motor voltage dip problem

[weressl]

Although it will be expensive, installing a Static VAR Compensator will solve your problem. One example is below.

http://www.converteam.com/majic/pag...atic-Var-Compensators--SVC----Converteam.html

..not to mention reducing your reliability by adding a multi-component system to your fairly simple rotating machinery.

 

[weressl]

A cheaper alternative is to start the big motor with a 480V pony motor. What this does is it gets the big motor spinning at full speed prior to energizing the terminals, so the starting torque of the motor is nil. Don't know of your design could be modified for that, just throwing it out there.

So how would a smaller motor be able to deliver the same starting torque? You can reduce it by adding a large gear, but then you would need to overdrive the small motor with an ASD multiple times over it's synchronous speed to get to the main motors' full load asynchronous speed and then have a mechanism to decouple it. Switching full voltage onto a rotating mass will have it's problems also until the fields aligned and the rotor is magnetized.

Somewhat complex, but not impossible.

 

[dbuckley]

Another possibility is some local generation plant, particularly if the motor doesn't have to start very often...

 

[kingpb]

So how would a smaller motor be able to deliver the same starting torque? You can reduce it by adding a large gear, but then you would need to overdrive the small motor with an ASD multiple times over it's synchronous speed to get to the main motors' full load asynchronous speed and then have a mechanism to decouple it. Switching full voltage onto a rotating mass will have it's problems also until the fields aligned and the rotor is magnetized.

Somewhat complex, but not impossible.

The pony motor concept will only work when the load can be decoupled such as through a gear, belt, or fluid drive, etc. The pony motor only needs to be able to overcome the torque of the larger motor since no load is connected. The pony motor would only need to be at 480V. To further reduce starting requirements you could still put it on a AFD, which would still be much cheaper then and AFD for the 8000Hp motor.

 

[Designer69]

No way to decouple this load but now I understand the 480V pony concept. Thanks

At this point it appears the prevailing option is to start up at deadhead (keeping all discharge valves and flow paths closed) until the motor is up to speed then open them using a TDR.

I personally wanted to try to stay away from that due to higher pressures and possible cavitation on the system, but the facility appears to be too cheap to purchase a static VAR compensator.

thanks for your help

 

[kingpb]

Don't forget you can bump any upstream transformer taps, at least -5% to increase the 13.8kV to 14.5kV. Gear should be rated 15kV.

Just watch your no-load voltage, as long as it doesn't overvoltage your bus by more than 10% you should be OK.

 

[weressl]

The pony motor concept will only work when the load can be decoupled such as through a gear, belt, or fluid drive, etc. The pony motor only needs to be able to overcome the torque of the larger motor since no load is connected. The pony motor would only need to be at 480V. To further reduce starting requirements you could still put it on a AFD, which would still be much cheaper then and AFD for the 8000Hp motor.

Not necessarily so.

Maybe I did not explain it clearly.

The proposal I was talking about is starting the FULL load without decoupling. Since the smaller motor would still need to deliver the breakaway torque, it can do it with gear reduction. That means that using the asynchronous speed of the small motor at 60Hz with FVNR start the main rotor would only rotate at the fraction of its own asynchronous speed, that ratio will be the same as the gear ratio. From this point on you have two routes:

1. If the man power can be applied and the motor can accelerate the load to full speed, all is well. (Don't forget to de-energize or decouple the small motor BEFORE .)
2. If the achieved speed is insufficient, then an ASD can be installed for the small motor and overdrive to 420Hz or whatever the ASD can produce, to achieve a speed of the main rotor closer to its asynchronous speed, or whatever speed is necessary not to stall the MV system.

I think that the cost of the mechanical system in my scenario would be considerably less than a main coupling, not to mention more reliable because the relative mechanical simplicity.

 

[weressl]

At this point it appears the prevailing option is to start up at deadhead (keeping all discharge valves and flow paths closed) until the motor is up to speed then open them using a TDR.

I personally wanted to try to stay away from that due to higher pressures and possible cavitation on the system, but the facility appears to be too cheap to purchase a static VAR compensator.

Well, you know what they say: pay me now or pay me - much more - later.

On the other hand the philosophy is reflective of the American approach, that made them so successful in the industrial age. If I pay today for a perfectly engineered system, it may bankrupt me in two years. (So there will be nobody there to pay 'later'.) If on the other hand I can have this solution work for me for two years, I can accumulate capital where I can afford to pay the extra maintenance cost or improve the installation WHEN it is needed. (Mercedes HAD to force their engineers to design cars that WON'T last 15 years, or whatever length.)