Softening Inrush Currents Seen By Power Company?

Putting a building in a rural area and will have a few machines out there. The PC indicates "I can have all the amps I want" but will only allow a maximum of 10HP on the line. I think this is mostly due to farmers trying to run big pumps across the line. My stuff is all on VFDs but there is still certainly some inrush.

Rather than testing the PC and having them knock at my door, I was wondering if I can snub or soften the inrush a bit? I am not looking to reduce my VFD accels, but rather add other electrics.

I also have to run phase converters and in the future, plan to build a rather big one and just run a massive DC bank which should snub things? But in the short term I will run a few 10HP rotaries. My machines have more than 10HP but I am getting 600A of 240V/1P so surely they can see reason? I will have my own transformer and only me and one other person on the primary for miles.

Probably they mean you cannot start anything bigger then 10 HP by just "dropping it onto the line" with a contactor. You would need to do a reduced current start for anything larger.

Reduced current starting several ways. they come down to reducing the voltage on the motor, which reduces the inrush. Added impedance also helps.

Resistor starting.... series resistor. not good for high torque starts. Motor current is reduced, so is torque.

Inductor starting... ditto using inductor. Inductor is less lossy than resistor, but more expensive.

autotransformer starting. This reduces the voltage AND the mains current significantly, most usual voltage ratio is output at about 60% of nominal. Less torque reduction than resistor start, costs more. Lowest mains current surge of any but VFD

Star-delta starting. If a delta motor is started with star connection,it amounts to a reduction to 58% voltage.

Obviously a VFD will do anything wanted.

Yes, they are being a bit confusing but do specifically stating "no motor larger than 10HP is permitted on single phase services". However, If you have 600A, the only way to "see" a motor as opposed to a big cooking oven is the power factor or reactive power. From what I gather, they will probably monitor that.

But, I suspect if I don't bother anyone and not knocking their voltages around, they will leave me alone.

Here is what I was thinking 10HP single phase is about 35A and maybe 8x for inrush so maybe 280A inrush. Even with my bigger motors, they run up to full speed in about 2-3sec so as long as I don't exceed that inrush, who would care? I also asked about "multiple 10hp motors" and he said "nothing bigger than 10hp, we are all good" so technically, what if you have 2-3, 10HP motors starting at the same time? Makes very little sense to me but I get that they don't have much power out there and apparently constantly dealing with fluctuation.

My digital 3ph converter should be able to snub all that inrush though, right? I realize the bank will have to be BIG though! Might have to figure out how to charge it without knocking out all the neighbors.

huleo said:

Putting a building in a rural area and will have a few machines out there. The PC indicates "I can have all the amps I want" but will only allow a maximum of 10HP on the line. I think this is mostly due to farmers trying to run big pumps across the line. My stuff is all on VFDs but there is still certainly some inrush.

Rather than testing the PC and having them knock at my door, I was wondering if I can snub or soften the inrush a bit? I am not looking to reduce my VFD accels, but rather add other electrics.

I also have to run phase converters and in the future, plan to build a rather big one and just run a massive DC bank which should snub things? But in the short term I will run a few 10HP rotaries. My machines have more than 10HP but I am getting 600A of 240V/1P so surely they can see reason? I will have my own transformer and only me and one other person on the primary for miles.

Click to expand...

If "all your stuff is on VFD's" there is so little inrush that it would be hard to tell you were even starting a motor. Set the start ramp to 3 seconds!

Back in the days of Thermite, and in the land before UPS's were common, one method of momentary energy storage was what was called "a ride through" generator. It was a motor/generator combination coupled to a flywheel. The flywheel was gradually brought up to speed and was only used when the line voltage sagged or went out and would only supply power as long as the flywheel had energy. It was intended to provide power long enough for an orderly shutdown. But it could be used here to lessen the affects on the power lines for starting large motor loads.

Tom

You can fix the power factor.... Just be sure to put the capacitors upstream from the motor protector.... you don't want it to see the corrected current, but the actual motor current instead.

