IT soft starter maximum current draw

Status
Not open for further replies.
we have a 400hp, 380v asynchronous motor supposed to drive a compressor (Full load current about 520A). We have installed Cutler-Hammer S801 series soft start for a smooth start. But starting current still remains a concern. How much amps our utility must supply in order to be sure that the starting current will not cause a problem when we use ramp start or kickstart mode?

Also Assume our utility can just supply about 500-600 amps. Can we use current limit option in soft starter to ensure that there will be no problems during start?

There is another question I didn't want to start a new topic for. Is distance a concern in selecting an ignition wire? Is there a standard for burial of ignition cables? And what other factors are required when selecting an ignition wire for a flare/ burn pit system?
 
Last edited:

kc8dxx

Senior Member
Location
Ohio
I would call the local Cutler-Hammer manufacturer's representative for the technical information on the motor-starter system. They will be able to advise you on the soft start current draw in it's various modes of operation.
 

charlie tuna

Senior Member
Location
Florida
this problem hopefully(?) was worked out between the power company and the specifier of the soft start unit before you bought the soft start?? by the amperages you listed, the full load is too close to the limitation of available power?? sounds to me that you would have been better off with a static drive than a soft start where you could control the maximum current draw.
 

hillbilly

Senior Member
Is this a air compressor?
If so, it seems that you would want to dump the air (bypass the discharge)until the compressor comes up to speed.
Also...a properly sized flywheel on the compressor input shaft (heavy sheave) would allow you to ramp up the compressor speed without overloading the electrical, and provide the torque required by the pump when you load it up (start compressing).
just a thought
steve
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The current the utility can supply is not fixed at some magic number. As you increase your current draw, the voltage drops, but you can still bring in a lot of current for short periods of time.

I think you are asking for trouble long term though if all your available power is being used by a single pieceof equipment.

The soft start will normally reduce the magnitude of the current inrush at startup. The manufacturer of the soft start should have some information on that.
 

robbietan

Senior Member
Location
Antipolo City
people,
it is my understanding that the utility supplies voltage, not current. current is load related. however, the amount of 'current' the utility can supply is usually the rating of the service transformer serving your facility.

i have experienced in another facility, the utility transformer fuses keep blowing everytime the plant's compressor motor starts. seems that the utility under-designed the transformer/transformer fuses and have to upgrade them to accomodate the starting current of the motor
 

hillbilly

Senior Member
Starting a large motor frequently can get expensive. I would find some way to ramp up the speed. Compressors are hard to start against a high (head/back) pressure so (IMO) a electronic soft start is not a good application without someway to bypass the discharge until the motor comes up to speed. Also, a flywheel is a old method to provide starting torque for large loads, but it is still very much applicable. Ramp the flywheel up slowly (low current draw) and when it's up to speed, use the kinetic energy of the wheel to start the load. The flywheel needs to be properly sized (weight and rim diameter) and balanced so that you have enough energy to start the load without twisting (or breaking) the shaft that supports it.
Hope this helps.
steve
 
Soft Starter Maximum Draw

Soft Starter Maximum Draw

Typically, at the Medium Voltage level, currents can reach 5 to 6 times FLA.
Assuming the same at 480V, you would see approximately 3120 Amps (6X540) with a L-L Start.

Depending on how electrically stiff your feeder circuit is, your Voltage drop may be significant, hence the need for the Soft Start. However, you may still see a problem even after the SS is installed.

I have seen cases where the Feeder is very soft, and the SS just drags the event out longer, and can actually make the Sag more perceptive to the eye, and disturb the Feeder worse.

The Load will play a major role also. Typically, any Compressor Load I have worked with will always start unloaded. Once, a customer tried to start a 10,000 HP Synchronous Machine pumping into a 1050 PSI Gas Transmission Line!!! That didnt work very well. We had successful starts at 720 PSI, but, because we were starting into a load, we didnt make the Utility's Voltage dip requirement.

The Customer then installed a recirculating bypass which allowed the Compressor to start unloaded. The Dip was acceptable to the Utility at that point.

I have Commissioned many Static Var Compensators for this problem.
With a 1 Cycle Response time, the Sag and Flicker is corrected to a calculated percentage.

A 300 HP Chiller on the end of a 17 Mile radial feeder can wreak the same havoc as an 18,000 HP Synchronous Machine sitting in a 230KV/12470V Sub.

With the correct information from your Utility on the Distrubution Feeder serving you, and your knowledge of your plants 480V Feeders, this can be calculated before you throw the switch and wonder why the Lights went out.

Good Luck!
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
While the starting current is in many cases 4 to 6 times the motor full load current, the inrush is often 10 to 20 times the FLA.
Don
 

ed_man

Member
Your starting current is still going to be 5-6 times FLA. Compressors can be a tricy application for a soft start. If not set up properly, it can make matters worse instead of better. I would set the initial voltage to 50% motor nameplate and set the ramp up to 3 - 5 seconds. Whatever you do, don't limit the current with the soft start. I tried that in my early days. It can cause the motor iron to saturate and the motor will run with a high slip at the max current limit until something gives. I've seen motors destroyed this way! In any event, you're pushing it with the utility supply.
 
Status
Not open for further replies.
Top