Motor Starting 1

[timm333]

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?

 

[dpc]

That's the whole point of soft-starting - it's just a fancy method of reduced voltage 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

 

[timm333]

Thanks DPC. Can I use capacitors across the motor terminals to compensate the voltage drop due to soft starter.

 

[dpc]

Well, I don't think you're going to want caps in the circuit downstream of the soft starter. There are a lot of harmonics associate with the output waveform of the soft starter.

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

 

[LionelHutz]

If you want full-voltage on the motor terminals then don't use a soft-starter or reduced voltage starter.

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.

 

[waross]

Hi Lionel.
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

 

[dpc]

I have seen starting capacitors used for large pipeline compressor motors.

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)

 

[jraef]

Using capacitors to get a large motor started is called "Capacitor Assisted Starting" and has been discussed several times here in the past. There are pros and cons, but the cons outweigh the pros in my opinion.

This thread has links I posted to other threads. thread237-224143 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

 

[lansford]

We had an application where a 700 HP mine pump required not less than 80% auto transformer tap reduced voltge starting. The utility has a soft system and required not greater than 50% autotransformer tap setting.
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

 

[LionelHutz]

I've seen it done with a soft-starter a few times. It works well once the cap bank is online, but there is a large transient voltage and current from energizing the capacitor bank.

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.

 

[antigfk]

OP,

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.