Bandsaw Blade and Drive Belt Tension

[derek labian]

Hello, I have a SCM S640P that I am still trying to get running. Currently, when it powers on, it starts bringing the blade up to speed. After about 3 seconds, the breaker trips. It's 3PH on a 30A breaker. Motor is rated at 22A. Motor spins up just fine with no blade and only bottom fly-wheel. Testing with a 1" blade setup as per manual.

I believe the trip is caused by either the blade or drive belt tension. Manual indicates 5-6mm deflection when pressure is applied from one side with your thumb. Originally I didn't push too hard and I thought it was good. After tripping, I tried it with considerably more tension, still able to deflect 5-6mm with my thumb.

I also set the blade tension as per the tension meter on the band saw that registers the tension in MM to match the blade width in MM. In this case, I've set it to 25mm. I also tried it at 15mm tension setting before I tightened the drive belts.

I read a few other threads on breaker trips here on SMC including one about a Rikon bandsaw. It seems like it must be related to resistance caused by tension increasing the amp draw on startup. My question is, should I be loosening the drive belts further than where I originally had them? That setting seems subjective, where as the tension meter seems very specific. In all cases, the flywheel doesn't seem hard to turn (IMHO). Its a 9HP motor and it gets warm even after just running for those few seconds. It's not hot, but I noticed it was warm when adjusting the drive belts.

Any other advice is appreciated.

[Bryan Lisowski]

I don’t think having the blade tensioned would cause the breaker to trip. Most bandsaws have a way to undo the tension while not in use. It would be very inefficient if you had to go through the tensioning process every time you turn on. Have you tried setting the tension, releasing and then starting. Also are you using a phase converter or is shop already wired for 3 phase?

[derek labian]

I don’t think having the blade tensioned would cause the breaker to trip. Most bandsaws have a way to undo the tension while not in use. It would be very inefficient if you had to go through the tensioning process every time you turn on. Have you tried setting the tension, releasing and then starting. Also are you using a phase converter or is shop already wired for 3 phase?

If not the blade tension, I would assume the drive belt tension. It seems to unlikely that the original tension I had was too much. I believe the way to de-tension the blade when not in use is to just reduce the tension. It only takes seconds to re-tension the blade using the tension meter.

I am using a Phase Perfect 15HP/100A phase converter outputting 60A of 3-Phase. A 30A 3Phase breaker is whats tripping. 3-Phase J/P works just fine.

Something is clearly driving up the amp draw as it spins up, just not sure what it is or what to do about it.

[Erik Loza]

Derek, what is the little amperage dial on the contactor (aka main start switch) set to? It may have been dialed in too low from the factory.

Erik

[Lee Schierer]

How tight are the blade guides on the blade. If they are too tight, the work like brakes.

Have you tried to spin the wheels by hand? They should turn easily even with the tension.

[derek labian]

How tight are the blade guides on the blade. If they are too tight, the work like brakes.

Have you tried to spin the wheels by hand? They should turn easily even with the tension.

Not touching, I triple checked and yes, it turns easily (enough) by hand. Thats the first thing I did when I installed the blade. It gets up to a pretty decent speed before the breaker (external to the unit) trips.

[derek labian]

Derek, what is the little amperage dial on the contactor (aka main start switch) set to? It may have been dialed in too low from the factory.

Erik

Couldn't find any dial for amperage. Even if the contactor had an amperage dial, that wouldn't cause the breaker (external to the unit) to trip, right?

470446 470448470447

[Rod Sheridan]

It could be the time/current curve of the breaker or a problem with the centrifugal switch.

I presume that you have a typo in your post and it’s 3 horsepower not 3 phase?

Regards, Rod

[derek labian]

It could be the time/current curve of the breaker or a problem with the centrifugal switch.

I presume that you have a typo in your post and it’s 3 horsepower not 3 phase?

Regards, Rod

3-Phase 9HP.

I tried two different breakers, same issue.

It's brand new, so I am assuming its just my configuration.

