AC electrical troubleshooting

I'm scratching my head.

I was just given a nice Grizzly table saw.  Upon plugging it in and turning it on, it starts right up, runs for about 3 seconds and then trips the breaker.  Motor is wired for 220v, and I know the circuit is ok because I use it every day on my bigger table saw.

Motor is rated at 8A at 220v
Breaker is 20A double throw
I replaced the male plug to a 6-50 so it worked in my outlet, so it's new and wired correctly (and yes, I know a 6-50 is a no-no on a 20A circuit, but blame the last guy, not me)
I tore the switch apart and everything was tight and looked brand new, but I tried another switch just in case.  Same problem.
I pulled the cover off the motor and all the wires are connected properly.
Took the belt off to verify that the bearings are all good and they're perfect.

So I suppose it has to be something in the motor because the rest is all kosher.  What typically goes first?  I could try the capacitor, but since it springs to life very quickly and dies after it's up to speed, could it be the problem?  I don't have a multimeter with a capacitance setting so I can't test it.

How old is the new one, and how old is the older one? I'm asking because  if your table saw is the only thing plugged in and it technically doesn’t exceed your amp rating of the outlet, there is something called “in rush current” that can still trip your breaker. Assuming your saw doesn’t have a feature to address this, your saw could actually be pulling 3x the current when it first kicks on. Newer tools (usually) address this, with older ones you want to google "soft start module" and go from there. 
 

I know you use power tools a lot, and you've seen the lights dim when kicking one on, it's this that causes the dimming. 

Yep tripping after only 3 seconds could be due to inrush current on startup, normally you could measure power with a cheap kill-a-watt unit to confirm, but for something with this kind of power you'd need a much beefier unit:

https://www.amazon.com/Poniie-PN2500-Professional-Wireless-Pass-Through/dp/B0C619YRRM/

Steve_Jones said:

How old is the new one, and how old is the older one? I'm asking because  if your table saw is the only thing plugged in and it technically doesn’t exceed your amp rating of the outlet, there is something called “in rush current” that can still trip your breaker. Assuming your saw doesn’t have a feature to address this, your saw could actually be pulling 3x the current when it first kicks on. Newer tools (usually) address this, with older ones you want to google "soft start module" and go from there. 
 

I know you use power tools a lot, and you've seen the lights dim when kicking one on, it's this that causes the dimming. 

They are both from the 90s.  I totally understand the startup current being higher, but it starts fine.  It's only after running a few seconds that it trips the breaker.

Single phase motors have a start winding and a run winding, there's a centripetal switch that kicks out the start winding once things are spinning. I wonder if your switch isn't kicking out and keeping that winding in the circuit and causing the high draw. That switch operates at a few hundred rpm so will kick in very quick on a table saw. You can often hear them click close when you turn off the motor as it spins down, maybe you've wondered what that noise was.

Could also possibly be a problem with one of the capacitors, they tend to be shorted when bad and short circuits draw lots of amps. I suspect this less as the motor is running and it needs good capacitors to do that. 

Pick-up a clamp meter on amazon to measure current and continuity, they are a pretty powerful tool these days for not a lot of money. I think you'll find your breaker is doing it's job and the motor is the problem.

Is there some kind of centrifugal motor starter circuit switch in the saw that might be gummed up?

I had a radial arm saw at one time that would do what you described after a lot of heavy use, and would have to disassemble it to clean the sawdust out of the starter switch to fix it.

adam525i said:

Single phase motors have a start winding and a run winding, there's a centripetal switch that kicks out the start winding once things are spinning. I wonder if your switch isn't kicking out and keeping that winding in the circuit and causing the high draw. 

Pick-up a clamp meter on amazon to measure current, they are a pretty powerful tool these days for not a lot of money. I think you'll find your breaker is doing it's job and the motor is the problem.

I have one.  Just checked.  Leg one shows a steady 11.9A, leg two shows the same.  Ground shows 0.01.  It doesn't spike or go higher than that, but admittedly the refresh rate on this meter is pretty slow.  It might be spiking but not showing.

