AC Motor

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rzimpala
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AC Motor

Post by rzimpala » Mon Jun 20, 2005 8:10 am

I have a 120VAC motor that will not start until you give it a little spin. There is an extra lead, is it for a starter cap or something?<p>3_________0 120 (3's represent windings)
3
3
3_________0 120
3
3
3
3___| |__0 unknown? (open to connect cap?)

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Chris Smith
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Re: AC Motor

Post by Chris Smith » Mon Jun 20, 2005 10:43 am

You could have some dead windings. That or lose solder connections that don’t conduct, until inertia makes them connect? If it’s a cap start motor, and you have bad windings, no field will be produced until a good one lines up, conducts with the cap and coil, and then tries to start. <p>If it were a DC brush motor, it would be a bad commutator or winding on the brush. <p>Start circuits for AC motors are “Slow start” which means they don’t have the torque to proceed from stop to Idle to full RPM under a load, but if you Free wheel This type motor with no load, they start by them self just fine. <p>They merely cant start under load similar to Large grinder wheels attached to their spindle.<p>These types usually have a spring loaded, centrifugal relay that kicks out the start circuit once the RPMs exceed a Certain point.

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jwax
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Re: AC Motor

Post by jwax » Mon Jun 20, 2005 6:20 pm

Perhaps some more data from the nameplate?
Induction motor, cap-start, HP, current draw?
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Dean Huster
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Re: AC Motor

Post by Dean Huster » Tue Jun 21, 2005 5:47 am

Most cap start, cap run or cap start/run motors have the caps mounted on the motor housings. But there are some applications where the cap is external (e.g., air conditioning compressors). If you pop the rear end shield off the motor, you should see a centrifugal switch that kicks the cap in and out of the circuit if a cap is involved.<p>Then again, with all those wires (there's either not enough or there's too many, depending) you could have a dual-voltage, three-phase motor on your hands. jwax has the question. What all is written on the nameplate?<p>
Dean<p>
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Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

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Robert Reed
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Re: AC Motor

Post by Robert Reed » Tue Jun 21, 2005 1:21 pm

Since you neglected to describe motor size and what it originally came off of, its hard to suggest a cure. But I will say this, if it is of a very lo starting torque such as syncronous or small fan motors -any buildup of crud or old lubricant on its bearing surfaces will prevent it from starting,until you give it a spin (just as how you describe in your case).I have seen this occur several times and in fact operated one for weeks like this because I was too lazy at the time to do a proper repair. Later a dissasembly and cleanup followed with new lubrication and the motor performed fine.

Sterling Martin
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Re: AC Motor

Post by Sterling Martin » Tue Jun 21, 2005 2:58 pm

Hi rzimpala,
Based on your description, it sounds like a PSC motor. A quick way to tell is to run the third winding to a 5 uf 370 volt oil capacitor and put the other side of the cap to one of the lines. If you get it across the right line it will develop some starting torque. (generally not very high torque) If you get the cap to the wrong line, it will sit there and buzz, just as before. Depending on what size motor you have, you may need a larger cap, like 10uf.<p>

3______________ L1
3
3
3______________ L2
3 ]
3 ]
3 5uf ]
3----][-------]<p>
or<p>
3______________ L1
3 ]
3 ]
3_______ L2 ]
3 ]
3 ]
3 5uf ]
3----][-------]<p>[ June 21, 2005: Message edited by: Sterling Martin ]<p>[ June 21, 2005: Message edited by: Sterling Martin ]</p>

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jollyrgr
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Re: AC Motor

Post by jollyrgr » Tue Jun 21, 2005 3:30 pm

More details about the motor would be helpful. Another possibility is that the third lead is for a START RELAY. These types of motors are found on most dishwashers. Here is a link to a PDF file showing different types of starting relays and a sample schematic of how it is hooked up:
http://www.zettlercontrols.com/products ... ay%20start'
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dacflyer
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Re: AC Motor

Post by dacflyer » Sat Jun 25, 2005 7:58 am

it might be a motor that needs a start/run switch..usually these are inside the motor..but some are external like the ones used on compressors.. but you might need a cap also..
a data plate info is needed for us to tell you more

thesprocket
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Re: AC Motor

Post by thesprocket » Sun Jul 03, 2005 7:31 am

Some cap-start motors have the capacitor internal to the housing, physically on the armature. It is connected via a normally closed circuit, for starting. Once the motor is running, centripetal force causes the circuit to open, for running. If the mechanical apparatus, the bit affected by centripetal force, has failed open(stuck in run mode), the motor will fail to start. But, will run properly once given a push, the failure of the starting circuit having no correlation with the serviceability of the steady state performance. Due to the small size and manufacturing method of this type of motor it is impractical to attempt to repair. More damage will be done to the housing during disassemble than benefits gained from the repair. <p>This type of motor is used by the friendly floks at GE in clothing washers and dryers, it is my belief that the design choice to do so was based on the prospective re-occurring revenue from the sale of replacement motors.

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