[Tom Trees]

How tight are the blade guides on the blade. If they are too tight, the work like brakes.

Have you tried to spin the wheels by hand? They should turn easily even with the tension.

Worth noting if not putting adaquete tension on the blade, it will walk back into the thrust guide and jam..
The tire cleaning brushes could be set too close or just a bit stiff yet also.

[derek labian]

Worth noting if not putting adaquete tension on the blade, it will walk back into the thrust guide and jam..
The tire cleaning brushes could be set too close or just a bit stiff yet also.

Hi Tom,

Thanks for the ideas. I don't think its the brake, it isn't moving at all. I guess it could be the brush, I could try adjusting that out.

It has a 2xV450 belts. The manual literally says "The correct tension is 5-6mm deflection when using your thumb on one side of the belt.". Of course I can deflect it to varying degrees based on how hard I press it. It sure would be nice if there was a spec of something more specific.

As far as electric goes, the manual says 13.3A, the motor says 22a/230v. It seems like 30A should be sufficient. No spec on the size of circuit to use though.

[Dan Friedrichs]

Was watching your other thread....what ended up being the problem? How'd you get it going?

This problem sounds like a great reason to buy a cheap clamp-on AC current meter to see what current is actually being drawn by the saw. I suspect the blade being in-place is adding some minor additional friction or inertia and you were already just on the verge of tripping the breaker without the blade on. I don't think it has anything to do with tension - although a clamp-on current meter would easily let you see for yourself.

Breakers have different curves for different time-delays. You likely have a breaker that is too small and/or too quick. The motor may be rated for 22A, but that's at full-load, and the motor will draw much more than 22A when initially speeding up (since the work required to get the heavy wheels up to speed is significant). The breaker is designed to tolerate this overload for some period of time, and different breakers are available with different delays.

The manual does not specify what size breaker to use because the breaker isn't intended to protect the saw (the overcurrent limiter internal to the saw is). You can safely use, say, a 60A breaker (provided you have a wire and receptacle rated for 60A), for instance.

[derek labian]

Worth noting if not putting adaquete tension on the blade, it will walk back into the thrust guide and jam..
The tire cleaning brushes could be set too close or just a bit stiff yet also.

I checked the brush, it's actually not even making contact with the lower flywheel. I tried really loosing up the V-Belts, no luck there either. Again I spin the wheel pretty easily with my hand.

[derek labian]

Was watching your other thread....what ended up being the problem? How'd you get it going?

It was the Foot Pedal microswitch. Just needed to be adjusted a bit to make better contact.


This problem sounds like a great reason to buy a cheap clamp-on AC current meter to see what current is actually being drawn by the saw. I suspect the blade being in-place is adding some minor additional friction or inertia and you were already just on the verge of tripping the breaker without the blade on. I don't think it has anything to do with tension - although a clamp-on current meter would easily let you see for yourself.

Breakers have different curves for different time-delays. You likely have a breaker that is too small and/or too quick. The motor may be rated for 22A, but that's at full-load, and the motor will draw much more than 22A when initially speeding up (since the work required to get the heavy wheels up to speed is significant). The breaker is designed to tolerate this overload for some period of time, and different breakers are available with different delays.

The manual does not specify what size breaker to use because the breaker isn't intended to protect the saw (the overcurrent limiter internal to the saw is). You can safely use, say, a 60A breaker (provided you have a wire and receptacle rated for 60A), for instance.

Ok, great points. I do have a clamp meter, I'll have to check the max draw. I hadn't considered the trip delay on the breaker. Thank you for the great ideas.

[derek labian]

Ok, great points. I do have a clamp meter, I'll have to check the max draw. I hadn't considered the trip delay on the breaker. Thank you for the great ideas.

Hi Dan,

I tested the max amps before trip, it was 92.4A, so your suggestion is almost certainly right (or if thats abnormal, its motor damage).

