Across-the-line motor starts vs VFD starts seen at Power Co.

[fastline12]

We have an issue with power and see no way around it but to bend the rules if we can. Our power company has provided us with plenty of "ampacity" but then goes on to say "nothing larger than 10HP on the line". This is a rural area. I have asked the question about VFDs as I assume their only real issue with bigger motors has to do with the inrush but not quite sure. I would think I could bring a 100HP motor up carefully on the line and apply a nice constant load and they would never know the difference? We maybe power factor but I seem to remember VFDs running darn good PF as well?

What I am proposing is that if 10HP across the line starts are fine, I would think a 20-25HP would be fine with a 1-2sec accel rate?

Anyone have any ideas or thoughts we have not considered that might throw a red flag with the power co? Only thing I can think of is the constant load/unload of the motor loads from industrial equipment. That might wiggle down to 100A hits on the secondary. This is a 7.2kv primary so about 3A delta on the primary.

I think this all comes down to customer calls from lights flickering or maybe upsetting their own regulators or something.


I think we have discussed maybe even beefing up the DC buss capacitor bank in the VFDs as a reserve or motor starts but not yet sure if we will need to install a delay device so they don't all try to charge at once. I would really like to figure out how much capacitance it might take to make things more stable.

 

[davidbeach]

Typically those motor size rules are about the largest motor that is permitted to be started DOL. Typically larger motors can be used if started otherwise. A soft starter may be all that's needed, no need for a VFD. Every utility is a bit different, and the answer for one won't apply for the next, you need to have the conversation with them.

 

[fastline12]

Thanks. I have tried to have the conversation and it ends with confusion. "10HP is the limit, we don't care how you start it....But you have have all the amperage you want"

This makes absolutely no sense at all to me so I think we have have to take the "try and see" approach with our own monitoring to see how everything on the line looks. The closest neighbor is 1/4mi away and everyone gets their own transformer.

VFDs are already designed into the equipment so that is an easy one, AND easy to configure if we need to slow down acceleration due to peak current draw BUT we don't really want to slow down progress.

 

[BrianPetersen]

If someone got persnickety about this, I'd use two 10 hp motors mechanically linked together to drive the load.

Makes absolutely no sense, but satisfies the bugger enforcing the rule as-written.

 

[jraef]

Yeah, it appears you may have come up against someone who only read a rule without comprehension of the principals behind it. Generally I've run into this where they described the limit as "x kVA per HP for starting", which can be challenging depending on the value of x. VFDs almost always work as the "nuclear option" (nuclear to your budget). But to say there is a HP limit but no amp limit? Makes zero sense. So you could connect 10 x 10HP motors, but not a single 15HP? Something is wrong here. Stealing a line from "Cool Hand Luke"...

What we have here... is a failure to communicate.

I'm wondering if he just said it incorrectly; as in it should have been "You are allowed all the starting power you need for up to 10HP, but 10HP is your hard limit." I thinking it something like they likely have an open delta transformer bank that will be marginal if there is any voltage imbalance caused by other single phase loads on that rural line. So the transformers are able to deliver whatever a 10HP motor can demand from it starting, briefly, but the continuous power could be problematic due to factors beyond the utility's control. I would appeal this to a higher level within the utility for clarification.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Artisi]

Question is, are you talking with the monkey or the organ grinder?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[waross]

Who is your utility. Someone may be willing to look up the tariffs online and see what the rules really are.
Possibly there may be a commercial service available with a larger motor limit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Well, you can't have all the amps you want because the service will have a limit there too. So, something seems off with both parts of the story.

With a typical 3-phase supply, you get X kVA of service supplied. There is a hard limit on current or power due to the service (usually transformer size limited). The utility typically only begins to take issue when you begin to cause voltage flicker issues for neighbors. But, they could also get grumpy if you overload their equipment and start calling because you're blowing fuses or damaging things.

Now, you're talking about a rural area so I'm doubting you have a true 3-phase service here. Possibly single phase or open delta as already mentioned. Any oddity due to the location and available service options can effect what can be done on the service.

 

[fastline12]

Yes, this is a single phase service. I only mention "all the amps we want" because the POCO is willing to install a 100kva transformer which is a lot more power than 10HP. They mentioned a willingness to install whatever size transformer we desire.

