Wire size question / VFD

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krussell@scs-usa.com

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OK,

I have a 3 phase 10 HP motor.

I have a facility with only single phase

They are using a 15kW VFD with the blocking diodes and the magic stuff to run a 220 volt 3 phase motor on VFD with 220 V single phase on the line side.

Do I size off the 15kW? or the 10 HP without the 3 phase power facor? or the 10HP withthe 3 phase power factor? or is there another calculation?

Thanks
 
My understanding is that conductor size needs to be according to the full load amps of the connected load, plus 25% of the largest single connected load.

The VFD will pull more amps than the actual motor due to miscellaneous losses. However, the current draw by the VFD will be proportional to the motor connected. It is difficult to say exactly how much current the VFD WILL carry, but we can say for sure how much it CAN carry, based on the drive's nameplate. Personally, I would size according to 15 kW @ 220VAC and check with another engineer to see what his thoughts are.

This type of setup will also create harmonics which can affect other circuits and reduce life span on the motors if the bearings and windings are not rated for vector duty. Personally, I would use input and output filters to minimize impacts. Here is an article that describes harmonics:

http://www.plantengineering.com/sin...systems/195ae63d6e6e0d6d87421de194bd2c88.html

What is this motor being used to control? A fan I presume? I can't imagine what type of facility would need drives and not have 3-phase power available.

Food for thought: A drive must be derated to operate a three phase load with single phase power. See the Schneider Altivar 71 Single-phase series for more details.
 
VFD is powering a small Lathe, the 10 HP is the only load, and that would be 7.5kw ish (if my calcs are right)

The drive supplier selected the oversize drive due to the phase blocking.

I will read up on the harmonics, Thanks.
 
No. Article 430.122 states that conductors to FEED a VFD must be sized at 125% of the rated INPUT current of the VFD, not the motor connected to it. This is because VFDs can be programmed to protect motors that are much much smaller than the VFD capacity. So if you sized for the motor, but someone changes it out with a larger one because they see that the VFD is rated for it, the conductors feeding the VFD can be forgotten and subject to failure. So they made that change starting in 2002.

Also when you feed a VFD with single phase to run a 3 phase motor, ALL of the motor power, regardless of the phase count, must come from the line. Since you only know the 3 phase amps of the motor, you determine the single phase current draw by multiplying it by the Sq. Rt. of 3, 1.732. That means that the diodes on the front end of the VFD must carry 1.732X as much current as the VFD was designed to carry. In addition, there is more DC bus ripple when you only feed 4 diodes instead of 6, so you need significantly more DC bus capacitance to smooth it. Therefore, the rule-of-thumb is to double the VFD size compared to the motor size. So you have a 10HP motor, they provided a 20HP (15kW) drive to safely run it.

Chances are that drive is rated for 65A, which means you must base your conductor size on 81.25A, so probably #4. The conductors from the VFD to the motor can be sized as per usual however, so 10HP 230V from the NEC chart, x 1.25 is 35A, so probably #10, maybe #8.
 
seems confusing...

You have a 10hp motor - don't care.

So you have a 20hp (15kw) vfd running it from 1ph input.

Good! Input diode, dc bus caps, all are good - no problem. Don't get confused...

Just size 1ph input as Jraef said: 125% of vfd input amperage nameplate rating. It is already oversized to handle your smaller motor.
 
ptonsparky said:
When all is done and you put a meter on the incoming lines and see it draws nowhere close to the ampacity of your branch circuit, do not dispair, you have done it correctly.
Click to expand...

Isn't that the truth!

I still remember doing one a few years ago that required 350mcm on the line side and only 4/0 on the load side.:blink:
 
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