750HP, 460VAC motor starting 5

[nightfox1925]

In the conceptual project I am working on, the power system I am into involves a lot of motors and the distribution voltage is 480VAC, 60Hz and we are designing the system to be catered by an in-plant steam turbine generator. The plant maximum demand is only 3MW and we are employing 3 x 1.5MW STGs + 500kW Diesel Generator for plant black starting. There is one "hump on the road" for me, the cane shredder motor is rated at 750HP at 460VAC which may pull off everything once it is started.

I am thinking of employing reduced voltage starting like Y-Delta or an auto-tranformer starting method.

Are there any other consideration I have look into before I employ this type of starting method?

I don't find a cost effective measure to employ a 4.16KV plant distribution because of this one big motor problem, because my switchgear, MCC cost will increase and employing individual step down transformer to nine locations will add up to the cost as well.

Our client has limited budget so I have to consider that but of course without compromising safe and reliable design as possible.

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[rbulsara]

Depending upon where you are located, I would personally prefer solid state reduced voltage starter. In this day and age the types you mentioned are fast becoming obsolete.

If support for electronic starters is not good in your area, I would prefer autotrasformers over Y-delta any day for this size motor.

 

[nightfox1925]

I am located in the Philippines.

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[jraef]

Condensed version:

Y-Delta; don't bother. Y-Delta is a source for more problems than you will solve, unless maybe you use closed transition Y-Delta, and in a controller that large, you are going to spend more for that version than Solid State anyway. it also limits your motor choices because you can ONLY use motors that are wound in such a way as to allow Y-Delta starting. BVottom line, never worth the trouble in my opinion.

Reduced Voltage AutoTransformer (RVAT); good only in that because of the transformer action in the circuit, the line current draw is the lowest per unit of torque of any of your choices. The downside is that they are not very flexible, in that they cannot be adjusted beyond the (typical) 3 tap points of the transformer at 50%, 65% and 80% voltage. If the motor needs 70% to avoid stalling, you must jump to 80% and that may not reduce your starting current enough. Conversely, if 65% is too much and you have to drop to 50% to avoid the voltage drop, the acceleration may take longer than the transformer thermal cutouts will allow, and then you have a big expensive controller that cannot be used. In addition, at 750HP it is going to be the largest, heaviest (that is a huge transformer) and has a lot of wear parts. It will also have a duty cycle capacity well below that of the motor it is attached to, maybe 2 starts every couple of hours, because the transformer will get hot and will have a thermal cutout. Lastly it is going to be the most expensive option, not just in the installed cost and plant floor space, but also because they are made of a number of very large and expensive components.

Reduced Voltage Solid State (RVSS) starters will be less complex, less moving parts, less expensive, smaller and much more flexible in terms of getting it set up to match your starting needs exactly. they are also usually designed to match the duty cycle capability of the motor as well. A bonus is that most name-brand solid state starters will come with built-in advanced motor protection features now, something that may cost thousands of dollars if added to an RVAT starter. The only downsides are that with solid state, the motor amps will equal the line amps, so your amps-per-unit-of-torque will be higher than the RVAT. There is also a "fear factor" to overcome if your maintenance people are afraid of things that they don't understand. A good local supplier with a training / service organization can go a long way to helping put that issue aside though.

Good luck with that.

 

[nightfox1925]

Thanks jraef for the very concise recommendation. I will look for a local vendor and coordinate with them. By the way, I have another interesting thread regarding cable sizing using NEC: thread238-190115.

Any further suggestions are all appreciated. God bless.

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[bigamp]

Nightfox1925,

Your biggest problem is that the system is weak. You can get excessive voltage dip whilst starting and quite often a voltage spike when the motor is shut down.

In an ideal world you would be able to start your motor unloaded. This would allow setting the soft starter to a low current limit plus it could have a gentle ramp to the current limit so the generator AVRs could keep up and the voltage dip would be low. However, your application is a shredder so quite possibly it may sometimes need to be able to start whilst loaded. This means that you can not reduce the starting current too much because the resulting reduced torque that the motor can produce may not be sufficient to run up the shredder. If this is the case, a weak system like yours is going to struggle and you may need to look at some other things....

I'm assuming that the motor is intended to be a cage induction type. Consider instead having the motor as a wound rotor type. Such motors have a rotor resistance starter and by controlling the external rotor resistance you control the motor torque and current. The resistance starts off high and is ramped downwards as the motor speeds up. You can start with a current that is little more than full load and have masses of torque available. These types of motors are ideal for weak systems and do not require a soft starter.

You may be able use a cage induction type motor and soft starter but it may be desirable to oversize the generators compared to the prime movers so that they have a better reactive power capability (less voltage dip). Also, make sure the generators are specified to have a fast acting AVR/excitation system. The motor itself would still need to be carefully selected so that it had low starting current and high torque (and still soft started).

 

[nightfox1925]

Thanks bigamp for the flag. What about if I will have a switched capacitor at the motor terminals. The Capacitor/s will switch on during starting via contactor. Is this a good option as well?