I would like to have the conversation on motor starts but I have already talked with the lead engineer that designs the entrances and he is not amendable to alternative solutions. I see no way around this other than to bend the rule and see what happens, thus my reason for asking about inrush and such. I fear prodding them for more info on VFDs will only lead to them monitoring the system more closely.

 

[BrianPetersen]

Two 10-horse motors mechanically linked and a VFD or soft-starter as the case may be. Done. Rule satisfied.

Yep, it's dumb. But when a dumb person in authority refuses to listen to common sense and reason ...

 

[fastline12]

Well, we are dealing with prebuilt machines with motors and VFDs so using multiple motors is not an option here.

As a simple test at another facility, we have a 7.5HP motor/VFD on a single phase service with a 1p/3p converter. I simply threw the Fluke 337 on a 1p leg to see what was happening. I could not trip the inrush test circuit in the meter during a motor start, so I take that to mean I did not have enough of a measurable inrush. I observed the peak current during acceleration at 39A/1p. This VFD is setup to accelerate the motor just as fast as it can within its own limits. I have tried to make accel faster in the VFD settings and it would seem stall the motor accelerate slower I believe because the V/hz ramp went way faster than the motor could handle and large amount of slip was experienced. This is an old scalar or open loop drive. A vector drive should not do this.


That 39A seems rather consistent with the FLA of a 7.5hp/1p motor.

 

[jraef]

You could have mentioned in the beginning that this was a single phase supply...

You don't have "inrush" in a VFD in the same was as you do with a motor started across-the-line, it's totally different and not really comparable. I have a LOT of trouble with utility engineers on this issue, you are not alone here. They have rules based on archaic data and no willingness to update those rules because it involves complexities that many of them simply don't care to deal with (in my opinion). In other words they are taking the lazy way out of having to engage their brains.

By the way, you can forego the phase converter by simply doubling the size of the VFD, are you aware of that? The VFD simply rectifies the AC to DC, then creates essentially a new source of AC for the motor. The VFD doesn't really "care" if the incoming power is 3 phase or 1 phase, other than the current draw through the rectifier components and the amount of bus capacitance to smooth out the ripple. So doubling the size takes care of both factors.

Now, considering your being at loggerheads with the utility guy, I still think you need to seek a higher level of authority. He's just being a blockhead if he will give you a 100kVA source but still insists on a 10HP artificial limit.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[wayne440]

How about this- Be sure to record who told you "10HP is the limit, we don't care how you start it....But you have have all the amperage you want". Get that in writing if possible. Then find someone, somewhere who will produce a motor identical to the one you want to use, but VERY conservatively rated at 10 horsepower. Be sure the rating plate says "10HP" in big letters and as little else as it has to. Then you can look bewildered and point at the rating plate and drop the name of the ""10HP is the limit..." person when the utility has second thoughts.

 

[fastline12]

Jraef, these machines are integrated CNC machines so simply replacing a VFD is much more involved. They are typically matched motor/VFD units with specialized parameter settings to integrate with the control. The RPC takes care of the problem, though I realize that is not ideal. I am still trying to dream up a digital converter similar to other commercial units, but with higher capacity and a HUGE cap bank to mitigate some of the peak draw issues.

I guess one good point for an RPC is actually the poor power factor. They have a lot of current draw due to poor PF but as load comes up, the PF improves and the current is more stable.


I am aware of the VFD DC buss and single phasing them. I seem to remember someone using a list of components to sneak some power over to that 3rd leg. I have also wondered if I guy could just tear into one and mod a few components to allow full power as I think the caps, rectifier, and buss bar might be the only real bottlenecks?

In any case, I am just trying to get a feel for what the peak current might look like for a motor start on a VFD? I think most are either limited or protected in any case to limit current within their IGBT's ability? If that is the case, am I safe for considering the highest rated overload amperage for accel?

 

[jraef]

99& of the "Constant Torque" (which is what you would have on a machine) rated VFDs out there are going to be rated for the continuous FLC of the given HP based upon the NEC charts, and are capable of 150% current for 60 seconds and 200-250% current for 3 seconds. The overload rating allows for accelerating current. Since the VFD is modulating voltage and frequency together, the output torque of the motor is going to follow the current, so at 150% of full load torque, the drive will deliver 150% current. 150% FLT is roughly the Locked Rotor Torque of a Design B motor, so what most loads are designed around. The actual PEAK torque the motor can deliver is called Break Down Torque (BDT) and is between 200-220% of FLT (again, design B). So that's what is behind the overload ratings of the VFD; the VFD can allow the motor to deliver the same expected torque you would get from an across-the-line start, but do it at any time, not just on start-up.

