VFD Question 1

[ChillerGuy]

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.

 

[dpc]

There is no problem putting disconnect on the output side of an adjustable frequency drive (AFD). I'm not sure the interlock is even necessary, although it is probably not a bad idea.

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.

 

[ChillerGuy]

Thanks for the quick and knowledgable response!

 

[thinker]

Also keep in mind that if you plan to open that disconnect
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.

 

[jraef]

"Thinker" is right. The best way to be sure is to look for a disconnect switch rated for use on DC power. Most cheap ones are not, and the contact surfaces will burn rater quickly. Think of the PWM output as looking like an arc welder when you open up the output circuit. Also, some drives cannot have the output opened under load without doing damage to the transistors. Check with your VFD manufacturer. That which does not kill me, makes me stronger... and pissed off!

 

[electricpete]

Maybe I'm just skiddish but I wouldn't open the disconnect on any motor under load....vfd or not. Am I off base?

 

[busbar]

FWIW,

http://ecatalog.squared.com/catalog/html/sections/03/17203005.htm

lists a few switches will assorted 250/600V DC-Hp ratings. DC-rail voltage may be applicable here, making it a though sell on 480VAC drives. A combover of NEC430 is probably in order. There is the qualification: “On 3-Pole devices, use two outside poles for switching DC.” Nota Bene: general-duty switches may not have comparable ratings.

 

[tmahan]

I think this is no big deal provided it is done correctly (sort of like "Golf is easy just hit the ball long and straight&quot. The key is that the disconnect should never be opened when the VFD is providing voltage for the motor. This is bad for IGBT's and raises questions about the switch.

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.

 

[wired1]

Be carefull! I once wanted to put a vfd on a small blower that had an oem motor on it. After numerous phone calls I found the motor manufacturer, who urged emphatically NOT to use the vfd. Oem motors use the cheapest insulation possible, being designed specifically for one set of operating conditions. A vfd might burn it up. I ended up using a two speed fan.

 

[busbar]

tmahan, you have a good point. I suppose that the switch should be labeled: "For isolating use only--Do not operate under load," to keep the safety people happy. This is speculation, but could it be that the voltage/current at the switching point is not smooth DC and cyclically approaches zero?

 

[jbartos]

Suggestion to ChillerGuy (Mechanical) Jun 11, 2002 marked ///\\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.\\

 

[dydt]

If you are going to install a VFD on existing motor, make sure that it is an "inverter duty"....latest VFD uses IGBT technology....in our area the price of equivalent inverter duty motor is not much...you can keep the OEM motor as a spare..

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

 

[jbartos]

Suggestion: When it comes to safety, opening of the disconnect is YES-YES no matter what will happen to the VFD. Have you noticed those suits for hundreds of millions?

 

[electricpete]

jbartos - If you want to open a disconnect under load that is your business. I hope nobody else tries it. The motor should be deenergized by another means before opening the disconnect.

 

[jbartos]

Suggestion to the previous posting: Read NEC very carefully.

 

[Energia]

ChillerGuy
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.

 

The Load side disconnect should NOT be used under load unless it is an emergency .....use early break contacts to switch your ASD off as well.

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 .

 

[vanskat]

I prefer the conventional method of VFD application. Main disconnect switch, then VFD, then load (motor). If maintenance is required the main disconnect can be opened. I think it would complicate things a bit and may have safety implications to have a disconnect switch at the VFD output.

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.

 

[jbartos]

Suggestion to David1953 (Visitor) Jun 20, 2002 marked ///\\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.