300HP Saw Mill Motor

[Sparky2791]

Good Day,

I am working on a project for a client who has a 300HP motor he needs connected for a saw mill. The power company is requesting a design from the customer (My client) to show that the motor will not produce a large power draw across the service in the event the motor is running and the saw hits something in the wood it is cutting which will slow the motor down bringing the motor closer to locked rotor current. We’ve all cut through wood with our 1 HP circular saw in the garage and if we hit a knot or nail in the wood the saw slows down or locks up and dims the lights in the garage, now imagine that time 300. I have been doing electrical design for 20 plus years and know how to spec a VFD drive to start this motor to avoid the in-rush current during start up in an effort to protect the power companies incomming service but this is the first time I have been tasked with insuring this does not happen after the motor has been started and is running at full capacity. I can understand the concern considering the locked rotor current of a 480V – 3Phase, 300HP motor is 2200A. That will likely cause quite a problem for the power companies customers. Is there a certain type of drive available to help prevent this from happening. Looking for some assistance as to ma manufacturer I can use as a resource for this. This is an older saw mill which is being connected at a new facility so apparently it is not ‘built into’ the saw.


Any assistance on this ‘learn something new every day’ moment for me would be appreciated.

Thanks for the input.

 

[Besoeker]

Good Day,

I am working on a project for a client who has a 300HP motor he needs connected for a saw mill. The power company is requesting a design from the customer (My client) to show that the motor will not produce a large power draw across the service in the event the motor is running and the saw hits something in the wood it is cutting which will slow the motor down bringing the motor closer to locked rotor current. We’ve all cut through wood with our 1 HP circular saw in the garage and if we hit a knot or nail in the wood the saw slows down or locks up and dims the lights in the garage, now imagine that time 300. I have been doing electrical design for 20 plus years and know how to spec a VFD drive to start this motor to avoid the in-rush current during start up in an effort to protect the power companies incomming service but this is the first time I have been tasked with insuring this does not happen after the motor has been started and is running at full capacity. I can understand the concern considering the locked rotor current of a 480V – 3Phase, 300HP motor is 2200A. That will likely cause quite a problem for the power companies customers. Is there a certain type of drive available to help prevent this from happening. Looking for some assistance as to ma manufacturer I can use as a resource for this. This is an older saw mill which is being connected at a new facility so apparently it is not ‘built into’ the saw.


Any assistance on this ‘learn something new every day’ moment for me would be appreciated.

Thanks for the input.

Are you going to use a VFD?
That would likely trip or go into current limit.
A solid state soft starter rated for continuous duty, no bypass, might do.

 

[Sparky2791]

Are you going to use a VFD?
That would likely trip or go into current limit.
A solid state soft starter rated for continuous duty, no bypass, might do.

Yes - Apparently the customer already has a VFD starter for the saw. I am waiting on the information on the starter the already own.
Thank You

 

[petersonra]

Yes - Apparently the customer already has a VFD starter for the saw. I am waiting on the information on the starter the already own.
Thank You

Chances are pretty good the VFD will just trip if the saw hits something that causes it to stall. Most VFDs will just go to current limit for some period of time and then trip. Current limit on most VFDs cannot exceed 100% for 60 seconds as a worst case and usually you can set it lower. There is no way a motor on a VFD ever draws anywhere near locked rotor current.

 

[Jraef]

A good quality Heavy Duty (aka Constant Torque) rated VFD will be able to allow the motor to deliver its rated Locked Rotor Torque (150-160%) for 60 seconds, some can even deliver Break-Down Torque (which is even higher at 200-220%) for 2-3 seconds, useful in re-acceleration issues like this. The problem is, torque and current follow each other, so if you need 150% torque, the motor will draw 150% current and if you need BDT for a few seconds, current will go to 200-220% for a few seconds. There is no way around that issue. You can program some VFDs for "Current Limit", but generally they accomplish that by folding back the commanded speed. But in a saw, that is not advisable because saw teeth are designed around a fairly narrow band of acceptable speeds and slowing down below that will overheat and ruin the teeth. Ask any experienced sawyer, they will confirm that.

So really, your only option to satisfy the utility requirement (which by the way needs more specific definition) is to program a Current Trip feature into the drive, meaning it will shut down if you "hit a knot". That's why you are going to need VERY specific details as to what the utility will allow, as in how high and for how long, because your customer will be looking at lost production and you are going to want to avoid that as much as is possible within the constraints.

MOST decent quality VFDs will have a Current Trip (aka "Electronic Shear Pin" or "Jam Detection") feature available, that's not the hard part.

