What size Start Capacitor to Use?

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Old 11-30-19, 05:39 PM
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What size Start Capacitor to Use?

I have a 5hp single phase 3450 rpm motor that the start capacitor was removed from.

What size should I use on it?

This is the motor label:

 

Last edited by bobinyelm; 11-30-19 at 06:19 PM.
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Old 11-30-19, 06:14 PM
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That picture is hosted on a site that DIY will not allow direct hotlinking to.
You can post the site and I'll grab it for you or post it directly to the board.
How-to-insert-pictures.
 
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Old 11-30-19, 06:40 PM
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Hi, what is the model # on the motor, cant read it
Geo
 
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Old 11-30-19, 08:08 PM
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Sorry...... I can't enhance it enough to read either.
180px120p isn't large enough.
 
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Old 12-01-19, 01:45 AM
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Let me try to post the label again (It shows on my Forum page, but evidently not on anyone else's).

I might mention it is 3450rpm, 220v Single Phase, 5 hp rated, but with only a 1/2" dia shaft, and has only 2 wires coming out for a capacitor. There is a place for a single "can" for the capacitor on the case and capacitor itself are missing, but that can would only be large enough for one round capacitor, and there are only 2 wires coming out of the motor case that would go into the "can". My other 5hp motors have 2 capacitors, BTW).

I tried Googling the Ingersoll Motor Part # 9701950 and Compressor series 4000 and nothing useful appears anywhere.

A call to IR some time back indicated that that model used what turned out to be a "junk" compressor pump (made by another company) for IR and no parts are available for it, and said while the light duty 5hp motor and 60gal tank were "quality parts," they advised owners of this series to just "junk" the whole thing as IR carries no repair parts for the compressor pump,, and have no longer has manuals or parts lists for the overall compressor assembly.

I want to refit the unit with a new Harbor Freight V-Pump made for the existing 5hp motor (I built another compressor using this pump and it works great).

Bob
 
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Old 12-01-19, 05:55 AM
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Hi, any motor shops in your area? I would think that any 5 HP with the same specs would use about the same size Cap.
Geo
 
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Old 12-01-19, 06:24 AM
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Roughly speaking a motor like yours would use in the range of 500 mfd @ 370 volts.
It should start your motor under load.

You should note that the voltage rating of the capacitor is due to induction voltages generated in the start winding, not the motor supply voltage.
If the cap voltage rating is too high that is ok but if lower than recommended the excess voltage can cause the insulator material in the cap to become perforated and it will not last long.
Another thing you might find is that the cap cover may not fit a replacement as some motor mfrs use undersized components.
 
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Old 12-01-19, 11:52 AM
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I had found a value of 440mfd @ 370vAC based on a similar knock-off IR motor.
That motor draws less than a typical 5hp motor. More likely a 3hp.

Typical ballpark values.....
3 Hp or 2.25 KW, 200-250VAC 500-580 F @370vac
5 Hp or 3.75 KW, 200-250VAC 750 F @370vac
 
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Old 12-01-19, 11:53 AM
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If the cap voltage rating is too high that is ok but if lower than recommended the excess voltage can cause the insulator material in the cap to become perforated and it will not last long.

Another thing you might find is that the cap cover may not fit a replacement as some motor mfrs use undersized components.



About the voltage rating-

It seems all of the motor start capacitors on Amazon are either 110-125v or 220-250v, which either means that they are referring to the line voltage, or I would have to go to a specialty motor shop to find one that would take 370 volts or so? The ones rated 110-125v must be for low voltage AC motors operated by transformers I guess.

Capacitor Cover Size
No worries about the cap cover. I mentioned in my write-up post that I received the motor with the cap AND the cover missing, so there's nothing for it to fit into. I was going to fashion one from a used tin can that would fasten via tabs into the existing screw holes.

The 400mfd caps even in the 250VAC rating are twice the size of the shadow (greater than 2" diameter) of where the existing (missing) cover sat, so you're right that they must use really small components from the factory!I would imagine 370VAC or higher rated caps are probably even BIGGER (thicker dielectric).

