reducing starting currrent in a 300HP motor.

[SparkyRules]

I have a 300HP (480V 3Ph) motor that table 430.250 tells me has a FLA of 361A and a start up (LR) per 430.251(B) of 2200A. What is the best way to reduce this start up current, VFD? Any other suggestions? :? And how much can this LR- amps be reduced? (as in the smallest size fuse that would be expected to hold after said starting current IS reduced)? Also per 430.6 the next standard fuse size is 2500A but table 430.52 says a 175% TD fuse will hold the start up current ( 2200 amps) Anyone have experience to know a dual element fuse at 700A will hold this motor? (361X 175=631.75A or 700A next standard size.). as I read 430.52(B)(1) exp does allow us to round up to the next standard size. Also am I correct that is the 700a does not hold then per exp 2 would be able to increase to an 800A TD fuse, using the same fuse holder. I mean really how expensive can it be if I get this wrong??? (Yikes)

 

[JoeStillman]

Use a solid state reduced voltae starter. You can program the starting and acceleration curve to your own liking. Once the motor is up to speed, a bypass contactor closes and you run like you're on a normal FVNR.

 

[kwired]

Actual starting current will also be limited by the source impedance as well as resistance of supply conductors.

The max LRC mentioned in NEC table kind of assumes infinite ability from the supply.

I don't do many motors larger than 100hp on a regular basis, but even the 100's I do frequently are irrigation well motors most of the time, with a pretty long supply conductor. The starting current on such long supply conductors doesn't even come close to what it may be if connected to a much shorter length of conductor.

If you are afraid of putting a lot of expense in this with trial and error approach, maybe you need an engineer or at least someone with a lot of experience with this size of a motor to be able to consult. Of course you have come to a good place for finding such people, but remember you get what you pay for and when using any advice you get here, there is no warranty. Good luck.

 

[Jraef]

All tabulations in 430 assume across-the-line starting. Those are all maximum protective device ratings, so if you reduce the current you can reduce the ratings too, as long as you are comfortable that they will not nuisance trip/clear on you. But you cannot reduce conductor sizes. Any kind of reduced voltage starting, except Wye-Delta*, will reduce the instantaneous inrush current to well below the curve on a reasonably sized fuse or breaker instantaneous trip.

I would never start a 300HP motor across the line even if allowed to, it's just too hard on everything. I agree on using a solid state soft starter, especially one with a bypass contactor. They (mostly) are fully adjustable to most starting conditions you will need to meet, but it will depend on the nature of the machine. Pump, fan, compressor, no problem at all. Rock crusher, loaded conveyor, large chipper or band saw, it will depend on the inertia in the load and the available utility source capacity, but most of the time those work too. When you use one, use the mfrs charts for selecting the fuses or CBs, they will have already tested them with those.

No need for the added expense of a VFD unless varying the speed has some other benefit to you, or the utility source is so limited that you cannot get to at least 250% of FLA during start, which is the typical minimum for a soft starter to work ( although it's usually 300-400%). A VFD is the only method that can accelerate a motor at 100% of FLA, as long as accel time is unimportant because a VFD can take as long as you like to accelerate.

* Wye-Delta, contrary to popular belief, rarely solves issues with instantaneous tripping or fuse clearing, because when it transitions from Wye to Delta, you still get that instantaneous change and under the wrong circumstances it can even be worse, i.e. 2200% of FLA!

 

[Besoeker]

I have a 300HP (480V 3Ph) motor that table 430.250 tells me has a FLA of 361A and a start up (LR) per 430.251(B) of 2200A.

As a matter of interest, what's the motor driving?

 

[kingpb]

As a matter of interest, what's the motor driving?

Yes, very good question.

Soft starters have limitations when starting high inertia loads and you need to get the manufacturers torque vs speed curves to determine if a RVSS will work. For high inertia loads a VFD may be necessary.

300Hp on 480V requires some serious design considerations.

 

[ptonsparky]

We did a 300 with a SS a few years ago. On a fan the owners were running at over speed to get the air flow they needed. It was a learning experience. They should have had the engineering help. The SS limited the number of starts and it started hard so getting it right (?) was a nerve racking experience for a first timer. Have fun.

