Solving Motor Braking Circuit Problem in Bowling Industry

[H012]

I think they will based on what little we know about the motor.

Test will be to feel of it after a few sweep cycles
if it's staying nice and cool i think it'll be fine

if it gets hot we need a bigger one

TomG and i are studying those curves in datasheet
depending on what assumptions we make predicted life comes out between a few hundred cycles and forever.

When it's that close, one test is worth a thousand opinions !old jim

Thanks Jim/Tom. I hope I'm not taking to much time away from helping others.

 

[Tom.G]

Heck, we do this (hope you don't mind me speaking for you, Jim) to relieve the boredom of retirement.
And we're having a ball on this one. It's getting us to actuallythink again!

 

[H012]

Heck, we do this (hope you don't mind me speaking for you, Jim) to relieve the boredom of retirement.
And we're having a ball on this one. It's getting us to actuallythink again!

LOL! I can't see myself just vegitating either.

 

[jim hardy]

yes I'm learning about fault current contribution from induction motors

We have long known that power engineers have to account for it in short circuit studies, but I'm no expert in those.
and i have taken oscillograms of bus voltage in the power plant during a transfer, those big pump motors (7000 horsepower for boiler feed and 6000 horsepower for reactor coolant) act as generators because the rotor field takes many cycles to decay away.

So
after your little motor has finished arcing the NO contact, which i would think is brief ,
its terminal voltage returns to some number not far from running voltage because the rotor field is still there.
So for the next several line cycles it is an induction generator making probably almost 120 volts.
Next thing that happens is your NC closes connecting the 1000 uf capacitors right across the motor main winding which is still generating ~ 120 volts because the rotor field hasn't collapsed yet.
What happens when you throw a completely discharged 1000 uf capacitor across a generator ? You get a surge of current.
How big a surge ? sixty four dollar question, I've never done that test.
After a lot of reading I'm finding not a real concensus, but the estimates that seem best presented say the surge is 2 to 3 times starting amps, and it's called "Subtransient Fault Current" and it lasts only a fraction of a cycle
starting amps for a 1/3 hp motor ought to be in the range of 5/3 to 7/3 KVA , which at 120 volts is 14 to 19 amps and you reported measuring 22 ..
so perhaps ~60 amps flows through your NC contact at the instant they close

Your resistor will knock down that fault current. I was afraid it'd stretch out motor coastdown but you report it's fine with the resistor.
So we tend to think that'll fix the NC burnup.

But -- that arcing at the NO contact ? How long does it persist ?
If that arc ever fails to go out before the NC contact closes, we then have a path from incoming hot through the arc then through the NC contact to that discharged 1000 uf capacitor. Incoming hot is probably capable of hundreds of amps.
Might that explain your "Gone" NC contacts , unlucky outcome of a relay contact race every now and then ?
Your 8 ohm resistor should limit fault current for that scenario too.

The MOV Varistor will help Mr NO contact extinguish the arc quickly.
and the 8 ohm resistor limits fault current from either source, subtransient from motor or direct from line through arcing NO contact

and diverse methods of protection is good design.

Guess I'm more recapping for my own clarity of thought than anything else
does this help?

Corrections welcome.

Now to figure out how big a varistor we need. ... your "touch test" will be imoprtant.

 

[H012]

yes I'm learning about fault current contribution from induction motors

We have long known that power engineers have to account for it in short circuit studies, but I'm no expert in those.
and i have taken oscillograms of bus voltage in the power plant during a transfer, those big pump motors (7000 horsepower for boiler feed and 6000 horsepower for reactor coolant) act as generators because the rotor field takes many cycles to decay away.

So
after your little motor has finished arcing the NO contact, which i would think is brief ,
its terminal voltage returns to some number not far from running voltage because the rotor field is still there.
So for the next several line cycles it is an induction generator making probably almost 120 volts.
Next thing that happens is your NC closes connecting the 1000 uf capacitors right across the motor main winding which is still generating ~ 120 volts because the rotor field hasn't collapsed yet.
What happens when you throw a completely discharged 1000 uf capacitor across a generator ? You get a surge of current.
How big a surge ? sixty four dollar question, I've never done that test.
After a lot of reading I'm finding not a real concensus, but the estimates that seem best presented say the surge is 2 to 3 times starting amps, and it's called "Subtransient Fault Current" and it lasts only a fraction of a cycle
starting amps for a 1/3 hp motor ought to be in the range of 5/3 to 7/3 KVA , which at 120 volts is 14 to 19 amps and you reported measuring 22 ..
so perhaps ~60 amps flows through your NC contact at the instant they close

Your resistor will knock down that fault current. I was afraid it'd stretch out motor coastdown but you report it's fine with the resistor.
So we tend to think that'll fix the NC burnup.

