Magnetic Starter Advice

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  #1  
Old 11-05-12, 03:00 PM
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Magnetic Starter Advice

I just purchased a new-used 7.5 hp 3 phase air compressor and am swapping the motor to a 7.5 single phase unit because I don't have 3 phase power. The motor coming of is a Baldor EM3311T, 7.5 hp, 3ph, 19.4 amps. I have included a picture of the motor's name plate if any more specifics are required. The motor I am planning on replacing it with is a Leeson 140155.00, 7.5 hp, 1ph, 36 amps. The magnetic starter that came on the compressor is an Eaton A27CGF45. A pic of it is also included. My research tells me that the starter is not going to be big enough for the single phase motor as it requires a higher capacity unit than a 3ph motor of equivalent hp. Is this correct? If I do in fact need a heavier starter, how much bigger do I need to go and what amperage do I need for the overload protection. I realize a 1ph and a 3ph starter are different, but I believe a 3ph unit can be rigged up to power a 1ph moter. Thanks for the advice, and let me know if you need more information about the new motor or whatever.
 
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  #2  
Old 11-05-12, 04:17 PM
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Yes, a three-phase magnetic starter can be used on a single phase motor but only up to the horsepower rating of the starter. I suspect that starter has a rating of no more than five horsepower and maybe as low as three horsepower for 240 volt single phase motors so from that you are out of luck and need a new starter.

Since starters are rated by motor horsepower, phases and voltage it is an easy task to select a new one. Simply go through the listings until you find one that will accept a single phase, 7-!/2 horsepower, 240 volt motor.

Note that starter you have has an adjustable "heater" (overload detection device) and more common starters will use separate discrete-rated heaters for the overload device. If you select the latter type of starter you will nned to purchase the heaters separately.

Here is one that would work. Note that Grainger is not always the best place to buy as their prices are often a bit high. You could check on eBay for a used model.

EATON DP Starter, 208-240V, 40A, Enclosed, 3P - Magnetic Motor Starters - 5PZY4|A25CGE40B - Grainger Industrial Supply
 
  #3  
Old 11-06-12, 10:25 AM
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Hi, Furd. Thanks for the reply. Here is a stupid question: why would the starter I have not be enough if it is rated at 45 amps and the one you suggested is a 40 amp unit?
 
  #4  
Old 11-06-12, 01:40 PM
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Forgive me and my tired eyes. That little yellow wheel on the overload relay (pictured) is the adjustment for the motor amperage. I at first thought it was adjusted to the maximum but upon looking closely it appears to be at the minimum. IF the label inside the cover lists the unit as being good for a 7-1/2 horsepower single phase motor then all you need to do is follow the wiring diagram on that label and adjust the yellow wheel for the full-load current, or up to 10% over the full load current of the new motor.
 
  #5  
Old 11-06-12, 02:49 PM
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Thanks for the advice, Furd!
 
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