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starting 100 kW AC motor using inductance

starting 100 kW AC motor using inductance

starting 100 kW AC motor using inductance

(OP)
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!

RE: starting 100 kW AC motor using inductance

First: A Wye/Delta starter is mostly not a good idea. Don't spend good money on that.
Second: A 100 kW motor usually starts willingly DOL. Did you try that?
Third: If the protection trips during start - which it shouldn't because the load (the ICE) isn't a high-inertia or high torque load - then you should consider a "start bypass" for the protection. But that will probably never be needed.
Fourth: Is it REALLY an asynchronous motor? It should not be necessary to bring it to synchronous speed if it were.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: starting 100 kW AC motor using inductance

I'm guessing the "fourth" bit is that they aim to connect to the grid at synchronous speed to minimise the currents they switch routinely, before ramping the speed above synchronous to start generating.

A.

RE: starting 100 kW AC motor using inductance

(OP)
Gunnar:
- Why is a Wye/delta starter a bad idea?
- what is DOL (I'm sorry for my ignorance, I'm a mechanic playing the electrician
- the protection that trips is upstream of the meter (it's not user-accessible, cannot be bypassed). When we trip it, we have to call the utility maintenance guys so they reset it... HUGE hassle!
- If we close the main contactor, wether the motor is stopped or spinning at synchronous speed, the 250 A protection on the utility side trips.
- we are 100% sure it is an asynchronous machine. The reason it needs to be brought to synchronous speed is to keep low the starting current, so the above protection won't trip. This particular motor has a slip of only 10 rpm at full load, this means that if we close the contactor at 1510 RPM, we are actually exporting 100 kW to the grid (180 A), plus magnetization current about 5 times that. At the beginning, we didn't have the inductor and the result was 100% protection tripping whenever we tried to connect the machine, no matter the speed.

RE: starting 100 kW AC motor using inductance

How long has it been since the injectors have been replaced? Has anyone checked the compression? Does the engine use a lot of oil and/or blow a lot of smoke?
When the starter and batteries start to fail regularly, it is usually a problem with the engine.
Is this a shop built machine?
!00 kW is quite large for a factory built gen-set. Induction generators don't use five times 180 Amps for exciting current. That sounds like a synchronous generator with a serious voltage mis-adjustment.
I would expect an induction generator to have a slip of about 40 RPM. At +10 RPM I would expect about 25% power delivered to the grid and not much magnetizing current.
I would expect a synchronous generator to have a droop of about 45 RPM. (No load speed, 103% x 1500 RPM)

Quote (OP)

plus it'll take about 10 kW to move it at 1500 rpm while it starts (about 10 seconds
Really- if your engine needs 10 seconds at 1500 RPM to start, fix the engine. Generators have little effect on the cranking power needed. Generators are one of the easiest direct connected loads to start.
There are millions of generators all over the world easily starting with no starter or battery problems. FIX THE MOTOR!!!
Star delta starting: There are serious transient torque and current issue when the starter transitions from star to delta.
Gunnar, I suspect that they are using an inductor in place of synchronizing gear.
PS, If I have forgotten to mention it; FIX the engine!!!

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

RE: starting 100 kW AC motor using inductance

Ebarba,

Wye/Delta doesn't really help much if not "trimmed" to avoid heavy transients when switching from Wye to Delta. Some use closed transition to avoid the transients. But I haven't seen an Y/D starter in practical use for a few decades. And one of them broke couplings between motor and driven machine when switching to Delta. That's why you shouldn't waste money on such a beast.

DOL means Direct On Line - that is just switch the motor on. Without any intermediate gear. That's how most motors are started. I have started MW motors like that. Need a strong grid. But otherwise no problem.

I understand your other points very well. Calling in an external guy everytime you trip the protection is something to avoid.

In the meantime, Bill (waross) has given good advice. He is THE expert on this kind of problems. He eats diesel generators for breakfast. Do what he says.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: starting 100 kW AC motor using inductance

Why would a diesel power an induction generator? Can you post the nameplate details of the generator?

As Bill says, it's an engine problem. Generators pose no torque issues.

Muthu
www.edison.co.in

RE: starting 100 kW AC motor using inductance

(OP)
Gunnar,

we have BEGGED the utility to change the protection with one having a longer reaction time curve. They just won't. Here's a photo of the plate of that protection:

Our original idea and the original switchgear was meant to connect the machine directly, but after weeks of asking the utility to change the protection we finally had to go with the inductance.