Most of those pocket electrical handbooks have PFC capacitor sizing charts. I think both the EASA pocket book and Ugly's do.

Thermite:

I always thought "dasn't" was a backwoods version of "doesn't dare".....

Nobody's yet mentioned the single easiest, most important item for reducing power inrush when
starting large rotary converters: bring them up to near sync speed before dropping them across
the line - using a small ac motor belted to them.

thermite said:

...A newer item is a VERY high-speed flywheel - the 'guts' of it resembling a 55 gal drum - but made of Aramid fibre. Cubic money substituted for cubic iron and copper, IOW...

Click to expand...

We have a customer with one of these. They run an apple processing/packing line in the PNW.

The flywheel energy storage unit is buried in the ground just in case something works loose. The unit also protects against the 'line recloser problem' where power is lost for upto 3 seconds.

That one is run up to speed using compressed air.

Thermite, did you use to work for Thomas Edison?

Tom

huleo said:

The PC indicates "I can have all the amps I want" but will only allow a maximum of 10HP on the line.

Click to expand...

This makes no sense whatsoever.

I think this is mostly due to farmers trying to run big pumps across the line. My stuff is all on VFDs but there is still certainly some inrush.

Rather than testing the PC and having them knock at my door, I was wondering if I can snub or soften the inrush a bit? I am not looking to reduce my VFD accels, but rather add other electrics.

Click to expand...

VFDs are already the solution for reducing starting current, there is nothing better than that. A VFD can accelerate the motor at 100% of FLC, nothing else can even do that, let alone better than that.

And... the PoCo is not going to show up at your door, they are going to send you a bill, or rather a MULTIPLIER on your bill. In fact they actually WANT you to exceed your peak demand, that way they CAN send you the higher bill. They just don't actually say that to you.

I also have to run phase converters and in the future, plan to build a rather big one and just run a massive DC bank which should snub things?

Click to expand...

Again, this doesn't make sense, not sure what you had in mind here. The only "bank" sort of thing for electricity is a battery. Batteries are actually not good at reducing energy bills.

But in the short term I will run a few 10HP rotaries. My machines have more than 10HP but I am getting 600A of 240V/1P so surely they can see reason? I will have my own transformer and only me and one other person on the primary for miles.

Click to expand...

No need to add rotary phase converters if you are using VFDs by the way, the VFDs will be capable of taking care of that aspect.

Jraef said:

Again, this doesn't make sense, not sure what you had in mind here. The only "bank" sort of thing for electricity is a battery. Batteries are actually not good at reducing energy bills.

Click to expand...

Batteries can reduce electrical bills in a number of cases when combined with a smart inverter that can offer peak shaving during premium rate hours while holding off on charging until off-peak times.

Lots of applications benefit from battery/inverter combos when conditions warrant.

But in the given OP's situation, yes . . . VFDs would be the solution for avoiding peak demand charges which is likely what the PoCo is talking about.

"VFDs are already the solution for reducing starting current, there is nothing better than that."

I think he's setting up a 20+ hp rotary converter. If he can find a VFD to replace that, it would be
a good way to go. Otherwise even if he could soft-start the 20 hp motor with a VFD, how can
he cut the motor over to the utility line so it can be running as a converter once it's up to speed?

Now running the pony motor off a VFD, that's a nice idea. Tune the pony speed for min inrush
when the jesus switch is closed.

Inductions motors have no fixed phase

thermite said:

Post # 3.

I suggested DC pony instead of AC 'coz you can run it off modest battery so it doesn't add even a little bit to the inrush problem being addressed.

OTOH, IF ..an AC pony could be mechanically arranged to always be in sync to the idler's phasing - such that it brought the idler not only to sync RPM, but ALSO 'actual' phase sync before the mains were switched over to it?

Probably overkill, but ..that could be about as 'gentle' as one could get as far as not putting rudeness back up the line.

Bill

Click to expand...

With an induction motor the armature phase is generated by the stator fields and there is not "out of phase condition". If near synchronous speed there should be little inrush current. Induction motors never run at sync RPM, they always slip a little.

