magnetic contactor bad? Help

Of course of all days, today my coolant pump goes out! Or maybe it is the magnetic contactor that is bad, how do you check them? How do I
check the circuit breaker? The Bestact relay appears to be fine.
Symptoms:
The pump is spinning at maybe 1/4 the rpm as normal, and then it shuts off after about a minute. Sometimes the motor runs backwards. If I press the contactor in, it won't stay. After a while, maybe 15 minutes I can get the contactor to work, the motor spins, but again too slow, and sometimes backwards.
Sounds like the contactor is only providing two legs of power. Is that correct?
Is it the contactor that is going bad, or is is the circuit breaker? Or the motor? Help!
ca
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Some things to check :
If your not sure the contactor is getting control voltage, then turn coolant on and hold a ruler, nail or similar mild steel object near the solenoid--it should be attracted to the pole piece if the coil is indeed energized.
To be sure you getting voltage to each line terminal at the contactor, use a voltmeter--you should see 230vac ( or 440 ) between each line terminal.....if not, then suspect a bad mains or distribution fuse, breaker, a loose screw or similar...find and fix.
Now then, if above looks okay, disconnect the motor leads and ohm out between each one, this tells you if the motor has an open winding, your readings should be relatively low...probly less than 100 ohms, likely even quite a bit less than that..a significantly high reading indicates a burnt winding...
Finally, measure voltage drop across ( through ) the contacts (with the contactor pulled in, motor recconnected and everything powered up)--should be none or very little to be seen...say a few volts tops....any significant reading here tells you that one of the 'points' in the contactor has gone bad...if so, likely there will also be found fairly obvious signs (sight and warmth to the touch) of overheating at the bad contact....
--
SVL



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PrecisionMechanicaL wrote:

thanks for the troubleshooting guide.
Hmmmm... So which am I hoping is going to be bad. the motor, the contactor, or mains?.........Hmmmmm.. From the description, it sounds like a contact is burnt in the contactor.
ca
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Your entirely welcome.
BTW, you have mail....
--
SVL





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Harry Cox wrote:

Burnt contacts in the contactor. Fixed, horray!
ca
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Hi list, this might be a good idea and at the same time I need a professional answer to my question here concerning methods of checking servo motor on field other than just using a good DMM to measure its internal winding? I am asking this because other than using a digital ohm meter to measure the motor winding for conformation of faults, then can I use an LCR meter to compare the inductance level of each internal stator pairs of the servo motor in order to justify its condition of fault? My theory is that since all motor is based on reactive inductance to work, then a measurement of its stator pair's inductive levels will tell the truth of its condition , and I right?
Thanks Choong
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Choong Keat Yian wrote:

That sounds like a good idea, except that as the rotor turns, the brushes span more than one rotor pair at a time. As the rotor is turned, you will likely see a jump in the readings as you pass from one rotor pairs set to another. Pete
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http://groups.google.com/group/sci.electronics.basics/browse_frm/thread / 7f64ea9cd78502a8/6b96e7f7aa7fb712?tvc=1&q=LCR+group%3Aalt.machines.cnc& hl=en#6b96e7f7aa7fb712
HTH Bart
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Assuming your garden variety dc servo, then check for leakage with a meter that reads in meg ohms...anything over a few meg ohms is negligable, otherwise blow out the carbon, etc...better yet to use a 'megger'...
Now, power it up with a car battery or even several in series....depending on size...upon powering down, short out the leads--the generated back emf should brake the motor to a halt within but a few milliseconds....be careful here, as the motor may launch off of your benchtop if it's not physically restrained.
--
SVL





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