Selecting a Starter for an Induction Motor

The HV motor can be started with Auto transformers of either oil immersed or cast resin types.The number of steps on the auto trans and in starting and at what percentage is dependent on the duty/load the motor has to perform.For example,mixing molten rubber,compressing gas,pumping water etc are all very different in torque/current,duration of each step.Also to specify is the number of start-stops per hour.

In this case 6.6KV vacuum contactors taking supply via VCB incoming panel are suitable for this kind of operation.

I think you are likely to buy the motor,auto-trans and the starter board seperately from different suppliers which will save quite a bit.It is very important to have sufficient electrical data and imformation on the job/duty ,motor characteristic,load,transformer's in rush current withstand,heat dissipation,transformer manufacturer's experience etc.You should give order to the manufacturer who has the sufficient experience to make such starter.I have seen ABB which is a big international company with many engineers, repeatedly burnt several ABB cast resin auto transformers in the ABB HV starter panels which were completely designed,supplied by ABB.It was for pumping of raw and treated water.

I personally had puchased a number of 11KV,600 to 1500 KVA cast resin auto trans and starter board for sucessful projects in oxygen liquification plants.

Many consulting engineers cannot design the specific starter for a specific duty confidently.If you are the designing engineer,kindly seek help from someone who really has the experience.

Regards,

Khor

750KW 6.6KV.Ok now what need the tis the driven equipment?is it apump,Compressor,etc?Youorque speed curve of the driven equipment, GDsq value and use them to calculate the acceleration time at 100% and 80% Or 60% depending upon the type starter and starting voltage.900KVA looks unsuitable for a DOL.This cannot be decided without detailed calculations .Hope u understand

Although technically nesbura is absolutely correct in saying that you have nowhere near enough information to even begin a determination, I'll just add that you must also determine your acceptable voltage drop on the system. If, for instance, this is the only load connected to the supply, a voltage drop of 10% may be completely tolerable and that can affect your decision on a starting technology. However, most other electrical equipment will not be able to tolerate that much without causing damage and/or nuisance tripping. Typical would be 2-3% drop tolerance.

With that in mind then, I will say from experience that it is highly unlikely that you can start that motor with that supply and maintain a 3% maximum drop using anything other than a variable frequency drive. You just do not have enough power. the VFD can be a PWM or CSI type, but often times on large medium voltage motors like this a common technology used is an LCI (Line Commutated Inverter) because it can be easily bypassed at full speed to avoid losses through the drive.

I work for www.aucom.com

We make soft starters for this type of application.

Talk to our sales people.

even with a soft starter where the minimum applied voltage at starting has to be sufficient to overcome the starting torque required by the driven eqipument.A soft start item like pump with 30% starting torque is ok.But if it is a fan or high inertia item the starting time calculation is a must .This should be within the locked rotor withstand time of Motor .Many palce the diff. between acceleration time and locked rotor withstand time is kept at 2 sec.which is minimum.so it is not all that simple.Further torque varies as sq of voltage.so at 80% Voltage you get 0.64 FLT.

I have been in the soft starter business for 20+ years, I for one have never seen a soft starter be able to start a motor with a load connected using anything as limited as this application. The rule of thumb I use, which has never failed me, is that the source supply kVA capacity must be AT LEAST 2x the motor kW, more if the load has high inertia or friction. In this case the source kVA is only 1.2x the motor kW. Not going to work with anything other than a VFD that can accelerate it with FLC over a very long time.

People often think that just because a soft starter can limit the current to 50% or 100% or 150% of FLC, that it will work. What they fail to realize is just what nesbura is saying; it has to accelerate in an amount of time that does not exceed the damage curve of the motor, unless you are varying the frequency at the same time.