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#4775 - 10/17/01 06:13 PM VFD's
Nevin Offline
Member

Registered: 05/28/01
Posts: 31
Loc: Lititz PA 17543
Variable Frequency Drives are not something I have worked with before so I am fairly ignorant when it comes to implementing them. My question is, If I have a 480volt 3 phase motor that I want to have variable speed, can I simply install a properly sized VFD in the motor circuit and control the speed right from the VFD? Or are these units dependant on a computer or controller to feed it the right information?

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#4776 - 10/17/01 06:52 PM Re: VFD's
NJwirenut Offline
Member

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
Depends on the drive, but every one I have worked with has had provisions for either local (keypad) speed control, or an analog speed signal input (0-10V or 4-20 mA). The analog voltage input is the easiest to set up, requiring only a potentiometer and a voltage source (usually provided right from the VFD).

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#4777 - 10/18/01 03:51 AM Re: VFD's
Redsy Offline
Member

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
Nevin,

Ditto NJwirenut.
Also(particularly if you are retro-fitting into an existing installation), be aware that there may be other issues to address. These include the insulation quality of the motor, the distance from the drive to the motor, the control(start & stop, not speed) voltage options, separation of power & low level control wiring, and input & output filters (line & load reactors) to help mitigate noise and voltage spikes.

[This message has been edited by Redsy (edited 10-18-2001).]

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#4778 - 10/18/01 03:55 AM Re: VFD's
ctolbert Offline
Member

Registered: 08/09/01
Posts: 33
Loc: Cumming, GA USA
Nevin:

Dirves are designed to be controlled independently, like what NJwirenut is stating. There have been vast improvements over the drives from even 5 years ago. Newer IGBT's, faster commutation..etc.

Sizing comes down to application mostly. The application depicts the derating (if necessary) and accessories. You may need line reactors, filters, and dynamic braking. If you want some more help, put down the size of the motor (HP, FLA, speed, voltage, freq.)and the application.

I do this stuff all day. I'm in tech support not sales...so I won't try to sell you something...promise.

Carl
_________________________
Carl Lee Tolbert
Technical Support
AC Drives
Automationdirect.com
Your source for the most practical automation products at almost-free pricing, delivered by 11 a.m., just by clicking a mouse.
PH: (770)844-4200
Fax:(770)886-3199
http://www.automationdirect.com

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#4779 - 10/18/01 09:29 AM Re: VFD's
WARREN1 Offline
Member

Registered: 05/09/01
Posts: 184
Loc: Greenville, SC, USA
As already stated, if the motor is existing, your insulation may have been damaged, so be observant. Remember, as the voltage and frequency change, so does the current, as well as harmonics. This can be calculated at any given frequency, maybe not so easy, but!!
One of our clients, a very large chemical company (worldwide), now always requires the use of oversized shielded cable on all VFD driven motors. They also require the VFD to be one size larger than required by the HP of the motor. Since I haven't worked on any of their projects in awhile, I think they always buy Allen Bradley drives. I know one of the largest brand snacks makers always specifies Allen Bradley. There are other good drive manufacturers out there, Reliance, Ge, to name a couple.
And there are several ways to control them. Get some literature from your local Allen Bradley rep or supplier.

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#4780 - 10/18/01 10:21 AM Re: VFD's
Redsy Offline
Member

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
We have about 10 Allen-Bradley 1336 drives and 2 Reliance. Based on my experiences, I will only use Allen-Bradley.

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#4781 - 10/19/01 06:28 AM Re: VFD's
ctolbert Offline
Member

Registered: 08/09/01
Posts: 33
Loc: Cumming, GA USA
Warren and Redsy, I respect your opinion and affection with AB drives… I used to use nothing but AB and GE Fuji a few years ago, I’ve just developed a unique perspective on this subject over the last year, which I would like to share.

I look at drives under 100 hp as Pop (can you tell I’m from Cleveland, Ohio.) The differences between Coke, Pepsi, RC cola, and the Sam’s club stuff (generic.) I’m getting smirks now…. seriously.

I deal with troubleshooting drives everyday, I handle our brand (Hitachi) and the interfacing of everyone else’s with our controls. I’ve dealt with just about every drive in North America…. even a few odd ones in South Africa and the UK.

The technology is converging…. A Coke is similar to Pepsi in the fact that it is a Cola, it’s just different. Here is what I mean:

1. My system voltage is 500 vac; I have a highly volatile blower that has to vary speed fast and often. I wouldn’t use a Japanese drive of any make…they are historically voltage intolerant. Unless I used a bucking transformer to 460vac and used dynamic braking to filter the dc bus from regenerative trips.
2. I may use ABB’s direct torque control on a lathe that had to have decent speed control with no ability to use an encoder for vector control. If I had a greater range of variation, most sensorless vector control drives would do as well.
3. Would I buy an AB or Square D drive with a NEMA 12 case? Sure, if it was in a location where I couldn’t fit another drive (most are NEMA 1) in a NEMA 12 enclosure and save at least half the cost overall.

Problems arise from misapplication and improper set-up and installation. Most manufacturers have very low failure rates (if all procedures were followed.) There are legitimate component level failures…you will find that the figures are low when you speak to EMA, Perfection servo, or any drive repair group. Here are a few recent examples of missapp/set-up issues that I’ve ran across:

1. I want to run a vfd with an open frame motor from 1970.
2. My motor is 1000 ft away from the drive and I have existing rag wire.
3. I have 15 plugging circuits on the same distribution bus; do I need a line reactor on the power side of the drive?

I may like Coke over Pepsi, but if Coke was $3 a can versus Pepsi at $1 dollar a can, and I’m a technically savvy guy who has done my homework, then I’m buying the Pepsi.

Carl
_________________________
Carl Lee Tolbert
Technical Support
AC Drives
Automationdirect.com
Your source for the most practical automation products at almost-free pricing, delivered by 11 a.m., just by clicking a mouse.
PH: (770)844-4200
Fax:(770)886-3199
http://www.automationdirect.com

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