In my opinion the advantage is cost effective variable control for multiple motors that need to run the same speed.
The disadvantage is that you can't vary the speed of the motors independantly and you may not get as good as performance as with independant drives. But... usually, this is ok for things like conveyors, fans, pumps, etc.
In some applications like overhead traveling cranes where two drive wheel motors must run at the same speed, a single VSD is better, cheaper way to match wheel speeds.
You need to remember a few things when controlling multiple motors from a single VSD. Most importantly is each motor needs to have an independent external motor overload. The outputs from these should be seriesed into the enable input of the VFD. Motor lead length also needs to be considered. The combined lead length should not exceed the manufacturer's spec. The VSD should also be oversized slightly. When controlling two 1.0 H.P. motors use a 3.0 H.P. VFD.
Responding to Mark's 11-Dec-06, 10:40 pm query... I note that this post is from the Humorous dept, but if you do it you won't be laughing all the way to the bank!
On a more serious note, if both motors are connected to the same driven-machine (in-tandem or mechnically in-parallel) and unless you consider having one motor "slip" (excuse the pun) relative to the other, then "Caveat Emptor!"
I strongly recommend that you search the List Archives for similar topics. One is identical to the one you posted!
Are you saying the output side of each thermal overload is wired in series with the next thermal overload resulting in only three wires (three phase) go to the VFD?
