600HP on 600V

[Eclair]

Hi everyone like me

I'd have a 600HP motor to connect on a 600V plant network.

It seems to me a too powerful motor to be connected to such a low voltage. I have thought about a VFD or a soft-start to start that motor.

The available power of the plant is enough for that motor in steady state mode.

Is someone here have encountered such a case? I figure that the starting is the big issue here but is there something else that I really should take care about? Could an autotransformer be adequate?

Thank you

 

[alehman]

In principle should not be a problem operating a 600hp motor on 600V. I've run up to 1000hp on 480V without difficulty (using soft starters). The main concern for selecting the voltage is the length of the feeder. If it is long, sizing for voltage drop may be costly.

If your system is relatively weak (regardless of voltage), reduced voltage staring (autotransformer) or VFD as you mention may be helpful to minimize voltage dip when starting. You should perform a motor starting analysis if there is any question.

 

[LionelHutz]

The motor is not too big for 600VAC. You just pay big $$ for wiring compared to using a higher voltage and lower current. We recently supplied a soft-starter for a 2000hp @ 480VAC = 2400A.

You can determine the starting impact with more details on the motor and load and power system.

 

[jraef]

Wow, 2000HP! The biggest I have ever done is 1500HP at 460V.

I'll chime in strongly on the suggestion of doing a motor starting analysis on anything 500HP and up. Nasty surprises can be very expensive in that realm. If you don't have some system analysis software such as SKM or ETAP, contract with a PE who does even if it is just for that job. Think of it as a form of insurance.

 

[alehman]

Lionel,
I've actually had the opposite experience pricing motors in the 1000 hp range. 480V came in somewhat less than 4160V. I suppose it depends on the mfg. The cost difference could be small in comparison to the feeder.

 

[LionelHutz]

Ya, you have to look at the whole cost of the installation. You also might not want to have a single 4160VAC starter on site when everything else is 600VAC.

One thing to consider with a VFD. It will start the motor without putting undo stress on the network but the installation will likely be around 96% efficient. So, if you just use the VFD to accelerate the motor to full speed then the VFD losses will be like adding a 18kW heater to the plant system. Running on 8 hour shifts that would be around 1000kWh a week or 50000kWh a year. No small amount of hydro - likely $7.5k to $10k of hydro depending on you rates.

The 2000hp motor was on a ship - bow thruster.

 

[jraef]

You also might not want to have a single 4160VAC starter on site when everything else is 600VAC.

Another good point. In most jurisdictions, service people must be specially qualified and/or licensed to be able to work on MV equipment. If that is the only piece of MV equipment you have, it may not be practical to have to have 2 or 3 service people trained and certified just for that.

 

[tin2779]

LionelHutz,

Interesting comment about VFD losses. I am working on 600 V, 250 HP Fan application but I never heard about losses being significant like you explained. Could you kindly elaborate why there are losses in a VFD case and not in a typical soft starter or DOL application.

Thanks

Sorry for interjecting in your question Eclair.

 

[ykee]

tin2779, have a look at the spec sheets, I'm sure they will list the drive efficiency for you. I think the losses are in the conversion of AC to DC and back again. Do a search here, there have been threads about this.

 

[itsmoked]

You always have switching losses when controlling power devices via rapid on/off switching.

A soft starter will have losses too. But because they are switching much differently the losses are far less. Often SS losses are completely stopped by having the system transition to a bypass contactor. So any switching is avoided.

ykee is correct in that there are rectification losses into a VFD and there are switching losses out of the VFD too. Hence the greatest loss of eff.

But as mentioned you could also bypass a VFD to avoid all these losses. You will need a VFD that can 'catch' a load if you want the load controlled on the way down too.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

A VFD has a rectifier stage, a DC buss and then an inverter stage. The rectifier stage devices have a voltage drop (~1V) which is resistive. The inverter devices have a voltage drop (~2V) which is resistive. There is a ripple current in the capacitors which causes losses. You will likely add a line and/or load reactor which has losses. All together, it adds up to about 96% efficiency. Most VFD's have the loss data and it is even often given as watts.

A soft-starter will have losses too. The SCR's have about a 1V drop across them so with 600A the losses are 3 phases x 600A x 1V = 1800W. However, most good soft-starters these days have a built-in bypass which lowers the voltage drop to <200mV.

A typical ATL will have a small voltage drop too, usually <200mV, meaning it will have 300W to 400W of losses.