What's the application? I think though, that 11kV and 'only' 2000kW is not a good combination for pricing from the MV drives players. Also, they will need to know if the drive is to have 12, 18 etc pulse retification or active front end. Environmental conditions - altitude etc. are required.
Hi Muthu
Is the VFD to connect to an existing motor or is it a new application?
If new, then MV VFD suppliers would rather match the motor with the drive whereas a retrofit would typically require quite a high degree of analysis of the motor to determine suitablility.
However, the questions drivesrock asks need answering first otherwise a ballpart figure could be 100% out.
The application is a direct driven air compressor. The user has chosen 11 KV supply from the utility and hence such a high voltage for 2000 KW (actually the motor is only 1850 KW, I added another 150 kw for cushion). The ambient temp is around 40 deg C max and altitude is 920 meters.
The problem is starting this motor. The compressor supplier supplied this motor with a soft starter as a package. The user's sanctioned demand is only 2500 KVA. Even with the SS, the in-rush current is around 500 A and the utility has set its O/L trip at 200 amps. The utility is not agreeing to increase the time delay to accommodate the inrush current.
So, the VFD has to start the motor with low uniform current till the full speed and then it has to be bypassed. So, the VFD doesn't need all the bells and whistles.
Any idea how much such a VFD will cost ? Or do you think it's a stupid idea to spend money such a starting scheme ?
The only other option is to for a pony motor but the main motor does not have an shaft extension on the ODE.
The major concern is if the existing motor is suitable to be fed off a VSD to begin with.
There's the problems of insulation and increased temperature rise, high dv/dt values damaging insulation and causing PD, and bearing currents causing discharge machining.
Retrofitting the existing motor may be more expensive than scrapping it all together.
At least from my experience, 2MW MV drive is typically around 200K US, plus phase shift transformer (50K US) if it has diode rectifier config.
The last 12.47kV drive I did was in 2005, about $165K USD, 18 pulse clean power, Robicon which means it includes the transformer.
Often for low current starting like this at that voltage, an LCI drive is considered as a form of "Soft Start" because it is relatively simple to bypass at full speed. 2000kW may still be a little small for that though.
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Just throwing out another idea - how about capacitor starting? Should in theory be able to reduce current drawn from utility without the adverse i^2*t discussed above for this particular motor. Maybe others can comment.
(disclaimer - I have never been involved with selection or installation of any of these things - reduced voltage start, cap start, fluid coupling... all of our large motors are DOL start)
I am verging towards a high-torque pony motor (11 KV, 600 KW, 36 Amps, 3000 RPM) coupled at the ODE of the main motor. (This would of course involve replacing the shaft and an additional motor bed, which is not a biggie). The pony motor LRA would be about 5 times i.e. 180 A, which would not trip the utility O/L relay, which is set at 200 Amps
I am also planning to have a 500 KVAR, 30 A, 11 KV PFC capacitor bank for the main 1850 KW motor. This PFC cap would be energized along with the pony motor, which should reduce the inrush current further to (180 – 30) = 150 A. This PFC cap will stay energized in the circuit as long as the main motor is running. When the pony motor reaches the full speed, the main motor will be energized (possibly with an inrush of very short period) and the pony motor will be de-energized.
Sequence of starting
1. Energize the pony motor along with the PFC cap with an inrush of 150 Amps app.
2. When the pony motor reaches the full speed, energize the main motor.
3. De-energize only the pony motor with PFC cap running in parallel with the main motor.
4. During this starting process, the compressor will be fully unloaded.
A 600kW pony motor on a 2000HP main motor seems pretty big - is there a large load presented by the unloaded compressor? I would have expected something more like 150kW or so if the main motor was running light.
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If we learn from our mistakes I'm getting a great education!
Maybe I'm being a bit dull here but if a pony motor at 600kW is able to start the unloaded compressor with no issues, why not just use the 2MW motor on the unloaded compressor and then bring the load on once started?
I'll make my excuses now as it's been a long day...
The main compressor motor (1850 KW) LRA is around 600 Amps, which is not acceptable to the utility and hence a pony motor, whose LRA, is only around 180 Amps. That 600 KW pony motor is the maximum possible cpaciity I could arrive at keeping the maximum starting current under 200 amps. The actual pony motor needed could be much lower KW.
I am waiting for the torque-speed curve of the compressor.
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I have the motor moment of inertia (64 kgm^2) and the compressor compressor moment of inertia (48.7 kgm^2). Is that enough for calculating the pony motor capacity ?
Yes, pete. It's the same motor we are discussing here. That soft-start was designed for the 1850 KW motor, which is now not working due to high in-rush current. Either VFD or a pony motor seem to be the only solutions left.
