2nd start cap in 2 weeks...

[fredhargis]

My DC has a 5 HP Leeson motor, and it just blew the start cap again, the 2nd time in less than 2 weeks. What do I need to look for? I should say, I had this same model motor on my last DC, and it did the same thing (different house/electrical/etc.) at one time, the it seemed to stop. So am I just having a bad run of luck, or do I need to check something?

[Sqpegs]

I'm a new member here and this is my 1st post, I just registered.  I have been lurking for a long time and been doing hobby wood work for many years.  I am glad to have this opportunity to contribute.  

Fred's post came up and even though I know little about motors,  I recall that the centrifugal switch can cause this problem.  As the motor comes up to speed the centrifugal switch switch's the capacitor out of the start circuit, it not doing its job, the capacitor then has currant flowing at all times.  It's not rated for that abuse and in a short while it will fail.

Blow the saw dust out of the centrifugal switch. All I have seen have been on lower horsepower motors, and the switch is  located behind the end cap on pully end of the motor.
Cleaning the switch assembly my help, it has moving parts that my be stuck. Maybe somebody can confirm this.

[TDKPE]

That 5 hp motor has two caps, doesn't it?  If so, what Sqpegs described above is the most likely scenario.

If it only had a start capacitor, then the start winding would have burned up long ago with a sticking centrifugal switch.  But with a run capacitor, the "B" winding (start/run) is always in the circuit, and only the start capacitor is switched out.

Crack it open and inspect and clean the centrifugal switch.  If you're reluctant to crack it open, slap a clamp-on ammeter on a start capacitor lead once you've replaced it, start the motor, and check to see if there's current flowing (there should not be any current).

[fredhargis]

Thanks guys, that gives me something to check. Tom, the motor does have 2 caps....so I'm guessing it's the switch....I'm really starting to admire 3 phase motors. Thsi makes more sense when I remember the cap popped after the motor had been running a minute or two after I started it up.

Sqpegs..Welcome to the forum!

[TDKPE]

....I'm really starting to admire 3 phase motors. 

They're a study in simplistic elegance.  One moving part.  And here's a tidbit - the power through the three wires to the motor is constant.  The magnetic poles are reversing 120 times per second with current flow direction reversal, but between the three of them, the power in is a constant.  

Single-phase motors are more like a bicycle, where there's a power pulse with each down stroke of a pedal.  Now imagine a bicycle built for three, with the pedals and crank arms rotated 60 degrees from rider to rider.  The resulting torque at the wheel would be smooth, assuming identical bike riders pushing exactly the same on the pedals on each down stroke.  Three-phase motors, and generators for that matter, are like that with constant (non-pulsing) mechanical power output or input which is probably the major reason 3-phase exists rather than 2-phase* or 4-phase.  

*OK, 2-phase 3- and 4-wire used to exist and still does in the NEC motor tables and a few odd places.

[daddo]

Once you get the start caps installed, see if they have a bleed resistor on at least one of them. This resistor will help protect the points/contacts in the motors start relay. When the relay opens, there is still high voltage in the cap and this can arc as the relay opens. It may not seem very noticeable, but after many starts, it will start to pit and carbon the points up.  This isn't necessary, but it can help the points last longer.

You can the see the resistor I installed on the upper start cap on this motor. The part number is 93007 and made by Mars.

(These caps were inside the old motor and very hard to get to. That's why I mounted them on the outside of the motor.)




 The part;

[fredhargis]

The OEM did have a resistor soldered in, the replacements I've bought did not. I had to buy a clip on resistor and put on it.

[tablesawtom]

I would pull the motor and take it to a motor repair shop. They can check it out and give you an estimate in short order. I would suspect something else is going on and the copastors are the final stopping place. 

Tom

[fredhargis]

If I pop another I'll have to pull the motor, being on a DC that will be a serious amount of work getting it down and apart. Then if the switch isn't obviously stuck in some fashion I'll get pro help.

[fredhargis]

Just to wrap this up. I went through the whole PITA process of dismantling my DC and pulled the motor. Then some stuff happened at home that kept otherwise occupied so I took it to a nearby motor shop. He replaced the switch....the contacts on the old one were pretty badly burned. While I wonder about that (he gave me the old switch back, or one from the pile he keeps for unsuspecting customers
) I'm back to putting things back together. Just a side comment. My old Oneida had the impeller "slide" onto the motor shaft and then tighten a pair of set screws. I had to take it off twice and it was nothing but a miserable job both times. The CV has the impeller installed wait a Taperlock bushing and man does that ever make the job easier. All the impellers should be put on with one of these.