Starting Current for 1200 HP, 600 volt motor. Loaded or unlaoded?

[11bgrunt]

Does anyone have experience showing different starting currents for an induction motor using straight start with motor loaded or unloaded?
Plant has a 1200HP pump motor served with 600 volts.
For a temporary maintenance period the plant source will change to a better transformer providing the power but a great deal farther away from the plant. Calculations suggest the voltage drop on motor start will affect other customers.
This motor does not have a VFD or soft start package.
The plant has suggested they can open all valves, allow the motor to start no load and significantly reduce the estimated voltage drop on the line.
I don't agree.
I have done many test demonstrating load affecting the running amps of a motor, but contend that starting currents will be the same.

 

[Phil Corso]

11bgrunt... you are correct!

Starting-current has nothing, repeat, nothing to do with load! Load only affects runup or starting time!

Valve-alignment does seriously affect the net acceleration-torque torque (torque produced by motor minus torque required by pump) which will impact runup-time!

All valves should be CLOSED during runup!

Regards, Phil Corso

 

[johnmeto]

you are right!

 

[Jraef]

Correct. Loaded or unloaded only affects acceleration time, not current. In some cases people mistakenly think that the current is lower when unloaded, because they use a cheap meter with a sampling rate too slow to catch the real peak, but just because you don't see it doesn't mean it didn't happen.

That said however, VOLTAGE DROP across a transformer caused by starting current is NOT just based on the current, it's also related to time. So think of it as a transient. A 600% current surge for 1 second has less of a noticeable affect on VD than a 600% surge for 1 minute.

If you really want to know for sure what will happen, do a Transient Motor Starting analysis (TMS) using SKM Power Tools or ETAP. On a 1200HP 600V motor, I would highly recommend it. If you don't own the software, it's probably too expensive for a one-off project, so I would suggest hiring a consultant who has it.

 

[11bgrunt]

What I expect is a live test on this 1200. Evaluate worst case, switch feed to the plant and start motor. Maybe we will find that it wasn't as painful as predicted.
Can we agree that starting current is the same number as locked rotor current and the only difference is time on line? Then why is this so difficult to understand?
Since I was a pup, a single phase compressor that created enough VD to affect residential lighting always had a hard start kit added to the compressor to help that motor start easier. When R22 was used, high head pressure was used as the reason for those high starting amps.
When scroll compressors came along there is no head pressure and we usually see VD when starting 4 ton and larger.
If there is a difference between getting motion started and run up to full speed then isn't it logical that lower amps will be pulled with no motor load and be reflected in VD across time?
I am thinking of a motor demo that may make this easier to see if I use a power quality analyzer. A 5HP 3 phase motor with a belt driven turbine that can be easily disconnected?

 

[Iron_Ben]

Correct. Loaded or unloaded only affects acceleration time, not current. In some cases people mistakenly think that the current is lower when unloaded, because they use a cheap meter with a sampling rate too slow to catch the real peak, but just because you don't see it doesn't mean it didn't happen.

That said however, VOLTAGE DROP across a transformer caused by starting current is NOT just based on the current, it's also related to time. So think of it as a transient. A 600% current surge for 1 second has less of a noticeable affect on VD than a 600% surge for 1 minute.

If you really want to know for sure what will happen, do a Transient Motor Starting analysis (TMS) using SKM Power Tools or ETAP. On a 1200HP 600V motor, I would highly recommend it. If you don't own the software, it's probably too expensive for a one-off project, so I would suggest hiring a consultant who has it.

Wow. That's a lot of motor at 460 volts. FLC over 1400 amps I would imagine. What's the upper limit on motor size at 460? I can't find my Red Book, but going from my admittedly dicey memory, I thought it was 1000 HP. Or is this a case where if you're willing to pay, Baldor or whoever will build you one as big as you want?