I'm using a Siemens 3-Phase panel with a Q330 breaker. Specs below:

Siemens Low Voltage Residential Circuit Breakers Miniature Thermal MagCircuit Breakers - Type QP/MP, 3-Pole, 240VAC are Circuit ProtectionLoad Center Mains, Feeders, and Miniature Circuit Breakers. Type: QP/MPApplication: Electrical Distribution Standard: UL 489 Voltage Rating: 240VAmperage Rating: 30A Trip Range: Thermal Magnetic Interrupt Rating: 10AIC Number Of Poles: 3Pnumber of poles 3suitability for operation Electrical Distributioncontinuous current rated value 30 Aproduct brand name SIEMENStype of electrical connection PLUG-INdepth [in] 3 indesign of the product QP/MPheight [in] 3 inwidth [in] 3 instandard UL 489

I guess you already made your suggestion, put in a bigger breaker, or a breaker with a different curve. I can install in a 50a or 60a breaker and see if that helps.

[Tom Trees]

So no contact of either upper or lower thrust guide due to inadaquete tension or tracking issues.
Could it be that the motor is configured to star/Y (high voltage) instead of Delta triangle (220v) ?

[John K Jordan]

I have no experience with large 3-phase motors but found this: "... motor control loads typically require more current when they are initially energized than under normal operating conditions.
This period of high current demand, referred to as inrush, may be as great as ten times the current required under steady state (normal) operating conditions, and can last up to 40 milliseconds."

The easiest thing to try might be to replace the breaker. Perhaps the existing one is defective or tripping too fast. If you have a storage oscilloscope you might rig it to watch the duration of the inrush current spike (across a shunt or with a current probe) to see if the time exceeds the breaker specs. If you do replace the breaker with a larger one, I assume you'll make sure the wire in the circuit properly sized for the breaker. (I didn't read all the details in the posts.)

As for sizing the breaker, I see lots of calculators and tables out there like this one, but some may be more appropriate than others:
http://wiresizecalculator.net/tables/threephasemotordata.htm

If stumped, might call an electrician familiar with 3-phase industrial applications.

JKJ

[derek labian]

A good article on the topic.

https://www.jadelearning.com/blog/understanding-motor-starting-inrush-currents-nec-article-430-52/

John, yes upsize to 6/4 and a 60a breaker.

[derek labian]

I did some more testing on this. The draw is a solid 90A until it trips, about 3 seconds. I'm not sure a larger breaker will solve this issue because it will still be over the 60A for a period longer than 3 seconds. I could also not find breaker that has a longer curve, but 3 seconds seems pretty long already. The Phase Perfect has the capacity for this but I'd have to put in a much larger breaker, perhaps 100A. I couldn't find any details on the profile for the thermal characteristics, but if it is thermal load, the 60a would hand the 90a inrush better, right?

I researched the wire further, it's more or less a waste to put in 6 or 4 awg wire because the wire is rated to handle that inrush load for a short period. I found a number of calculators that show my cabling would be fine for much longer than I need that extended load. NEC accounts for this as well. The current cord is 10/4 SOOW, and the internal wiring on the saw is, at best, 10AWG, both of which should be fine even with the 90A inrush.

I suppose I could put a slow blow fuse inline with a 100a breaker behind it..

I'm open to any other suggestions.

[derek labian]

It seems like I could put a RK5 current limiting time delay fuse inline which are designed for motor starts/inrush. It should provide 60 seconds at 30A Fuse with peak current of ~600A. So I guess I could put in a 100A breaker and wire it to the fused saftey disconnect with RK5 30A slow blow fuses, and then run the existing 10/4 SOOW wire to the saw.

[Dan Friedrichs]

90A seems awfully high. You're putting the clamp meter around one leg of the 3phase supply?

How much current do you get with just the motor driving the bottom wheel (no blade)? You mentioned that without the blade, you can get it running - what does it draw for current once running (at speed)?

[Dan Friedrichs]

Oh, I missed that you had the 9.0HP, not the 4.8HP version. 90A still seems high but I suspect your breaker is just undersized. Breakers trip based on both the time and current, and if you look at the curve for a breaker, it will, for instance, tolerate 10s of 2x the rated current, or 5s of 3x the rated current, etc. So I think if you switch to a 40A or 60A breaker (along with appropriate wiring and receptacle), you'll be fine.

