starting 100 kW AC motor using inductance 1

[ebarba]

Hi all,

is there a way to use a 3-phase inductance to start an AC motor?

The whole picture: the motor is in reality a 110 kW asynchronous 3-phase generator (400 V, delta connected, 50 Hz), moved by an internal combustion engine. Because the generator is SO massive it tolls the engine starter way more than it's designed for and we keep burning it (not to mention constantly ruining the battery).

Our "brilliant" solution: use the generator as a motor (is it a brilliant solution???). Problem is there's no starter for this large "motor" on site.

The usual procedure to get this generating plant online is to start the engine, ramp the generator to sync speed and then connect it to mains. To avoid tripping the 250A protection on this line, there is a 3 phase inductance and a smaller contactor in parallel with the main one. First the inductance branch is closed (so we get the current peak "filtered" by the inductance) then the main contactor is closed. Finally we open the inductance branch.

Since this is an Asychronous machine, it is essentially a standard AC motor. If we had a delta-star or a soft starter we could use it to move the generator and start the engine... but we only have the inductances in place. Is there a way to use them to "soft start" the generator in motor mode?

I had the idea to close the contact in the inductance branch with the engine and motor stopped: the inductance hummed and the motor moved about 1 degree. During the test, current in each phase was 210 A and the phase-to-phase voltage in the motor side of the inductance was 50 V... too little to make it spin.

What would happen if we close the main contactor while keeping closed the inductor branch? Will this make the motor run or trip the 250A protection?

The starting power of the IC engine at near-zero rpm is less than 3 kW, plus it'll take about 10 kW to move it at 1500 rpm while it starts (about 10 seconds).

Any ideas to avoid buying a new engine or a delta-star starter?

Thanks!

 

[waross]

I believe the tripping happened because of the huge instantaneous power the generator tried to deliver.

The inductor will hold the generator in sync when the speed is close to synchronous speed.
The inductor will also be supplying magnetizing current.
At 1515 RPM the machine could be expected to be exporting about 150 kW.
However if the speed surge pulled the generator out of sync and the contactor closed against opposing back EMF then you can get the high transient currents.

This was the reasoning given to us by the generator manufacturer

I am probably not the only one on this forum who has won technical arguments with sales engineers.

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

 

[LionelHutz]

I wouldn't count out wye-delta starting yet. You would basically start the generator in wye and then once the engine is started and you are running at 1500rpm you would transition the generator to delta with the reactor in the circuit. Then, short the reactor. Offhand, I don't know that breaker but the wye connection would reduce the current by the square root of 3 compared to trying it full-voltage. Why don't you first reconnect the generator in wye and give it a try by closing the main contactor to see if it would start?

 

[catserveng]

When I was involved with a pretty good cogeneration market many years ago, we had a number of units in this size range, mostly gas engines, most everything below 150ekW was an induction machine, most everything above 300 was synchronus, and we didn't have much product at the time in between. We did have a customer who wanted to reduce package cost insist on using the synchronus generator as the starting means.

First off, I think the assumption that "Since this is an Asychronous machine, it is essentially a standard AC motor." may not be correct, most generators I work on in this size range are lousy motors. I am not familiar with the OP's brand of generator but I'd be surprised if it was constructed to be a suitable motor for starting an engine.

Back to my old customer, he faced a number of issues in the attempts, if he tried to bring the engine up to speed too fast we had problems with the generator couplings, and in a couple of cases crankshaft issues due to the end thrust being changed (mostly a result of the coupling problems). The couple of units he did get the start and ramp up all had winding failures within 2 years. These were units that started and stopped once a week, start on Monday morning, shutoff on Friday evening. Those units had magneto ignitions and hydra-mechanical governors, so they had no DC voltage system on the engines, and the available DC power in the controls was less than 15 amps. Several of the units were taken out of service after about two years, due to multiple operational issues mostly, in my opinion, caused by trying to save way too much cost on the front end. Two units did stay in service, one was retrofitted to DC starters and a battery/charge system, the other had a Techno AC starter installed.

I also think the assumption about how much powered is required to turn the engine is wrong. if it takes less that 1%, then I'm assuming the reverse power protection is set to that point? In general I think most IC engines parasitic power is about 5% of rating, usually a bit more, otherwise that default 5% for 10 seconds I see a LOT of 32 devices set for probably would be much use, would it? Now on some newer diesel engines that are pretty highly turbocharged the parasitic power may be less, but I wouldn't think that is the case with this unit.

As so many folks above have already said, in most cases a properly sized set of batteries, with the right sized cables, good connections and a properly sized starter motor should start just fine with 12 or 24 VDC. But with biofueled engines they don't always start right away, and if they go thru multiple crank cycles without a proper rest, or pull the batteries down too low, then the problems you're seeing are pretty typical.