 

[Sparky2791]

A good quality Heavy Duty (aka Constant Torque) rated VFD will be able to allow the motor to deliver its rated Locked Rotor Torque (150-160%) for 60 seconds, some can even deliver Break-Down Torque (which is even higher at 200-220%) for 2-3 seconds, useful in re-acceleration issues like this. The problem is, torque and current follow each other, so if you need 150% torque, the motor will draw 150% current and if you need BDT for a few seconds, current will go to 200-220% for a few seconds. There is no way around that issue. You can program some VFDs for "Current Limit", but generally they accomplish that by folding back the commanded speed. But in a saw, that is not advisable because saw teeth are designed around a fairly narrow band of acceptable speeds and slowing down below that will overheat and ruin the teeth. Ask any experienced sawyer, they will confirm that.

So really, your only option to satisfy the utility requirement (which by the way needs more specific definition) is to program a Current Trip feature into the drive, meaning it will shut down if you "hit a knot". That's why you are going to need VERY specific details as to what the utility will allow, as in how high and for how long, because your customer will be looking at lost production and you are going to want to avoid that as much as is possible within the constraints.

MOST decent quality VFDs will have a Current Trip (aka "Electronic Shear Pin" or "Jam Detection") feature available, that's not the hard part.

Thank You Jraef......I assume the hard part is to get the power company to agree to the acceptable parameters if they will provide this at all . I spoke briefly with the power company last week and he made the suggestion of a flywheel design but I was not sure what he was referring to so I thought I would do a little digging. My understanding of a flywheel used in the electrical world is to use the inertia of the flywheel to maintain power to a given object for a brief period of time. Not sure how that would prevent a motor from drawing excessive current. Any idea what he was suggesting?
Thank You!

 

[Jraef]

Thank You Jraef......I assume the hard part is to get the power company to agree to the acceptable parameters if they will provide this at all . I spoke briefly with the power company last week and he made the suggestion of a flywheel design but I was not sure what he was referring to so I thought I would do a little digging. My understanding of a flywheel used in the electrical world is to use the inertia of the flywheel to maintain power to a given object for a brief period of time. Not sure how that would prevent a motor from drawing excessive current. Any idea what he was suggesting?
Thank You!

That would have to involve redesigning the saw! Some saws, like Band Saws, already have a flywheel effect; the large rotating wheels that the blade runs on act as a flywheel. But if this is a circular or gang arbor saw, the only mass is the blades and motor rotor. Adding a flywheel then becomes a major redesign.

 

[Besoeker]

A good quality Heavy Duty (aka Constant Torque) rated VFD will be able to allow the motor to deliver its rated Locked Rotor Torque (150-160%) for 60 seconds,

I presume you mean at reduced volts and frequency? Rated volts will give LRT in the region of six to eight times FLT.

 

[Jraef]

I presume you mean at reduced volts and frequency? Rated volts will give LRT in the region of six to eight times FLT.

I think you mean LRC, not LRT. Locked Rotor Torque is determined by the motor design and as most motors are Design B, LRT is 150-160% of FLT.



LRC is 5-6x FLC, but that's because at Locked Rotor, most of that current is reactive,so it is not actually producing torque yet.

 

[Besoeker]

I think you mean LRC, not LRT. Locked Rotor Torque is determined by the motor design and as most motors are Design B, LRT is 150-160% of FLT.
View attachment 20594


LRC is 5-6x FLC, but that's because at Locked Rotor, most of that current is reactive,so it is not actually producing torque yet.

Yes. I do know how SCIMs work.

 

[winnie]

I presume you mean at reduced volts and frequency? Rated volts will give LRT in the region of six to eight times FLT.

That would depend on the operating speed of the motor at the time.

LRT is simply a torque value, similarly BDT is simply a torque value. Of course in a fixed frequency situation both values also imply speed and slip, but in the VFD world you can get away with using them as torque values.

I read Jraef as saying that a suitably rated VFD could maintain a torque 150% of nominal for about 60 seconds, at near full speed. This would mean 150% nominal current between drive and motor, and 150% power if the motor is operating at near full speed.

It seems to me that the simplest electrical design is to limit output torque to 100% nominal. If the blade hits an excessive load, then it will stall, with the VFD dropping frequency and limiting current. This would probably not be optimal from a production point of view, but without knowing the mechanical capability of the saw itself you don't know what sort of mechanical overload it could tolerate.

Say the VFD is selected to permit short duration 200% torque. Does this mean that when you hit something hard the motor simply shears a shaft or rips off blade teeth?

The blade on a saw this big probably also has lots of angular momentum, which will give a flywheel effect powering through small hard spots. You could add energy storage to the VFD to power through larger hard spots without loading the utility supply, but this would probably not be cheap.

Even if you never exceed 100% torque, what will the utility see as the load cycles from 0 to 100% torque?

-Jon

 

[Sparky2791]

That would depend on the operating speed of the motor at the time.

LRT is simply a torque value, similarly BDT is simply a torque value. Of course in a fixed frequency situation both values also imply speed and slip, but in the VFD world you can get away with using them as torque values.

I read Jraef as saying that a suitably rated VFD could maintain a torque 150% of nominal for about 60 seconds, at near full speed. This would mean 150% nominal current between drive and motor, and 150% power if the motor is operating at near full speed.