So... there isn't really a clever way to coax that switchgear to soft start the motor, right? What if we crank the engine while the inductance branch is connected to the grid? will this help the starter have a less demanding load?


Waross,

the engine is not to blame, we just performed a major scheduled maintenance and everything is ok. In any case, we've had this problem since the very beginning. This engine cranks about twice a day and after 6 months we need to replace the starter... then once a year the battery needs to be dealt with.

If this was a synchronous machine, which is what a usual genset has, it would be like you say. This particular asynchronous machine has a polar inertia of 2.6 kg*m2, about 3 times your usual synchronous rotor (the motor weighs 1 metric ton...).

My comment about the 10 s to bring the engine on is because we don't know how the governor will react to the engine cranking at 1500 rpm. Hopefully it will be running in less than a blink, if the motor spins the engine

RE: starting 100 kW AC motor using inductance

(OP)
oh, I forgot: the engine is NOT diesel, it's a gaseous fuel, so it's spark-ignited

RE: starting 100 kW AC motor using inductance

OK, Time to stop fighting the problem and start solving it.
Some pics of the generator would be nice.
Some pics of the inductance would be nice.
Some background on the set may help.
WARNING ----SUGGESTIONS FOR AN INDUCTION GENERATOR MAY DAMAGE A SYNCHRONOUS GENERATOR.----
An asynchronous induction motor uses a squirrel cage winding on the rotor to develop torque. A synchronous generator uses a damper winding on the rotor that is very similar to a squirrel cage winding except that it may be much lighter and may be damaged if it is used for starting duty. We recently had a post about a synchronous generator being shop tested by being started as a motor. This seemed to be causing damage to the field winding by high induced voltages.
OK, You have an asynchronous machine.
That breaker shown will probably not support DOL starting.
How about a compromise?
If you cut out half the inductance, or replace the inductance with one of one half the inductive reactance you should get twice the current through the motor, from about 50 Amps to about 100 amps. That means 22 or four times the starting torque.
Now try the starter and energize the generator through the inductance both together. See if that will make enough difference to starting. If you need more torque than that, then reduce the inductance to 1/3 of the original value and try again. That should give you around 9 times the original torque.

The sizing of the inductor is a trade off between starting torque and connection surges.
Once you get the motor to start dependably, we have to be concerned with the no load speed of the engine before shorting out the inductance.
I am concerned with your comment:

Quote (OP)

, this means that if we close the contactor at 1510 RPM, we are actually exporting 100 kW to the grid (180 A), plus magnetization current about 5 times that.
Can you elaborate on this a little?

And on the topic of starter replacements; There are some very poor quality electrical components on the market.
The alternator on my old car failed a few years ago. I bought a replacement and it failed in about 15 miles. I bought another replacement and it failed in about 7 miles. I took one of the failed alternators to a repair shop with a good reputation and had one of the alternators repaired with quality parts and good workmanship. It has been running ever since. The owner of the shop told me that the market was full of bad quality replacements.
You may want to consider taking one of the failed starters to a good shop and having it repaired. It is possible that you have been the victim of shoddy replacement starters.

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

RE: starting 100 kW AC motor using inductance

I'd look into a better starter and double the battery bank size. There is often a surprising selection of alternative starters available. Look for a geared solution.

It is also very possible for a crummy battery charging circuit to be the root cause of your entire problem.

Keith Cress
kcress - http://www.flaminsystems.com

RE: starting 100 kW AC motor using inductance

(OP)
Thanks for all your help, guys!

here is the info requested:
generator

generator plate

inductance



Quote (waross)

Once you get the motor to start dependably, we have to be concerned with the no load speed of the engine before shorting out the inductance.
I am concerned with your comment:
Quote (OP)
, this means that if we close the contactor at 1510 RPM, we are actually exporting 100 kW to the grid (180 A), plus magnetization current about 5 times that.
Can you elaborate on this a little?
What I meant is that, given the protection in place, we have to be very careful on when we close the contactor. Even with the inductance in place, once connected at about 1515 rpm, tripping the protection.

Spare parts: I can't swear they are problem free, but at least they are of the right brand and sourced from an authorized dealer... It is a geared starter and to me it looks very good quality, even after disassembling 3 of them, trying to fix them.

Quote (itsmoked)

It is also very possible for a crummy battery charging circuit to be the root cause of your entire problem.
Yes, this is a possibility, the battery charger generator is bit too small for my taste, but voltage is ok and she battery is new, there doesn't seem to be a problem. When the battery starts aging it's possible that a larger charger would be better.