MikeL.

 

[ebarba]

Why don't you first reconnect the generator in wye and give it a try by closing the main contactor to see if it would start?

We tried that in the early days and the motor indeed starts without tripping the protection.

Your suggestions are all clear now to me. I think the best is to find an AC starter and that's what I'll try to do today... But for the sake of curiosity, what would happen if - with engine/generator at rest - we close the inductors and after a couple of seconds the main switch? What's the risk of tripping the protection?

As I said before, last Friday I closed the inductor contact, the motor does not run but it doesn't either trip the protection. Would the subsequent direct contact look like a short and trip again the protection, or would the current be low due to
- [the inductors keeping the motor more or less in sync]
- [magnetization having occurred before]
and the motor would start rotating?

 

[LionelHutz]

If you do that the current will be locked-rotor levels, which could easily be over 1000A.

 

[waross]

A decently sized battery may be cheaper, but it's your circus.
There are a lot of standard motor designs with different starting torque and different starting currents. Most recommendations are based on design "B" which is by far the most common.
The characteristics of a purpose built induction generator used as a motor, particularly when the focus is on both starting torque and starting current, are relatively unknown.
Have you forgotten the suggestion to use an inductor with less inductive reactance? More torque and more current but not enough to trip your breaker. A compromise between what you have now and DOL starting.
OR
A decent battery.

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

 

[ebarba]

Have you forgotten the suggestion to use an inductor with less inductive reactance? More torque and more current but not enough to trip your breaker. A compromise between what you have now and DOL starting.
OR
A decent battery.

Din't forget about those two at all. I'm asking the guys that manufactured that inductor if they can rewind it and/or make intermediate connectors, as well as a quote for a new one.

As for the decent battery, there are lots of brands available locally and they all have good reputations. Other than increasing the rating (we have gone from 125A to 140A in the last change, Friday) it's very difficult to make a "deterministic" improvement on the batt.

Today our good resident mechanic took a much bigger and tougher starter and modified it to fit our engine. There seems to be an improvement, measured in voltage during cranking (not less than 10V). This might be a quick fix, but I think our best shot is an AC starter, both money and time wise.

 

[waross]

A lot of highway trucks and heavy equipment use two of these to get started.

Group 8D 12-Volt Commercial Battery is a heavy-duty power battery for heavy trucks, buses, coaches, farm applications and more.
1100 cold-cranking amps, 1355 cranking amps, 325 minutes reserve capacity
Group Size 8D

These aren't cheap but still may be cheaper than an inductor.
Go big or go home.

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

 

[edison123]

MikeL nailed it. An induction generator is not a straight forward induction motor. The air-gap is larger in induction generator and hence if run as motor, it is going to draw a much higher inrush current.

The generator starting the engine is like the tail wagging the dog. Fix the starting system properly as many have suggested and get it over with.

Muthu

http://www.edison.co.in

 

[ebarba]

Well, things are becoming hairy. Talked to the starter manufacturer quoted by catserveng and they stopped manufacturing starters for these engines, according to him because the size of the gear's tooth is so small, and the load is so big, compared to the size of the starter, that they shattered when driven by their AC starter... He said he would pass on my request.

I have no difficulty in believing him, as these engines use some passenger car parts, albeit being industrial. I have personally seen broken teeth in the flywheels of these engines in the past.

I think the same problem would arise again with a pneumatic starter, so changing the starter to a different power source seems dead.

On the other hand, the inductance manufacturer wasn't very straightforward about possible solutions and officially he "will think about it".

Given the situation, two things will happen:
- we'll keep using my mechanic's creation and see it this improves anything
- when and if it will fail, we'll just repeat the infinite loop of fitting a new starter/batt

... Unless groundbreaking ideas arise!

Thanks everyone anyway!

 

[LionelHutz]

Well, you posted to me yesterdays seems to indicate that generator works fine in wye for starting the system. So, why not just do that? You have everything you need except for the contactors to switch between wye and delta.

 

[waross]

Further to Lionel's suggestion, if it works in wye, then it should work with a lower impedance inductor. Either way you have to increase the current through the generator above what the inductor will now allow. More current but less than DOL current.
The advantage of changing the inductor is two fold. You will not have wye:delta transition issues, and you may be able to use the existing contactors.
However I don't disrespect Lionel's suggestion. It may be a valid solution.
On the other hand we have all seen the effects of a failing battery or an undercharged battery:
Poor starting.
Overheated starter.
Overheated cables.
Further degradation of the battery.
Eventually burned out starters.
What do you mean by a 125 Amp battery and a 140 Amp battery.
Is that Amp/hours? Surely not cold cranking amps.
Cold cranking Amps is a better indication of battery starting capacity.


