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700kw Motor Starting On Dg Set


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I am dealing with a vendor for Water Treatment plant. He has to supply complete equipments for Treatment plant. Actual load requirement is around 1000KW. Among this, 350KW @ 415Volt level is for aux equipments which are having small capacities of motors –ie- less than 45 to 55 KW and only one motor, for water pump, is having 700KW capacity @ 6.6KV level. MV starter and a step down transformer for LV motors are also included. He has selected this 6.6KV for this particular motor only because of higher rating. I want to know what is the problem in having this 700KW motor @ 415Volt level.


Now, I have to arrange a DG set power for the operation or treatment plant since there is no state electricity power. While discussing with a DG vendor he prefer to have 3 nos. of 415Volts DG of around 630KVA each with paralleling option and a common transformer of 3000KVA capacity for 415/6.6KV conversion to feed power to my MV board.


My worry is whether these DG sets can handle the starting load of 700KW motor (Water pump) or not without any voltage dip or any damage to DG sets over a period of time.


I am just awaiting the 700KW motor starting details from the vendor.


Forum’s suggestions in this regard will be very much help full






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Generally, it is a common practice to use MV on motors over 500HP (roughly 670kW) because of the starting current requirements and the general issues of connecting that much load current, i.e. cable sizes, voltage drop etc. Starting current on a 415V 700kW motor if DOL is going to be approx. 7000A! Even with a soft starter or autotransformer it is still a lot of current for a genset to handle. Even after it is running, 1100 FLA is going to take a lot of cable to transmit, and is also very vulnerable to voltage drop.


In your case you have 2 additional issues to contend with; availability of MV generation, or energy costs.


If you try to find MV generators and you cannot contract with a readily available source in your area, the consequences could be dire if you ever have trouble with it. The good thing about this however is that you can most likely do it with one generator (ignoring the issue of back up redundancy for now), and I mention this because I have not had good experiences trying to start large single motors on paralleled generators. Slight differences in voltage regulation can have big consequences in starting.


On the other hand, your generator supplier's suggestion of using transformers to go up for the largest amount of your load is going to cost you in energy efficiency in the long run. There will be losses in the transformer which will directly affect your generation costs. If you go with an MV generator and a LV transformer for the remainder of the plant you have losses too, but the remainder of the plant is 1/2 the size of that big motor.

"He's not dead, he's just pinin' for the fjords!"
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Hey jraef,


I know you do things different over there but

500HP (roughly 670kW)
Don't tell me that your HP are different from ours, just like your gallons??

500HP = 373KW over here!


I agree however, that in many regions, once the motors get beyond a certain size, they are operated from MV rather than LV.




The major reason is to reduce the copper loss in the feeders to the motors and also reduce the minimum transformer size required. If the motor was LV, the transformer would need to be large enough to supply the start current without a major voltage drop.


With generator systems, there are two major components and a number of aux components to consider.

The engine(s) supply KW and must supply sufficient KW to start and run the equipment. This includes overcoming the losses in the system. When you start a motor, there is a high loss in the motor, plus there is a high loss in the cable. If the cable is size for a 5% voltage loss during run, then under DOL starting, there could be as much as 30% voltage loss due to the high start current. This loss must be taken into account when sizing the engine(s).

The alternator(s) supply the current and must supply sufficient current to start and run the motors and equipment.

Have a look at http://www.lmphotonics.com/genset.htm

I would expect that the generator(s) will need to supply in the order of 5200 amps at 415 V to start the 700KW motor (assuming that a soft starter is used). This is approx 3588KVA. If we add this to approx 700KVA (estimated) for the rest of the load, then we need a short term KVA capacity of 4288KVA.

If the alternators are fitted with three phase averaging AVRs, and PMG excitation, you can usually achieve a short term overload capacity of 300%. In this case, the minimum alternator rating is in the order of 1430 KVA. This could easily be handled by 3 x 650KVAgensets provided that they have the short term overload capacity.

Self excited alternators typically have a short term overload capacity of only 120% and this would need more than three x 650KVA alternators.

Engine sizing is based on the KW demand during start of the main machine plus the losses at the overload current plus the remaining load from the other equipment. If the figures are known, this can be calculated in a similar manner to the KVA calculations.


Best regards,

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630 kva X 3nos for 700 kw is undersized...i would go for 1250 kva X 3.


If the 700kw motor is not a frequent start stop motor then....use all three for startup and then shut down..other 1 or 2 depending on the load.


Load monitoring module will help in cutting off the gens.


Having a undersized power supply ( capacity) is much painful than having a powersupply with some meat for the future expansions....( and water treatment plants are known to have multiple expansions..)


Good luck,








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Hello chaterpilar


630 kva X 3nos for 700 kw is undersized

Not necessarily. It depends on the short term overload margin of the gensets. Standard botom end gensets typically have an overload margin of 20% (120% load) and in that case, I would agree, however with pmg excitation, three phase averaging AVR, turboboost etc, it is possible to achieve 200% short term overload margin (300% load) and in that case 3 x 630KVA may be sufficient.


Best regards,

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Thanks for all for your technically justified comments and solutions..


Now somewhat I can manage this issue and I want some technical suggestions on the following..


What will be the impact on the 3000KVA 415/6.6KV transformer and the alternator while handling the starting current of the 700KW motor? and what is the capacity of the transformer I should select.??


If at all any problem what is the remedial thing I have to undergo??



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Hello Pragathees...


The 415/6.6KV transformer will in effect add an impedance in series with the supply. This will cause the voltage on the 6.6KV to droop during start.

Typically, the transformer will have a rated impedance at full load of around 5%. This impedance is both resistive and inductive. What this means is that at the transformer rated output, the voltage will droop by 5% (or whatever the impedance of the transformer is) If you draw twice the rating of the transformer, the output will droop by twice the value, i.e. 10%

If, in this case, we assume that the transformer has an impedance of 4.7%, and there is a reduced voltage starter used enabling the motor to start at say 4300KVA, then the voltage drop on the output of the transformer (due to the transformer only) would be 4.7 x 4300 / 3000 = 6.74%. If you DOL start the motor, the start current will be higher and so the voltage droop will be higher.

I would suggest that as there will be nothing else connected to the 6.6KV output, you could get away with a much smaller transformer.


During start, the alternators will possibly be driven into a short term overload, depending on the size selected. This will possibly introduce some voltage droop depending on the current draw and the excitation.

NB if you use DOL starting, the AVRs and governors will take time to respond and there could be a change in frequency and voltage while they recover, assuming that they are operating within acceptable overload margins. Using a soft starter on the large motor allows for the current to be gently applied to the gensets and they can compensate as it is applied. This can allow the gensets to be more marginally sized without voltage and frequency droops.


During start, there is a high current draw. This will result in additional I2R losses in the alternator, transformer, cables and motor. This needs to be taken into account as this is a significant KW loading on the engines. Reducing the start current from 600% to 400% will reduce this loading by over 50%. Another good reason to use a soft starter!

If we assume that the "coper" loss of the motor is around 2% at full load, then during start (DOL at 600%) the copper loss will be 36 x 2% = 72%. .72 x 700 / .95 / .95 = 558KW (assume pf = 0.95 and eff - 95%)

If we now drop the start current to 400% using a soft starter, the copper loss in the motor = 248KW

The same applies to the alternator and the transformer. We could reduce the starting KW demand on the engines by 900KW by reducing the start current.


Hope this answers some of your questions.


Best regards,

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