It seems to me that the simplest electrical design is to limit output torque to 100% nominal. If the blade hits an excessive load, then it will stall, with the VFD dropping frequency and limiting current. This would probably not be optimal from a production point of view, but without knowing the mechanical capability of the saw itself you don't know what sort of mechanical overload it could tolerate.

Say the VFD is selected to permit short duration 200% torque. Does this mean that when you hit something hard the motor simply shears a shaft or rips off blade teeth?

The blade on a saw this big probably also has lots of angular momentum, which will give a flywheel effect powering through small hard spots. You could add energy storage to the VFD to power through larger hard spots without loading the utility supply, but this would probably not be cheap.

Even if you never exceed 100% torque, what will the utility see as the load cycles from 0 to 100% torque?

-Jon

Interesting. I did place a call to the power company requesting the acceptable parameters for the motor in-rush. I am dealing with a power company I am not familiar with in West Virginia (I am locate in PA) but they have been pretty responsive thus far. I'll update as I know more but please....keep the ideas coming as I read each of them to gather intel on the subject.

Thank you!

 

[GoldDigger]

Interesting. I did place a call to the power company requesting the acceptable parameters for the motor in-rush. I am dealing with a power company I am not familiar with in West Virginia (I am locate in PA) but they have been pretty responsive thus far. I'll update as I know more but please....keep the ideas coming as I read each of them to gather intel on the subject.

Thank you!

Just a final nail in the coffin of the flywheel idea: Anything that is likely to actually jam the saw and bring the motor to locked rotor conditions, you might be able to power through with the added mechanical inertia of a large flywheel, but IMHO you are much more likely to break something. Preventing that is what shear pins are for, and "virtual shear pin" mode in the VFD will be a lot safer all around.
When the energy in a large flywheel lets go all at once the result is often not pretty!

 

[drktmplr12]

Thank You Jraef......I assume the hard part is to get the power company to agree to the acceptable parameters if they will provide this at all . I spoke briefly with the power company last week and he made the suggestion of a flywheel design but I was not sure what he was referring to so I thought I would do a little digging. My understanding of a flywheel used in the electrical world is to use the inertia of the flywheel to maintain power to a given object for a brief period of time. Not sure how that would prevent a motor from drawing excessive current. Any idea what he was suggesting?
Thank You!

It figures the POCO would suggest that you redesign a (minimum) quarter million dollar piece of equipment. :lol:

could an auto transformer soft start with an automated capacitor bank and help comply with the utility's constraints during locked rotor condition, since the power in locked rotor condition is largely reactive.

perhaps the bank would have to be impractically large?

just an idea.

 

[Jraef]

It figures the POCO would suggest that you redesign a (minimum) quarter million dollar piece of equipment. :lol:

could an auto transformer soft start with an automated capacitor bank and help comply with the utility's constraints during locked rotor condition, since the power in locked rotor condition is largely reactive.

perhaps the bank would have to be impractically large?

just an idea.

They already have a VFD. You can't get better than that as far as reducing start-up current.

... Apparently the customer already has a VFD starter for the saw. ...

LibertyEngineering,
In cases like this where I have had to deal with somewhat extreme limitations from utilities*, they have quoted it to me as a maximum kVA/HP limit. See if they will give it to you that way, it makes it much simpler when looking at your options, especially with a VFD in place.

*Side story:
I once had to start a 350HP compressor at a small sawmill in Idaho that was near Patty Duke's estate, where she had a digital post production studio (for you youngsters, Patty Duke was a movie and TV star in the 60s and 70s, who went on to be a producer in the 80s and 90s, also the mother of Sean Astin, who played Sam in Lord of the Rings). Sometimes when their compressor started, the VD would cause P.D.'s studio to lose data. Rather than just tell her to get a UPS, the local star-struck utility forced the sawmill to limit their compressor starting to 1.5kVA/HP. We managed to get it to work with a soft starter, but just barely and depending on the line voltage level, sometimes it would stall at those settings. So we called the utility for the inspection, then shut down everything else in the mill while starting the compressor with their meter connected. We passed, then after they left we tweaked the current limit up just an RCH to help avoiding it stalling.

 

[retirede]

We passed, then after they left we tweaked the current limit up just an RCH to help avoiding it stalling.

There’s a unit of measure I haven’t used in quite awhile.

 

[Besoeker]

I think you mean LRC, not LRT. Locked Rotor Torque is determined by the motor design and as most motors are Design B, LRT is 150-160% of FLT.

Yes. At six to eight times LRC. That was my point.

 

[GoldDigger]

Yes. At six to eight times LRC. That was my point.

Six to eight times LRC? FLC?

Sent from my XT1585 using Tapatalk

 

[GoldDigger]

There’s a unit of measure I haven’t used in quite awhile.

Let's keep it that way!

Sent from my XT1585 using Tapatalk

 

[Besoeker]

Six to eight times LRC? FLC?

Sent from my XT1585 using Tapatalk

Oops. FLC of course.