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Old 02-24-2019, 08:50 PM   #1
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Default Electronic Soft-Start

Can an electronic soft-start 'catch' a spinning motor or does it need to be at a standstill?

This one is pretty good-size; 850HP 2 pole 460 volts.

It's installed but won't be turning for several months.

The basic scheme is for the turbine to reach a certain speed (I don't know what it is yet) then the controller gives a 'ok to close' light then the operator closes a 1000 amp breaker.

Obviously, it won't be in synch, the only speed indication it has is a magnetic pulse pickup on the turbine shaft which is directly coupled to the motor.

Once the breaker is closed (assuming it's still in one piece.......), the steam valve opens fully and overdrives the motor thus making it a generator.

I'd much rather soft-start it than slam it across the lines.

The motor is 6 lead single voltage so I could build a Y-∆ starter if I need to.

On the other hand, it might be just fine the way it is.........I just want to know my options if the rest of the system (2000 amp switchboard that's pretty much loaded when the gen comes in) can't handle the surge current.

The manufacturer claims the breaker can be closed at any time the 'ok to close' light is on.
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Old 02-25-2019, 03:12 PM   #2
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A soft starter is only altering the RMS voltage, not the frequency. The frequency is determined by the line source. So it's not like a VFD that has to catch into a rotating field and match it first.

But in your case if the turbine is spinning it up with NO energy on the motor yet*, the line frequency becomes irrelevant. It's no different than what's called a "pony motor" starting scheme. The motor, when energized, will still have inrush and will still be pulling current to get to slip speed, it's just that the length of time will be shorter because the turbine has already taken it most of the way there. So in reality a soft starter may not be necessary.

*In a perfect world... The reality is that there WILL be residual magnetism in that motor and that means it will have a magnetic field that will be out of sync with the line frequency. So connecting two rotating magnetic fields that are not in sync can have very bad consequences. Having a soft starter WOULD dampen any nasty effects of that connection by having it take place with a low initial voltage and ramping up.

If it were me, I'd rather have the Soft Start and not need it that need it and not have it.

Whatever you do, DON'T do this in Wye-Delta, the inherent phase shift between Wye and Delta will be the worst thing you could do.
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Old 02-25-2019, 09:33 PM   #3
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Thanks, I hadn't thought of the phase shift from Y to ∆, yes, it would slam the rotor a bit.......

Also, I wasn't sure if the excitation of the motor stator at a speed other than zero would backfeed the soft-start SCRs to the point of destruction or not.
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Old 02-25-2019, 10:57 PM   #4
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Originally Posted by micromind View Post
This one is pretty good-size; 850HP 2 pole 460 volts.

Damn Micro......850HP?!?!?


Maybe you should start experimenting with something small like a 10HP before moving on to the big ones......
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Old 02-25-2019, 11:20 PM   #5
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If I had a small one and a soft-start, I certainly would.

I've done plenty of co-gens but this is the first one I've ever seen that didn't have any synch relaying. I was quite surprised when I looked at the drawings.
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Old 02-26-2019, 07:25 PM   #6
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did you say 850 HP at 480 volts ???

that is pretty good size sob ya got there.,

first of all do NOT use Y-D connection this due the phase will shift and can cause a bit of headache ( most are open translation so that why I am not too keen on close verison so just stay clear of it )

some soft starters can catch the spinning motor mode but the phase angle will not line up correct when you run in pony motor mode but the soft starter can soften up and will try to get close to the motor phase sequence and go from there.

and whatever you do .,, do not slam it on direct on line at all especially if you do not have phase or synchronous gauge or light indacitor there. too big a risk on that.

JRaef make few good points there so that one way he suggest on that.

If you get the steam turbine up running near sychronous speed (3600 RPM) and get it pretty close to it .,, it will be easy translation to generator mode but you may have to overdrive just a hair to get it on generator mode. unless you got sychronous motor then just lock it in specfic speed.
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Old 02-26-2019, 09:48 PM   #7
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Originally Posted by JRaef View Post
A soft starter is only altering the RMS voltage, not the frequency. The frequency is determined by the line source. So it's not like a VFD that has to catch into a rotating field and match it first.

