Motor cap sizing help...?

[mayanees]

Follow-up on this .....

Motor was returned this morning; they said it had some issues they fixed along with replacing the motor starter and heaters. I was told the capacitors I installed are the right size.

So this motor has a legit starting current of 255A. I had a 60A cb on this circuit and it won’t hold. I connected directly to the line side of the incoming feeder unfused, and after 6 seconds of 255A, it dropped to 40A.

The nameplate on the blower calls for a 45A circuit. Never seen this in my life.

I just ordered some sub-feed lugs for the loadcenter to see if it will hold on the 200A MCB.


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I think you need a 150-amp breaker, per Table 430.52, with the option of going as high as 400% of 55 = 220 (200) per exception 2c if the 150 doesn't hold.

 

[ptonsparky]

A big centrifugal blower with large inertia might take that long to come up to speed. Sounds like a good application for a soft start, but could one be applied to a single phase motor successfully?

If this was working before, and now it isn't, you have to ask: what changed? Did someone change sheave sizes on the fan's belt drive in an attempt to move more air through the grain, for instance? Are bearings somewhere else in the system going bad? What's the inrush when the motor starts unloaded and mechanically disconnected from the fan; does it still take six seconds for the current to drop?


SceneryDriver

Most likely this is an axial flow with the blade connected directly to the motor shaft. Could be a centrifugal, but the blower is still shaft mounted. I would suspect bearings were replaced, or should have been while it was at the motor shop.

 

[brantmacga]

What is the VD as you are starting this motor?

10% VD when starting at 255A. Normalizes after 6 seconds.




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[brantmacga]

If this was working before, and now it isn't, you have to ask: what changed? Did someone change sheave sizes on the fan's belt drive in an attempt to move more air through the grain,


SceneryDriver

Motor came to us inoperable; fan is connected directly to the shaft.



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[topgone]

Motor came to us inoperable; fan is connected directly to the shaft.



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Blowers should have its outlet dampers closed or partially open when starting. Your motor is okay but you have to start your blower "unloaded" (outlet dampers closed). That way, your motor can speed up quick and will lower its amperage before the breaker trips on short time trip.

 

[brantmacga]

Blowers should have its outlet dampers closed or partially open when starting....

No dampers on this setup; just a chute going straight under the floor.

He does have other grain bins with dampers but only because a single fan is


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[brantmacga]

Blowers should have its outlet dampers closed or partially open when starting....

No dampers on this setup; just a chute going straight under the floor.

He does have other grain bins with dampers but only because a single fan is used for two bin pairs.


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[ptonsparky]

Blowers should have its outlet dampers closed or partially open when starting. Your motor is okay but you have to start your blower "unloaded" (outlet dampers closed). That way, your motor can speed up quick and will lower its amperage before the breaker trips on short time trip.

That is true for centrifugal fans. Not axial flow.

 

[Cow]

Follow-up on this .....

Motor was returned this morning; they said it had some issues they fixed along with replacing the motor starter and heaters. I was told the capacitors I installed are the right size.

So this motor has a legit starting current of 255A. I had a 60A cb on this circuit and it won’t hold. I connected directly to the line side of the incoming feeder unfused, and after 6 seconds of 255A, it dropped to 40A.

The nameplate on the blower calls for a 45A circuit. Never seen this in my life.

I just ordered some sub-feed lugs for the loadcenter to see if it will hold on the 200A MCB.


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I would be concerned about burning the buss out of the panel with those subfeed lugs if your motor starting current is indeed 255 amps for 6 seconds every time it starts.

You can't be the only one in the US that has encountered this. I can't help but think an oversized VFD that can take single phase input and a 3 phase motor might be a lot easier on your existing electrical system, than hard starting a 10HP single phase motor and potentially burning your panel up.

The bin fans I deal with over here are all 480v 3 phase, so it's not a big deal.

 

[brantmacga]

I would be concerned about burning the buss out of the panel with those subfeed lugs if your motor starting current is indeed 255 amps for 6 seconds every time it starts.

Maybe next year.... started cutting corn today. It’ll get started only twice this year though; once for my dad to dry some of his corn before trucking off, and once more in a few days when I put my corn in.


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[topgone]

Maybe next year.... started cutting corn today. It’ll get started only twice this year though; once for my dad to dry some of his corn before trucking off, and once more in a few days when I put my corn in.