But -- that arcing at the NO contact ? How long does it persist ?
If that arc ever fails to go out before the NC contact closes, we then have a path from incoming hot through the arc then through the NC contact to that discharged 1000 uf capacitor. Incoming hot is probably capable of hundreds of amps.
Might that explain your "Gone" NC contacts , unlucky outcome of a relay contact race every now and then ?
Your 8 ohm resistor should limit fault current for that scenario too.

The MOV Varistor will help Mr NO contact extinguish the arc quickly.
and the 8 ohm resistor limits fault current from either source, subtransient from motor or direct from line through arcing NO contact

and diverse methods of protection is good design.

Guess I'm more recapping for my own clarity of thought than anything else
does this help?

Corrections welcome.

Now to figure out how big a varistor we need. ... your "touch test" will be imoprtant.

I was thinking a lot about the 1000 micro farad's that's a lot! Necessary I don't. Know. Been doing some research and far as small HP induction motors go and braking there isn't much out there, a lot of crap about. Induction motors as generators. Putting that much capacitor charge on a 1/3 HP motor seems very inefficient! In my opinion.

 

[jim hardy]

you could experiment and find out at what capacitance it no longer slows down quickly enough to park where you wnat it. Start capacitors cost around 3 to 8 bucks at my local parts house.

My guess is 1000 uf is way more than you need at 60 hz but what about when motor slows down and is generating 20 hz or 10 ? Without knowing how they chose 1000 uf i'd have to start by experiment.

...... in the spirit of ruling out what seems unlikely ...

Can you think of an experiment that would rule out possibility of that 'unlucky outcome of a relay contact race" ? You're a mechanical, so you are familiar with electric welding and arc voltage, freeze frame your thinking single step it through ...

Got a spare motor and relay you could put on the bench ?

..........

Well your varistors will be here soon and that's a good experiment, and good next step. Use all your senses, observe arc intensity, listen to motor, and feel the varistor's warmth.
I think arc intensity should vary randomly depending where in the line cycle contact opens.
Look for nuances like - arc is big at NO when it's small at NC ; arcs same size all the time, ; one white one blue,, you never know what little thing will turn into an AHA ! moment .
I like to have my long-shots proven wrong because then i quit worrying about them.

old jim

 

[H012]

you could experiment and find out at what capacitance it no longer slows down quickly enough to park where you wnat it. Start capacitors cost around 3 to 8 bucks at my local parts house.

My guess is 1000 uf is way more than you need at 60 hz but what about when motor slows down and is generating 20 hz or 10 ? Without knowing how they chose 1000 uf i'd have to start by experiment.

...... in the spirit of ruling out what seems unlikely ...

Can you think of an experiment that would rule out possibility of that 'unlucky outcome of a relay contact race" ? You're a mechanical, so you are familiar with electric welding and arc voltage, freeze frame your thinking single step it through ...

Got a spare motor and relay you could put on the bench ?

..........

Well your varistors will be here soon and that's a good experiment, and good next step. Use all your senses, observe arc intensity, listen to motor, and feel the varistor's warmth.
I think arc intensity should vary randomly depending where in the line cycle contact opens.
Look for nuances like - arc is big at NO when it's small at NC ; arcs same size all the time, ; one white one blue,, you never know what little thing will turn into an AHA ! moment .
I like to have my long-shots proven wrong because then i quit worrying about them.

old jim

True. As this motor calls for a. 250uf I'll try 2 of those.

 

[Tom.G]

Putting that much capacitor charge on a 1/3 HP motor seems very inefficient!

They are using the caps as a load to brake the motor, not as a charge source.
BTW, the 1000u caps with the 10mH motor inductance have a resonant frequency of 50Hz, suspiciouly close to Line frequency. Don't know exactly why, but the designer probably had a reason.

 

[Tom.G]

True. As this motor calls for a. 250uf I'll try 2 of those.

But two 250uF in series yield 125uF, not what the motor wants. The present circuit has the caps in series during Start and in parallel for Brakeing. The motor may not start under load with only 125uF.

