Starting an air compressor SLOWLY?

[GoldDigger]

Since both the supply and the return conductors (as well as the EGC I hope) are wrapped around the iron there will be no net current and therefore no inductance.
If the EGC does not follow the same path, then fault current will see the inductance and the short circuit fault current might be limited, potentially delaying the breaker trip.

 

[ActionDave]

How much does voltage drop at the generator though when the air compressor is starting? Very likely you have some drop in the generator itself, then some more drop in the conductors between source and load. You can increase conductor size all you want, it will not change the impedance of the source.

:thumbsup:

 

[moonshineJ]

How much does voltage drop at the generator though when the air compressor is starting? Very likely you have some drop in the generator itself, then some more drop in the conductors between source and load. You can increase conductor size all you want, it will not change the impedance of the source.

Ok, I saw voltage drops from 240 VAC to 207-215 VAC both while on shore tie, or one of generators. Each generator breaker is rated at 360 A. Shore tie breaker is rated at 200 A.

Here is my next question: while observing starting current for air compressors (they both behave about the same), I saw starting current in a phase from 175 to 222 A. I guess this depends whether compressor starts upstroke, downstroke, etc. I saw such difference on compressors before. Of course, 222A lasts probably less than 1 sec, while shore tie breaker (200 A) is continuous rating. But, are there any implications...200 A vs. 222 A, per se??

By the way, the tech from air compressor company suggested among other things (soft start, namely) changing the motor for more HP, while keeping the same compressor + changing the pulley size. He thinks this can reduce this inrush current. I told the guys from this vessel that changing both motor AND compressor for say 10 HP can perhaps be a better solution.

kwired, I absolutely agree: the shore tie AWG is huge (3/0), compared against a minimum AWG 8 required for this motor. I checked the bus work - no problems! But yes, both the x-former's secondary supplying the shore tie and generators' winding have copper wire much smaller than AWG 8.

 

[don_resqcapt19]

... Insufficient conductor size jumps to the top of the list, meaning all of the conductors connecting to the starter and the motor. Your conductors for that entire circuit should be no smaller than #8, larger if the distance is longer.
...

Since the voltage drop is at the power source, why wouldn't undersized conductors act like a reduced voltage starter and actually make the voltage drop at the source less when the motor is starting?

 

[GoldDigger]

Since the voltage drop is at the power source, why wouldn't undersized conductors act like a reduced voltage starter and actually make the voltage drop at the source less when the motor is starting?

That might be true for a motor starting without load. But if the voltage drops far enough a loaded motor (especially a constant torque load like a piston compressor) simply will not start and will draw the LRA until the OCPD or overload opens.

 

[bob]

" Sometimes it becomes a nuisance, especially when voltage drop shuts some LVP controllers. "


If this is your main concern you might consider using a ferroresonant transformer for the supply to the controllers.
It would maintain the voltage level during the startup.

 

[Besoeker]

FWIW, we did a reduced voltage (auto transformer) for a fairly large gas compressor. It started on the 80% tap. Direct on line, it pulled the station supply down and that of much of the surrounding area.
Reduced voltage worked in that case.

 

[kwired]

Ok, I saw voltage drops from 240 VAC to 207-215 VAC both while on shore tie, or one of generators. Each generator breaker is rated at 360 A. Shore tie breaker is rated at 200 A.

Here is my next question: while observing starting current for air compressors (they both behave about the same), I saw starting current in a phase from 175 to 222 A. I guess this depends whether compressor starts upstroke, downstroke, etc. I saw such difference on compressors before. Of course, 222A lasts probably less than 1 sec, while shore tie breaker (200 A) is continuous rating. But, are there any implications...200 A vs. 222 A, per se??

By the way, the tech from air compressor company suggested among other things (soft start, namely) changing the motor for more HP, while keeping the same compressor + changing the pulley size. He thinks this can reduce this inrush current. I told the guys from this vessel that changing both motor AND compressor for say 10 HP can perhaps be a better solution.

kwired, I absolutely agree: the shore tie AWG is huge (3/0), compared against a minimum AWG 8 required for this motor. I checked the bus work - no problems! But yes, both the x-former's secondary supplying the shore tie and generators' winding have copper wire much smaller than AWG 8.

The 8 AWG required for this motor is a minimum size to prevent overheating of that conductor during run time. The motor still draws a high current for a short time at startup that will result in higher voltage drop during that time then what you will see once current drops after the motor reaches speed. No matter what is in the path that voltage drop during starting won't hurt the motor much as long as it don't drop to the point it causes the motor to fail to accelerate, but if the same conductor is supplying other loads - that is where you notice the effects, especially if lighting is supplied by same feeder.

 

[Phil Corso]

Gentlemen,

Starting current and starting torque of the typical induction motor depends solely on its design parameters! The Driven-Load, itself, has absolutely no impact on breakaway-torque, runup-torque, pull-out torque, or even full-load torque!

The Driven-Load, however, does materially affect run-up or acceleration time... because it depends how much net-acceleration torque, i.e., the torque produced by the motor, less the torque required by the driven-machine, is available!

Regards, Phil Corso

 

[Besoeker]

Gentlemen,

Starting current and starting torque of the typical induction motor depends solely on its design parameters!

Not so. It depends also on applied voltage which is why reduced voltage starters work.

 

[Phil Corso]

Besoeker..

Please note... I purposely addressed only "The LOAD"!" I call your attention to the "typical" induction motor model: Stator and rotor impedances are unaltered at start!

Yes, the magnetic-circuit can be affected by voltage! I said, can, because while altered using the "T" model, it is unaffected using the "Pi" model!

Phil

 

[Phil Corso]

Besoeker,

"Yes, the magnetic-circuit can be affected by voltage! I said, can, because while altered using the "T" model, it is unaffected using the "Pi" model!"

Oops... I should have said "Yes, the magnetic-circuit can be affected by terminal-voltage! While the "Pi" model is affected by only terminal-voltage, the "Tee" model is affected by both terminal-voltage, and starting-current!"

Phil

 

[Besoeker]

Besoeker,

"Yes, the magnetic-circuit can be affected by voltage! I said, can, because while altered using the "T" model, it is unaffected using the "Pi" model!"

I use the Steinmetz model. It has served me well for many years. Getting performance figures wrong would have resulted in swingeing financial penalties.

 

[Phil Corso]

@ Besoeker... so the "T"!

 

[mivey]

As of logging events, I thought about this, actually having on mind a different case. Problem is they have no electrician....unless something is going on. I found some good stuff on market, namely Hantek 365. It has models A thru F, with a price range $100-150. Going to get one.

It is only good up to 10 amps so might not help you with this case.