I found LOTS of capacitors rated at 370VAC but they were all "run" capacitors and not "start" capacitors. Since my motor had only s single can and 2 wires, I would imagine the wires are for a "start" capacitor, but was never equipped w/ a "run" capacitor?

I don't care how it looks (home made cap cover can) as long as the motor STARTS and is SAFE

Thanks,
Bob
 
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Old 12-01-19, 12:04 PM
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The biggest problem you'll have is locating the proper sized cap. A motor shop would try different values and pick the best performer. You don't have that luxury. You may end up experimenting with several different sizes.

Use the chart located here..... Grainger supply
 
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Old 12-01-19, 01:06 PM
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If the motor has a centrifugal. switch, the start capacitor is only in the circuit until the switch opens. The motor manufacturer chooses a common mfd rated capacitor such that the start current leads the run current by approximately 90 degrees electrical to maximize starting torque. Expensive equipment is required to make this measurement.If you can take the motor and its load to a motor repair shop with the expensive equipment, they can determine the best mfd rated capacitor. If not try one in the ballpark range listed in an earlier post or the other 5 hp motor with the dual capacitor. The start capacitor is the one with the higher mfd rating. If the motor runs fine your done. If not, its trial and error. The capacitor voltage rating should be equal to or greater than the line voltage, 240 vac in your case.
 
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Old 12-01-19, 04:29 PM
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I have used this formula several times with excellent results.

Start capacitor sizing formula
2650 X Full Load Amps Divided by Supply Voltage = MFD
If the full load is not the locked rotor amps then increase MFD by 1/3
For hard start loads such as compressors or augers.


Your example should have at least a 370V capacitor with 200-220mfd.

Best of luck,

RR
 
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Old 12-01-19, 05:58 PM
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Great-

I think I now know what to do/try.

I sent an email inquiry to IR a few days ago so maybe someone will answer w/ more info than when I called some time back. I gave them the label information.

Sounds like I will need a very large cap (400 +/- start capacitor). Most I looked at were smaller, but 5hp is a fairly big motor.

On the voltage rating, virtually none I can find in that capacitance range are 370v, so maybe I need to get one w/ a 370v rating from a specialty shop. Run capacitors seem to TYPICALLY be 370v in catalogs, but start ones seem to carry supply line voltage ratings for the motor concerned (one poster said line voltage rating is OK, however).

Thanks all for the education!!

Bob

BTW, the pulley on the 3450 rpm motor is 7" diameter, and on the pump is 12" meaning they spun the pump at 2012 RPM!!

No wonder why the super light weight all aluminum pump failed with great regularity! Pump rotational speeds on other compressors are typically 750-1050rpm.

The comment about the motor being 5hp rated, but likely 3hp is not unfounded as this one weighs less and has a smaller shaft than the Century 5hp I bought for my IR T-30 80gal.

If I get the motor running I will likely try spinning the V-Pump rated for 5hp at less than the 1000rpm I use on my bigger unit. Even with the heavier motor and the switch cutout set to 145psi, I can hear the motor start sounding a tad different at 140psi. Not slowing, but just working harder.

A slower rpm should be like a lower gearing in a car I'd think, allowing the lesser torque of the lighter duty motor to be sufficient.

Comments welcome!
 
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Old 12-02-19, 01:15 PM
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If you don't have 4 leads coming out of the motor ,(L1 to start winding and centrifugal switch input, L2 to start and run coil common, wire 3 is centrifugal switch output to capacitor input, wire 4 is capacitor output to start winding) you will have to remove the rotor and try to find the physical points to attach missing wires that need to go out of the motor housing.
 
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Old 12-02-19, 05:55 PM
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I left you a link back in post # 10 of all available size caps for your application.
If it's not in that chart..... it probably isn't made.

It looks like you may need to use a 220-250vac cap.
 