CarlBoyd

I have a large variac that would pop the breaking as soon as you put power to it because of inrush.....the solution was simple, a large varistor or a variable resistor. Has whatever resistance cold but quickly drops to an ohm or two when warm a split second later. Not sure if it'll work for what you want, but its a nifty way to get around the spike of inrush current

What is at question here is that when a magnetic device is brought on line the residual flux conditions have to be reset to match the incoming power. That could be accomplished by connecting a power resistor to the fields to provide some excitation and then shorted out when going to full voltage when the rotor is at speed.

Same as a primary resistance reduced voltage starter. This the only way to mechanically switch over to running conditions without momentarily disconnecting power. That small period of time is enough to allow the phases to get out of sych which will result in a pulse of current to restore the phase synchonization.
Probably could be done with electronics, but I am from the silver-cad oxide and copper era.

Tom

Let's make a distinction here, concerning inrush currents. There are two being discussed in parallel and they are NOT the same.

1) The reasonably long high amperage draw of an induction motor when started. This is anywhere from a half second to several seconds (30 cycles to several hundred cycles

2) The short surge that charges the magnetic circuit of the motor. This is sub-one-cycle.

And, lets not get overly concerned about phase, because an induction motor has no particular synching to do. It has no rotor poles until there is a magnetic field, and that field automatically synchs it by "writing" the poles onto the rotor in the correct phase as per the stator supply. No fuss, no muss, no bother.


Starting surge:
If you connect full line volts to the motor when it is stopped, It first draws the current necessary to establish the magnetic field, then it draws locked rotor current, and then, as the rotor accelerates, the current drops. That takes a bit of time before the current is down to a normal amount, because the motor has to accelerate to near synch speed from zero.

This surge can be objectionable because it is high current AND sustained over many cycles. There IS a magnetic surge as well, at the start, but it is almost never noticed with the other currents.

Magnetic surge:
If the motor is pre-accelerated to near final speed, then when voltage is applied, there is heavy current flow until enough energy has been input so that the magnetic field is established.

The field then develops rotor current, and a rotor field by transformer action. Then the rotor field, which is turning at essentially full speed, generated a back EMF of essentially normal amplitude. The only surge is the one necessary to input the required energy to establish the magnetic field, and the rotor current/field. That takes less than one cycle.


General conclusions:
Since the same exact magnetic surge occurs if a transformer is switched on, it is mostly unavoidable. The avoidable surge is the longer starting surge, which is what the reduced voltage starters (and VFDs) fix.

A pony motor start to nominal motor RPM will also avoid the longer starting surge, but NOT the magnetic surge.

You would have to consult the tariffs, or the power user guide to determine what is allowable and what is not, but I suspect the magnetic surge is not much of a problem for the powerco, they will be concerned with the start surge mostly.

If the magnetic surge is an issue, a 2 stage start with series resistors that are shorted out right away will cut the magnetic surge also. Or a VFD, which will fix all of it (and has internal inrush limiting for its own supply).

Well said.

Tom

Way back in about 1980 I built a 30HP RPC.
Small AC 1/3HP pony motor to spin it up...faster than synchronous...and then disconnect power to pony motor.
Coast down TO synchronous via time delay relay, and then close the
main contactor. Arranged as an automatic event initiated with 1 momentary push button. With some adjustment of the TDR, inrush was quite minimal,
transition was smooth.

Yes....

I should mention something.....

Motor speed is an issue, even if phase is not so much. If your pony drives the motor lower than nominal speed, there will be *some* surge to accelerate the motor to full nominal speed. If it is driven closer to nominal, the surge should be much less than a start from a stopped condition, but it is there.

If the pony gets it going OVER synch speed, the motor will begin to generate when power is applied. That will give a short OUTGOING surge as the motor slows to nominal, the total energy output being whatever is stored in the motor system due to the overspeed.

Depending on motor size and stored energy, either surge can still be objectionable to the powerco. Or not.

Would the best switching speed be at synchronous or at the slip speed, if one were coasting down from an over-speed condition? In order to have just the magnetic surge.

SAF Ω