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!

 

[Skogsgurra]

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

http://www.gke.org

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

 

[zeusfaber]

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.

 

[ebarba]

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.

 

[waross]

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)

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

 

[Skogsgurra]

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

http://www.gke.org

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

 

[edison123]

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

http://www.edison.co.in

 

[ebarba]

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

 

[ebarba]

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

 

[waross]

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:

, 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

 

[itsmoked]

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

 

[ebarba]

Thanks for all your help, guys!

here is the info requested:
generator

generator plate

inductance

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.

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.

 

[itsmoked]

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

 

[waross]

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

 

[ebarba]

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)

 

[catserveng]

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-ac-dc-starters.html

MikeL

 

[catserveng]

This site has a better selection chart,

http://www.favoritevendor.com/page17.html

MikeL.

 

[waross]

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

 

[MatthewDB]

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.

 

[ebarba]

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.

What is failing in the starters?

There are cables burned, both in rotor and stator. They get over 130ºC when they fail.

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!)

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.

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.

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.

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 . 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.