Variable frequency drives
- Thread starter blt
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
The operation of disconnect between the VFD and the motor is not recommended, however this disconnect is often required by code rules for safety purposes.
Don
Don
necnotevenclose
Senior Member
It is common design practice where I work that if a VFD is provided with a UL Listed approved disconnect then a disconnect is not required between the fan and the VFD.
- Location
- Illinois
- Occupation
- retired electrician
necnotevenclose,
Don
What about the disconnect that is required by 430.102(B)?
Don
necnotevenclose
Senior Member
430.102 states that you need a means of disconnect ahead of the controller this is provided only if the vfd is not in sight of the disconnect or over 50ft.
- Location
- Massachusetts
You are required to have a disconnect between the VFD and the motor if the controller disconnect is not in sight at the motor.
It is common to have the controllers at a central MCC supplying motors in locations that you can not see the MCC from. In that case you must install a disconnect at the motor.
It is common to have the controllers at a central MCC supplying motors in locations that you can not see the MCC from. In that case you must install a disconnect at the motor.
- Location
- Illinois
- Occupation
- retired electrician
There are two disconenct rules in 430.102. One for the controller (VFD) and one for the motor. If both the motor and the controller are within sight of the controller line side disconnect, then you only need that disconnect. Any time the motor is not within sight of the controller disconenct, you need a motor disconnect, unless you can comply with one of the exceptions to 430.102(B).
Don
motorcontrol
Member
- Location
- Plainview, ny
I would say, if the mcc is not in site of the fan, then, install disconnect to comply with nec. if the disconnect on load side of the vfd is opened while the fan is in operation, the vfd will trip anyway. If the fan operation is part of hvac then, you can integrate either the current output or fault output from the vfd to rest of the controls.
Blue Flame
Member
Variable frequency drives
My expirience has always been to have it before feeding the VFD, due to possible damage the other way. Opening / closing between VFD and motor can cause damage??
Blt: I think the manufacture or the manual could answer that question as far as the drive being harmed.
For the code I agree with Iwire it has to be in site or locked out.
My expirience has always been to have it before feeding the VFD, due to possible damage the other way. Opening / closing between VFD and motor can cause damage??
Blt: I think the manufacture or the manual could answer that question as far as the drive being harmed.
For the code I agree with Iwire it has to be in site or locked out.
- Location
- Massachusetts
rmiller0248@charter.net said:
Which is a good idea but only possible if the VFD is within sight of the motor.
motorcontrol
Member
- Location
- Plainview, ny
Line side disconnect is must regardless of load side service disconnect.
Blue Flame
Member
iwire said:
So your saying even if you lock it out it still has to be in sight?
motorcontrol
Member
- Location
- Plainview, ny
NO. if the vfd is in line of site, disconnect should always be on line side. installing a load side service diconnect on vfd application is not healthy....Only as necessary evil to comply with AHJ......
Blue Flame
Member
motorcontrol said:
So you say its not health explain? Besides the drive faulting because of the loss of line voltage.
- Location
- Illinois
- Occupation
- retired electrician
Rob,
Don
The controller disconnect must always be within sight of the controller. There are no exceptions to 430.102(A). The motor disconnect must be within sight of the motor unless you meet one of the conditions in the exception to 430.102(B). There was a major change in the wording of the exception a few years back. In the older codes, a remote lockable disconnect was always permitted to be used as the motor disconnect. The current wording has placed major restrictions on the use of a remote lockable disconnect in place of a "within sight" disconnect for the motor.
Don
Blue Flame
Member
don_resqcapt19 said:
Thanks Don I reread 430.102b are plant must fall under exception B
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
motorcontrol said:
why would the vfd trip if you opened the output side?
a vector drive will, but most vfds are not vector drives.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
The danger in having a disconnect or contactor downstream from a VFD is in OPENING said disconnect or contactor while the drive is running the motor. There is no problem having a safety disconnect at the motor to lock it off when servicing the equipment as long as the drive is off before opening the disconnect. Should the disconnect need to be opened in an emergency (i.e. life-safety), the damage it will do to the transistors in the drive is inconsequential in comparison.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Why would the drive care?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
petersonra said:
Well, "care" would impart some sentience to an inanimate object :wink: , but I know what you mean.
When a set of contacts are opened under load, an arc is drawn across the air gap. The arc oscillates between the load inductance and stray capacitances and each successive oscillation is at a higher voltage until the widening gap has sufficient dielectric strength to interrupt the current flow. This creates a severe voltage spike in the source, in this case the output transistors of the VFD, which can immediately cause them to short from dV/dt (too rapid of a rate of change in voltage), or at the very least do incremental damage to the silicon substrate of the transistor. The drive doesn't really "care", but the owner will when the repair and downtime bill comes in.
- Status