200-kW Mill Motor on a 250-kVA Genset

"Do soft starters actually limit the startup current?" - yes, see here.

Is your purpose to have a soft start instead of a hard start? What type of motor do you have? There are several kinds of soft start devices available. What is the mill you speak of... is it a ball mill? What does your motor drive directly... a reduction gear, a belt drive... what? What is the inertial mass of the machine you need to accelerate from standstill? What is your present acceleration period from standstill to full operational rpm. 3 seconds... 4 sec? What is your normal full load working amperage? What is your starting acceleration amperage?

One big issue you may possible have is that when factoring in a 200 KW motor running at its full load plus its efficency losses plus the VFD or Starter system losses you may be well past the gen sets working KVA rating.

If your motor was 90% efficient it would need around 220 Kw. If your VFD unit was also around 90% efficient that puts the system at 244 Kw without factoring in any power factor adjustments or service factor head room.

It would probibly work fine up to a point but you may find that you have a working capacity limitation considerably below the motors rated capacity.

Its just a thought based on loose math and numbers.

msamad, I'm not quite the PC type so thank you very much for those thread links,they were satisfyingly informative. By the way, it is a ring roller mill and the motor will draw after a 30-40 sec. startup and then at full milling capacity, +-120-150A. I will post the outcome but I am pretty sure now that a vector drive will suite best. Thanks to all again.

I will not debate wether a VFD is a better solution (#5) but it is certainly NOT the only solution. I recommend to abandon an electrical solution such as VFD or Soft Start in any format and to consider a hydrodynamic drive, commonly known as a fluid drive. These are superior machines for smoothly and almost effortlessly accelerating high inertial loads. This forum is not suitable to go into lengthy technical explanations and descriptions. Consult Rexnord-Falk, Twin Disk, Voith (of Germany) or others. There are quite a few on the market. On the practical side, I have used these drives on numerous occasions, to drive ball and rod mills, to start and run heavily loaded long conveyer belts, to start and run very large blowers etc. In all cases, the performance was flawless. On one occasion we were talked into trying a VFD to start a medium size ball mill and it did not work. We rapidly replaced it with a Twin Disk fluid drive. Even so, the down time was very costly. Later we used that same VFD to run a large liquid vacuum pump and it worked fine. Shows that primary inertial load must be carefully considered. Hope this helps.

Keep in mind that the generator set must have a capacity of at least 2.5 times the capacity of the VFD.

If capacity of the generator is too small, the voltage drop is too big, the capacitors inside the VFD will not get enough voltage and you get a low voltage error.

The RMS value of the generator output voltage will be correct, but it will have a trapezial in stead of a sinus shape. This means the peak voltage will be less the sqrt(2). The capacitors are only loaded as far as the peak of the sinus ….

Rudy, I think that on the programming of the drive parameters, the error messages can be eliminated. WEG CFW090...... YES? NO?

I would recommend starting with no load using VFD to accelerate to full speed, then changeover to generator output at the same frequency, then you will have minimum starting current using the VFD, then sinusoidal motor current of the motor when you start to load.

The soft start is unlikely to be able to start at motor rated current only, even at no load, and if you leave the VFD in circuit when you apply load, the harmonics will overheat the generator windings if you use a standard 6 pulse design. The accelerating current at the VFD input should be quite low relative to genset capability as you have no load, only inertia and you can set the acceleration time as you wish.

Some VFD's have this logic in-built, for example FUJI Electric, furthermore, you only need a VFD for light starting as main duty will then be d-o-l with the motor when under load.

"I would recommend starting with no load ..." - 'fraid you've lost me there.

Starting at no load means that the main process is not operating during starting phase, e.g. press uses the clutch after you reach full speed only; compressor, only start compressing when beyond a minimum speed,; fan; only allow air flow when up to speed etc.

Sorry for any misunderstanding, I work in VFD technology and this is a common term (jargon?) when describing running conditions.

