What load must be available for motor while starting in Y/D?
What load must be available for motor while starting in Y/D?
(OP)
Hi!
On board a ship we are having a diesel-generator set which supplies 44 [E] kW.
I do have a basic [E] load of about 35kW.
Q:
How do I calculate if I can start a compressor using Y/D starter in this configuration?
TIA
-Bart
On board a ship we are having a diesel-generator set which supplies 44 [E] kW.
I do have a basic [E] load of about 35kW.
Q:
How do I calculate if I can start a compressor using Y/D starter in this configuration?
TIA
-Bart





RE: What load must be available for motor while starting in Y/D?
Keith Cress
kcress - http://www.flaminsystems.com
RE: What load must be available for motor while starting in Y/D?
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(2B)+(2B)' ?
RE: What load must be available for motor while starting in Y/D?
Best guess 9 kW/2 = 4 or 5 kW motor.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: What load must be available for motor while starting in Y/D?
=====================================
(2B)+(2B)' ?
RE: What load must be available for motor while starting in Y/D?
Off the top of my head though, you are nowhere close to being able to start a 35KW motor from a 44kW generator with anything other than a VFD, and even that is going to be tricky depending on the TYPE of compressor and how quickly it must accelerate. The general rule of thumb is that a generator kW rating needs to be 3-4X the motor kW for Across-the-Line (DOL) starting, then everything else is based on acceleration time, voltage drop tolerance, load torque requirements etc. etc.
If I have misinterpreted your question, start over and describe EXACTLY what you are trying to do, don't make us guess.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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RE: What load must be available for motor while starting in Y/D?
RE: What load must be available for motor while starting in Y/D?
If the problem is that you do not know where to start, this is not the place. But if the problem is something you are stuck on or do not understand, then give us as much to work with as you know.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: What load must be available for motor while starting in Y/D?
Thanks for your time so far. Glad to find others here who are willing to help.
Some more explanation:
I am working at a shipyard where we are building relative small vessels (size 12-40 mtr.)
The vessel I'm involved in now has a generator installed which might have a rated load of 44kW.[Accepted peakload 1,1*44kW for 1 hour /each 12hour]
There are lots of other consumers on this vessel such as small pumps, lighting, kitchen machines, navigation aids a.s.o.
During normal sailing the total expected load is somewhere 35kW.[Intermittent rate factor s has been taken into account as well]
In fact there is in this situation a 9kW 'spare' [E]-power availble.
The vessel is also equipped with a hydraulic crane.
The rated load of the E-motor for the hydraulics of that crane is 7.5kW. We intend to start that motor wit Y/D starter.
Now I want to calculate whether or not the gen-set will be able to allowe that.
My calculation looks like:
E-motor 7.5 kW assumed cos.phi = 0,85 Boardnet voltage 440V-60Hz.
Inom = 7500/(440*V3*0,85) = 11,6 A
Istart in normal = about 3*Inom = 3*11,6 = 34,8A
kVAstart = U*Istart*V3 = 440*34,8*1,73 = 26,4kVA
Assumed cos.phi when starting = 0,3
Power what genset must be able to deliver [on top of load already supplied by the genset]
kVAStart*cos.phi start = 26,4*0,3 = 7,92 kW.
Total load during starting the hydraulic motor = baseload + startload motor = 35kW+7,92kW = about 43kW.
So In theory this goes well.
But....
Is my theory OK?
Thanks in advange for your time again.
-Bart
RE: What load must be available for motor while starting in Y/D?
I am aware of less torque when starting in "Y" (in theory 33% of rated torque). Also efficiency should be taken into account but I left that out for reason of simplicity.
-Bart
RE: What load must be available for motor while starting in Y/D?
Motor size = 7.5 kW
Available capacity = 9 kW
All of our rules of thumb say that the motor won't start.
To start a motor when it is the only load generally requires a generator capacity of 2.5 times the motor capacity.
To start a motor when there are other loads which may be affected by voltage and frequency dips requires a generator capacity of 3 times the capacity.
BUT you have a few things going for you.
1: The star delta start will give you an advantage but possibly less than you expect. You may have issues when the starter transitions to the delta connection.
2: The low starting power factor and the sizable base load will work in your favor. Be aware that you may have voltage dip issues and if the starting load pulls the frequency down, the UFRO feature of the AVR will drop the voltage.