 

[H012]

But two 250uF in series yield 125uF, not what the motor wants. The present circuit has the caps in series during Start and in parallel for Brakeing. The motor may not start under load with only 125uF.

I understand. It probably won't start. Motor plate does say 250 uF.

 

[H012]

About 6 yrs. Ago I took one of these motors with 5 20 uF run caps to just see what this motor would produce as a generator. The best it will do is 40 volts at 1500 rpm if I tried to push the motor any higher it experienced locked rotor killing the 3 HP gas motor. Just wanted to share.

 

[jim hardy]

this is how we learn !

have at it.

 

[H012]

this is how we learn !

have at it.

Yes I did it more to add more proof to tell others not to believe everything on the internet.

 

[Tom.G]

If you want to check the need for motor braking versus coasting you could just disconnect the NC contacts. Just be prepared to kill power if something goes wrong.

Still hoping for some video of the situation. If that's not possible detailed observation and notes will have to do. Of course some 'scope photos of both voltage and current at both the NC and NO contacts is the Gold standard. Probably not practical in your present environment though.

I'm beginning to feel like we are trying to shoot fish in a barrel... blindfolded and we don't know where the barrel is!

 

[jim hardy]

About 6 yrs. Ago I took one of these motors with 5 20 uF run caps to just see what this motor would produce as a generator. The best it will do is 40 volts at 1500 rpm if I tried to push the motor any higher it experienced locked rotor killing the 3 HP gas motor. Just wanted to share.

i seem to remember a similar experiment described someplace
i wonder what went on there ? A clamp around ammeter on motor lead would be interesting... and voltage across cap

 

[H012]

If you want to check the need for motor braking versus coasting you could just disconnect the NC contacts. Just be prepared to kill power if something goes wrong.

Still hoping for some video of the situation. If that's not possible detailed observation and notes will have to do. Of course some 'scope photos of both voltage and current at both the NC and NO contacts is the Gold standard. Probably not practical in your present environment though.

I'm beginning to feel like we are trying to shoot fish in a barrel... blindfolded and we don't know where the barrel is!

I did disconnect NC circuit somewhere at beginning of our posts. Anyway the NO braking causes the sweep to come down to hard on the bowling lane also the momentum of of the motor and gearbox causes the sweep to keep moving a couple of inches after hitting the lanes causing sweep and table to be out of time.

 

[H012]

At this point guys because of not being able to give you guys enough info due to a lack of scope readings ect. I can try the MOV's and resistor and leave it at that.You guys have been more than helpful.

 

[jim hardy]

At this point guys because of not being able to give you guys enough info due to a lack of scope readings ect. I can try the MOV's and resistor and leave it at that.You guys have been more than helpful.

i remember you said early on you didnt want a research project

If those (edit -) varistors reduce arcing at NO contacts then you will have addressed both of the things i worried about,
1. transient current capability of motor exceeding NC contact rating during bounce period
2. persistence of arc at NO causing contact overlap maybe one out of ten braking cycles, that should show as a really big arc on both contacts at same time but only once in a while.

svein's resistor helps with both 1 and 2, limits fault current
varistor addresses 2, arcing at NO and removes energy from motor during contact flight time which might help with 1 as well

i really like the combination
and i hope you write an article for Bowling World or whatever is your trade journal.

so - let us know what you see and if the varistors stay cool.

thanks,

old jim

 

[Tom.G]

A few of thoughts if you continue to have contact life problems.

I can't tell if the contacts on the relays you are using can be replaced without replacing the relay. If not, then a lower cost approach may be to use either front mounted or side mounted contacts to avoid replacing the whole relay. Maybe even for just the NC contacts since they seem to cause the most trouble.

Another alternative is use a larger, higher rated, relay.

Regardless of results and whatever followup you do, we would love to be kept up to date. Be sure to check in on progress & attempts. And of course any article you decide to write!

If you are using the same relay on most of the alleys, try getting a Field Engineer from either the manufacturer or the outfit from which you buy the relays. If that doesn't work try getting a Field Engineer from a different relay manufacturer, I bet they would be delighted to steal a competitors business!

 

[H012]

A few of thoughts if you continue to have contact life problems.

I can't tell if the contacts on the relays you are using can be replaced without replacing the relay. If not, then a lower cost approach may be to use either front mounted or side mounted contacts to avoid replacing the whole relay. Maybe even for just the NC contacts since they seem to cause the most trouble.