RE: starting 100 kW AC motor using inductance

In the time you mention between starts I can believe there should be enough time to recharge. However a battery that's just a little too small will torture everything including the starter and itself. It can keep the torque low and extend cranking, drawing the battery down excessively. In the service you describe a correctly sized and charged battery should easily last 5 years.

Keep in mind some brands of LA batteries are absolutely crap.

Your charger could be mis-adjusted or broken and not even fully charging so the battery quickly sulfates losing capacity that results in slower cranking and longer cranking which cyclically increases stress on the battery in a rapid circle around the drain.

I'd get a voltmeter on the battery before a normal starting. Write it in a log.
Then keep on the meter during the start and note the lowest voltage seen and how fast it drops from float to that low setting; instantly?, over 2 seconds?, or 6 seconds?

This will tell us a LOT. It will tell us if the charger is right and if the battery capacity is viable.

Oh and what is the battery chemistry? Valve Regulated Lead Acid (VRLA) like gel cells or absorbed glass mat, or just classic Lead Acid.


Keith Cress
kcress - http://www.flaminsystems.com

RE: starting 100 kW AC motor using inductance

What is failing in the starters?

If you are tripping that breaker when you close the main contactor we may ave to consider a sync check relay.
A lower impedance reactor as suggested may hold the generator closer to synchronism and avoid tripping the breaker.
In fact, you can probably monitor the current and safely close the contactor when the current is at the lowest as the generator accelerates.
Note that it is common for induction generators to have some residual magnetism. If the induction generator is generating even a small voltage and the contactor is closed out of phase you may experience high transient currents.

A note on batteries; If the battery is too small for the starter, the voltage will be too low and the starter will overheat, leading to early failure. At the same time the battery will be aged rapidly.
Too small a battery may cause early failure of both starter and battery.
If you watch the charging ammeter after starting you will see a high rate of charge, pretty much the full output of the charging alternator. After a fairly short time the ammeter will drop to near zero. At this point the battery is recharged. Use this to judge the size of the charging alternator.

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

RE: starting 100 kW AC motor using inductance

(OP)
Prior we burned the last starter, voltage during cranking dipped to 8 V, old battery. On a new battery and prior the starter failed, the voltage dip was to 9 V. The battery was a 125-Amp and now it's a bigger one (140 A)

RE: starting 100 kW AC motor using inductance

Is this being driven by your wood gas fired engine from your other posts?

Maybe another solution could be an AC powered engine starter. I have used these on projects with low btu fueled engines with long crank and purge times, here is one company I know of who provides them , http://www.usenergycorporation.com/explosion-proof...

MikeL

RE: starting 100 kW AC motor using inductance

Well if your battery starts with 12 Volts, then at 9 Volts cranking you have 75% of the power going to the starter and 25% lost as heat in the battery.
If you have 13.8 Volts, then you are losing almost 35% in heating the battery.
Get a decent battery. Your cranking voltage is too low.

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

RE: starting 100 kW AC motor using inductance

I would look for a gear reduction starter. They are sometimes available for engines, and are more durable because the higher rotation speed makes the motor efficiency and power capability higher. They're better for both the starter and the battery. I'd also switch to the highest voltage starter you can find for the engine.

Barring that, I would check to see if there is a pneumatic starter available for that engine. They deal with heavy use far better than low voltage DC starters.

RE: starting 100 kW AC motor using inductance

(OP)

Quote (waross)

Get a decent battery. Your cranking voltage is too low.
that, or the starter was already toast. Monday we'll have a new starter, let's see what the voltage does.

Quote (waross)

What is failing in the starters?
There are cables burned, both in rotor and stator. They get over 130ºC when they fail.

Quote (catserveng)

Is this being driven by your wood gas fired engine from your other posts?
oh, yes... the dreaded application... the one and only (thank God!)

Quote (catserveng)

Maybe another solution could be an AC powered engine starter. I have used these on projects with low btu fueled engines with long crank and purge times, here is one company I know of who provides them , http://www.usenergycorporation.com/explosion-proof...
Now, this is sweet! I didn't know they existed! I have pondered many times the use of a standard AC motor with a reduction gear and the like, but honestly it sucked for many reasons, space was the main one. These should work, tomorrow I'll try to find a local supplier... hope they don't cost a fortune.