Edit to correct.  After watching the video, it's drawing 119A

Yikes.  Check this out:

In reply to Curtis73 (Forum Supporter) :

That's high for an 8 amp motor but it should not trip the breaker. If it's a steady 11.9 amps and doesn't jump up/short after a few seconds when the breaker trips it sounds like it is time for a new breaker then. They can definitely fail. 

In reply to adam525i :

Yeah... I edited my other post.  It's 119A.  I wasn't paying attention to the decimal when i was videoing.

shorted winding?

If it's not the starter circuit or something totes obvs in the wiring you are gonna need a megger to get much further.   Checking windings and insulation and whatnots.

If it's not the starter circuit or something totes obvs in the wiring you are gonna need a megger to get much further.   Checking windings and insulation and whatnots.

I did pull the fan off the back and found the centrifugal switch.  It was clean and appeared to be functioning properly.

TL;DR: I was in a similar boat and wish I'd just replaced the motor.

It's not useful for troubleshooting your saw, but it all sounds (minus the amperage measurement, which I didn't do, though I did trip a lot of breakers) like attempting to troubleshoot the Grizzly 7x12 horizontal band saw I got off CL.

I tried for a while, then I took it to pros. Who looked it over, then got puzzled, then one guy set it aside as a pet project, then there we moved and I decided that slow service was better than getting it back just to move it into storage to move it into a new shop and then there was a pandemic... then it turned out the shop had shut their repair side down and sent it for scrap. They claimed to have tried to get a hold of me. Oh well.

Anyhow, I wish I'd just bought another motor from Grizzly. It was overall a much nicer saw than the HF 4x6 I ended up with; hydraulic feed control, coolant pump... *sigh*

Could be one of the capacitors is fried. Usually they burst and stink.

Clean the points on the start switch?

Is that 119A with the belt off and the motor free-spinning? My tablesaw started blowing breakers when the arbor bearings were in the process of seizing up.

I do have a couple motors sitting around the shop that may work.  I don't think any of them are 1.5 hp like the one that's on it, but worth a look.

ShawnG said:

Could be one of the capacitors is fried. Usually they burst and stink.

Clean the points on the start switch?

Is that 119A with the belt off and the motor free-spinning? My tablesaw started blowing breakers when the arbor bearings were in the process of seizing up.

I could clean the points.  It would require taking the counterweights off and they don't look very easy to put back on with the tiny little springs and stuff.

I didn't try running it with the belt off, but when I had the belt off, the blade spun darn near forever.  No noise, no play, and spun for at least a minute.  I only saw one capacitor, but maybe there's one tucked in the mounting carriage side.  The one I looked at appeared good, but I don't have a capacitance mode on any of my meters.

Edit to add:  The motor also spun very freely by hand much like the blade arbor

In reply to Curtis73 (Forum Supporter) :

Just for E36 M3s and giggles, try starting the motor with the belt off and see if it stays running.  If it does add me to those guessing the start cap is bad.  They're typically short money, rating in voltage and microfarads (uF) on the side of the capacitor, match those when looking for a replacement.  

In reply to 11GTCS :

Will do tomorrow and update

And I discharge the old capacitor by touching it to my tongue, right?  

The switch points are usually mounted in the end bell of the motor housing. Just pull the end bell opposite the shaft and clean them.

The weight assembly will be on the motor shaft.

In reply to ShawnG :

I did find them in the butt behind the fan.  They look squeaky clean, and pulling on them allows smooth operation and the switch lifts off the contact.  I want to run it and watch the switch to make sure it's doing its thang.

I have a nail file that I can put between the contacts potentially without removing the counterweights, but in all honesty, this thing looks like it maybe ran a dozen times.  The blade is a Grizzly-branded blade and the label isn't even smeared, the teeth are razor sharp, and it had very little sawdust in the nooks and crannies.  If it's a bad centrifugal switch, it's definitely from underuse and not overuse.