Another alternative is use a larger, higher rated, relay.

Regardless of results and whatever followup you do, we would love to be kept up to date. Be sure to check in on progress & attempts. And of course any article you decide to write!

If you are using the same relay on most of the alleys, try getting a Field Engineer from either the manufacturer or the outfit from which you buy the relays. If that doesn't work try getting a Field Engineer from a different relay manufacturer, I bet they would be delighted to steal a competitors business!

I will know more when MOV's get here. UPS got held up due to the flooding North of me. Tracking information shows Wednesday or Thursday.

 

[jim hardy]

I bet they would be delighted to steal a competitors business!

You should've seen the stir it caused when we replaced Copes Vulcan feedwater valve actuators with Fisher...

 

[Tom.G]

Yeah, when all else fails, call in the competitors!
I suspect that was a larger financial hit for someone than a couple dozen relays.

 

[H012]

Yeah, when all else fails, call in the competitors!
I suspect that was a larger financial hit for someone than a couple dozen relays.

Yea I was doing a little research on these relays. They all use basically the same contact material. I looked at the next step up in relays and size mainly and the big increase in price to changing type of relay off the table.

 

[jim hardy]

your fixes might let a SSR survive, if the owner will let you experiment

offer him co-authorship on your article ?

 

[Tom.G]

Just as an FYI, this popped up in a different thread referring to shunt wound motors. Seemed like it would also fit here.

Back EMF = (1/2) * (angular speed) * (number of turns) * (rotor radius) * (average magnetic flux) * (conductor length)

 

[H012]

Just as an FYI, this popped up in a different thread referring to shunt wound motors. Seemed like it would also fit here.

Thanks

 

[jim hardy]

To close the loop..

H012 sent a PM that adding the resistor and MOV suppressors have cured his arcing, it's no longer visible .

Installed the MOV's and resistors on table and sweep. The arcing is completely gone! Not even a little arc. Wanted to say thanks to everyone who helped. In coming week I will make tech video for bowl Tech.com.

Everybody loves to hear of a victory and he's had one.
I hope he writes that article for his trade journal magazine and posts a link to it.Thanks to all who chipped in and helped. It's what we do here !

old jim himself

 

[H012]

The NC contacts at relay # 31& 32 are sticking closed. Took relay apart found contact welded I was able to tap on relay and it released Jim Tom 32 on relay is the C1 23 then the other side at 31on relay ois jumpered to the NO C1 22 if you guys can remember I installed a 8 ohm resistor at C1 23 to center tap TS 27 of factory resistor Hopefully you still have diagrams. I have a suspect that resistor I put in might be an issue. The inrush current at NO C1 22 when energized to start motor because its also jumpered to the NC 31 this current for a micro second is maybe a problem when NC open. Let me know you guys thoughts.

 

[Tom.G]

Is this happening on just one machine or on several?
Does the arcing occur when the contacts open or when they close?

So far, three possibilities come to mind.

1) The Surge Suppressor (MOV) at the motor may have become disconnected or failed open
2) The 8 Ohm resistor needs to be higher value (wire another 8 Ohm in series with the existing one for testing)
3) The relay contacts may have been contaminated with something, causing poor contact

(for those following this thread, the schematic is in posts #65 and #66)

 

[jim hardy]

(for those following this thread, the schematic is in posts #65 and #66)

Sweep motor ?

The contact right above where it says "Fig 3" ?

Took relay apart found contact welded I was able to tap on relay and it released

refresh me - what is "make current" rating of that relay ?

What does contact look like under a magnifying glass? Melted, or just little volcano looking metal deposits ?

 

[Tom.G]

Sweep motor ?

I think so, that's where all the problems were.

refresh me - what is "make current" rating of that relay ?

30 Amps Make
3 Amps Break

 

[H012]

Sweep motor ?

The contact right above where it says "Fig 3" ?



refresh me - what is "make current" rating of that relay ?

What does contact look like under a magnifying glass? Melted, or just little volcano looking metal deposits ?

I will look up make current. The contact melted.MOV is OK at motor

 

[H012]

I think so, that's where all the problems were.30 Amps Make
3 Amps Break

Data sheet says make capacity ( A) 160

 

[H012]

Check this out for spec. On this relay

 

[Tom.G]

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?