Quote (waross)

A note on batteries; If the battery is too small for the starter, the voltage will be too low and the starter will overheat, leading to early failure. At the same time the battery will be aged rapidly.
Too small a battery may cause early failure of both starter and battery.
If you watch the charging ammeter after starting you will see a high rate of charge, pretty much the full output of the charging alternator. After a fairly short time the ammeter will drop to near zero. At this point the battery is recharged. Use this to judge the size of the charging alternator.
When both batt and starter are new, the charging phase lasts 15 minutes or so. When the starter is near it's end of life, then this period can last for an hour. Don't take this times as super accurate, this is what I remember by heart.

Quote (waross)

If you are tripping that breaker when you close the main contactor we may ave to consider a sync check relay.
Well, we have never tripped again the protection since the inductance has been in place. The one time we tripped it happened because the governor was dirty and the engine surged to 1515 rpm exactly at the moment when the main breaker closed. If this machine has a 10kW/rpm slip, I believe the tripping happened because of the huge instantaneous power the generator tried to deliver.

Quote (waross)

A lower impedance reactor as suggested may hold the generator closer to synchronism and avoid tripping the breaker.
In fact, you can probably monitor the current and safely close the contactor when the current is at the lowest as the generator accelerates.
Note that it is common for induction generators to have some residual magnetism. If the induction generator is generating even a small voltage and the contactor is closed out of phase you may experience high transient currents.
I will check what is the open circuit voltage of the generator. I'm not convinced this is the cause, but since I'm not an electrician, my opinion is just that: an opinion smile. Our current understanding of the problem is the issue wasn't the starting current (at sync speed is near zero), but rather the magnetization current. The latter is huge even if it lasts only half a cycle. This was the reasoning given to us by the generator manufacturer, the only one that found a solution that didn't involve a lot of money and fancy gear. Wether or not his explanation is correct, we never tripped the protection again ever since.

RE: starting 100 kW AC motor using inductance

Quote (OP)

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.

Quote (OP)

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

RE: starting 100 kW AC motor using inductance

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?

RE: starting 100 kW AC motor using inductance

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.

RE: starting 100 kW AC motor using inductance

(OP)

Quote (lionelhutz)

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?

RE: starting 100 kW AC motor using inductance

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

RE: starting 100 kW AC motor using inductance

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

RE: starting 100 kW AC motor using inductance

(OP)

Quote (waross)

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.

RE: starting 100 kW AC motor using inductance

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

Quote (Battery)

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

RE: starting 100 kW AC motor using inductance

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
www.edison.co.in

RE: starting 100 kW AC motor using inductance

(OP)
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!

RE: starting 100 kW AC motor using inductance

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.

RE: starting 100 kW AC motor using inductance

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

RE: starting 100 kW AC motor using inductance

(OP)

Quote (lionelhutz)

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.

Quote (waross)

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.

Quote (waross)

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.

Quote (waross)

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

RE: starting 100 kW AC motor using inductance

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

  1. We start the generator using a Y connection, this will make the engine start.
  2. Once the engine is running, we open the Y connection to the generator
  3. After the engine has reached a stable operating speed, we perform the connection to the grid just like we've been doing so far.
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:
  1. voltage downstream of the inductor is 50V, current is 200A (mesasured friday, see above)
  2. knowing that the protection trips at 2.5 times nominal current and that it's a 250A switch
  3. 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.
Am I right?

RE: starting 100 kW AC motor using inductance

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

RE: starting 100 kW AC motor using inductance

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?

RE: starting 100 kW AC motor using inductance

(OP)

Quote (lionelhutz)

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.


Quote (waross)

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.

RE: starting 100 kW AC motor using inductance

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.

RE: starting 100 kW AC motor using inductance

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

RE: starting 100 kW AC motor using inductance

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?

RE: starting 100 kW AC motor using inductance

Quote (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.

You need to leave the motor de-energized for several seconds during the transition.

The problem with Y-D starters is that when the motor is de-energized in wye, significant magnetic flux is left in the rotor circuit. The rotor will continue to roll below synchronous frequency. When the motor is re-energized in delta quickly, the flux is still high but the phase angle has rotated well out of phase. At reapplication of power, there is a huge torque pulse and current draw because the two fluxes are too far out of phase.

In many applications, the motor can't be de-energized for several seconds because it would slow too much while coasting.

Since you have the engine running, you can wait several seconds for the flux to die down and it will be the same as energizing it after a normal start.

