What load must be available for motor while starting in Y/D? 2

[nifrabar]

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

 

[itsmoked]

How big is said motor?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

I’m not aware of any minimum load for wye delta starting. Maybe others will educate me.

=====================================
(2B)+(2B)' ?

 

[waross]

44 kW minus 35 kW = 9 kW.
Best guess 9 kW/2 = 4 or 5 kW motor.

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

 

[electricpete]

Disregard my comment - I misunderstood the question based on title (thought you were looking for minimum mechanical load... minimum excess load capacity of generator makes a lot more sense).

=====================================
(2B)+(2B)' ?

 

[jraef]

If I interpret your question correctly, you have a 35kW motor and a 44kW generator, you are asking if Y/D starting will work? The answer, if that is your question, is that this is not enough information, and it is not really something that can be done haphazardlyover an anonymous internet forum. What you need is a Transient Motor Statrting (TMS) analysis to be performed by someone qualified to ask ALL of the pertinent questions and who knows how to do the complex math, or is in possesion of one of the TMS programs that will do it for you, such as ETAP or SKM Power Tools.

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)
For the best use of Eng-Tips, please click here -> faq731-376

 

[electrical62]

i have problem in hoist vfd control setting can any body help me

 

[jraef]

electrical62, you need to start your own thread and you need to provide all of the available information. Describe the PROBLEM you are having. Don't hijack someone else's thread it gets too confusing.

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

 

[nifrabar]

Dear all,

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

 

[nifrabar]

In addition:
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

 

[waross]

Let's just look at the big numbers.
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

 

[waross]

Let's just look at the big numbers.
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

 

[itsmoked]

I can also see a crane being turned on/off/on/off/on/off/on/off when someone is trying to position a load. Star-delta? That's crazy in that service!

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

 

[potteryshard]

On a hydraulic crane I'd expect the motor to start unloaded, and to run continuously.

 

[nifrabar]

Thanks so far for all your thoughts.
But who is going to dig into my calculations?
-Bart

 

[LionelHutz]

The kW number may make it seem OK but what about that genset supplying 26kVA during the start?

 

[itsmoked]

nifrabar;

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

 

[jraef]

I'll comment on your calcs.

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://caterpillar.lithium.com/t5/...educed-Voltage-Starters-When-Sizing/ba-p/2967

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

 

[davidbeach]

The closed transition version of the S-D starter eliminates the second hit of an open transition S-D without the cost of the transformer of an RVAT starter.

 

[jraef]

Yes, but with the added cost of a 4th contactor and a resistor bank. I never even consider that for larger motors, it's less expensive to go Solid State than it is to go Clodes Transition S-D. In this case however you may have a point. That might not be so bad because it is so small, the resistors will not get too hot and can possibly be put in the box with the starter.

"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