 

[SparkyRules]

drving a hammermill

drving a hammermill

I sense this a gonna get complex, it is driving a hammer mill, which seems to have a lot of initial inertia so soft starting may be an issue. Before I do anything I will get an engineer to review, but I want to do as much of this 1st so when I work with an engineer and manufacturer I have a handle on what they are suggesting. Unlike everyone else s experience with engineers who always get it right the 1st time for all you folks my unfortunate experience is engineers have not always got it right for me the 1st time. My advantage is I have no clue what I am doing here, so I will figure it all out one atom at a time. Why not a giant bank of starting capacitors??? I also wonder if the mill could be programmed to stop with the hammer in the hi position so the weight would work with the start up to reduce inertia? All this really for me is to determine what size switch I need to feed this. The FLA=361A x 175% ( dual ele fuse)= 631 A so this means I need at least an 800A switch fused as low as 700A but could go up to 800A fuse if the 700A would not hold (exp2). Since it is bigger than 600A ( the switches they have) I need to replace a switch in the board anyway so the only question then is do I change it to an 800 or a 1200( next size bigger?). After that it is really not my issue so much as the plant will do the rest, I just want to be as informed as I can be. If there was any chance this could run off a 600A switch, which seem doubtful, then I save the cost of the replacement (to 800 or 1200) but that is the question can I do something ( like a reduce voltage soft starter ) that would allow me to use one of the existing 600 Amp switches? This is the only question I really need to answer. The rest I simply want to know

 

[masterinbama]

Recently did 2 300HP hammermills and I used RVAT starters in 2 separate MCC's. I also time interlocked the 2 so they could not start within 2 minutes of one another.

 

[SparkyRules]

2 RVAT

2 RVAT

Let me understand what you saying you used 2 separate RVAT to start the same hammermill so one could "cool down" while the other started the hammer mill? There by increasing the starts per hour the mill could be run?

 

[kwired]

I sense this a gonna get complex, it is driving a hammer mill, which seems to have a lot of initial inertia so soft starting may be an issue. Before I do anything I will get an engineer to review, but I want to do as much of this 1st so when I work with an engineer and manufacturer I have a handle on what they are suggesting. Unlike everyone else s experience with engineers who always get it right the 1st time for all you folks my unfortunate experience is engineers have not always got it right for me the 1st time. My advantage is I have no clue what I am doing here, so I will figure it all out one atom at a time. Why not a giant bank of starting capacitors??? I also wonder if the mill could be programmed to stop with the hammer in the hi position so the weight would work with the start up to reduce inertia? All this really for me is to determine what size switch I need to feed this. The FLA=361A x 175% ( dual ele fuse)= 631 A so this means I need at least an 800A switch fused as low as 700A but could go up to 800A fuse if the 700A would not hold (exp2). Since it is bigger than 600A ( the switches they have) I need to replace a switch in the board anyway so the only question then is do I change it to an 800 or a 1200( next size bigger?). After that it is really not my issue so much as the plant will do the rest, I just want to be as informed as I can be. If there was any chance this could run off a 600A switch, which seem doubtful, then I save the cost of the replacement (to 800 or 1200) but that is the question can I do something ( like a reduce voltage soft starter ) that would allow me to use one of the existing 600 Amp switches? This is the only question I really need to answer. The rest I simply want to know

Basic soft starters probably will not be able to stop your machine in a specific position, but this shouldn't be much of a problem with just about any VFD, you will need sensors on the machine to indicate positioning though, and possibly a mechanical brake to prevent it from moving before the next start.

Selecting the right engineer can be tricky, make sure you get someone that is not only an EE, but also has good mechanical knowledge. Someone that only designs office, retail, and similar installs may not have much experience in industrial matters.

May not be a PE there, but a good place to seek advice is your motor shop - you have to have a reliable motor shop if you deal with many applications even close to what you have here.

 

[Jraef]

I sense this a gonna get complex, it is driving a hammer mill, which seems to have a lot of initial inertia so soft starting may be an issue. Before I do anything I will get an engineer to review, but I want to do as much of this 1st so when I work with an engineer and manufacturer I have a handle on what they are suggesting. Unlike everyone else s experience with engineers who always get it right the 1st time for all you folks my unfortunate experience is engineers have not always got it right for me the 1st time. My advantage is I have no clue what I am doing here, so I will figure it all out one atom at a time.

