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Electric Motors/motor start capacitor sizing


I have a Marathon Electric, model cb 184tbdr7003aawccw, 7.5-9 hp, 184t frame, 1 phase, 230 volt, 38 amp, type bdr-le, continuous duty, 210 starting amps, 170 minimum starting volts.  Someone has installed a 440 volt 35 micro fared run capacitor in place of the start capacitor.  I need to know what size start capacitor it should have.  Thanks

Wow where did you get that monster?   Where about in Missouri,  I am from Kansas City,  [don't be specific if you reply]

Will the motor start with the "run" cap?  are there just two wires from the motor to capacitor area?I assume this motor is 3600 RPM?

Here is one cross reference with frame first in the model:

Here is a 7009 not 7003   yours might be older,

I really need to know if there are more wires available for the caps,   and just what is going on here,    you don't want to blow a cap that large,  for sure,  any cap for that matter,   

You realize that start capacitors are not always plastic enclosed,  they can be metal just like a run capacitor. That is why I ask if the motor will start with the existing cap.  It makes no difference what the case is made of, it is the voltage and farad rating.

I am surprised it is not a cap start, cap run motor just for pF reasons,  at that size,    

7.5-9 is a "different" rating, I say different as compared to off the shelf NEMA Motors.It is likely an ag duty or agricultural duty motor,  [just another sales tool]

Most often these ratings are found for motors driving fans and grain dryers.

Those sort of applications and are built and specified for that kind of fan-blower duty   which is really blower duty  a entire complex issue, and very common,    fans are not blowers and blowers are not fans,  not even close..... but are often thought of as interchangeable or the same thing,  and that is way far from reality.

A grain dryer in a tube is really a blower not a fan and requires certain restrictions or loads on both ends,  to be run wide open is dangerous to the motor, and anyone around as the load is critical in the entire operation.

This thing is a beast, again MOST likely a grain dryer duty motor,     I am coming up blank with Marathon  other than it is some sort of OEM motor     

I know you have looked but please look again,   SOMETIMES in real small print   is the cap rating stamped on the nameplate  or data plate.

That is what we really want.  The rest are formulas and we have a range of horsepower,  not a fixed horsepower,   I don't see a service factor, if I used a .25 service factor  at the low end of 7.5    it would be 9.37 HP  so real close to a 1.25 service factor 7.5HP motor.

So lets try 1.15 service factor   from the low end  [7.5 x 1.15]:

At a very typical 1.15 we are looking at 8.62 HP  at rated voltage, ambient conditions, frequency so on,     constant=continuous duty.

It is going to be a guess unless I can find the original specs for THIS motor,

 it has to be OEM built for some kind of specific application with a HP rating with a "-" in the middle. It is possibly ag duty NEMA   with variable HP ratings,   a dash versus a forward slash  means in between inclusive,  while a slash   5/2.5 HP would normally be a two speed motor of some sort.

Yours is saying this motor was designed to handle this RANGE OF LOAD  7.5 to 9 HP continuous duty,   and the odd thing  is  MINIMUM 170 volts starting     so while it is a minimum it is not likely the starting volts would reach 440 as the cap is rated for,

It could well be the cap that should be there is a range of capacitance   like 35-42   or some other range  but 35 is not out in space as far as a rating for a start cap on a 7.5 HP motor regardless if it is in a metal can,    and many outside duty motors do put metal cans on the start caps for weather related durability        

I know this seems like it should be a simple answer to a simple question,  I am still not sure the metal cap is not the right cap,    again minimum starting voltage,    170  no max   

Is the motor on the original piece of equipment as far as you know, or do you just have the motor by itself?

Many times the motor is just a part number for the driven machine.

I would size this as an 8 HP motor, the suffix after the motor is OEM to a manufacturer of something,  right now I say it is a grain dryer blower of some type.

These links get complicated but without any OEM info from the device the motor drives,  I am finding zip on this motor model as it starts with the frame size,

Marathon parts:

As you see 35 MFD falls in this 7.5HP range  but again until I can find the exact model and the exact Marathon cap specs  this metal can might be the right cap,   if it runs and the current is right on the motor,     

Tak a look at the nameplate really really close  again,   and then let me do some digging,   I would like to know if the motor runs as is,  

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