starting current of a 500 hp motor

Status
Not open for further replies.
C

cordova64

Member
Location
Vancouver Washington
I worked on a 500 hp compressor motor. It was 480 VAC. When the initial start up began the motor feeders in the pipe shook so bad the pipe actually vibrated violently. The motor was set up with solid state starting equipped with capacitors. Does any one know why this may have occured
 
I am pretty much sure you mention soft start device but what the setting you have it on now ??

the reason why the wire viberated pretty hard in the pipes due large amout of current running in the motor during start up stage.

Is this Compressor started on load or unloaded ?? it will make the diffrence there.

Merci, Marc
 
We are just getting ready to start up a 250hp. The instructions for the softsart we are using has special requirements for use with PFC capacitors. First was not on the load side of the softstart. Line side, they had to be at least 10' away from the drive, plus the use of contacors designed for switching capacitors was recommended.
 
Yep. That's a big no-no.

If those caps are downstream from the soft starter, chances are they have caused a short in one or more of your SCRs. If that happens you get a high DC component in the starting power output while ramping. The motor fights itself until the bypass contactor kicks in or the ramp time expires and the soft starter goes to full voltage. Might be time to rethink the application. Did someone not read the instructions?

It's possible that you got lucky and the caps merely caused self-commutation of the SCRs (meaning they turned on by themselves when they weren't supposed to), and they will be OK when you remove the caps. Check the SCR stacks with an ohmmeter going line-to-load on each phase (L1-T1, L2-T2, L3-T3, power off of course). If you have anything less than 10k ohms, you have a short. If you do, replace them all. If the caps damaged one, they probably stressed the rest to the point of near failure.
 
Jraef said:
Check the SCR stacks with an ohmmeter going line-to-load on each phase (L1-T1, L2-T2, L3-T3, power off of course). If you have anything less than 10k ohms, you have a short.
Click to expand...
Being SCR's and not triacs, shouldn't he read each stack with the ohmmeter leads in both directions?
 
LarryFine said:
Being SCR's and not triacs, shouldn't he read each stack with the ohmmeter leads in both directions?
Click to expand...
Each SCR "stack" will consist of 2 anti-parallel SCRs and a common heat sink. You can test the entire stack and if one is shorted, the entire stack conducts. Then you can sometimes split the heat sink on a few designs to determine which of the 2 SCRs is bad, but the directional test is still irrelevant because again, what you are looking for s the SHORTED SCR, not the good one.
 
cordova64 said:
In hind sight I am wondering what size capacitors would have been required with such a large motor. Assuming a original power factor of 65%. It is a 480VAC compressor motor
Click to expand...
Most capacitor mfrs have simple charts or a formula for determining the correct cap value.
 
The cap value is easily calculated. 65% pf? Seems quite low for a 500Hp motor. 500Hp is the largest low voltage motor recognized by NEMA MG-1 so the pf should be better than 65%.
 
Status
Not open for further replies.
Top