But in your case if the turbine is spinning it up with NO energy on the motor yet*, the line frequency becomes irrelevant. It's no different than what's called a "pony motor" starting scheme. The motor, when energized, will still have inrush and will still be pulling current to get to slip speed, it's just that the length of time will be shorter because the turbine has already taken it most of the way there. So in reality a soft starter may not be necessary.

*In a perfect world... The reality is that there WILL be residual magnetism in that motor and that means it will have a magnetic field that will be out of sync with the line frequency. So connecting two rotating magnetic fields that are not in sync can have very bad consequences. Having a soft starter WOULD dampen any nasty effects of that connection by having it take place with a low initial voltage and ramping up.

If it were me, I'd rather have the Soft Start and not need it that need it and not have it.

Whatever you do, DON'T do this in Wye-Delta, the inherent phase shift between Wye and Delta will be the worst thing you could do.
Wouldn't back feeding a soft start make it go boom?

Is this really engineered, or have Fred and Barney been out in the barn?

It would seem that it would work better to start the motor first and then open the steam valve (If the motor will turn the turbine). The motor should be in sync and as the turbine steam drives the turbine up to speed it would overtake the motor and allow the motor to become a generator. Even this way there could be a frequency mismatch to cause trouble.

I would REALLY like to know how well this works.

It still seems like there is some synchronization controls missing.
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Old 02-26-2019, 10:29 PM   #8
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It's not synchronous, it's just a basic standard motor, the rotor isn't wound so there's no place to inject the DC.

It does have a pulse pickup on the gen shaft, it's possible that it waits until around 3600 RPM before it gives the 'ok to close breaker' light.

Even then, it'll still be out of synch and when the breaker closes in, there's going to be a pretty serious jolt.......both mechanical and electrical.

There is a 4-20 steam valve on the inlet but it's just a basic 8" model. I doubt it can control speed accurately. Others I've seen have a small valve in parallel with the big valve; those can get the speed really close.
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Old 02-27-2019, 01:11 PM   #9
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With it being an induction machine, there is no rotating magnetic field when it is not connected yet because there is no flux in the stator (other than any aforementioned residual magnetism in the steel from the previous energization). So when a contactor or breaker closes into it, if the rotation speed is sub-slip, the machine just pulls current to (a) magnetize the windings and (b) pull it up to slip speed. If the rotational speed is already OVER synchronous due to the turbine, it's just going to pull magnetizing current.


My concern is the residual magnetism, which on a large motor can be substantial, but it really depends on the design of the motor; mostly the magnetic permeability of the steel used in the laminations for the stator frame. YMMV.
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Old 02-27-2019, 08:35 PM   #10
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Is this really engineered, or have Fred and Barney been out in the barn?
It came as a factory assembled skid plus a pretty fancy control panel..

the panel has a Beckwith 3410 generator protection relay, a Woodward 505 turbine controller and some other stuff.

There are PTs and CTs on the gen side but not on the utility side. Both go to the Beckwith and a power meter, nowhere else. The Beckwith doesn't know what the utility is doing so it cannot synch with it.

It looks pretty well designed but it bothers me a lot that it just slams across the lines rather than synch to the lines.

There is factory start-up available but it's expensive and I'm pretty sure they won't buy it........so guess who will do the start-up and commissioning........
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Old 02-27-2019, 10:30 PM   #11
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Aside from a good dose of theory and hands-on of 50 HP and less, I'm out of my league here.



So my question is: How much will a motor like this naturally act as an alternator when driven at or above its rated RPM? From a standstill, I wouldn't think it to have enough residual magnetism to self-excite.


But if it does have an induced voltage from residual magnetism, how much stress, theoretically, could be caused by applying current while the induced voltage was at maximum phase differential on a motor this size?
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Old 02-28-2019, 06:53 AM   #12
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This sounds like this is configured as a stand alone or back up system to be used NOT connected to the utility. The synchronization control would be another piece of equipment that would connect to the turbine controls and the utility power and match the two before closing the breaker.

Maybe some one did not order this option?
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Old 02-28-2019, 09:19 AM   #13
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The one-line plainly states that one side of the breaker connects to the gen and the other connects to the utility.

The specs state that the unit is designed to cogenerate 630KW back into the utility.

