OT – (1) 50HP VFD + (2) 20 HP Motors & Cycling One Off

Searched the archives and did not find anything directly on this:

I have a facility designer that want to drive two exhaust fans possibly with a single VFD and then be able to cycle one of the fans off on a new application (this is due limited wall space for all the electrical disconnect, enclosures, etc. required). Separate motor protection will be required with klixons in the motor windings since the ambient can be up to 130 F. When they open, I was planning on shutting the drive down but could a contactor be installed just before each fan's motor to just take the overheated fan offline until it cools down? And since there will already be contactors there, can we cycle off a fan when there is too much exhaust with both fans running at minimum speed? If required, we could stop both the fans with the drive, open the contactor, and then restart only with one fan.


What are your thoughts/ concerns/ caveats.

Disconnecting a running fan, while the drive is running would probably not harm anything. Connecting a motor to a running drive on the other hand is a very bad idea.

Stop the drive before switching motor configurations.

See this thread for a similar application:
http://www.plctalk.net/qanda/showthread.php?t=52949

A few observations, for what they are worth.

First, it is my experience than in almost all cases, by the time you get done with the interlocks and engineering and extra wiring and relliability/redundancy you are almost always ahead to put in a separate VFD for each load.

Second, if you are worried about motor temp you should consider RTDs in the motor windings. These are fairly common, not too expensive, and will give a lot more data than just a temp switch.

Third, the Klixon alone may not provide complete protection, since you could have shorts in the power leads or ground faults in the motor windings etc. You still separate need overcurrent protection on each load.

While cutting off a motor while running can be done, starting a stationary motor into a running drive cannot be done. You will have an overcurrent or short circuit fault every time. You would need to stop the drive to reconfigure, just as rdrast says.

Tom Jenkins makes a really valuable point tho. It probably isn't worth it especially at 20hp. Shucks, the 460V 20hp drives probably won't cost any more than about $1500 each, these days.

I am puzzled at Tom's comments about Klixon switches. For my money, they are the best choice for inverter motor thermal protection. It is certainly true that RTD's and thermocouples provide more information but it's information that doesn't matter. For example, I really don't care how warm the motors are. My only concern is if they are TOO warm and then the Klixon's operate. Perfect and simple! I like that.

As to shorts and grounds, those protections are not part of the drive's overload protection. Shorts and grounds are separate faults which operate completely independently of the motor thermal protection, even when its turned off.

For me, if the customer has spent the money to bury Klixons in the motor, I would want to use them. There are two added benefits: first, the protection is actually better since the temp is being measured, not estimated; and second, experience shows that you can actually get more out of your motor without putting it at risk since the software overload calculations tend to be rather conservative. That can be valuable if you are running the motor on the "edge" and you don't want to go over!

Thanks For The Inputs

Gents,

Thanks for reinforced my original gut feelings that this was a bad idea: life's too short to design something that's probably going to cause issues on startup or randomly at a later date.

Not sure why I didn't catch jrwb4gbm referenced post when I searched, it was posted when I was on vacation so I didn't read it last month or I would have remembered it.

Starting another motor while one was running I knew was a likely a cause for unintentional excitement.

The added cost of separate protection equipment, extra software to handle sequencing, and time to design and install the extra items will outweigh any possible benefits of putting additional motors on a single drive.

DickDV said:

I am puzzled at Tom's comments about Klixon switches. For my money, they are the best choice for inverter motor thermal protection. It is certainly true that RTD's and thermocouples provide more information but it's information that doesn't matter. For example, I really don't care how warm the motors are. My only concern is if they are TOO warm and then the Klixon's operate. Perfect and simple! I like that.

Click to expand...

On small motors a Klixon is probably the way to go. I like RTDs on the larger hp because in addition to seeing if you are running into thermal problems at low speed you can use them for limiting starts. A lot of the consultants specify MultiLins and their starts per hour limits don't take into account the reduced thermal impact of starting on VFDs. That means I get to spend a lot of time waiting for the MultiLin to time out. If I can convince the counsulting engineer to let me take RTDs into my PLC instead of using a MultLin that issue goes away. If the motor's not hot I can start it.

The other minor issue with a Klixon is that they are usually wired in series with the E-Stop circuit. Most drives have a single fault contact. When the motor drops out you have to figure out if someone hit the E-Stop, if the over current tripped, or if the motor got hot. Of course, if I've got a comm link to the VFD and a second contact on the E-Stop the diagnostics get simpler.

So, if I have a choice between RTDs or Klixon then it isn't a big deal, but I have a mild preference for the RTDs. If it is a choice between anything and a MultiLin then I prefer anything!