Yeah, all our new drives are actually dual drives. Really quite awesome... but expensive. Our control schemes wouldn't really work without variable speed, so bypass wouldn't have even been an acceptable backup mode anyway.
Originally Posted by mechdorn
Yep. A decent VFD of modern design does a lot to help protect a motor.
Originally Posted by crab master
I have several customers who use them even in applications where speed control is not really a factor, for just that reason.
On a current job site, doing work for a University, their in-house facilities engineering/maintenance staff want VFD's on virtually everything. For instance, for a new building we're working on for them, there are a number of fixed speed MAU's. The system design engineer, who worked for the hired architectural firm that designed the building, did not call for VFD's for those units.
The University staff, as soon as they saw the units, with only standard motor starters, immediately issued a change order to have VFD's installed. We were happy to accommodate them. For a fee, of course. Their reasoning?
Extra motor protection.
Plus soft ramping which would produce less wear and tear on the equipment.
Control of high inrush currents during start-up.
And they figured that come balancing time, if the balancers needed to adjust speed, it was a labor cost savings to simply adjust the speed with the VFD than to have to go in and change sheaves/pulleys.
As to running at constant speed all the time, the usual loss induced by a VFD in line, using modern VFDs, is something like 2 to 3 %. The University's chief engineer figures this is acceptable in view of the added protections and such which the VFD's provide.
He also likes that, even in constant speed applications, with the VFD's (which all have BacNet interfaces) their central monitoring/control station can monitor and trend energy usage, amps, voltage, motor/VFD status, and so forth. Just attach a network cable.
Besides this University, we have a number of other facilities which take much the same view on this.
The only places, any more, where we see bypasses put in are for critical applications. Hospitals, certain manufacturing facilities, some applications at airports, some big data centers, and some organizations dealing with public safety/Homeland Defense.
Several of our customers interested in protection against long term down time, simply keep one each for each power range, of spare VFD's at some central location.
VFD's waste a small amount of electricity internally so it would be slightly less expensive to run bypassed. I think the number is like 3%. They will run forever a 100%.
They that can give up essential liberty to obtain a little temporary safety deserve neither safety nor liberty.
If they add a VFD on a MAU what will control it? static Pressure, CO2 levels? if this is a school they most likely have to maintain a percent of outside air at all times. To me the MAU will run all the time bring in outside air at a constant rate.
Originally Posted by osiyo
It most likely would. He was talking about the other merits a VFD have, such as motor protection, ease of air balance adjustments, etc.
Originally Posted by dlove
CraziFuzzy is correct, the MAUs run at constant speed.
Originally Posted by dlove
Altho they benefit from soft ramping upon start and stop.
The VFD's also provide low and high voltage protection, phase loss protection, over amperage protection, etc. In addition, are set up to automatically attempt 3 restart tries after such events before locking out and requiring manual reset. In case the cause was a transient condition. The restart tries are set to be 3 seconds apart. All this is settable and adjustable. And if you are curious about what'd caused it to stop, but wasn't around at the time, just look at it's internal alarm history, easily accessible from the LCD panel. It's not only tell you what ... it'll tell you the time and date of the event. Try that with a standard motor starter.
Around here, a sheave and belt change out on a sizable unit is usually quoted (in the commercial world) at somewhere around $600 to $800. With the VFDs installed, it's a simple matter of just adjusting max speed at the VFD's panel. In the case of the job I'm talking about, 2 of the 3 MAU's needed slowing down, one needed speeded up to run at 67 Hz. (No, it won't hurt the motor)
Haven't done it yet, but I'll be putting the VFD's on the local Bacnet Network. VFD start enable is via one of the BAS controllers in the machinery rooms concerned, which are also doing other tasks. The BacNet connection will simply be so that customer's in-house facilities engineering department can monitor and collect data. Such as current power draw, accumulated KW/hrs, and so forth.
Over the years I've seen VFDs catch a LOT of issues and protect the motors they're attached to.
On a constant volume system you can utilize a VFD to maintain a constant airflow based off or duct static pressure to account for filter loading. We have utilized this many times. More often where they need to maintain a certain airflow but it is a good application in general. I agree also with the soft starting features. You can also integrate into most drives (BACnet and Modbus) so you can have feedback for Voltage, Amperage and other data. Most drives have analog inputs with setpoints that can be used to let it run stand alone.
It has been pretty 'cool' since 2007 watching heated air temp controlled by smart sensors in conjunction with/ from the 'condenser' side.
Programmed set points are just like in a bonnet-air-temp of the programmable heat pumps heating outputs by a selected degree.