Since it's a basic motor being driven by a steam turbine, it can't be a stand-alone since it doesn't have any excitation of the stator, it has to cogenerate.
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Old 02-28-2019, 09:50 AM   #14
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I’m still trying to wrap my mind around 850HP @ 480VAC!! Wow!!

Can you get some pictures of this at some point?
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Old 02-28-2019, 09:35 PM   #15
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I'd love to post pics but given my computer skills (next to none......) and the complexity of posting a pic, I'd be seriously surprised if I would be able to.
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Old 02-28-2019, 09:44 PM   #16
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Aside from a good dose of theory and hands-on of 50 HP and less, I'm out of my league here.



So my question is: How much will a motor like this naturally act as an alternator when driven at or above its rated RPM? From a standstill, I wouldn't think it to have enough residual magnetism to self-excite.


But if it does have an induced voltage from residual magnetism, how much stress, theoretically, could be caused by applying current while the induced voltage was at maximum phase differential on a motor this size?
I posted this earlier but it failed to appear........

The specs state that it'll cogenerate 630 KW, which is not too far from 850 HP.

I'd be surprised if it had enough residual magnetism to self-excite.

Regardless of whether the motor is generating a voltage or not, when the breaker closes, it'll be a pretty serious jolt, even it it's turning right at 3600 RPM.

It's the jolt that bothers me.......
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Old 02-28-2019, 10:52 PM   #17
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I posted this earlier but it failed to appear........

The specs state that it'll cogenerate 630 KW, which is not too far from 850 HP.

I'd be surprised if it had enough residual magnetism to self-excite.

Regardless of whether the motor is generating a voltage or not, when the breaker closes, it'll be a pretty serious jolt, even it it's turning right at 3600 RPM.

It's the jolt that bothers me.......
Since the motor is running when the circuit is closed, would it be any worse than the transition from wye to delta with a starter?
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Old 02-28-2019, 11:31 PM   #18
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Since the motor is running when the circuit is closed, would it be any worse than the transition from wye to delta with a starter?
It's likely slam a bit harder than Y-∆ because with Y-∆, there's no magnetizing current, just a phase shift.

Not sure though.......
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Old 03-01-2019, 02:33 PM   #19
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The one-line plainly states that one side of the breaker connects to the gen and the other connects to the utility.

The specs state that the unit is designed to cogenerate 630KW back into the utility.

Since it's a basic motor being driven by a steam turbine, it can't be a stand-alone since it doesn't have any excitation of the stator, it has to cogenerate.
It's what's called a Grid Tied Induction Generator. It can't generate anything unless it is tied to the grid because the grid is what provides the excitation (flux). This is what most of the large Wind Turbine generators are now as well. No synchronization is necessary, because when first connected there is no flux in it and as I said if it is below slip speed, it will draw current FROM the grid like a motor until the prime mover (steam turbine in your case) takes is super synchronous, or if already at a super synchronous speed when connected, the only thing you see is the magnetizing inrush for less than 2 cycles.



With a steam turbine, it's unlikely that they will ever connect when it is still sub-synchronous. That happens with wind turbines though because at low speeds, the mechanical stresses on the blades are really high. So they will connect them first with the blades turned to offer no wind resistance, run them up to slip speed as a motor, then turn the blades and have the turbine take it super synchronous.
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Old 03-01-2019, 09:49 PM   #20
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It's what's called a Grid Tied Induction Generator. It can't generate anything unless it is tied to the grid because the grid is what provides the excitation (flux). This is what most of the large Wind Turbine generators are now as well. No synchronization is necessary, because when first connected there is no flux in it and as I said if it is below slip speed, it will draw current FROM the grid like a motor until the prime mover (steam turbine in your case) takes is super synchronous, or if already at a super synchronous speed when connected, the only thing you see is the magnetizing inrush for less than 2 cycles.



With a steam turbine, it's unlikely that they will ever connect when it is still sub-synchronous. That happens with wind turbines though because at low speeds, the mechanical stresses on the blades are really high. So they will connect them first with the blades turned to offer no wind resistance, run them up to slip speed as a motor, then turn the blades and have the turbine take it super synchronous.
Thank you. I am always glad to learn something new.
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