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

 

[ebarba]

So, why not just do that? You have everything you need except for the contactors to switch between wye and delta.

Because earlier in this post it was explained that the Y/D transition would be harsh and damage to the coupling is common, something we definitely want to avoid.

More current but less than DOL current.
The advantage of changing the inductor is two fold. You will not have wye:delta transition issues, and you may be able to use the existing contactors.

This is something we're working on: we asked again for a quote for new inductors or a modification of the one in place, they will likely respond shortly.

What do you mean by a 125 Amp battery and a 140 Amp battery.
Is that Amp/hours? Surely not cold cranking amps.

Sorry, I was very inaccurate... here's the full data:
OLD battery: 125 Ah, 940A (EN)
NEW battery: 140 Ah, 900A (EN)
Cold cranking amps should be the second figure.

If the induction generator is generating even a small voltage and the contactor is closed out of phase you may experience high transient currents.

Today I measured phase to phase voltage at 1500 rpm and open contact (no grid connection) and it was between 3.70VAC and 4.01VAC

 

[ebarba]

taking on Lionel's suggestion, but with a twist... what if we do a "delayed" Y/D start?

[ol 1]
[li]We start the generator using a Y connection, this will make the engine start.[/li]
[li]Once the engine is running, we open the Y connection to the generator[/li]
[li]After the engine has reached a stable operating speed, we perform the connection to the grid just like we've been doing so far.[/li]
[/ol]

Even control of this maneuver would be extremely easy: we'll just use the relay of the 12VDC starter to close the Y DOL connection for the sync machine. The control system of the generating set will automatically open the Y connection once the engine has gone past starting speed, just like it does right now!


On the other hand, the inductor manufacturer said the price is the same regardless of size (not exactly cheap)... but they need to know the size. Doing some crappy math:
[ol 1]
[li]voltage downstream of the inductor is 50V, current is 200A (mesasured friday, see above)[/li]
[li]knowing that the protection trips at 2.5 times nominal current and that it's a 250A switch[/li]
[li]we could -at least- increase current to twice the actual value ---> half the inductance we have now should provide twice the current and 4x the starting torque.[/li]
[/ol]

Am I right?

 

[waross]

OK Now you have 50 Volts dropped across the motor and 350 Volts dropped across the inductor. A ratio of 1:7 Cutting the inductor in half will give an impedance ratio of 1:3.5. That would be a motor voltage of 89 Volts. Current ratio of 89V/50V or 1.78
That should give a torque increase of a little over 3:1
Hit the starter at the same time and it may work.
By the way, my 130 HP tractor uses two batteries in parallel, each with 1000 CCA.

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

 

[LionelHutz]

I think you get it now. I did post that, but it seems I needed to spell it out clearer or differently. You have full control of the contactors. You can do the switching at whatever timing you want to get rid of any bad things that would otherwise happen during the transition. You already have the reactor that limits the current when you connect the generator in delta @ 1500rpm. All you need to do is add some contactors for the wye start, be smart about the starting sequence and you're good to go. Gunners hatred of wye/delta starters on motors, where they can cause issues and generally should be avoided, was wrongly applied to this case where you can easily eliminate the transitional issues with a smart sequence.

At this point, why bother designing another inductor that might work when you already have a solution that will work?

 

[ebarba]

At this point, why bother designing another inductor that might work when you already have a solution that will work?

Costs issues: a new inductor doesn't add more gear, wiring, labour, other than -of course- paying the new one and switching the existing.
On the other hand, the Y starter needs contactors, wiring for control and power, a new box because we don't have the extra space and the like.

We have to receive final quotes, but I will not be surprised if changing inductor is going to cost less.

By the way, my 130 HP tractor uses two batteries in parallel, each with 1000 CCA.

Well, I can connect the old one in parallel to the new one and see if noticeable improvements arise. It might not be perfect, but it'll give some extra power.

 

[LionelHutz]

OK, if you can connect in wye and actually start without tripping then I'd put a much smaller inductor than just 1/2 of what you currently have. 1/2 of your current inductor will still provide quite a bit less current than what typically works for starting.

 

[Skogsgurra]

LH: "Gunners hatred of wye/delta starters on motors, where they can cause issues and generally should be avoided, was wrongly applied to this case where you can easily eliminate the transitional issues with a smart sequence."

What does that smart sequence look like? I really want to know. Never seen that. Didn't know that it exists.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

Did you bother reading the read the post 2 prior to what you are quoting?

You do understand that powering the generator in wye starts the engine?

And do you further understand that once the engine is running the generator can be completely disconnected from line power and then the operators can take however long is required to re-connect as delta and then re-synchronize the generator to the line?