater than the sum of its parts !

old jim
I really don't know, just one more bit of data gathering.

Thanks Jim yea sounds easy enough. You still want me to lift the wire at TS20 first before I try the inductor? It sounds as if all you guys are
on the same page in a round about way. Can I ask a stupid question? How come put my DC meter on the caps

 

[Tom.G]

How come put my DC meter on the caps

Yea, counter-intuitive isn't it? With the motor and relay in the Stopped condition, the only place to store energy is in the caps, and they can't store AC. Ergo, if they are storing energy it must be DC.

The resistors across the caps are there to bleed off any stored charge. If there is a sustained voltage across the caps then the resistors are not doing their job. With the conditions you described, this would show up only after the relay contacts are damaged.

Another useful test is measure both the AC and DC voltage across the caps while the motor is running. Both AC & DC should be zero. If there is a sustained DC voltage, then the resistors are not doing their job. If there is an AC voltage then either the motor Start winding is not disconnecting, or the NC contacts at C1-33K to C1-31A are not opening.

@jim hardy Don't bail out on us! You are making valuable contributions.

 

[H012]

Yea, counter-intuitive isn't it? With the motor and relay in the Stopped condition, the only place to store energy is in the caps, and they can't store AC. Ergo, if they are storing energy it must be DC.

The resistors across the caps are there to bleed off any stored charge. If there is a sustained voltage across the caps then the resistors are not doing their job. With the conditions you described, this would show up only after the relay contacts are damaged.

Another useful test is measure both the AC and DC voltage across the caps while the motor is running. Both AC & DC should be zero. If there is a sustained DC voltage, then the resistors are not doing their job. If there is an AC voltage then either the motor Start winding is not disconnecting, or the NC contacts at C1-33K to C1-31A are not opening.

@jim hardy Don't bail out on us! You are making valuable contributions.

Sounds logical. I have a suspicion one of these is occurring. Will do this when I return to work tomorrow. Will let every one know. I work 3 PM-12.00 post AM Wednesday. Thanks!

 

[jim hardy]

You still want me to lift the wire at TS20 first before I try the inductor?

no, not for this test
we'll check for those voltages
and then hopefully see just how much the current shakes your coil
hopefully you can do it on a machine with open _contacts relays where you can see the arcing
if this little bit of added impedance affects arc intensity that's another clue

Don't bail out on us! You are making valuable contributions.

Thanks.. i tend to get scrambled
that's why i stick to basics

how's this for a hypothesis to check out, if those voltage tests on the caps rule out gross malfunction like sticking contacts ?
Just a hypothesis at this point...

<<<<<<<<<<<<<start hypothesis>>>>>>>>>>>>>>

The initial braking current might be quite high .
The motor becomes an induction generator that gets very nearly shorted out by those 500 uf capacitors in parallel.
We think of those capacitors as 2.65 ohms of Xc, but that's at 60 hz steady state.
At the instant of NC contact closure the capacitor (if it's discharged) is very nearly a short circuit
so change your thinking from steady state 60 hz to millisecond by millisecond

Instantaneous AC is a lot like DC
if the voltage at motor terminals if high on the sinewave at instant T's NC contact closes, we have maybe 150 volts applied instantaneously to a discharged capacitor
and the only thing limiting current is the internal impedances of the motor and the capacitor (well, plus the wires)
We'd like to think current would only be as much as LRA , probably thirty amps or less
but the motor is likely capable of a lot more

i'm no expert at fault current calculations
so have searched for general info on induction motor fault current

http://ee.lamar.edu/gleb/power/Lecture%2012%20-%20Symmetrical%20faults.pdf
4/29/2010

Fault current transients in machines
It is possible to observe the three periods of fault current if the rms magnitude of the AC component current is plotted as a function of time on a semilogarithmic scale.
It is possible to determine the time constants for the three periods...

The AC current flowing in the generator during the subtransient period is called the subtransient current and is denoted by I”. This current is caused by the damper windings of synchronous machines. The time constant of the subtransient current is denoted by T” and it can be determined from the slope. This current may be 10 times the steady-state fault current
Fault transients in induction motors
An induction motor is an AC machine that has only damper windings on its rotor. Since damper windings are major source of current during the subtransient period, the induction motors in a power system should be considered during calculations of subtransient currents flowing in faults.
Since currents in a damper winding are of little importance during the transient and steady-state periods of faults, induction motors may be ignored in fault current analysis after the subtransient period

I believe your motor is capable of 100 amps of fault current for a fraction of a line cycle and that "subtransient current" may be what is wrecking your contacts..That much current will make an impressive arc like you report seeing .
Your clamp on ammeter will not report that transient, instead it'll average a few cycles and report that smaller number.
That transient is significant to the poor little relay contact though, because relay contacts don't just close and stay closed, they bounce for a millisecond or two.
Every bounce interrupts that subtransient current and makes sparks
after a few milliseconds the current settles down and the contact quits bouncing and the motor slows to a stop as intended.

Svein and TomG you guys pointed the way to this one
if it turns out this is the trouble the fix should be easy Limit subtransient current.

That 4 inch 100 turn coil i think should oppose instantaneous current rise , it's around 1.8 millihenries i think which is not quite 1/5th of the 10mh TomG measured on a similar size motor
TomG measured the motor's steady state reactance . Subtransient reactance will be several times smaller. So that coil might roughly halve subtransient current? It's hard to judge an arc, but maybe we'll be lucky and the difference will be noticeable. Again there'll be some randomness to it depending where in the line cycle the contact closes so watch several sweep cycles.
Hopefully we can play with combination of inductance and resistance to reduce contact arcing and still give desired coastdown.<<<<<<<<<<<<<<end hypotheses>>>>>>>>>>>>>>
If testing proves me 100% wrong, well, won't be first time !

sanity check :
TomG measured 10mh
which at 60 hz is 3.77 ohms, 120 volts / 3.77 ohms is 31.8 amps and HO12 reported 29

If X'' is 1/10 of X_{steady state}, then X'' is ~0.3 ohms
and our 1.8mh coil is 0.69 ohms ..
What will be rate of current rise through our 1.8 mh coil and (guess at motor's subtransient reactance of ) 1 mh X'' ?
E = L X rate of change of current
so rate of change of current = E / L
without coil 150V / 1mh =150 amps per millisecond
with coil 150V / 2.8mh = 54 amps per millisecond
we might need a core...
maybe a ferrite toroid from an old CPU power supply?
What do you guys think ?old jim

ps make sure all the turns go same way on that coil - don't use lampcord with the two wires twisted together at far end, that cancels the inductance

old jim

 

[H012]

Yea, counter-intuitive isn't it? With the motor and relay in the Stopped condition, the only place to store energy is in the caps, and they can't store AC. Ergo, if they are storing energy it must be DC.

The resistors across the caps are there to bleed off any stored charge. If there is a sustained voltage across the caps then the resistors are not doing their job. With the conditions you described, this would show up only after the relay contacts are damaged.

Another useful test is measure both the AC and DC voltage across the caps while the motor is running. Both AC & DC should be zero. If there is a sustained DC voltage, then the resistors are not doing their job. If there is an AC voltage then either the motor Start winding is not disconnecting, or the NC contacts at C1-33K to C1-31A are not opening.

@jim hardy Don't bail out on us! You are making valuable contributions.

So measure voltage of both caps at tmpZ and return [neutral] while motor is running?

 

[H012]

So measure voltage of both caps at tmpZ and return [neutral] while motor is running?

Rockwell sent me this. Must admit symbols are you guys expertise.

 

[H012]

Forgot to add that AC-15 is the line on the photo for my relay.