electricity cost of single vs three phase

[petroremedy]

I am designing a groundwater remediation system in a town that does not have three phase power. The closest three phase line is 0.5 miles away. Because the system would not be there for more than about 5 years, the power company wants about $15k to install three phase to the site. I have always used three phase in the past because the motors have to be XP (Class I Div I area) and there are usually a couple motors above 5 hp. The total horsepower of the system will probably be about 25 horsepower. If it was a three phase system, there would be one 15 hp blower motor, one 7.5 hp blower motor, and a couple 1 to 2 hp transfer pump motors.

I'm trying to determine whether to use phase converter/VFD, limit motor size to 5 hp (requiring multiple blower units), or pay to get three phase. Some people have told me to get the three phase because the motors will last longer, especially with frequent start/stops. I am also trying to evaluate the electrical cost of operation of the system in single phase versus three phase. Is it more expensive to operate the same sized equipment single phase?

Thanks in advance.

 

[DickDV]

One potential problem that often comes up with the use of VFD's for phase conversion is that the start/stop control of the motor must be on the VFD. A VFD will fault on overcurrent every time if a stopped motor is contactor-connected to the drive when the drive is up to speed.

This pretty much precludes running multiple motors on the VFD too, if the motors are to be started and stopped at different times. In that case, an inverter for each motor would be required.

And, yes, single phase motors will always be a few % less efficient that equivalent three phase motors. But, then, an inverter can be expected to waste about 2% of load energy as heat also.

Makes a tough choice!

 

[hoy7t]

First topic
Consider a motor-generator set. If you can find a single phase driver at that capacity (doubtful) then it can drive a three phase generator.

Second topic:
ANYTIME you use a VFD you should use shielded cable. ground the shield on both ends. (Ground loops are a worry only in instrumentation). You should also follow the manufacturer's recommendation on filtering or reactors.