I understand that when a customer presents you with a vessel with installed equipment, it may be too late to spec a larger genset.
I would investigate the possibility of load shedding to allow the motor to start. If you have any type of electric heating that is a good place to start. Most heating applications will accept a 5 or 10 second interruption of power with no ill effects
If you are able to curtail enough load I would consider going to direct on line starting. Once started the motor may run well.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: What load must be available for motor while starting in Y/D?
Motor size = 7.5 kW
Available capacity = 9 kW
All of our rules of thumb say that the motor won't start.
To start a motor when it is the only load generally requires a generator capacity of 2.5 times the motor capacity.
To start a motor when there are other loads which may be affected by voltage and frequency dips requires a generator capacity of 3 times the capacity.
BUT you have a few things going for you.
1: The star delta start will give you an advantage but possibly less than you expect. You may have issues when the starter transitions to the delta connection.
2: The low starting power factor and the sizable base load will work in your favor. Be aware that you may have voltage dip issues and if the starting load pulls the frequency down, the UFRO feature of the AVR will drop the voltage.
I understand that when a customer presents you with a vessel with installed equipment, it may be too late to spec a larger genset.
I would investigate the possibility of load shedding to allow the motor to start. If you have any type of electric heating that is a good place to start. Most heating applications will accept a 5 or 10 second interruption of power with no ill effects
If you are able to curtail enough load I would consider going to direct on line starting. Once started the motor may run well.
3: If the generator is Prime Power rated as opposed to Standby Rated you may consider it to be a 48.4 kW set for purposes of motor starting. That extra 4.4 kW will be a big help.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: What load must be available for motor while starting in Y/D?
S-D starters can trip breakers and hammer the supply unless they're closed transition. Will it be OK if the main generator breaker trips occasionally?
I can see this wreaking havoc with other devices being powered on that totally marginal generator. If that happens you're going to have an angry customer. At sea, is bad place to have marginal anything. Marginal there - can often equal disaster.
I'd suggest a properly sized generator OR use a VFD to run the crane so it can smoothly pickup loads and smoothly bring them to a stop where they're wanted. The VFD would greatly reduce the starting hits on the generator.
Keith Cress
kcress - http://www.flaminsystems.com
RE: What load must be available for motor while starting in Y/D?
RE: What load must be available for motor while starting in Y/D?
But who is going to dig into my calculations?
-Bart
RE: What load must be available for motor while starting in Y/D?
RE: What load must be available for motor while starting in Y/D?
I doubt anyone is going to comment on your calculations. It boarders on pointless. You see the success of an installation or a positive outcome of the system is dependent on a large number of variables we don't have the answers to. Likely you won't either.
If your marginal setup allows the hoist motor to start - and it might - that alone isn't a successful outcome. If all the navigation gear, computers, displays, and control systems all reset every time the crane starts, your installation is still a failure.
Your estimate that the available power might be 9kW(inevitably it will be less) and you want to start a 7kW motor violates our 'experience' pretty badly. We've said you need available generator capacity to be 3X of the motor's normal running current to start the motor.
3 x 7kW >> 9kW
As for your 'calculations' I guess I will comment. They're wrong. You need to multiply the motor kW by 3X. Does the result still come in below 44kW? No.
Put in a bigger generator or use a VFD. Those are the only reasonable alternatives short of a big gamble and/or a very annoyed customer coming back for a bigger generator.
Keith Cress
kcress - http://www.flaminsystems.com
RE: What load must be available for motor while starting in Y/D?
At first I thought it was incorrect because of the Istart = 3* Inom, then I realized you were calculating the starting at Y-Delta. Although this looks good on paper, the reality is, Star-Delta is not what it appears to be, it's kind of a cheat. The INITIAL current is reduced when in star to being 300% of normal, but when it transitions from Star to Delta, there is a second current surge that will in all likelihood still be as high as 600%, sometimes WORSE if the conditions are right (wrong). The only real difference between DOL and Star-Delta starting in my opinion is the length of time that the current is high, being that with S-D the motor is already moving so the second surge is of comparatively short duration. If your generator cannot handle that second hit, you will still see flicker, or worse yet, trip the generator main breaker. For this reason alone, you will rarely see Star-Delta recommended as a starting method behind small generator supplied systems.
