Mag Starter Chatter

[brianwelch]

Tried to turn on my unisaw yesterday morning. Heard some chatter/crunching and quickly turned off. Tried to turn on again, nothing...breakers, cords, connections all tested good. Opened the starter box and checkiing for power to/thru the starter. NCO probe touched one of the terminals and the saw came to life. I had used the saw maybe 1/2 hr earlier and for the past year or so with no issues.
What appears to be happening is the contact struggling to engage. It arcs/chatters for a moment and then either starts, or shuts off. By manually pressing the contact with an insulated tool, the motor engages and all is good. This seems to be an intermittent issue. 
It is a single phase 5hp 240volt motor and the Shihlin brand starter is rated just for that.
Suggestions? Replace the magstarter unit? 
Thanks in advance

[TDKPE]

Start with tightening all the connections.  Including and especially the small control wires, AND the circuit conductors all the way back to and including the panel.  It sounds like the voltage is being pulled down to where the coil can't hold it.  The locked-rotor current is usually 5 to 8 times the rated current, so a weak connection somewhere will allow the voltage to drop too much for the coil to pull the contactor in.  Also check the plug and receptacle for burns and loose connections, if it has them, any disconnects, breaker, junctions, etc.

If the push button manually pushes the contactor in, as some of those small controls do, then holding the button until the motor is up to speed may get past a voltage (under starting current conditions) problem, but that's not a solution, and the root problem should be found and corrected.

It can also just be the control is going bad, but I wouldn't go to that without checking everything upstream first.

[brianwelch]

Start with tightening all the connections.  Including and especially the small control wires, AND the circuit conductors all the way back to and including the panel.  It sounds like the voltage is being pulled down to where the coil can't hold it.  The locked-rotor current is usually 5 to 8 times the rated current, so a weak connection somewhere will allow the voltage to drop too much for the coil to pull the contactor in.  Also check the plug and receptacle for burns and loose connections, if it has them, any disconnects, breaker, junctions, etc.

If the push button manually pushes the contactor in, as some of those small controls do, then holding the button until the motor is up to speed may get past a voltage (under starting current conditions) problem, but that's not a solution, and the root problem should be found and corrected.

It can also just be the control is going bad, but I wouldn't go to that without checking everything upstream first.

Thanks...did check most, and all seemed good, but will check again.
Thank you...

[brianwelch]

Thanks...did check most, and all seemed good, but will check again.
Thank you...

TDKPE,
I rechecked all the connections, and sure enough, several of the pre-wired connections had worked loose, one extremely so...
Once all were tightened she started like a champ.
Thanks!

[TDKPE]

You’re welcome.  It’s actually a fairly common problem, one that’s showed up here many times in the past.  So it was still in the front of my mind, more or less.