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Old 12-04-19, 06:46 AM
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Ok.
You can use ANY voltage capacitor to test your motor as long as the mfd rating is high enough to give your start winding the power to start the motor.
As I said you need to select the correct voltage to ensure the capacitor will not prematurely fail.
By all means, select the capacitor based on supply voltage and again I'll say it WILL start your motor.
But again as said, this is not neccesarily the voltage the capacitor is working with.

The induced voltage is somewhat based on how much boost the motor mfr wants the capacitor to provide to the motor.
If they want the motor to have a slightly higher amount of starting torque they will increase the MFD rating but in doing so will increase the induced voltage.
This higher induced voltage will require an increase in the voltage rating of the capacitor.
In your case I would suggest you get a 400 mfd for your size of motor in whatever voltage rating you can get.
If it just sits there and hums try a larger say 550 mfd or so and if it works use it and see how long the capacitor lasts.

In commercial refrigeration work a capacitor is a very important component .
When you service a system you need to make sure the system is reliable and can not afford to experiment.

This capacitor MFR can supply pretty much any rating you need and has the sizes listed as well.
This brand also tends to be somewhat smaller than other makers..
 
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Old 12-04-19, 02:40 PM
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beelzebob
If you don't have 4 leads coming out of the motor ,(L1 to start winding and centrifugal switch input, L2 to start and run coil common, wire 3 is centrifugal switch output to capacitor input, wire 4 is capacitor output to start winding) you will have to remove the rotor and try to find the physical points to attach missing wires that need to go out of the motor housing.



Unfortunately, the grommet the 2 existing wires come out through the case, and the hole through the steel case probably couldn't support 2 more wires, so I guess you are saying I should drill a bigger hole and bigger grommet for the other 2 new wires after disassembling the motor and locating 2 more wires to solder to some inside my motor? I could find a large enough tin can to house the 2 capacitors and drill/tap another hole for the tab I fashion on the bigger can will have a place to attach to easily enough. (See Photo)

What wires inside the case should I attach the new wires through? Are they color-coded?

I wonder why my motor didn't come with 4 wires for 2 different capacitors?

I guess you're saying if I modify my motor it will work better or more reliably?

Sorry if my questions seem elementary, but I've never actually disassembled a large electric motor before to add wires and components. I'm now tempted to take my motor to an electric shop for this modification, though I can buy a new 5hp motor online for about $235 with free shipping, and I wonder about spending a large sum on my old motor that at this point I only SUSPECT will work (As I mentioned, I got the motor on the IR 60 gal compressor with a bad pump, and with a motor missing the start capacitor and can that covered it , so I never saw it run.)

When I began this thread I only thought I needed a start capacitor, but if I need to modify the motor for TWO capacitors by disassembling the motor, I am questioning the whole idea of trying to re-use it.
Thanks,
Bob

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Old 12-04-19, 03:23 PM
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Another point...

Looking at the back of the motor, (the cover plate was also missing), the only power connections are Black, Yellow, and Case-Ground (Green) (the wires were cut off inside the opening, but I can see the power cord White was attached to the Yellow motor wire).

Technically White means neutral, but in this case I imagine White (Yellow in the motor) is just a "second black" or a red, though for a power cord, I guess I only need a 2-wire with ground cord to the appropriate 220v plug (and obviously a 2+ ground power cord is intended for 110v so will have a black, a white, and a green) since I don't need a "neutral" wire. My shop has a floating neutral (not bonded to ground) so I cannot connect the neutral to ground if I elected to use a 3 + Ground cord anyway)

The wires that are attached to the power terminals inside the case are #14, but I imagine I should use a #12 power cord since the motor says 15 amps, but that's the max for a #14 solid wire, and I imagine marginal for a flexible stranded cord especially on motor start?

Bob

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Old 12-04-19, 05:04 PM
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Sorry my latest post caused confusion. After rereading the posts, I realized it was not established if the motor was a start capacitor motor or a permanent capacitor motor. My latest post described a start capacitor motor (most common and has a centrifugal switch) and has the 4 wires as I described. Two of the wires go to the capacitor as shown in the picture of post 17. The other 2 wires are the input power and go to L1 and L2 terminals shown in the picture of post 18 even though the wires have been cut off. Your statement about only 2 wires exited the motor didn't seem correct. If you know what a centrifugal switch looks like, can you see one inside your motor? Your motor is a single capacitor motor. You need to know the motor type to get the correct type capacitor.
 
