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paul renshaw

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I don't do a lot of residential, but saw this installation and wondered if it was code legal There is a well controller, VFD, on a three phase submersible pump that is fed with 240V single phase input. The well guys ran a piece of #10-2 romex from the disconnect to the VFD. There is a 40 amp breaker in the disconnect, and there is a #8 run to the line side of the disconnect with a 40 amp breaker in the panel. I know conductors get fused higher sometimes on motor installations in commercial, but is that compliant with type NM cable in a house?
 

GoldDigger

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You did not say what the rating of the pump is. That, along with the overload setting (in VFD or other) is what will determine required ampacity.
 

paul renshaw

Senior Member
I would have to check to be sure 100% but I believe the motor load was around 10 amps or so, and the drive will be programmed to correspond with the motor amperage. I was just questioning the legality of fusing the #10 at higher then 30 amps, as in 240.4(D). Does article 430 override in this situation and allow it? Plus, they ran #8 down to the motor because of the voltage drop, seems odd to put #10 in that circuit, when the jumper is literally shorter than 3 feet. The number 8 from the panel to the disconnect is only about a 15' run.
 

Ingenieur

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#10 is limited to 30 A unless an exception applies
it appears to: reference 2011 240(G)
motor circuits are exempted from the 30 A CB limitation so 430 Parts III, IV, V, VI and VII apply
sounds like the #10 with 40 CB might be code compliant in this case
without knowing motor and associated controller specifics hard to say
 

Ingenieur

Senior Member
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Ditto.

Jraef explained it a few times, but I still get confused. Static vs, rotary converter or some such.

VFD has 3 main sections
-rectifier, makes DC
-DC bus, self explanatory
-inverter (w/firing controller), a digital controller triggers SS switching devices, connected to the DC bus, based on some algorithm based combination of width/duration, freq and amplitude to simulate a sine wave
it has 3 sections (or phases that can be fired 120 deg apart)

the DC bus voltage is 2 x (3 V sqrt2) / (2 Pi)
eg 480 (2 3 480 1.414)/(2 Pi) = 648 VDC or +/- 324 VDC
simplifies to 1.35 x 480 = 648 or +/-324
single phase is (2 sqt2 V) /(Pi)
if V = 480 then (2 1.414 480) / (Pi) = 432 or +/-216 VDC

the invertor section does not 'care' what supplies the DC buss, 1 ph, 2 ph, 3 ph, as long as it has a DC reservoir to power its IGBT (or ???)
the controller/software will adjust the firing parameters to achieve the desire output setpoints
 
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VFD has 3 main sections
-rectifier, makes DC
-DC bus, self explanatory
-inverter (w/firing controller), a digital controller triggers SS switching devices, connected to the DC bus, based on some algorithm based combination of width/duration, freq and amplitude to simulate a sine wave
it has 3 sections (or phases that can be fired 120 deg apart)

the DC bus voltage is 2 x (3 V sqrt2) / (2 Pi)
eg 480 (2 3 480 1.414)/(2 Pi) = 648 VDC or +/- 324 VDC
simplifies to 1.35 x 480 = 648 or +/-324
single phase is (2 sqt2 V) /(Pi)
if V = 480 then (2 1.414 480) / (Pi) = 432 or +/-216 VDC

the invertor section does not 'care' what supplies the DC buss, 1 ph, 2 ph, 3 ph, as long as it has a DC reservoir to power its IGBT (or ???)
the controller/software will adjust the firing parameters to achieve the desire output setpoints

Magic. Not Black Magic or Voodo, but magic.
 

Saturn_Europa

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Fishing Industry
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Electrician Limited License NC
I don't do a lot of residential, but saw this installation and wondered if it was code legal There is a well controller, VFD, on a three phase submersible pump that is fed with 240V single phase input. The well guys ran a piece of #10-2 romex from the disconnect to the VFD. There is a 40 amp breaker in the disconnect, and there is a #8 run to the line side of the disconnect with a 40 amp breaker in the panel. I know conductors get fused higher sometimes on motor installations in commercial, but is that compliant with type NM cable in a house?

It sounds fine to me. I've used the NEC plenty of times to upsize the OCPD device to handle motor starting current and never had to increase the size of the conductors going to the motor. On motor control circuits the OCPD is mainly for short circuit protection and the motor overloads are to protect for lock rotor or over current draw.

To size the conductors use table 430.250 then multiplying it by 1.25 for continuous load. Sounds like it will work though.


Single phase input to VFD and three phase out to motor. The pool pump and well pump at my house are both this set up.

I love that VFDs can do this now days. Back in the dark ages we would add roto-phases which basically ran a motor and a small generator that shifted a phase 120 degrees. Loud and prone to failure.
 

iwire

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Location
Massachusetts
If it is on the supply side of the VFD the conductors and overcurrent protection are based on the VFD rated input current and the OCPD must not exceed the ratings of the conductors.
 

Ingenieur

Senior Member
Location
Earth
Magic. Not Black Magic or Voodo, but magic.

Been playing with them since the 80's
a 50 hp was the size of a refrigerator and required dedicated cooling
now the size of a large shoe box lol

took a course last fall on FACTS
lot of i and v sourced converters
the theory and modelling in matlab/plecs
actual built them up using basic components, igbt's, etc

still FEM to me
Freakin'
Electrical
Magic
:)
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Registered Professional Engineer
... the DC bus voltage is 2 x (3 V sqrt2) / (2 Pi) ...

If the DC buss contains filter capacitors, its voltage can be up to √2·Vin.
(680 volts with 480 input, 340 volts with 240 input, regardless of the number of phases)
Without filter caps, a single-phase-input VFD probably wouldn't work.

One of the configurations for a solid-state static phase converter is basically a variable-frequency drive that just doesn't vary in frequency.
 

Ingenieur

Senior Member
Location
Earth
Most I've measured at 480 are 650 or +/- 325

page 3 https://library.e.abb.com/public/5da6c7e7c230318b852575de0072821c/ACS500-programming.pdf

Note that the Motor Control Card of the ACS 500 is at DC bus voltagepotential.
The DC bus capacitors contain dangerous DC voltage levels (1.35 x VIN).After disconnecting the supply, wait at least five minutes after the displayreadout on the control panel has disappeared before taking any measurements






Basically the 1/2 wave integrated

if you google vfd dc bus 1.35 pops up everywhere
 

Ingenieur

Senior Member
Location
Earth
Example: 480v AC Nominal Voltage
1.35 x 480v = 648v

drive trouble shooting guide
pg 24 http://literature.rockwellautomation.com/idc/groups/literature/documents/tg/pflex-tg001_-en-p.pdf

 

paul renshaw

Senior Member
It sounds fine to me. I've used the NEC plenty of times to upsize the OCPD device to handle motor starting current and never had to increase the size of the conductors going to the motor. On motor control circuits the OCPD is mainly for short circuit protection and the motor overloads are to protect for lock rotor or over current draw.

To size the conductors use table 430.250 then multiplying it by 1.25 for continuous load. Sounds like it will work though.




I love that VFDs can do this now days. Back in the dark ages we would add roto-phases which basically ran a motor and a small generator that shifted a phase 120 degrees. Loud and prone to failure.

The good old roto-phase, you needed hearing protection around those, lol.
 
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