In addition, the VFD can be programmed to LIMIT the torque to the FLT value and essentially use no more than FLA to do it. In many cases this is sufficient to accelerate a motor, as long as time is not a factor. In other words if I limit a motor to FLA with the VFD, it might take 3 hours, 3 days or 3 weeks for it to get to full speed, but it CAN be done and the VFD allows that to take place because if you never exceed FLA, the motor can do that forever. So if you have a 20HP 230V motor with a FLC of 54A, you can program the VFD to limit the current to 54A and it will do that forever. It limits current by artificially limiting speed / acceleration, so the accel time becomes an unknown, but if you can live with that, you can do it. I've done this countless times on centrifugal pumps and accelerated in 10 seconds or so. Ive done it a dozen times on refrigeration reciprocal compressors, works great but took a minute or so to get to full speed in one case. So you do have to watch out for things like mechanical oilers and such.

Now, I picked 20HP 230V 3 phase because it is 54A FLC, and a 10HP 230V single phase motor is 55A. So there you go, it would have the same effect on the power system either way. There is your argument (for 20HP anyway).


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

It doesn't matter what the Engineer says. The tariffs are the rules.
There is possibly a clause in the tariffs that is simplified to "10 HP".
You need to check the tariffs to find out what the rule actually is.
A question for Jeff (jraef)
A hypothetical motor is running at full torque and full rated current at 50% speed. The HP output is 50% of rated HP.
At 50% HP, what is the current into the VFD?
Can the motor be fed more than rated current at less than rated speed without the VFD input current going too high?
Thanks Jeff.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Input current = output current less the PF difference (because the caps in the drive are supplying the VARs to the motor, essentially correcting the input PF vs the motor PF). So for example if, at 50% load, the motor is at a .70 PF, the output current from the drive has to supply those VARs so it IS going to be higher than the input current, because the input is corrected to a .95PF.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[fastline12]

Jraef, I have looked through my books and cannot find any of my VFDs spec out the 3s overload capacity, they only give the 60s which is about 150% of max continuous amps. Are Vector drives typically setup to access the 3s capacity for accel? I know we have one machine that will indicate 180% load during accel but I have not yet tested it to see what it pulls.




I think I chatted with you before on a subject and I want to revisit this. The DC buss in a VFD is effectively a storage tank. I remember us talking about maybe even tripling the capacity to reduce the accel current seen on the line. However, you mentioned that the rectifier circuit probably won't handle charging all that on start up so I figured we could probably set up a couple banks with delay relays. So lets say a VFD has a factory installed bank of 2Farads, I might seek to install 2 more 2Farad banks, each timed about 5 seconds apart so the drive would first charge the factory bank, then bank 1, then bank 2.

Do you see any additional issues with this?

 

[LionelHutz]

Input current is not output current less the power factor difference.

The VFD is a power conversion device. This means input power is output power plus VFD losses. With the motor at 1/2 speed and rated current, the motor is operating at half power. This means the input power will be about 1/2 of rated motor power. Since the input voltage is constant, the input current will be approximately half the full-speed & full load current.

 

[jraef]

Oops, right. I conflated current and power. (again...) Sorry.

The PEAK output current capacity is not a universal design spec, it varies from mfr to mfr (caveat emptor). It is the HARD limit of the output transistor overload capability, meant to protect them from thermal damage. It's often not listed in performance specs, because some mfrs worry that users "push the envelope" and if your components are marginal, they might fail. You can't get around that by boosting the capacitance or anything like that, it's the limit of the power devices themselves.

You can sometimes indirectly find out what it is however by looking at the Fault Codes and finding one that says something like "Hardware Over Current Trip", which means it is a separate chip looking at current without regard to anything else going on in the drive and overrides all other programming. Somewhere they usually tell you what that fault means in terms of current overload %. With Rockwell drives for example, the "3 second" rating is 180% of the VFD rating every 1 minute, but the "Hardware Trip" level is another 143% of THAT value.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington