Starting Problem with Soft Starter

From my (perhaps faulty) memory, circuit breakers are allowed to be 2.25 x FLA. (?)

Tornado:

Depend on the breaker, Tonado. If it is a MCCB with intergrated current setting, it just can carry the current which is on its nameplate or let's say 65%, 85%...

Changing the breaker size is only acceptable if the cables supplying the motor are adequately sized for the higher current. Refer to algorithms within British Standard 7671 for an assessment protocol.

Is the upstream service adequately sized also?

The upstream ACB is rated at 3600A. The cables from the Incoming MCCB to the compressor panels is 2 run X 4C X 240 sq mm

Does that comply with BS7671?

nrmahadeokar:

300kW air compressor operated at 415VAC? Hmmmm...how you get that FLA at 493A?

The key performance of a soft starter is to reduce the initial voltage applied to motor terminals. The starting current thus reduced accordingly. So, you should adjust the voltage rather than current. Correct me if I am wrong.

Things would be a concern now is you need to get the graph of Starting torque vs applied voltage (100%, 80%, 60%...) to ensure if the motor can start and accelerate. Then check it against the current draw up. What is the motor manucaturer?

What do you mean by double ramp up? Is that "torque boost", a method to enhance torque? If it is, the current will increase.

The manufacturer of the motor is Baldor frame size 449TSC, SF 1.15. You are right Hien Double ramp up is torque boost for short period (like a pulse) to overcome the lock rotor torque.

I will check with AB if voltage adjustment is possible.

That sounds right. But please note that you need to work at the same time with Baldor for the graph of starting torque vs voltage.

Update us what you have.

I can send you the torque Vs voltage curves from Baldor. Can you send me the email id

My email id has been sent to you. Thank you in advance.

Increasing the ramp-up time you can solve the problem of the system inertia load but you can do nothing about the -shouldn't be there at startup- load of the compressor. Depending on working pressure and if technicaly applicable one can choke input or blow-off output pressure at startup. S.M.

the compressors are equipped with modulating inlet valve. When we are starting the compressors the inlet valve is 'crack' open. It is not fully closed due to development of vacuum and it is not fully open as it will not start in loaded condition

Let me be more specific. If your working (output) pressure is over 1 bar (2bar absolute) it's advantageous to fully close the input valve, and let full vacum occur. That minimizes the starting load. (unless there is some deficiency in the input plumbing that could give implosion danger etc) S.M.

What does the equipment manufacturer say about the incoming supply requirements, and why has this not been followed?

JRaef:

I respect what you pointed out and that it is very valuable. But may I ask you how the thermal tripping will react? Is that it will react according to the current in the system? If you decrease the applied voltage, the starting current thus reduced. How to activate the thermal tripping curve? I remember that most breaker trips by current not temperature. Moreover, where is the OL function? Is that it inside the soft start? How much OL current you want to adjust? 100? 125? or 165? There is not an issue.

The issue here is that the OP has the limited power source to his air compressor. We are finding the solution to start the motor with that source. All we need to do is to get the starting torque vs voltage or current. If it is impossible to start, the OP must request his customer to change the larger source.

I don't think that our last project done last year with 3500kW Cameron air compressor was a wrong way. Please be informed that the factory is under commissioning at the moment.

Standard circuit breaker trips are either thermal, by which I mean the thermal effects of current flow in the sensor element, and/or magnetic, which differentiates instantaneous high current surges such as short circuits. In a circuit breaker the thermal tripping is a curve that is loosely based on the thermal damage curve of the conductors it is protecting. Motor Overload curves are similar but offer closer protection to be able to protect the windings in the motor based on their thermal damage curve, which is generally lower than the conductors feeding it. That is why in most cases it is allowable to have the motor overload relay be the thermal protection device for the conductors, because it is better (lower) than a CB or fuse curve in that regard. But there are also rules on sizing of conductors and their protective devices that sometimes conflict with this allowance and you end up with a CB curve that is too low for the application.

When starting with a soft starter, all you can really do is control the voltage directly, and thereby control the current indirectly by measuring current and altering voltage to control it. However, the amount of ENERGY that it takes to accelerate a motor is always the same. So I like to teach that you have to look at a curve of energy that goes into an accelerating motor and if you start across-the-line (DOL), that curve is very steep, goes very high, but has very narrow (short) duration. Using reduced voltage starting does nothing more than trade height of the curve for width. You can alter the shape of the curve, but never the total area of the curve.

A Solid State Soft Starter can ALSO have the effect of lengthening the angle of attack on the leading edge of that curve, with the idea of gently increasing torque and posing the least mechanical stress on the drive train components. While this is very desirable in many applications, there is a price to pay for it. As you soften that increase, you are also increasing the length of time it takes to accelerate, essentially moving the other end of that energy curve out.

In any case, when starting a motor you are always looking to keep the thermal effects of that accelerating energy curve below the thermal damage curve of the motor. If you trip the OL relay (or solid state OL protection curve in the soft starter), then you have failed to meet that condition. A circuit breaker curve may be even less forgiving if not selected correctly. So when you already know you have that situation, the best approach is to forgo the softest starting and immediately apply as much torque as you can so as to keep the thermal effects of the acceleration energy from crossing over the damage curve of the motor, as supported by the OL tripping curve.

Can you check what minimum opening of "suction throttle valve" or "inlet guide vanes" is prescribed in compressur operation manual. Compare it with actual (phisical opening). More opening also give such trouble.

Is the compressor starting off load which for a compressor of that size ( or for any size come to that) should do?

Thank you all very much for your detailed explanation, today AB soft starter engineer is on site to evaluate the problem. I will update ASAP we get any solutions from them.

Oh, I thought that your project is still on paper?

Hi NRmahadeokar, with this type of soft starter there are many options available, you can soft start, kick start, current limit start, dual ramp start to name a few.

I would look at using the kick start option, because you are tying to achieve the required break-away toque to start the motor rotating.

For this kind of application I have had more success with a conventional star - delta starter, the star (starting) will supply enough break away torque to get the motor running up to about 60% of full speed and this way you will only draw about 2 x full load amps in the star condition, when the starter switches to delta there will be a rise in the current but this will only be for a very short duration.

Example: 75kW CNG Compressor; Main supply fuses 150 amp I was able to start the compressor every time without fail, except when the ring main load of about 12 other units was high, in this case we took out the entire neighbourhood, but this was not my problem. It was the supply company charging the other customers for 3 phase 60 amp supply's that is 12 x 60 amp + our 150 amp supply of 315 amp transformer fuses. They had to change them a few times before they upgraded the supply.

But going back to soft starters, it has been my experience when using a soft starter, in soft starting applications like this they are draw about 3 times the full load current during the start up, but this of coarse reduces as the motor reaches full speed.

Another application we had was on a smaller motor it was a 7.5 kW water blaster pump, full load 14.7 amps. The customer was trying to start this DOL but blowing fuses, we changed the starter to star delta and tuned the starting time so that as the pump got the max achievable speed in the star con-fig it then switched over to delta, now they can operate the system where ever they want of 16 amp supply fuses. Star Delta Systems are all about the tuning of the starting system.

But going back again to soft starters, it may be that you have to look at the circuit breaker that is installed is it "C" or"D" curve does it have adjustable trip time and current. The SMC unit can handle longer trip times because the current overload is controlled by the soft starter and not the circuit breaker, the circuit breaker is only there to protect the cable connected to the soft starter.

Have a look at the manual, try a few different options, you may also try the current limiting option but I don't think you would get the break-away torque required to start the compressor, but I may be wrong, look at all options, be different and assert yourself.

Best Regards

Joe

Dear All

In continuation of the previous discussion I thank everybody for the advices lended to me. We have started the compressor temporary by using DG as customer was in need of the oil free compressed air.

The only solution is starting the compressor with VFD (drive) as it is very difficult in this part of world (GCC) to change SLD once it has been approved by the local electrical authority.

we are still awaiting confirmation from motor manufacturer if the motor supplied to us is suitable for drive application and checking the customer back end if their system will be affected by harmonics procuded during the operation

Best regards

Nilesh

Nilesh,

What kind of temporary starting you are using? How much is the FLC?

Thank you,

the problem is with the breaker sizing from the customer. now we are using a dedicated feeder from DG with breaker rating of 1250A. This is temporary arrangement, we are still using softstarter with current limit setting of 350% for ramp up time of 20 secs