Motor Starting
Motor Starting
(OP)
I am performing motor starting study using ETAP. If I don't use the soft starters on the motor to be started, the voltage across the generator switchgear drops down. If I use the soft starter, the voltage drop across the generator switchgear is improved, but now the voltage is dropped across the terminals of the motors to be started. Does someone have idea which type of soft starters should I use that will not cause a voltage drop across the terminals of the motor?






RE: Motor Starting
If you put full voltage across the motor, it will draw full current.
If you can't live with this situation, you may need to think about installing a VFD instead of a soft starter.
"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
RE: Motor Starting
RE: Motor Starting
Generally the soft starters are used in situations where the motor can start successfully at a reduced voltage. This is most of the time for fans and pumps.
"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
RE: Motor Starting
If you want full voltage then investigate switching a capacitor bank online with the motor to compensate for the reactive current the motor draws. Switch the capacitor bank offline again as the motor reaches speed.
If you don't think you can start the motor directly or with a soft-starter then look at using a VFD.
RE: Motor Starting
I read a paper on that technique thirty some years ago but I have never seen it in practice. I am wondering if anyone has seen the technique in use. As I understood it, the capacitors were cut out in steps as the motor accelerated.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Motor Starting
But it takes a lot of capacitance to get the voltage back close to full voltage.
"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)
RE: Motor Starting
This thread has links I posted to other threads. thread237-224143: Series Capacitor Help Diminish Vdrop Due to Motor Starting??? Read carefully before jumping into that.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Motor Starting
We purchased a motor starter that energized a large block of capacitors to compensate for the reactive inrush requirement at starting then droped out.
It worked well and all parties were happy.
JIM
RE: Motor Starting
There are a few companies that sell faster capacitor switching compensation systems. Basically, they use a diode/SCR anti-parallel and charge the capacitors to the peak of the line voltage. Then, just turn on the SCR at the right time to get transient free switching.
RE: Motor Starting
Did you plug the speed/torque/PF versus time curves of the motor plus the speed vs torque curve of the load into ETAP? With the softstarter using Dynamic motor acceleration, did it accelerate to full speed? If so, then don't worry about the voltage drop at the motor terminals caused by the soft starter. That is the whole point of the soft starter - to reduce starting voltage to the motor.