VFD versus Magna Drive

[andytang]

Like to know if anyone may have some comparisons (pros & Cons)for using Magna Drive (magnetic coupling) and VFD. Comparisons such as cost, efficiency, maintenance, ease of operation, technical support, typical problems, etc. I am considering an varibale mean to drive three 250 Hp motor for water pumping application. Thanks.

 

[CB2]

Urge you carefully read the installation manual, I liked the plastic wedges they supply. Will cost you more to purchase and alot more to operate (energy) in spit of the marketing fluf.

 

[DougMSOE]

When you evaluate the equipment, be sure to include the people who will have to repair it.
Just about anyone can fix a magnetic drive with simple tools, any motor repair shop.
What does it take to repair the VFD? Much more than a VOM

 

[jbartos]

Suggestion: Sometimes, references are available since they handsomely sell the product. Try to obtain some.

 

[niallnz]

IMHO
I expect the most cost effective solution (both in terms of initial capital and ongoing energy costs) for this application would be to use a single drive that has the capability to control the flow or pressure (using an internal PID system to set the speed set point) and also "synchronise" the motor to the utility. With this scheme, as the demands of the system vary the drive can control the speed of the motor then as the system demand increases the drive can synchronise the first motor and be switch on to the second motor and continue to control the pressure/flow. I know that ABB dirves have this capability, and I expect that AB/Relience, Siemens, etc do to.

While it can cost alot to repair a VFD, when a reputable manufactures drive is correctly sized and correctly installed the reliability is exelent and annual maintenace is minimal.

 

[mdonner]

If all 3 pumps are pumping into the same system header, you may use one VFD (with PID and PLC) sized to control one motor, and the other two will kick in to assist (per PLC control) whenever demand is beyond the controlled pump capacity (motor at full speed and PID demand not met).
The controlled pump may be changed daily by interlocked contactors feeding the motors. This way you only need one VFD rated to single motor power.
Note: to run ("synchronize&quot 3 motors one one VFD you may want to size it for one motor starting current on top of 2 motors running at full load amps, using the VFD data sheet. This will yield a very expensive VFD!!!
For power cost savings consider the power losses on controllers. VFD typical is 3% but you shuold consult data sheets. Also consider initial cost for equipment (Magna Drive may need cooling). From my experience, the VFD reliability is usually very high, but repair cost may be high depending on the type of repair. You may have to check the service in your area. Magna drives are much simpler and easy to repair, but not every shop will be able to repair it, its eficiency may be to low, and it may be trickier to control. Another aspect to consider is the pump characteristic. If it is too flat, speed controll will be very unstable for accurate output, and very sluggish for stable speed (and pressure).

 

[electricpete]

One thing I have heard from users at magnadrive.com is that the Magnadrives tend to operate hot when there is a large difference in speed between motor and load.

 

[CB2]

Electricpete,

It's called a "linear efficiency curve", standard for Eddy Current Couplings. Also that heat can be an issue for the motor depending upon the installation environment, VFD's and motors are both designed for a 40 degree C ambient.

 

[jbartos]

Suggestion: Visit

http://www.ohiowater.org/oawwa/Newsletter/Winter 2002/winter 1-15.pdf

for two articles:
1.
by: John Primeau, Hughes-Primeau
Controls, Inc.
The MagnaDrive Adjustable Speed
Drive: How Does It Work?
2.
Reprinted from Industrial Week,
published December 2001-January 2002
MagnaDrive Adjustable Speed Drive

 

[SolDirJoe]

No question an AC Invertor is best. Mag couplings do the job and are 'simpler' to figure out. However, they are inefficient, especially if idling and at low output speeds.

Given the power in this case you can calculate the time it would take to recoup capital expenditure if you have realistic duty cycles and MTBF's etc.

Support ? with over 80 different brands to choose from its likely you will find at least one local to your site unless its in the middle of Somerset Lake - even then !!!!!
SoldirJoe

 

[jOmega]

Takes a couple of millwrights about 2-days to set one up.

VFD ... Three wires in... three wires out.... apply power...hit the start button ...and you're moving water.

Unlike the Eddy Current drives or Eddy Current Clutches... you do not apply a small dc current to the control coil. The Magna Drive is all mechanical... ergo, you need to have an actuator cylinder that is electrically controlled in order to effect a speed change.... Interface to your control circuit or PLC or whatever needs to be custom designed (and by whom ???) ... VFD.... plenty of common I/O .... 0-10v.... 4-20 mA ... ditigal logic commands.... plus outputs for speed (freq) current, power... etc.... Magna Drive has NONE.... Sure its cheap enough... but it is not energy efficient. Consider that a VFD with a square law V/Hz curve reduces the excitation to the motor as flow is reduced...(speed) and that saves energy..... as opposed to the ac motor with the Magna Drive contraption... motor receives full excitation...regardless of the output shaft speed....

Pumps in water applications seldom operate below 40-50% speed (harmful to pump).. Easy to set MIN SPEED or MIN FREQ in a VFD... a couple of keystrokes on the keypad get the task done.... How do you effect a minimum speed on the Magna Drive ? ... you need a specially designed controller for the ACTUATOR on the Magna Drive...

If you look at their marketing materials... and the referenced studies... you will note that the studies were done with OLD INVERTER TECHNOLOGY ... and are nowheres close to todays technology... but they forget to tell you that... Oh, well .....

Caveat Emptor.

 

[GusD]

Despite their faults ,new VFD technology offers the flexibility and reliability that today's industry demands.
All other means of varying speed,torque control under all load conditions,cannot match today's VFDs capabilities.
It is true that the initial costs are higher,repair costs may be higher as well,however;industries are not turning their backs on VFDs despite the apparent higher cost.On the contrary,check around your work place,the answer to this question should be obvious.

thanks GusD