 

[GoldDigger]

What I expect is a live test on this 1200. Evaluate worst case, switch feed to the plant and start motor. Maybe we will find that it wasn't as painful as predicted.
Can we agree that starting current is the same number as locked rotor current and the only difference is time on line? Then why is this so difficult to understand?
Since I was a pup, a single phase compressor that created enough VD to affect residential lighting always had a hard start kit added to the compressor to help that motor start easier. When R22 was used, high head pressure was used as the reason for those high starting amps.
When scroll compressors came along there is no head pressure and we usually see VD when starting 4 ton and larger.
If there is a difference between getting motion started and run up to full speed then isn't it logical that lower amps will be pulled with no motor load and be reflected in VD across time?
I am thinking of a motor demo that may make this easier to see if I use a power quality analyzer. A 5HP 3 phase motor with a belt driven turbine that can be easily disconnected?

When the rotor is not turning, the current will be the Locked Rotor Amp (LRA) or something higher if there are single cycle effects of residual magnetism in the motor somewhere (much more common for transformer inrush.)

When the shaft is not turning it does not matter whether or not anything is currently preventing it from turning initially.
What the load effects is how long the rotor stays close to stationary with the current at/near LRA.

If the voltage drop is only a few cycles an incandescent bulb or an LED or CFL with filtering will not show a visible decrease in brightness. The voltage drop as measured with a scope or peak-catching meter will still be there. And electronics with poor power supply regulation or with line voltage monitoring could still drop out.

 

[11bgrunt]

Wow. That's a lot of motor at 460 volts. FLC over 1400 amps I would imagine. What's the upper limit on motor size at 460? I can't find my Red Book, but going from my admittedly dicey memory, I thought it was 1000 HP. Or is this a case where if you're willing to pay, Baldor or whoever will build you one as big as you want?

Their transformer serves their 1200HP motor with 600 volts. Changes things a little but not like 4160 would.

 

[Jraef]

Wow. That's a lot of motor at 460 volts. FLC over 1400 amps I would imagine. What's the upper limit on motor size at 460? I can't find my Red Book, but going from my admittedly dicey memory, I thought it was 1000 HP. Or is this a case where if you're willing to pay, Baldor or whoever will build you one as big as you want?

People use Low Voltage for big motors sometimes because they don't have people on staff who are qualified to work on MV.

The biggest 460V motor I have ever worked on was 2,000HP for a wood chipper. I put a VFD on it to use just as a soft starter, nothing else would work. I did a regular solid state soft starter for a 660V Bow Thruster on a ship that worked out to be 1809HP (1350kW). On that application, I didn't get to do anything on it, but there was also a main Thruster motor that was 1850kW @ 660V, which is around 2400HP. I've done several 1000HP and 1200HP motors for irrigation pumps in the Columbia River basin that turn on once per year and ran continuously for 6 months or more. Those were started Across-the-Line!

As the saying goes, someone will build you anything you can describe on the back of a big enough check... (or for the Canadians, cheque)

 

[GoldDigger]

All valves should be CLOSED during runup!

Regards, Phil Corso

Except for positive displacement pumps, which are not likely in the OP's case. More likely to be found in air compressors and hydraulic prime movers.

 

[Iron_Ben]

People use Low Voltage for big motors sometimes because they don't have people on staff who are qualified to work on MV.

The biggest 460V motor I have ever worked on was 2,000HP for a wood chipper. I put a VFD on it to use just as a soft starter, nothing else would work. I did a regular solid state soft starter for a 660V Bow Thruster on a ship that worked out to be 1809HP (1350kW). On that application, I didn't get to do anything on it, but there was also a main Thruster motor that was 1850kW @ 660V, which is around 2400HP. I've done several 1000HP and 1200HP motors for irrigation pumps in the Columbia River basin that turn on once per year and ran continuously for 6 months or more. Those were started Across-the-Line!

As the saying goes, someone will build you anything you can describe on the back of a big enough check... (or for the Canadians, cheque)

Very good answer, thanks much!

 

[wwhitney]

That said however, VOLTAGE DROP across a transformer caused by starting current is NOT just based on the current, it's also related to time. So think of it as a transient. A 600% current surge for 1 second has less of a noticeable affect on VD than a 600% surge for 1 minute.

