Startup current

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
FLA of 250 HP fan is 268. Nameplate. SF of 1. It has been rewound at sometime in the past.

New softstart inrush is 1600 to 1700 amps for about 18 sec then quick drop to run current of 325 with cold air. Normally it moves hot.

Any idea what the accross line start current would have been?

It was auto transformer start in past and needed replaced.
 
Is there a NEMA locked rotor code given either on the nameplate of the motor? Looking for something that might correspond to Table 430.7(B) in the NEC. This method however, will only give you a range of values.

If you have specifications of the motor, that should tell you what the locked rotor current is, but I am guessing you don't have that and since it has been rewound, the LRA is probably different.

Or you could bypass the softstart and take a measurement ;)

What do you mean by, "it normally moves hot?"
 
ptonsparky said:
FLA of 250 HP fan is 268. Nameplate. SF of 1. It has been rewound at sometime in the past.

New softstart inrush is 1600 to 1700 amps for about 18 sec then quick drop to run current of 325 with cold air. Normally it moves hot.

Any idea what the accross line start current would have been?

It was auto transformer start in past and needed replaced.
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That is an awfull long time to draw that high current for that long. It soiund like the setup of the soft start needs to be adjusted. (Especially with a centrifugal load where the breakaway torque is usually not that high. Unless for some reason the fan has a large flywheel attached:D .)
 
Operators have said it took about 20 sec or so to hit full speed in past.
They are also the ones that failed to tell me that after a redesign a few years ago the fan moves to much air and they have to run the damper at one notch above minimum flow or it overloads.

The loud "Thunk! Thunk!" you hear is my head beating against the building steel.

I will contact tech suppport again once we get hot air, the production process started, and determine just how many CFM we have. District manager knows how much air flow is needed. Slowing the fan down should help all the way around.
 
ptonsparky said:
Operators have said it took about 20 sec or so to hit full speed in past.
They are also the ones that failed to tell me that after a redesign a few years ago the fan moves to much air and they have to run the damper at one notch above minimum flow or it overloads.

The loud "Thunk! Thunk!" you hear is my head beating against the building steel.

I will contact tech suppport again once we get hot air, the production process started, and determine just how many CFM we have. District manager knows how much air flow is needed. Slowing the fan down should help all the way around.
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Damper? Sounds like you should have installed an ASD instead of just a SSRV starter.
 
ptonsparky said:
This is a hay mill that makes pellets. In season the green chopped hay takes more CFM. Off season they use bailed hay that is much dryer and takes less. Even field to field takes some adjustment. Damper has worked for 30 - 40 yrs. Should work a few more.
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With the damper you loose efficiency when compared to the Adjustable Speed Drives. They(ASD's) come close to pricing when compared to a SSRV and would have helped with the overloading without needing to fuss with the damper and the adjustment is finer than with the damper.
 
Amazing what 1" will do.

Amazing what 1" will do.

Talked them into the fact that this motor has always been overloaded. After initial protest they reduced the drive sheave 1". Smallest available. $1000.

Calculated reduction in RPM was 7% which dropped us to 115% of FLA. Adding heat and product in the system dropped it to 98% of FLA.

Starting time is now 12 seconds with reduced torque requirements.

I also threw in a adjustment to their analog % current meter to indicate "0" when the motor was off...instead of resting against the far left stop.

They could get by with a smaller motor if they would adjust the sheaves further but they have to make that decision.
 
Pton - What is the voltage? It seems it is probably 480vac, then the nameplate FLA do not seem right, but the 325 amps do seem within reason. You also said something about 40 years old plant, is the motor that old? You also stated it had been rewound.
Any changes made to the intake or discharge on that fan other than those you just made?

With an old frame motor from the 50's I once had a 100 hp motor rewound to 125 hp. It ran just fine.....those old 1950's motor ran cool to the touch, after rewinding to 125 hp that motor ran hot - but not as hot as the new frame motors.
I've also had rewound motors get the incorrect wire sizes when the coils were wound. They ran way over FLA!! In that case I was lucky the rewind shop realized their mistake when I talked to them.

Edit to add: What about the motor bearings. I was asked to check out a fan motor? one weekend in the old part of the plant where I was doing start-up. Turned out they were having trouble keeping it running the night before, had tripped OL"s twice, they called the engineer and he told them to go ahead and restart it and he'd find someone to look at it the next morning. When he showed up I was having my help disconnect the motor leads.........
He got a little upset, saying he had to get it going right away - I then showed him where his orders to restart the night before had caused the endbells to get to hot they cracked, and the motor shaft was froze. This on a 200hp? motor....They had to buy a new one, and were down for quite a few days.
 
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Motor has been reworked not to long ago. Originally SF was 1. What it is now is your guess. Plant design was changed some years ago that allowed more airflow but nothing done to the fan. Damper has had to be at min setting to allow motor to start for as long as anyone there can remember(20yrs+), then opened slightly for production.
 
I've done a number of blowers like this with soft starters.

  1. The damper needs to be CLOSED at startup, not even open a little bit. In a centrifugal blower, load is related to flow. No flow = no load. The mass and inertia on these is usually pretty high so ANY load will impede starting. For most people they think of this as counter intuitive and they feel they must "crack it a bit or it will back up", but that doesn't apply here. That only applies to positive displacement pumps and compressors, not centrifugal.
  2. With the dampers fully closed, you should be able to set your current limit (assuming your soft starter has it) for between 350 and 400%. You were at 590%, which is, for all intents and purposes, Across-the-Line. If your soft starter does not have Current Limit, extend the ramp time to effect one. But realize that the amount of line current necessary using a soft starter is almost always more than it would have been with an autotransformer starter. Reduced line current due to the transformer action is the only advantage the RVAT has.
  3. If they truly use varying air flows via damper control, a VFD may have been a better choice for sure. But if they run pretty much the same speed all the time, then the VFD would actually waste energy. But another advantage of a VFD in this case would be a much better soft start than what a soft starter can do. With a VFD you can usually accelerate the motor with little more than FLC. The only possible problem is that if you have a bunch of cave-man types running it and they barely understand the damper, a VFD might be asking for trouble.
 
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