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As mentioned above, it would be easy to run your motor quick if that motor of yours is a centrifugal blower (impeller looks like a water pump) because you can just block either the discharge port or the air inlet port. Motor starts quickly with less amps during starting. Also try a bigger capacitor than the standard cap used to make the motor starting torque stronger. How about the starting caps size you are using now?

 

[retirede]

Maybe next year.... started cutting corn today.


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Regional terminology is interesting. In my neck of the woods, we cut wheat, but we pick corn.
This is the first time I’ve run into “cutting corn”.

 

[ptonsparky]

Regional terminology is interesting. In my neck of the woods, we cut wheat, but we pick corn.
This is the first time I’ve run into “cutting corn”.

I read it and thought something was off.

My daughter drivers her hunting husband nuts with 'Did you catch any brds today?'

 

[Besoeker]

Regional terminology is interesting. In my neck of the woods, we cut wheat, but we pick corn.
This is the first time I’ve run into “cutting corn”.

Regional.......
What you call corn would be maize to us.
In Scotland corn is often the term used for oats. I remember being told, at school I think, that corn is a term used for the most commonly grown cereal crop in any particular region.

Back to the blower motor. At 10HP (7.5kW) it is on the limit of what I would consider for single phase but, if that's the only supply available......

 

[ptonsparky]

Regional.......


Back to the blower motor. At 10HP (7.5kW) it is on the limit of what I would consider for single phase but, if that's the only supply available......

We had a customer with 3 15 Hp single phase centrifugal fans on a bin. They were rough on start capacitors. Special order to boot. Last time, we replaced them on all three motors. Good or bad. Kind of like relamping the whole room or building at the same time.

 

[Jraef]

Biggest 1 phase motor I've seen was 50HP. On a farm of course...

 

[MattS87]

Regional terminology is interesting. In my neck of the woods, we cut wheat, but we pick corn.
This is the first time I’ve run into “cutting corn”.

Well if you are using a harvester that would be "cutting" corn in my book

 

[retirede]

Well if you are using a harvester that would be "cutting" corn in my book

We have no “harvesters”, either. We pick corn with a combine or chop it into silage with silage chopper.

 

[ptonsparky]

Typical axial flow grain aeration fan. Inlet.
1.7 amp is with no restriction.
2.1 amp is with inlet blocked with sheet of plywood. Blocking output, as best we can, is 2 amps.

VFD set at 60 HZ.

Nameplate: 1.5 HP, 3450 rpm, 460v, 2.1 fla.

 

[kwired]

Regional terminology is interesting. In my neck of the woods, we cut wheat, but we pick corn.
This is the first time I’ve run into “cutting corn”.

Trying to get this dryer fan going on a grain bin at the farm. The capacitors were shot. The markings were gone.

I guessed at the start capacitor size using the 30-50 rule and struck out I think.

The motor SOUNDS fine but it’s pulling 215A to start, and trips the breaker after a few seconds.

It’s a 10-12HP 215TZ frame motor.

Any help on sizing the capacitors?

Pic of nameplate ....

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Such a motor is pretty common around here, on older bins anyway. A two pole motor (~3400 rpm) doesn't have high starting torque and will take time to accelerate the large blower attached to it, even cast aluminum (which is most common) blower is heavy enough it takes time to accelerate to that speed. 100 amp breaker will typically hold long enough to start it though, that current does decrease as it accelerates but easily is pretty high for 3-5 seconds.

such 2 pole motors are typically only seen on axial fans, and the blower is directly attached to the motor shaft and the motor is located in the air stream. 10-12 HP "air over" is common design. Motor is basically built like a 10 hp, the "air over" allows the 12 HP rating because of cooling by the driven blower. This motor is not suitable for "general purpose" duty.

Failed capacitors and centrifugally operated starting switches seem to be biggest maintenance issues with these. They are trying to get all they can out of a small package single phase motor, and it doesn't help that the OEM's specify "drip proof" motors instead of totally enclosed motors - which allows dirt, debris, insects to get into the motor and cause problems with the centrifugal switch - which can lead to failed capacitors as well.

Next common problem with these is vibration and mounting of the motor controller on the same assembly - vibration just gets to it over time.