Variable Voltage / Variable Freq. Soft Starter
Variable Voltage / Variable Freq. Soft Starter
(OP)
Does anyone know of any manufacturer of a variable voltage/ variable frequency soft starter in the medium voltage range, 5.5, 6.6kV? Are there more advantages in using a 50% rated VFD for soft starting duties than using a real soft starter that varies the voltage and limits the current?





RE: Variable Voltage / Variable Freq. Soft Starter
A soft starter controlls voltage and does not control frequency. A VFD controls both voltage and frequency.
There are a number of manufacturers of MV soft starters, Motortronics, Benshaw, AuCom, Solcon and others. There are also a number of manufacturers of MV VFDs, ABB, Toshiba Fuji, Rubicon and others.
One advantage of the soft starter, is price, another is eficiency, another is load torque capabilities.
Generally, if you do not need to vary the speed in operation, i.e. fixed speed operation, then look at a soft starter. If you gain an advantage from varying the speed, then look at a VFD. There are times when the motor start characteristics are ery bad, and the supply is very weak, when there is an advantage in using a VFD to start the motor. This is because the motor never operates in high slip with a VFD and so the start currents are very low compared to a reduced voltage starter.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Variable Voltage / Variable Freq. Soft Starter
A VFD can be used for starting a very large inertia requiring a longer ramp-up time than would be acceptable using other methods. An application requiring frequent starting and stopping might be another good VFD application.
RE: Variable Voltage / Variable Freq. Soft Starter
At these voltages, I'm not sure there would be any significant difference in VFD costing between the 50% load and 100% load units. The bypass would, of course, have considerable cost, as well.
About the best you can acheive with a reduced voltage starter is to reduce the inrush currents to one-half the across-the-line value. That would leave consider inrush current and the reduced voltage softstarter would have to be sized to handle this inrush current. On the other hand, an inverter could start the same load with no inrush at all, in most cases.
So, if you are soft-starting simply to reduce shock levels and mechanical stresses in the load, that can be acheived most economically with a reduced voltage starter. If you are trying to start with minimum inrush, then the VFD will give the best result.
RE: Variable Voltage / Variable Freq. Soft Starter
I'll confirm the issue of a "50% rated VFD" being an unlikely savings. The biggest cost on MV VFDs in in making the initial leap to MV. After that, current ratings make little difference in the overall price of the VFD part. Most of the VFD power systems are really sized in big "chunks" such as 200A, 400A, 600A etc. The difference for instance in a 200A vs a 400A is usually only a few thousand dollars (<5%). The associated switchgear would be more expensive, but you would have no savings there because at some point you still need to run the motor through it, so the switchgear is the same, or in fact more because of the transfer components. The only time it makes some sense is when you have multiple motors and you want to sequentially start them with the VFD, then switch them over to ATL (DOL). Still, in that case it makes better economic sense to use a fully rated VFD and run the last motor in the sequence directly from it.
Why don't you post more details as to what problem(s) you are trying to overcome and maybe we can offer better suggestions?
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Variable Voltage / Variable Freq. Soft Starter
respectfully
RE: Variable Voltage / Variable Freq. Soft Starter
I've heard of people overspeeding the motor with the drive just prior to an open-transition in order to have the motor speed at reclosing be as close as possible to rated slip speed, but that's not 1st hand knowledge for me.
JRaef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376