If you are fearful of the idea of using electronics such as a VFD or Soft Starter, a Reduced Voltage AutoTransformer type starter would be a better choice. Because of the transformer action, line current is much lower compared to other starting methods. If the motor will start with S-D, it will likely start at 65% taps on the RVAT, it may even start at 50%. It's still a two step process, but if you use what's called a Closed Transition (Korndorfer) starter, you will not have that 2nd high peak. 7.5kW is pretty small for an RVAT starter however, you would likely have to build your own. I would go with a soft starter or VFD rather than go to that trouble.
Take a look at this website from Caterpillar though, it's a good quick way to look at sizing generators for motor starting. It gets more complicated for your situation, but basically if you apply these rules to being for your REMAINING capacity above and beyond the static existing load, you will see that it is not likely that your situation will work. https
By the way, I don't necessarily agree with their assessment of over sizing the generator by 250% for a VFD, but the VFDs need other careful considerations, so I'm thinking they are assuming the worst case here and covering themselves for someone who may apply one without proper engineering.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: What load must be available for motor while starting in Y/D?
RE: What load must be available for motor while starting in Y/D?
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: What load must be available for motor while starting in Y/D?
Thanks for your comprehensive information which real makes sense.
I think the Korndorfer starter may be a good solution in this case.
I agree with others that still some 'factors' may disturb the proper work of the mains o/b the vessel.
Would there be some more info available for Davidbeach solution:
The closed transition version of the S-D starter
Maybe a scheme somewhere what tells more than a hubdred words?
TIA and thanks for your time so far.
-Bart
RE: What load must be available for motor while starting in Y/D?
It may be possible to turn off selected equipment to allow enough capacity to start the crane. Any heating loads are a good starting place.
The KVA of the motor start will probably be at a very low power factor. The power factor of the base load will probably be at a much higher power factor. Due to the different phase angles of the starting current and the base load current the total current and the total KVA will be less than the sum of the parts. This is to your advantage but also complicates the calculations as to whether the set will start.
Take a look at the base loads and see what may be safely turned off for a few seconds to allow the motor to start.
I have found that the Cat programs are quite conservative. I ran the numbers of a few marginal installations that I had been stuck with that involved motor starting. (Residential with large A/C units) All of the installations were working adequately but the Cat software said that all would fail.
The Cat software is good for initial sizing but when you are stuck with a large motor to start on an existing set, it is discouraging.
Based on direct experience starting motors on existing gen-sets with existing base loads, your installation is a little worse than marginal. There are a few factors such as the ratio of the motor size to the base load and the phase angle difference between motor starting and the base load that work in your favor. If the gen-set is prime rated rather than standby rated that is a big factor in your favor.
At the end of the day, the installation is still marginal. I have found that load curtailing is the most dependable and cheapest solution. This may be manual with suitable instructions, semi automatic or fully automatic.
In residential applications I have had good luck with semi-automatic schemes. The selected loads are tripped off automatically and manually restarted.
This is a case where you must make several assumptions in order to calculate the possibility of starting the motor. Then you go out in the field and see which assumptions were erroneous if the motor doesn't start.
You may consider a trip to the vessel and some testing to determine what conditions are.
I don't think that a generator will be happy with either a VFD or a soft start under marginal conditions. Both of these devices have input current waveforms that are challenging to the AVRs of generators. We are back to assumptions as to whether there is sufficient reserve capacity for these devices to function properly.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: What load must be available for motor while starting in Y/D?
The comments about the second current surge during transition were not fully explained. There are actually 2 different causes of current surges during the transition.
The first is caused by closing the power onto the motor when the motor has residual back EMF and is also out of phase with the line power. In this case, you can see a few cycle current that could be 10 to 20 times the rated current. The closed transition starter helps with this.
The second is caused by the motor speeed when the transition occurs. Most typical motors have a high current draw until the motor reaches around 90% to 95% speed and then the current quickly drops off. The motor current will jump back to the full-voltage levels after the transition so the transition needs to be performed after the motor has reached about 95% speed. Otherwise, the motor current can easily reach 400% plus levels after the transition until the motor finishes accelerating to full speed. The motor needs to have a small enough load it accelerates close to synchronous speed before transitioning to avoid this issue.
RE: What load must be available for motor while starting in Y/D?
Is just a remembering of an old engineer:Weak back up system,
motor starting problems,transition peaks, etc and a wound rotor motor getting a starting kVA around 1,5 times running kVA.
Simply the easiest type of motor starting for a weak gen set.
Old memories...