starting a 700 kw with a VFD of 180 kw

[gaux]

hi, thanks for your always good ideas, now i just thinking on a new project on my table, that´s a motor that moves a propeller, that start on pitch 0, and i have calculated the mechanical losses.. and they are 116 kw´on 1600 rpm, the idea is to run the motor without load till 1600 with VFD (with VFD rated to 150 or 180 kw), (motor is 700 kw and four poles) and then, disconnect VFD and after a litle delay, something like 0,5 s, connect on direct connection the power supply to the motor, 400 V, but my question is What do you think about idea? i think the current will be really low, and is it possible to calculate the time for accelerating..... if we have a load if we have a load that starts with 0 and changes with the formula
181+ (0,0002*n^2) where n is speed rpm
thanks for your time.....

 

[itsmoked]

This to use cheaper VFDs?

Can you keep the props in neutral up to speed?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[gaux]

yes, i could keep propulsion on 0 pitch till i have the full speed,

 

[DickDV]

You have to magnetize the motor before you can produce any torque in the shaft. Magnetizing amps at full voltage and frequency run between 25 and 30% of FLA.

In view of that, you will be able to start the motor and accelerate very slowly up to some speed but it is going to be very questionable whether you can get to full speed before you run out of drive amps.

One way to get some useful numbers to work with would be to set the pitch to zero and run the motor up to full speed using some other power source, maybe across the line. Measure the no-load amps at full speed and no-load. You will need a drive with at least that output amp capacity to do what you are planning.

 

[LionelHutz]

The drive sounds too small. Don't forget you have to support the full kVA into the motor, not just the real power. A 700kW motor will have a significant kVA draw with no load compared to a 116kW load.

This sounds like a bow thruster. The voltage is probably 440VAC so I'll assume that for the calculation. 700kW = 939hp =~ 1125A. I would estimate the motor will draw approximately 30% current at idle with no load = 337A.

This 337A is about 257kVA at likely around 0.3 power factor which would be around 77kW and 245kVAR. Now add your real power requirement to get 193kW and 245kVAR.

So, the drive has to support about 312kVA or about 410A.

Hope with that data you can figure it out for your specific case. Hopefully, you have more motor data to help too.

I would accelerate the motor to just above synchronous speed. Not sure where you get 1600rpm because that would be for something like a 54hz supply. Anyways, go just above the line frequency with the drive and then disconnect the motor. Use a synchronizing relay to energize the line contactor when the motor and source are in phase. The transfer should be quick and low transient.

 

[CJCPE]

The reactive kVA requirement of the motor determines the current carried by the anti-parallel diodes and bus capacitor ripple current in the VFD. Those components are usually sized assuming that the motor power rating is equivalent to the nominal power rating of the VFD. That should be taken into consideration when connecting a VFD to a motor with a power rating that is much larger than the nominal power rating of the VFD. Advice from the VFD manufacturer is probably required.

 

[rbulsara]

Just buy a proper reduced voltage starter. Why do you think a 700kW motor (and its associated load) is something to toy around with? or may be the application is not important, business wise?

Take pride in doing business the right way and not rig something up just because you know something you can toy with.

 

[gaux]

yes, thanks a lot, i´m just a mechanical and i don´t know so much about this questions...for this i appreciate your opinions
thanks

 

[mccla5214]

The way your proposing will result in a hugh stress on the mechanical system when you go across line because you cannot syncronize the phase angle between the motor and the untility.

Use a soft start to ramp up to full speed and then the bypass contactor will pull in and the soft start will drop off line. this results in a no stress transfer to full load.

 

[Skokian]

You have to be very careful to connect the motor in phase with the line voltage, or close to it to avoid a large switching transient due to the motor back EMF which will be around 85% of the line voltage if the motor is up to speed.

For starting an unloaded motor, you may be better off with a soft starter, if it's of the type which can be paralleled with a contactor once the motor is up to speed. Also, the soft starter would have to be of the current limiting type, and the manufacture should be contacted to make sure the soft starter won't fault out on the reduced motor impedance.

Hope this helps.

Regards . . .

Jim S. Nasby