[derek labian]

90A seems awfully high. You're putting the clamp meter around one leg of the 3phase supply?

Yes.


How much current do you get with just the motor driving the bottom wheel (no blade)? You mentioned that without the blade, you can get it running - what does it draw for current once running (at speed)?

I don't know the current from no-blade/flywheel. From what I've been reading in my research, draw can be many times nominal load at startup for motors and transformers. These fuses are designed for motors startup. I could test again without the blade on to see though. Since these are Thermal MagCircuit Breakers, if the load was half at startup (1 flywheel), it would be 45A instead of 90A. All things being equal (and I know its a curve, but just for illustration), that might be 12 seconds of draw, and the saw spins up in about 5-6 seconds, thus it doesn't trip.

[derek labian]

Oh, I missed that you had the 9.0HP, not the 4.8HP version. 90A still seems high but I suspect your breaker is just undersized. Breakers trip based on both the time and current, and if you look at the curve for a breaker, it will, for instance, tolerate 10s of 2x the rated current, or 5s of 3x the rated current, etc. So I think if you switch to a 40A or 60A breaker (along with appropriate wiring and receptacle), you'll be fine.

Hi Dan,

I failed to find the curve specs for the Siemens QP series breakers.

Edit: I looked again. All the spec/datasheets I could find don't have trip curve data. In any event, I don't have a lot of choice in breaker selection that will fit this panel. Unless the 22KAIC breaker has a longer curve...

[John K Jordan]

...

I'm open to any other suggestions.

For peace of mind I'd take the motor to a local motor shop and ask them to test it.
It might not make sense to design around the symptoms if a problem exists in the motor itself.
Might ask if they would test the switch at the same time.

The times I've taken a local motor to a motor shop for inspection or repair they were quick and reasonably priced. Also, I like to buy switches for motors from them to save me the effort of verifying the claims of those I find online. Some lie.

JKJ

[derek labian]

For peace of mind I'd take the motor to a local motor shop and ask them to test it.
It might not make sense to design around the symptoms if a problem exists in the motor itself.
Might ask if they would test the switch at the same time.

The times I've taken a local motor to a motor shop for inspection or repair they were quick and reasonably priced. Also, I like to buy switches for motors from them to save me the effort of verifying the claims of those I find online. Some lie.

JKJ

its brand new, if its got a problem, i should contact the manufacturer, from what ive read this is not abnormal. Once its running st spead, if nominal load doesnt drop to 22a, ill contact the manufacturer.

[Malcolm McLeod]

Might have missed a detail or 6, as I didn’t read this all, but did read the OP…

Didn’t see anyone state the obvious, but when starting with no blade, the motor only has to overcome its internal inertia and that of the lower wheel (plus some friction). Adding the blade will (nearly) double the inertia. I don’t know what your wheels weigh, but I’d bet its a lot.

FWIW, I’d estimate my 18” BS takes at least 3-4 seconds to spin up to ~85%(?) with a blade, but <1 second without. (Never had issues, so never tried to get more precise).

A larger CB may be the only reasonable solution (with wire to match). Other options could be:

a VFD to slow the accel and so inrush current;
a soft start (Wye-Delta dual starter setup);

[Dan Friedrichs]

This seems specific to air compressors, but it shows needing a 40A breaker for a 3ph, 240V, 10HP motor: https://eatoncompressor.com/wp-content/uploads/Amp-draws.pdf

I think this is the curve for your Siemens QP breaker: https://assets.new.siemens.com/siemens/assets/api/uuid:046fbb46-cb38-482b-9d02-21b7eab7cb54/sie-tc-qp-bq-bl-qt-15-70a-1p.pdf

So if you're using a 30A breaker, your 90A load is drawing 3x the rated breaker load, which the breaker should be able to handle for a minimum of 4.5 seconds.