I fully understand the terminology, I just couldn't imagine that this mill would have any way of doing it. I don't think it's common practice to fit a clutch on this kind of electric motor driven machinery. Please correct me if I'm mistaken.

There's no clutch, you introduce the material into the mill to be processed, this then introduces the 'load'.

I explained my rationale in another post so I'm not sure which part isn't clear. The mill 'load' itself will be a significant part of the total load, so starting without it is much easier on the motor and the VFD only draws the active power needed for this phase. Changeover to mains then relieves the harmonic burden the VFD presents to the generator windings when adding the 'load'.

Come on John, just common sense and experience, surely?

Please see #5 & #15

But Malcolm, I DO need the gen to start as its the only power source available.

Leonf, please give us some more information:

1) Is it possible to start up the generator without any electrical load or with only a very small electrical load compared with the current needed by the motor? ( You need a kind of switch between motor and generator )

2) Is it possible to disconnect the mechanical load from the motor output shaft by means of a clutch, a coupling or what ever mechanically possibility their is.

3) Is it possible to reconnect the mechanical load to the motor if the motor is already turning at full speed ? Or is it important that the mechanical load accelerates slowly ?

4) Is it possible to start the motor with its mechanical load already connected to the motor output shaft, but without the power, torque or energy needed to do the proces that the motor is intended for ?

Hi Rudy, 1-yes, 2-no, 3-no the inertial load would be there from the beginning and once product is fed the process power requirement will only be slightly higher

For those who apparently don't understand, a "comminution mill", i.e. a Ball Mill, is a large drum with a load of steel balls in it. The balls roll around and are lifted slightly by ridge bars so they fall on the material being processed. You have to accelerate the drum and balls to full speed, then you start to introduce the "load" material to be comminuted (ground to a fine powder). The difference between "loaded" and "unloaded" is small because the grinding media, the balls in this case, are always there. You theoretically can remove the balls, but not practically; they weigh tons in some cases. So the concept of starting unloaded is somewhat moot as even if it is always started without the process material (also somewhat impractical), there is still a lot of weight in the drum.

Accelerating with a VFD and switching to DOL would be VERY dangerous in this case as the load (and thus the motor) would slow down very quickly. When reconnected DOL, the motor would need to be off-line for at least a second or so until the filed collapsed or a severe voltage spike would occur. In that time, the slower motor will require acceleration torque again and the high slip will create a large current spike.

Use a VFD, leave it on the VFD full time, buy a VFD that has harmonics under control if you are concerned about it, i.e. 12 or 18 pulse.

Alexander M. Berlin has some very good suggestions to limit damage that can result, usually in maintenance repair, from DOL (Direct On Line) start-up. This creates huge stresses and straining of everything that is mechanical. VFD or [Triple VF's] work well in applications they are suited for but as previously commented, there are enormous amounts of mass that has to be rotated and electrically, there has to be an onerous over power supply for the system to work and be started under a fully loaded situation which is not uncommon in these types of applications. Fluid drives such as the Phelps Dodge CST[Controlled Start Technology] allows an electric motor to start in DOL, but this is only for the motor to get to full running state, in about a second (Draws about 400% of the power spike) for >1 second and settles to no-load running current, the oil fluid shear between the clutch(Brake) plates then allows the full torque load to be transmitted to the output and start the mill. This gentle starting has proven its worth in long conveyors >20kms length and reduces the damaging forces on the belting and structural to minimal figures. It is also used on autogen, rod and ball mills for decades. In the Philippines, changing to CSTs reduced the chain and sprocket or open gear drive torque applications and other [rod and ball] mills mechanical parts that used to be damaged by OEM designed DOL starts to minimal maintenance and costs were also reduced dramatically. The power requirement was reduced from the 60% over power requirements necessary for general fully loaded starts and was reduced to the normal fully loaded running parameters. One of the 4 diesel Merlees powered generators was de-commissioned and sent to another location.

The application you have mentioned would not be a workable situation from my experience.

Hope this helps.