Best VFD to control 20 parallel motors ?

[JesperMP]

Dan Bentler, I dont think you have read all that I write.
I repeat this one:
How is it that many small motors looks like a dead short, in relative to a single motor with the same FLA as the total of all the small motors FLA. ?

Using overload capacity I think is bad business.

I am going by the manufacturers recommendations, and it specifically describe how going above the drives nominal power (for constant operation) is acceptable depending on the load cycle (which in my case is a long pause, then run for a short time).
edit: "overload capacity" is a bad description of what it is. It is the same as for the motor. Motors also have a nominal capacity (thats what FLA refers to), but if the load profile varies it becomes more complicated. There is no law that you cant go above the nominal constant load capacity for a short period. Every time you start a motor by direct start you go way above FLA. Frequent direct starts may therefore be a problem, but a certain number of direct starts per hour is OK.

Has it been mentioned that with parallel motors and/or long cables, an output filter should be mandatory ?
REPLY with longer cable from what I have read more than likely. Multiple motors I am not sure. Good question for Dick DV or the factory.

It was not a question. I said that it is mandatory.

 

[Skyba]

Has it been mentioned that with parallel motors and/or long cables, an output filter should be mandatory ?.

How long is long ? My VFD->motor cables would be from 5 to 10m.

 

[balash]

How long is long ? My VFD->motor cables would be from 5 to 10m.

I think that manual reffers to 150 but i don't know is it ft. or m. Anyway the border is much longer than you need.

 

[JesperMP]

But for 20 parallel motors, you should use an output filter.

 

[Skyba]

But for 20 parallel motors, you should use an output filter.

Does that really have to be ? I mean such a thing costs around $1000...

We'll use the motors in the most simple way ever. Normally there will not be any change in motor speed. And if there is, the change is slow, not sudden.

I suppost that depends on the type of cabling, too ?

90% of the time we'll use less than 15 of the motors. We are going to try a system with 6 motors in one month. Don' t you think we could try without filter first, and then add one if we notice the necessity ?

 

[balash]

The thing with vfd is not the speed they work yet the way they work. Vfd chop voltage so you have +/- 585 volts dc with different switching time. Switching frequency is very high, cca. 3-4 khz so that create harmonics, and very short voltage pulses. Filters are there to lower the influence of higher harmonics and to limit the voltage change du/dt i.e. limit voltage peaks.

 

[Skyba]

Don't get me wrong, it's not that I don't trust you, I am just trying to get some weighted opinion before the investement.

Because my cables are obviously not so long and I will not have any sudden voltage shifts during the operation. And when I google "output filter for parallel motors", oddly this thread comes up first, with no other significant result...

 

[balash]

You will have sudden voltage shifts as all vfds have. Output of vfd consist of six igbt work in 3 pairs. So you have rectified voltage that is on/off in very short times. If the speed is nominal pulses are longer but they are still there. That's why motors should be named inverter rated if intended to work with vfd.
Now for cables. Length raises up voltage peak and that is why you use filters. For your worst case you would have to sum all cables between motors to cable lenght total. Maybe thatis on whatothers reffer to. Don't know for sure... Will see what will they say...

 

[DickDV]

Twenty motors each with 5 meters of lead length adds up to 100 meters. This is in the range of "long" motor leads especially in the lower motor hp ranges. I'd add a 3% motor lead reactor.

The reactor has the additional benefit of limiting the starting impedance and starting current. You could partially pay for the reactor by not oversizing the drive quite so much.