If you switched to a 40A breaker, you'd get at least 15 seconds; a 60A breaker, at least 50 seconds.

[derek labian]

This seems specific to air compressors, but it shows needing a 40A breaker for a 3ph, 240V, 10HP motor: https://eatoncompressor.com/wp-content/uploads/Amp-draws.pdf

I think this is the curve for your Siemens QP breaker: https://assets.new.siemens.com/siemens/assets/api/uuid:046fbb46-cb38-482b-9d02-21b7eab7cb54/sie-tc-qp-bq-bl-qt-15-70a-1p.pdf

So if you're using a 30A breaker, your 90A load is drawing 3x the rated breaker load, which the breaker should be able to handle for a minimum of 4.5 seconds.

If you switched to a 40A breaker, you'd get at least 15 seconds; a 60A breaker, at least 50 seconds.

awesome, i have a 40, 50, 60, and 100 on the way, along with the slow blow RK5’s. Ill start with the 40! Thanks Dan!

[derek labian]

Didn’t see anyone state the obvious, but when starting with no blade, the motor only has to overcome its internal inertia and that of the lower wheel (plus some friction). Adding the blade will (nearly) double the inertia. I don’t know what your wheels weigh, but I’d bet its a lot.

Correct.


A larger CB may be the only reasonable solution (with wire to match). Other options could be:

a VFD to slow the accel and so inrush current;
a soft start (Wye-Delta dual starter setup);



Or the slow blow fuse I mentioned. It seems the larger breaker is the obvious answer however I don't see a need to change the wire. The 10/4 SOOW i'm using, in my configuration is rated at 39a, at least, in my application. I will check the thermals on it though once I get it up and working. The wire in the saw is also 10awg.

[derek labian]

Derek, what is the little amperage dial on the contactor (aka main start switch) set to? It may have been dialed in too low from the factory.

Erik

I found a motor protection circuit breaker designed just for this purpose. I assume this is what you were looking for. Seems like this would be the ideal kind of device, something designed for motors that is. It has an adjustable dial.

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[Dan Friedrichs]

I think we're getting two different things confused:

The Siemens breaker you have in your breaker panel is intended to protect the wiring between the breaker and the receptacle. Not the saw motor. Think about all the other circuits in your house: you have an outlet in your bathroom that you plug your electric toothbrush in to - do you need to go change the breaker on that circuit to a 0.12A breaker to match the label on the toothbrush? Of course not. The 15A breaker on that circuit is sized to protect the 14AWG wire feeding the outlet. Should your electric toothbrush develop a short circuit, it may start on fire, but the 15A breaker will trip before the 14AWG wiring in your walls gets hot enough to start a fire. Protecting the wiring feeding a receptacle is the purpose of a breaker. An appliance plugged into that receptacle needs its own protection.

The "motor protection circuit breaker" you linked a picture of - there is one of those in your saw, already. It may look different or be hidden somewhere in the starter enclosure, but there definitely is one. It is intended to protect the motor from over-current. It does not go in a load center (breaker box) - it goes in the enclosure on the piece of equipment that also contains the contactor.

You are tripping the Siemens breaker, so the setting of your saw's over-current protection device is not relevant (you are not tripping it).

You need both of these devices (the breaker and the motor over-current device) to protect different elements of the system.

[derek labian]

I think we're getting two different things confused:

The Siemens breaker you have in your breaker panel is intended to protect the wiring between the breaker and the receptacle.

Hi Dan,

Erick, as quoted, was talking about a breaker in the bandsaw itself with a dial for amperage that might have been tripping. I was just posting the picture of the same kind of breaker that would be inline between the motor and the panel. I was not suggesting it was a replacement for the breaker at the panel.

Derek

[Dan Friedrichs]

Gotcha. Just to be clear - you do already have one of those in your saw.

[Erik Loza]

470473

Yes, that's it. Again, I haven't worked on a ACM saw but all the Centauros had that as the main start contactor. Perhaps the ACM's are wired totally different? No idea. This seems like an inordinate amount of hassle for such a simple machine as a bandsaw.