RE: starting 100 kW AC motor using inductance

How about throwing an auto-transformer into the mix? It can give even more torque vs. current draw since the stator current is increased above the line current.

RE: starting 100 kW AC motor using inductance

(OP)

Quote (mathewDB)

You need to leave the motor de-energized for several seconds during the transition.

In this particular case, we can de-energize and keep it that way even a couple of minutes, it's something we do right now to allow for even heating of the engin, so it isn't a problem.


Quote (mathewDB)

How about throwing an auto-transformer into the mix? It can give even more torque vs. current draw since the stator current is increased above the line current.
Can you elaborate a little more, for the electrically challenged (i.e. Me)? Do you mean a 1:1 auto-transformer?

RE: starting 100 kW AC motor using inductance

AUTO TRANSFORMER STARTING.
All it takes is money.
The motor to be started is energized through an auto transformer. The taps are set at about 80% and 65%, pick one.
When the motor is energized through one of the taps, the starting current is reduced by two effects;
1/ The impedance of the auto trans reduces the starting current similar to the effect of the inductance that you are now using.
2/ The reduced voltage to the motor results in a lower line current.
The downside is reduced starting torque; 80%2 = 64% when the 80% tap is used and 65%2 = 42% when the 65% tap is used.
The cost of the starter is similar to the cost of a wye:delta starter plus the cost of the auto transformer.
Both methods use a starting contactor and a running contactor. The wye:delta starter also uses a shorting contactor, but the autotransformer needs a larger enclosure.

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

RE: starting 100 kW AC motor using inductance

The big advantage of the autotransformer is that it also reduces the line current by the transformer ratio which can help on limit power sources. Using the 80% tap may reduce the current to 80% and the torque to 64%, but it also reduces the line current to 64%. A soft-starter or reactor would still have the 80% line current.

RE: starting 100 kW AC motor using inductance

MatthewDB

If THAT is what you call a "smart sequence" I must say that I know more about that than most people that don't even know about the phasing of Counter EMF when switching from Wye to Delta.

I thought that you meant a device that, taking contactor delays into account, waited for the correct phasing and then closing the Star contactor in the correct moment to minimize the transient.

I think that I shall make one so that Y/D starts can be of some use again.

BTW - still discussing starting? Wasn't the problem more on the engine/starter/battery side?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: starting 100 kW AC motor using inductance

Gunner. This IS NOT a simple motor driving a load. Why all the harping on about the typical wye/delta starter issues that don't apply in this application? Everyone else already understands that this application doesn't require a fast transition and in fact should not use a fast transition, specifically to eliminate any transition transients that would occur.

BTW, the thing you are going to invent to fix the wye-delta starters already exists.

RE: starting 100 kW AC motor using inductance

Where? Are you sure you understand what I am saying?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: starting 100 kW AC motor using inductance

(OP)
Just wanted to let you all know what the conclusion was: we decided to go with the lower inductance instead of the "delayed" D/Y starter, for simple economic reasons.

After discussing with the inductor manufacturer, he realized that the original one was oversized for the short period of time it works. After re-sizing, a new inductor with taps at about 66% and 50% the actual reactance has a price practical equal to that of a new starter motor, no particular hardware is needed and the labor to replace the existing one is the same as replacing the engine starter. Controls issues are minimal.

On the other hand, the cost of labour and hardware for a D/Y starter is 3 to 4 times the price for the engine starter.

Right now everything is working fine and it was decided that the stringent law of "don't fix what ain't broken" will be applied. In any case, we'll keep an eye on the battery health and have a set up a monitor of the voltage during cranking. Maybe the new starter will solve the problem, but if we burn another one we'll go for the smaller inductance.

Thank guys!

RE: starting 100 kW AC motor using inductance

In my opinion, if the locked rotor current will be 8*Irat at full voltage [400 V] at 65% will be 4*Irat. MCB of 250 A will trip at 970 A in not less than 5 sec.
The starting time for 65% of rated voltage could be less than 1 second.
You have to check the motor inertia moment [for generator rotor-could be around 2-3 kg.m^2 and for DG around of 20 kg.m^2 in my opinion] and the starting torque and maximum torque[Ts=1.7-2.2*Trated and Tb=2.4-2.6*Trated].
Then a series reactor of 0.085 ohm and 650 A for 8 second could be useful, in my opinion.
However, the actually voltage drop will take into consideration the entire supply system impedance-transformer and cables.

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