25+ years of experience applying and commissioning soft starters has taught me that most engineers have very little experience with them and many have no clue at all, but will not admit it.

Why not a giant bank of starting capacitors???

It's called "Capacitor Assisted Starting" and was tried for years with varying degrees of success. The problem is, you have to REMOVE the caps at exactly the right time, the point in the acceleration process when the flux density in the rotor core is such that they no longer provide a benefit and begin to cause saturation in the stator windings, which causes you yo heat up the motor. That point is between 85 and 90% speed, which is difficult to see, let alone react to quickly enough. If also requires an addition contactor, speed sensors and control logic to pull off, which makes the whole arrangement too expensive for what you get.

I also wonder if the mill could be programmed to stop with the hammer in the hi position so the weight would work with the start up to reduce inertia?

That would only be possible (with difficulty in braking it into the exact position) if there was only one hammer. Most hammer mills have multiple hammers arranged around a cylinder or on a crankshaft so some are always at the apex, it's those that are not that are the problem. You usually cannot have them all there.

All this really for me is to determine what size switch I need to feed this. The FLA=361A x 175% ( dual ele fuse)= 631 A so this means I need at least an 800A switch fused as low as 700A but could go up to 800A fuse if the 700A would not hold (exp2). Since it is bigger than 600A ( the switches they have) I need to replace a switch in the board anyway so the only question then is do I change it to an 800 or a 1200( next size bigger?). After that it is really not my issue so much as the plant will do the rest, I just want to be as informed as I can be. If there was any chance this could run off a 600A switch, which seem doubtful, then I save the cost of the replacement (to 800 or 1200) but that is the question can I do something ( like a reduce voltage soft starter ) that would allow me to use one of the existing 600 Amp switches? This is the only question I really need to answer. The rest I simply want to know

I've done plenty of hammer mills on RVSS starters, it is entirely possible. Usually you will need 400-450% current limit for 15-20 seconds, so you must make sure you do not get a cheap soft starter that will artificially limit current lower to protect itself, and ABSOLUTELY do not use a "2-phase" soft starter, one that cuts cost by putting SCRs on only two of the 3 phases. Those will damage your motor. By using a proper heavy duty soft starter, you can definitely stick with a 600A disconnect, that's what size most of them will use in their combination starter packages. Personally, I might be more inclined to leave the 1200A switch in place and use 600A fuses in it though. Or just buy a factory built combination soft starter with the disconnect built-in. The package is safer that way, casual users cannot open the door with the disconnect closed.

 

[kingpb]

Jraef, although you have provided some great posts and responses regarding motor issues over the years, I would just like to caution anyone on utilizing RVSS that it is not possible to make a blanket statement that a certain type of application may or may not work. Yes, many RV starters may have successfully been put on hammer mills, however as you well know every case is not the same.

Anyone that does know how to properly apply these devices, will certainly gather the motor characteristic curves as a first step. Without these curves you would not know for certain if it will work, or not.

 

[Besoeker]

Anyone that does know how to properly apply these devices, will certainly gather the motor characteristic curves as a first step. Without these curves you would not know for certain if it will work, or not.

I think you also need load torque curves.

 

[kingpb]

I think you also need load torque curves.

Agree.:slaphead: Motor curves don't do much good if you don't know what your starting.:blink:

 

[topgone]

Recently did 2 300HP hammermills and I used RVAT starters in 2 separate MCC's. I also time interlocked the 2 so they could not start within 2 minutes of one another.

Did your motor survive having to deal with so many starts?:?:?

 

[Besoeker]

Did your motor survive having to deal with so many starts?:?:?

Might you have misinterpreted this?
I read it as two starters and two motors.

 

[broadgage]

Might you have misinterpreted this?
I read it as two starters and two motors.

So did I, sounded to me like two machines each with its own soft start, possibly starting at infrequent intervals. I presume that the 2 minute delay was to avoid tripping the thermal or time delay element of the MCCB protecting the feeder or service.

 

[masterinbama]

Did your motor survive having to deal with so many starts?:?:?

The interlocks were for first of shift startups and power failures. The plant only ground grain on two of their three shifts.