CJCPE said:
If you use one VFD for all of the motors, you need to have a time delay between opening the VFD contactor and closing the line contactor. The delay needs to be long enough to let the motor field decay, perhaps 0.5 to 1.5 seconds. Otherwise, you get a current surge and torque impulse at transfer. If the speed drops too much during the delay, you will still get a current surge and torque impulse.
This is indeed a problem, but there is an alternative. There are few high-end VFDs on the market which offer what is called "synchronous transfer", wherein when you approach full speed, the VFD synchronizes it's output with the line supply phase angles so that the DOL bypass contactor can be closed without first opening the VFD output contactor. This does NOT reduce the number of contactors of course, it just eliminates that surge or speed drop on transferring the motor to DOL. I know for a fact that Siemens has this feature in their Sinamics G120 product line (because I work for a different division of Siemens and have seen it), but not in their other VFD products, so be careful. I also heard that ABB claims to have it as well but only in their ACS800 series, and probably one or two manufacturers. In general, this feature seems to be available in VFDs that can have a "line regenerative front end", even if that option is not selected. It makes sense though, because in order to use line regen, the VFD would need to be able to sense the incoming line phase angles. These are, as you might expect, the highest cost VFDs available in their respective product lines, but certainly they would be a lot less expensive than 6 individual lower-end VFDs.
In my previous job, I used to get this request a lot and worked up dozens of proposals to prove / disprove the economic viability on this scheme over the years. Here is my experience on costs:
1) It does indeed work out to be less expensive to use 1 VFD and multiple interlocked motor starters, UNLESS you must also be capable of reversing the motor starters. In that case, the 3rd contactor and relatively complex logic pushes the cost over the edge to where individual VFDs make more economic sense. One trick that a lot of people miss however is the motor overload protection. Make sure that the OL relay is in the circuit DOWNSTREAM of the common connection point of the VFD and DOL contactors, otherwise the motor starting current is not seen by the OLR and it could result in motor damage.
2) Multiple soft starters are sometimes less expensive, especially if it is very large motors, i.e. over 150HP (110kW). But this is not absolute and it depends on a lot of other mitigating factors. But certainly if you have limitations on starting power and/or you need long acceleration times, then the VFD method is a better choice.
3) In low voltage, it NEVER works out to try to do this with 1 soft starter and multiple motor starters; the soft starter costs are just too low to be worth it. Multiple LV soft starters will always end up costing less. However it DOES work out when discussing MV soft starters, because the soft starters are a lot more expensive. The caveat there however is starting duty. You usually cannot use a MV soft starter more than 2 or 3 times per hour. I know none of this was previously discussed here, I only bring it up for future reference
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