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Old 12-04-19, 08:37 PM
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Sorry my latest post caused confusion. After rereading the posts, I realized it was not established if the motor was a start capacitor motor or a permanent capacitor motor. My latest post described a start capacitor motor (most common and has a centrifugal switch) and has the 4 wires as I described. Two of the wires go to the capacitor as shown in the picture of post 17. The other 2 wires are the input power and go to L1 and L2 terminals shown in the picture of post 18 even though the wires have been cut off. Your statement about only 2 wires exited the motor didn't seem correct. If you know what a centrifugal switch looks like, can you see one inside your motor? Your motor is a single capacitor motor. You need to know the motor type to get the correct type capacitor.

Not sure what a centrifugal switch looks like but I can try to take pictures through the grille on the back end. I only see windings in there, but maybe I wasn't looking carefully enough.

I gather if it's a start capacitor motor it will have two wires for the capacitor, and the other two for the power input. I was thinking for the 4 you mentioned, you meant two for a start capacitor, and two for a run capacitor (also up on top), but the other two you meant were for the 220v power input!

I had a pool pump motor years back that would periodically stop running and just hum like crazy, and the case would get burning hot, and it had only 2 wires up top like mine. Replacing the capacitor every couple of years kept it happy. They called it a "start capacitor," but I don't remember the value. It was a 2hp unit.

My OTHER 5hp compressor motor has two caps on top (4 wires) in one big can (vs 2 cans as some motors have) plus the two at the back for power input (6 wires total).

You say " You need to know the motor type to get the correct type capacitor."

So it's established mine is a one capacitor motor, but we're not yet sure it's a START capacitor (it is a start capacitor ONLY if it has a centrifugal switch* visible through the grille). If there's not centrifugal switch in there, it's a "permanent capacitor" motor (meaning the capacitor is a "run capacitor"?

Is it likely that a larger motor (5hp rated) would not have a start capacitor? Just asking.

Bob

*I assume a centrifugal switch is a fairly large assembly mounted around the rear part of the motor shaft, and has flyweights and contacts that open at a certain cut-out rpm? If so, that should be obvious when I examine the motor. I'll report back after I re-examine the back end of the motor for a centrifugal switch, and trace where the wires up top, and at the connection board go inside the motor.

I'll also try to get a look into the 2-capacitor Century brand compressor motor I have for comparison, since it MUST have a centrifugal switch in there since it has 4 leads up top.
 
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Old 12-05-19, 07:17 AM
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The internet is a good source of info. Search on "electric motor centrifugal switch".
 
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Old 12-05-19, 04:44 PM
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Looking into the end of my motor, and comparing what I saw online (Only 1 image per post so I am putting it on the following one ), I am confident that my motor does have a centrifugal switch mounted in the gap between the back of the windings and the motor's back end plate.

I suspect that what I see is the flyweight assembly that move a disk closer to a switch contact set on phenolic board on the back of which the power connects through the access opening.

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Old 12-05-19, 04:52 PM
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This is a Google sourced photo of a centrifugal switch that fits around the motor shaft

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Old 12-05-19, 05:05 PM
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We know what a centrifugal switch looks like. beelzebob wanted you to look it up so that you know what it looks like. You should be able to see that inside the back of the motor.

You need to make your pictures smaller. You, as a member, only has a 10mb storage area. You were hitting that. You can actually post many pictures in a single post but not if you hit your limit. I resized your pictures and you're at 2.3mb now.
 
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Old 12-06-19, 06:06 AM
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A start capacitor is used on a single phase electric motor with a centrifugal switch. It's mfd rating is usually larger than a run capacitor. That is why it was important to establish the type of motor you have. Taking the motor to a local trade or STEM school in your area to repair as a lab exercise may be an alternative if you feel unsure about any of the suggestions being offered.
 
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