VFD Question
VFD Question
(OP)
Is there a problem with having an electrical disconnect on the load side of a VFD? We're adding a VFD to an air handler motor. The VFD will be located inside building approxmately 35 feet away from motor. Code requires local disconnecting means at air handler which will put it on the load side. I'm in the process of checking motor to make sure its VFD compatible but aren't all new motors that way these days? Disconnect will be interlocked with VFD so when disconnect is open the drive cannot start.





RE: VFD Question
As the motor, it depends on what you mean by "compatible". Nearly any induction motor will operate on an AFD. However, a standard motor is still designed and rated for operation on a sine wave. Anything other than a sine wave and the motor will not perform per specifications.
If you specify an "inverter-duty" motor, you may get a motor with heavier insulation on the stator windings to protect against voltage stresses that can occur.
I'd advise making sure your motor has a service factor of 1.15, then size it so it never exceeds 87% of the motor nameplate horsepower.
Also bear in mind that motor will not operate at stated efficiency on an AFD.
RE: VFD Question
RE: VFD Question
switch under load, the arc suppression system could fail
at low frequencies on VFD output. You might need to look for
a specially designed disconnect switch which would normally operate under load at frequency well below 60 Hz.
RE: VFD Question
That which does not kill me, makes me stronger... and pissed off!
RE: VFD Question
RE: VFD Question
RE: VFD Question
Motor disconnects are usually there for maintenance reasons, If you want to stop the motor from running you need to have an emergency stop located at the switch or at the very least have a normally closed contact coming off the switch that is tied in with the drive's run permissive that operates sooner than the switch.
The actual switch should never need to make or break any current so it can be used in a standard 3-pole configuration.
RE: VFD Question
RE: VFD Question
RE: VFD Question
Is there a problem with having an electrical disconnect on the load side of a VFD?
///No. The disconnect should be properly selected to open the motor hp load. Notice that the disconnects (A-B, Square D, etc.) have Standard HP ratings and Max HP ratings. The hp range is huge. Also, notice that NEC 2002 provides numerous exceptions to the invoked disconnect.\\\
We're adding a VFD to an air handler motor. The VFD will be located inside building approximately 35 feet away from motor. Code requires local disconnecting means at air handler which will put it on the load side.
///Yes, it does. However, there are also exceptions cited.\\\
I'm in the process of checking motor to make sure its VFD compatible but aren't all new motors that way these days?
///No. You have to specify an inverter duty or spike-proof motor.\\\
Disconnect will be interlocked with VFD so when disconnect is open the drive cannot start.
///Notice, that the code also exempts you from having the motor disconnect if you install a disconnect on the inverter input side, which is possible to lock in the open and closed position.\\\
RE: VFD Question
regarding the disconnect on the load side of VFD....turning off a disconnect on the load side of the drive while it is running is a NO NO...it is like turning off your car ignition key while you are driving in a freeway...i don't think the IGBT's will like that as mentioned on the earlier post...you can put a disconnect for maintenance purposes only...better put a lock so that nobody will turn it off intentionally or accidentally.....
...there is no better and comfortable than you put a Lock Out Tag Out (LOTO)on the disconnect while you or anybody working on the motor or on the load connected to it...
....sometimes it is a good practice to "idiot proof" our design
nice series of thread guys...
dydt
RE: VFD Question
RE: VFD Question
RE: VFD Question
RE: VFD Question
The expected motor life of the existing unit will drop. Just buy and inverter duty one and have it in stock for when that happens. Actually the VFD will fail to Ground Fault, but the motor will still work on by-pass (w/out VFD).
Regarding to the disconnect, as suggested by somebody install an interlock with the VFD. Drives has an input for emergency stop, this will enable/disable the unit when shut. Place a sign indicating that the motor shall be turned of by maintenance an VFD.
RE: VFD Question
Large Arcs can be drawn when opening Load Break switches .....also all three phases do not always open simultaneously.
Although some ASD manufacturers claim that contactors and disconnects on the load side are OK,and might even do a demonstration in controlled circumstances .......beware , they can easily blow the IGBT's on the ASD output bridge .
RE: VFD Question
Load break or not, even with arc tips and arc chutes, from my unfortunate experience this can be very dangerous, even if the load current is small.
RE: VFD Question
The Load side disconnect should NOT be used under load unless it is an emergency .....
///Agreed. Many items cited (the motor disconnect switch, protective devices, etc.) in NFPA 70 (NEC) are for safety including emergency. That is why NFPA is in existence. It is obvious that the upstream power distribution and loads may experience the impact of some safety/emergency act(s) implemented downstream, e.g. oscillations, tripping, malfunctions, etc.\\\
use early break contacts to switch your ASD off as well.