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Old Keystone Compressor motor just hums....

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Truck

05-20-2002 07:25:21




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My big ol' Keystone compressor stopped working a while back, and I suspect maybe the starting capacitors, but don't know how to test them. It was tooling along fine, although the pump is getting a little slow in its old age, and then the next time when called upon to start it wouldn't.
IT just sits there and hums ominously. I snapped the breaker off as quick as I could, checked that the unloading valve had worked and the pump turned freely, and it still wouldn't start up.
Any ideas what to check for? I have an oldtime electrical shop an hour away, but I'd like to have an idea what's wrong before I bring it in. BTW it has a big starter unit that hangs on the wall. ANd I donm't see a capacitor box on the motor, unlike the modern motors. It is a 7.5 or 6 HP motor.Single phase, weighs a TON. I do have a couple of VOM's including one as old as the compressor.
Thanks!

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Robert Travis

08-29-2004 17:02:03




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 Re: Old Keystone Compressor motor just hums.... in reply to Truck, 05-20-2002 07:25:21  
Do you know if they still make Keystone Air Compressor? Looking for customer who has one he bought from auction. Can you help. Thanks Bob



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Joker

05-22-2002 17:41:13




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 Re: Old Keystone Compressor motor just hums.... in reply to Truck, 05-20-2002 07:25:21  
The reason it hhhhhuuuummmm's that it doesn't know the words.



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Truck

05-23-2002 06:57:39




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 Re: Re: Old Keystone Compressor motor just hums.... in reply to Joker, 05-22-2002 17:41:13  
It's just copying its owner, I know!
At least it ain't flat!



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G Taylor

05-20-2002 22:04:05




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 Re: Old Keystone Compressor motor just hums.... in reply to Truck, 05-20-2002 07:25:21  
If this motor is single phase it and used to to start very old piston pump, she maybe an "repulsion start/induction run". At startup they operate as a series motor. At 1/2 speed or so, the brushes lift off the commutator & a shorting ring snaps down. Now the rotor is induction only with AC only on the stator windings. To this day the repusion start/induction run motors has about the higher starting torque with the lowest in-rush current. Effiency when running is lower than todays cap start/cap run/ low resistance rotor motors however. Brushes do wear down, linkages get sticky with dried lub/dirt or rust.

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Truck

05-21-2002 06:42:40




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 Re: Re: Old Keystone Compressor motor just hums.... in reply to G Taylor , 05-20-2002 22:04:05  
Sounds like you may have hit it, I kinda remember a two step start, with something clicking in once it got up to a reasonable speed. So maybe It isn't something I'll have to take it into the motor shop for...That's GOOD news!
Once I get into it I'll post further.



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jake

05-20-2002 21:15:37




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 Re: Old Keystone Compressor motor just hums.... in reply to Truck, 05-20-2002 07:25:21  
If the motor humms the starting windings are not being activated which could be a defective starting switch, broken wire or the capacitor, the start cap which only costs a few dollars, Check the start points by removing the rear housing. the cap can be motor mounted or remote as you might have.



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PCC-AL

05-20-2002 17:58:16




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 Re: Old Keystone Compressor motor just hums.... in reply to Truck, 05-20-2002 07:25:21  
Hi Truck,
I am probably way out of line answering this post as my electrical knowledge is very limited. However, I had a friend that had a mower repair shop and he had a bench grinder that had a bad starter capacitor on it. He still used it by starting the wheel spinning by hand and then turning on the juice. Just a thought, but if your compressor has a pump separate from the motor(mine does), maybe you can very carefully spin it by hand and turn the current on as you spin the pump wheel. If it then goes, you have found the problem. Be careful, don't get your hand caught in the belt. Good luck.

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