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[waross]

In theory, capacitors for staring a large motor is a good idea and as a teaching example it will give students a bit of a workout.
However in real life it is not as good as it looks on papaer.
I have an old text book that proposes using capacitors to reduce the starting current draw of an induction motor.
In theory capacitors may be used to supply the KVARs that a motor draws when starting.
In practice, it may take 6 or 8 times as much capacity to supply the starting KVARs as it does to supply the running KVARs. If this much capacity is inadvertently left on the line when not needed it may well cause over voltages and generator stability issues.
The capacitors should be cut out in steps as the motor accelerates.
A failure in the control system may leave too much capacity connected to the line after the motor has started.
The cost of the capacitors and controls may be more than other types of reduced current starting.
In summary, a large capacitor bank will educe starting current seen by the supply system however the cost of installation is high and there is a risk of overvoltages and instability on the system if the controls fail to properly remove the capacitors from the line.
respectfully

 

[nightfox1925]

Thanks waross, I will keep that in mind. I guess I am left with the options of using a solid state starter or a wound rotor motor or increase the motor voltage to 4160V.

One thing more I was looking into is to use a captive motor with a step up transformer 480/4160VAC. However, during starting, the same proportion of current will be drawn by the motor on the 480VAC primary side, right?

Thanks for the advises guys!

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[jraef]

Actually a little more because of losses in the transformer.

Another option may be to use a "pony motor" to spin the load up to a lower speed with a smaller motor before energizing the Soft Starter or RVAT on the main motor. The arrangement is mechanically more complicated, but it serves to cut down on the acceleration time. Don't be mislead by those who say that it reduces the starting current, it does not really, but by reducing the acceleration time it does permit some leeway in how you do start the main motor, i.e. lower current limit for longer, but without tripping on overload because a lot of the starting energy was absorbed by the pony motor.

 

[waross]

What are the load characteristics of the shredder? Can it be cleared at the end of each use and then started up with no load?
I am thinking of a hog or chipper at a lumber mill. Both machines are typically run empty before being stopped. Product (waste wood) is not introduced to either until it is up to speed. The only load is bearing friction, belt friction and windage. Such a machine may be accelerated up to running speed with quite a small motor. Because of the high inertia of these machines, the small motor must either be fed from a Variable Frequency Drive, or a horsepower rated magnetic slip clutch must be used. jraef, for a pony motor, is it possible to drive a 230 volt rated motor to double speed and voltage with a 480 volt fed VFD? This may further reduce the size of the pony motor.
respectfully

 

[nightfox1925]

The system is under study based from the proposal of the ethanol plant vendor's technical bid. We are simulating their proposed power system inorder to verify the integrity of their proposed system, check for any concerns, find ways to reduce the cost of equipment and offer localized parts or equipment.

I tried to simulate the new plant system using the proposed 480VAC distribution, with 3 x 1.5MW steam turbine generators rated at 480VAC, 60Hz in ETAP and the 750HP motor is pulling of 0.6MW and 1.1MVAR during starting. Will it be possible to say obtain generators with MVAR limits and capability sufficient to accomodate starting of the highest rated motor of the plant?

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[bigamp]

Nightfox1925, just to answer your question, I am dubious of having capacitance in circuit for motor starting and concur with waross.

I believe that for your application if you went for an MV motor - captive transformer combination you would draw somewhat less current during starting because the transformer impedance would act as a sort of reactance starter. There would be some impedance voltage drop across the transformer and this would reduce the voltage available at the motor hence starting current would go down. Certainly during steady state operation you would draw a bit more current from your system due to transformer losses. I don't think an MV motor - captive transformer combination would work for you.

If you don't have to start loaded then you may be OK with a soft starter and cage induction motor but you would need to do some analysis to confirm this. If the shredder gets shut down whilst loaded can it be cleaned out prior to being restarted?

I don't know how ETAP is modelling the 750HP motor (560kW)but 0.6MW and 1.1MVAr sounds way out for a DOL start. Perhaps it is modelling a reduced voltage start? For DOL start, assuming full load current of about 780A and locked rotor current of 600% the current drawn from an infinite bus would be about 4700A which equates to 3.9MVA, most of which would be reactive.

 

[Rodmcm]

I would definitely be looking at the pony motor option on such a week system.We used this on refiner with a 2MVA 11kV motor on a weak system. The drive can be connected to the mill side of the crusher rather than the motor. This will be cheaper. A soft start or better still a VSD will control the ramp up. Inrush as noted above is almost the same but the duration cures the voltage drop problem.
Alternatively put two motors on the mill, one on each side. We have done this on a chipper, with one soft starter and on DOL on the second motor.

 

[LionelHutz]

If this shredder is not loaded, ie a spinning weight, during the start then I would say it's very possible to start that motor with one 1.5MW generator using a good softstarter.

However, without knowing more details on the motor and load there's no guarantees.

 

[nightfox1925]

Thank you gentlemen for all your technical views. I will take this up seriously with our plant equipment vendor to ensure that what they are offering in their bid is something feasible and require hem to submit more detailed data of their proposed system.

On the other hand, I will see two options: Maintain the 480VAC distribution and handle the 750HP motor with special starting equipment....or make all the STGs rated at 4160V with a 4160V paralleling switchgear connected to a double ended 480VAC switchgear with NO bus tie breaker. I also predict that the fault level contribution at 480VAC will be enormous that will make my switchgears and MCCs very expensive (assuming a provision for a utility interconnection later on will be realized to export/import power).

Best regards

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