Is that because the I^2*R losses heat the conductor, increasing the resistance, thereby increasing the voltage drop? Or is there some other effect going on?

Thanks,
Wayne

 

[templdl]

Wow. That's a lot of motor at 460 volts. FLC over 1400 amps I would imagine. What's the upper limit on motor size at 460? I can't find my Red Book, but going from my admittedly dicey memory, I thought it was 1000 HP. Or is this a case where if you're willing to pay, Baldor or whoever will build you one as big as you want?

Right on. Let's go one step farther. A motor of that HP and that voltage it become impractical to wind the stator.
One has to wonder if this is a real life application of theoretical. I would think that a motor of this HP would be a minimum of 2300v.
How fast (big) would motor need to be wound 480 or 600 volts?

 

[11bgrunt]

Right on. Let's go one step farther. A motor of that HP and that voltage it become impractical to wind the stator.
One has to wonder if this is a real life application of theoretical. I would think that a motor of this HP would be a minimum of 2300v.
How fast (big) would motor need to be wound 480 or 600 volts?

Real life application. Natural gas processing plant.

 

[Fulthrotl]

What I expect is a live test on this 1200. Evaluate worst case, switch feed to the plant and start motor. Maybe we will find that it wasn't as painful as predicted.
Can we agree that starting current is the same number as locked rotor current and the only difference is time on line? Then why is this so difficult to understand?
Since I was a pup, a single phase compressor that created enough VD to affect residential lighting always had a hard start kit added to the compressor to help that motor start easier. When R22 was used, high head pressure was used as the reason for those high starting amps.
When scroll compressors came along there is no head pressure and we usually see VD when starting 4 ton and larger.
If there is a difference between getting motion started and run up to full speed then isn't it logical that lower amps will be pulled with no motor load and be reflected in VD across time?
I am thinking of a motor demo that may make this easier to see if I use a power quality analyzer. A 5HP 3 phase motor with a belt driven turbine that can be easily disconnected?

my experience has been that starting inrush current is 10 or 11 times running current,
with no load, and a 10ms sampling width. try it with a small motor, and get the proportions
for yourself, and i'll bet it scales up pretty similarly for the large motor. i wouldn't even put
a load on it, and cloud the issue.

once you have that inrush number for the big motor, you can probably punch in your conductor
length and whatnot, and get a proper voltage drop. not knowing your nameplate code letter,
a SWAG for locked rotor gives you 3623 amps at 600 volts.
yes, the TV up the street may flicker a bit.

starting current IS locked rotor current. full voltage is applied, and shaft is a no spinny. :happyyes:
doesn't matter if it's frozen or just sitting there. the electrons don't know the difference,
for three cycles anyway.

high head pressure will usually just pop the overloads, hence the pressure switch that keeps
an air compressor from starting until the head pressure bleeds off.

the largest motor i've ever seen started across the line was a 5,000 hp on a hydrocracker
at texaco in wilmingon. voltage drop, you say? the plant operator had to call DWP and get
a clearance to start it, or it would drop the entire south bay.

 

[templdl]

Real life application. Natural gas processing plant.

Damn!!! I would like to see how they made that motor. I can't imagine the size of the wire they used to wind the stator. Then the losses?

 

[Tony S]

the largest motor i've ever seen started across the line was a 5,000 hp on a hydrocracker at texaco in wilmingon. voltage drop, you say? the plant operator had to call DWP and get a clearance to start it, or it would drop the entire south bay.

Our plant PLC’s did that automatically, the start up sequence would hold until confirmation that “spinning reserve” was available. On shut down the PLC notified the generating company that load would drop. The grid system would slow down without the reserve being up and running.

Conversely I had to do a crash shut down and 23MW suddenly vanished off the system causing it to speed up. The generating company was soon on the phone wanting to know what the hell had happened. There’s not a lot you can do when a MV board is smoking.

http://www.dynamicdemand.co.uk/grid.htm