Starting Current

Worst case, 5-7 times the FLA rating.But beyond that, no.

Starting current is totally load dependent at anything less than Locked Rotor Current (500-700%). It also depends on the starting methodology. You can use devices such as soft starters, electro-mechanical reduced voltage starters and variable frequency drives to reduce starting current, but again, there is no way to predict it in advance without a lot more information about the load and motor. You need Wk^2 of the load and motor, torque-speed requirement curves of the load, torque-speed delivery curves of the motor, gear reduction (if any), power supply limitations, allowable voltage drop etc. etc. There are complex and expensive programs to predict it for you and they work well, but it's not something you can do off the top of your head.

Programs I know of which provide what is called "Transient Motor Starting Analysis"; SKM Power Tools, ETAP

Hi Jraef,

Thank you for the input, i forgot to tell you this compressor is supplied by 100kw,380volts, 50 Hz. Base on what i know this generator will drop 25 to 30% reduction on total kw capacity because it was 50 Hz.. As soon as i start the compressor the generator shuts down automatically and module shows low voltage.

Do you think increasing the voltage parameters of the generator will help?

Hi freewill,

If you increase the voltage of the generator, it will effect to other equipments that are supplied power by generator. so you can install a soft starter to prevent generator shuting down such as Mr.Jrafael's above solution.

But here, I want to discuss with you about your compressor. Any medium or big compressor must have unloader unit for compressor starting and stopping, it controls compressor will run with no load when start then after few seconds it closes load for compressor. This will protect your compressor's parts: bearing, crane,......

Therefore the optimize solution is you have to contact compressor manufacturer for installing the unloader.

Best regard.

negative, just play for time

you need to add some capacitors in a starting circuit. That is too large a load relative to generator nameplate to simply connect. Also may need to adjust the generator controls to allow a longer low voltage period for starting this motor.

INRUSH CURRENT AT DIRECT-ON-LINE DOL starting will be~300amps.

You do not calculate that. It happens so due to motor designer's inability to make any less.

If the generator is diesel driven , during the starting period the speed shall drop but not as much as gas .

As a quick calculation the starting current is between 5 to 7 times the normal running current but the power factor drop drastically, so I do not see any major problem

Using the variable frequency controller as mentioned will help you eliminate the need for a capacitor. It will also help you control the amount of power in terms of amps to be supplied towards the induction motor thus also eliminating the need for knowing the right amount of current needed.

This is based on the pressumption that you are properly powering your compressor with the right motor.

Your starting current of the compressor shall be around 385 Amps. Generator shall stall. Your generator gives only around 193 Amps at 380V. You cannot have any additional inductive loads at the time of starting. If you have one or two lights in the vicinity you will notice that go dark. If you dont have any options of having 200kW Gen then increase fuel maximum at thetime of starting the compressor with conventional starter. Usually 30kW has unloaders. If not you have to add a unloader. Also go for a soft starter. That's really expensive stuff but not as a 200kW gen.

Even you are successfull you cannot connect any more big inductive loads. Your voltage go down everytime you try that. good luck . Just give us your feed back

You may already have the information to solve your problem. Seems there's been a lot of good advice posted. Just thought I would throw in a couple more possibilities. Since I don't know what your budget is the capacitors are cheap and would not only store energy for the initial surge but also help with your power factor both when starting and running . Mechanically unloading your compressor may be quite simple and cheap also. A couple solenoids and three-way valves, I'm making an assumption that were talking about an air compressor,. We do similar things with refrigeration compressors, in the case of an air compressor cycling to maintain whatever its preset pressures are isolating the pressure in the system from the compressor, and venting the discharge should help reduce the starting load. Restricting the suction side should also help during startup. I don't know your piping size but imagine it's quite large judging by the size of the compressor, so the valves could be expensive, or possibly you have some lying around. I've done similar things manually when I've had to start an air compressor in the field off a generator as a temporary solution.

I don't know of any of this will be of assistance to you just thought I'd throw it in. The VFD, soft start devices, all will fix your problem, without knowing your budget and parts availability or timeframe just thought I'd add some low-tech solutions.

I ran out of time to edit my original comment, if it is possible to isolate your suction side there may be some other even simpler solutions. I would need to know more about your system to say. Good luck

starting current is design related.During design stage using special designs u can limit to 5 times but becomes expensive.Standard cage motors have 6-7 times rated current at the start.However the duration of the staring current depends upon the type of starting a s well on the acceleration time which depends on load inertia.I

Thank you to all Gentlemen all of your input are all good and considerable. These will help me to solve my problem.

Regards to all,