Erik

[derek labian]

This seems like an inordinate amount of hassle for such a simple machine as a bandsaw.

Erik

I agree +100

Similar wiring with an Eaton cont actor, but no breaker/dial that I could see. Perhaps the magnetic switch is the breaker though.

[Earl McLain]

I’m recalling a damaged bell housing on that motor found while unpacking, and that you had to do some straightening to get it spinning freely. Any chance that the damage could have gone deeper than just the outer shell, and contributing to the current issue? Just thinking out loud.
earl

[derek labian]

I’m recalling a damaged bell housing on that motor found while unpacking, and that you had to do some straightening to get it spinning freely. Any chance that the damage could have gone deeper than just the outer shell, and contributing to the current issue? Just thinking out loud.
earl

I thought that too, but the housing was only touching the plastic fan blades, and they seem (mostly) undamaged. It now spins freely (and so do the wheels) so I don't think thats the culprit. No other damage on the motor. Load is a constant 90A, but if it doesn't fall off to 22A or less once at full speed, I've got an issue.

[Erik Loza]

Derek, I'm looking at the operator's manual for our FB640, which I believe is the same machine as yours, and they are mentioning something about an "air brake" on the motor. The only ACM I've ever worked on was an LT16HD with a Baldor motor, so this is all somewhat new to me but our documentation mentions a protocol for calibrating or adjusting it. I wonder if yours has this feature and is set too tight, thus tripping the breaker? Here is a screen grab from the operator's manual:

470501

Erik

[derek labian]

Derek, I'm looking at the operator's manual for our FB640, which I believe is the same machine as yours, and they are mentioning something about an "air brake" on the motor. The only ACM I've ever worked on was an LT16HD with a Baldor motor, so this is all somewhat new to me but our documentation mentions a protocol for calibrating or adjusting it. I wonder if yours has this feature and is set too tight, thus tripping the breaker? Here is a screen grab from the operator's manual:

470501

Erik

Interesting, the manual doesn't say anything about a motor break. If you had an air brake like this though that brought the motor to a halt in 6-10 seconds, it would probably negate the foot brake, so maybe thats an upgrade. Still, I'll give the motor a good once over to be sure. I'd contact SCM support and ask them about that, but I haven't figured out how other than a generic contact us form.

[Bryan Lisowski]

I would reach out to Sam Blaisco. I don’t have his contact info, but maybe pm Jim Becker, he probably has it memorized as he often provides to people.

[Rod Sheridan]

I thought that too, but the housing was only touching the plastic fan blades, and they seem (mostly) undamaged. It now spins freely (and so do the wheels) so I don't think thats the culprit. No other damage on the motor. Load is a constant 90A, but if it doesn't fall off to 22A or less once at full speed, I've got an issue.

Sounds like the motor centrifugal switch isn’t opening when the motor hits 70 to 80% of rated speed……Regards, Rod.

[Dan Friedrichs]

Sounds like the motor centrifugal switch isn’t opening when the motor hits 70 to 80% of rated speed……Regards, Rod.

3 phase motor :)

[Greg Quenneville]

In the 415v three phase world a motor that size would have star/delta starting circuits to reduce starting current. Do you have similar in the 240v three phase schemes?

Or, on edit, do you have a dual voltage motor which can be wired for either 240v (star, or wye windings) or 415v? (Delta connection)

[Brian Holcombe]

With regard to the brakes, the machines that have them often have a switch for manually activating the brake so that it can be adjusted. There must be an air gap between the fan and the clutch material. The manual will describe how large the gap must be.

That said, I doubt you have one since you would not be spinning the wheels by hand if you did and you mentioned a plastic fan.

[Rod Sheridan]

3 phase motor :)

Yeah, I assumed it was a typo and he meant 3HP (which turned out to be wrong)

Time to increase breaker size for motor starting or measure line voltages in case the phase converter is collapsing…. Rod

[derek labian]

40A breaker did solve the issue. Current falls off quickly after about 3-4 seconds and normalizes to about 13A.