480v 3 phase compared to 2400v 3 phase

[quarryman33]

so, at the quarry I work at in the pit they have a 3 phase pit pump motor that is feed with 2400v. They are asking me to give them the comparisons between a 2400v and 480v motor. I am sure there are many differences but, the main differences are the safety aspect as well as the availability of parts. Can anyone help me out with there opinion? Thanks in advance for responding.

 

[charlie b]

I can't speak to the availability of spare parts, for that depends on the motor manufacturer and the age of the motor. A 2400 volt motor would be more efficient, both from the standpoint of energy used to drive the motor and energy lost in the branch circuit conductors. So it would cost less to operate. Someone else would have to address maintenance costs. A 480 volt motor would be physically much larger for the same horsepower. So too would the conductors that fed the motor. For example, for a 200 HP motor, the 2400 volt version would draw 49 amps, and could be fed with #8 conductors. A 200 HP motor fed at 480 volts would draw 240 amps, and would require conductors of 250 KCMIL.

 

[kingpb]

What size motor?

 

[quarryman33]

350 HP

 

[kingpb]

Well that is a big cable. Probably using a 750KCMIL, and that is not consider any VD due to location of pump from source.

I assume it's not a fixed pump, so you would need armored 3-core cable to move it around.

From a motor standpoint, I would have to say a 2400V (2300V) motor at 350Hp is probably more available than at 480V (460V), due to Hp rating. But that can be easily checked by calling some of your suppliers and see what they stock.

Might want to consider a motor driven pump, and bag the electrical connection. Now you don't need an electrician around to use it.

 

[Jraef]

I can't speak to the availability of spare parts, for that depends on the motor manufacturer and the age of the motor. A 2400 volt motor would be more efficient, both from the standpoint of energy used to drive the motor and energy lost in the branch circuit conductors. So it would cost less to operate. Someone else would have to address maintenance costs. A 480 volt motor would be physically much larger for the same horsepower. So too would the conductors that fed the motor. For example, for a 200 HP motor, the 2400 volt version would draw 49 amps, and could be fed with #8 conductors. A 200 HP motor fed at 480 volts would draw 240 amps, and would require conductors of 250 KCMIL.

Overall circuit efficiency will be lower for a LV motor, but the motor efficiency itself has to do with the motor design, not the voltage per se. So if I have a 460V motor that is 95% efficient*, and a 2300V motor that is 94% efficient*, the LV motor ITSELF will be more efficient. But to charlie b's point, that is not the whole picture.

The voltage for that motor (whatever it may be) will have to come from a transformer at some level. So if the site already has 2300V brought in by the utility and the meter is on the 2300V side of the PoCo HV transformer, then THEY own the associated transformer losses. The user no longer needs another transformer for the motor and does not see added losses. But to run it at 480V, that means adding a 2300-480V transformer of at least 750kVA to feed it (depending on how it is started), and the user WILL see losses in that transformer, likely somewhere in the neighborhood of 2%*. Even if they have a 480V transformer with enough spare capacity to start that 350HP motor, the losses through that transformer will still be there. Plus all of the cable I²R losses too of course, which follows the conductor sizing issue already mentioned.

So in the scenario I proposed above, the combined effects of having lower total circuit losses would then make the OVERALL cost to operate that 2300V version lower.

* Note: the efficiency / losses info I used is based on NEMA standards.

 

[augie47]

Obviously a lot of variables, but if there are any distances involved I think the difference in the cost of supply cables might seal the deal with the 480 volt motor requiring a suplly cable 5X the size of the 2400.