3 Phase induction motor starting

How much time do you have on the TD relay? It may not be enough.

Try a closed transition starter

Thanks for your advice.

We have tried the close transition starter as well. It works effectively, but still there is a problem of still high starting current(I think, this may be because of high starting torque requirement) . We would like to reduce it.

Can you please suggest will it be useful to have soft starter OR VFD OR any other mechanism.

Regards,

Manish

Could you start the motor on no load and apply the load gradually?.

Yes, we are starting the motor at no load, we want to reduce the starting current from what it takes in closed transition starter.

It is Horizontal impact type crusher. Which is powered by Utility and in case of power cut it is on Generator. I would like to mention here that just because of high starting current we had to install higher rating generator, which is really a shame. We need to start machine first. Once this is started then all other motors get started.

We can live with upto 20% voltage drop.

We have used closed transition Star delta starter, but still the current is quite high. If I have understood correctly, the purpose of close transition Y-delta is to reduce the voltage across motor, so it will draw less current? Won't it be the same effect if we use soft starter? OR Soft starter will be better as we can ramp up the voltage smoothly to regulate the current?

Cheers

Transition Star-Delta does not reduce the starting (Star) current. It only reduce the chock from star to Delta, reducing the peak at the Delta reconnection. Therefore, your problem is the same if you are looking to reduce the STAR current!

If the fuses are blowing only when starting with a generator power supply, it means that the voltage drop experienced by the Genset is too high, creating a higher starting current than when using the Utility supply. For this, the suggestion from One of the contributors, to use Power factor correction capacitors on the motor might help if the generator size is not increased OR you want to start the crusher at any time other than the first item in sequence when on Genset.

Otherwise, Bigger genset, or Bigger fuses/CB (Watch the Heating problem on the motor winding...?), Soft Starter properly connected to be removed after starting completed, Auto-Transformer starting system (more expensive and bulky...).

At the end of the day, you have to live with some compromise and reduce the running cost / breakdowns but keep the machinery running!

It sound like the motor is not getting up to sufficient speed before the transition from star to delta. This is not just bad because it blows fuses, it also generates some fairly savage torque transients that can damage both the motor and the plant.

Star-delta starters are a very poor choice of starter for applications needing high starting torque, largely because of these issues.

A better starting method would be to use a soft-starter.

Consider installing a "large" (1000 Lb or more) flywheel that is at least the motor's diameter on the motor shaft. Isolate the motor and flywheel from the drive shaft of the crusher with a magnetic clutch.

When the motor / flywheel is up to speed- less than 2 seconds by my guess so give it a 10 second timer control- then engage the clutch. The inertia of the flywheel, coupled with the full speed motor's torque, should get you there easily.

Hi Mjain,

I have found strar delta starters have been better than soft starters, for use in high torque applications. But you have to design the power around the application, fuses must be the correct size with the correct curve ratting "D" Curve for motor starting.

I would recomend that you replace the fuses with a D Curve Circuit Breaker of the correct rating, for the motor load you are trying to start. The recomended current ratting of the circuit breaker may be a lot more than you think.

On starting the motor try to tune is so that the time delay between star and delta allows the motor to get to the maximum speed in star and then switches to delta, this may take a few startups, but this is what we call commisioning.

I have found that reduced voltage starting (star delta) in the applications that we have used it in draws less current than soft starting, but there is a down side to this and that is the transition from star to delta, if you dont get the Star wind up time correct you will have a greater inrush current on the transition from star to delta, this has to be fined tuned. The circuit protection has also to be off the correct ratting.

Cheers

Joe

GA

In the environment of a stone crusher, a Star-Delta is cheaper to maintain, IF properly designed with the appropriate fuses/C.B. and Timing delays.

It all depends on the inertia of the moving parts and the type of torque/load ( Only rotating, or Cyclic). If rotating, the delay to reach a reasonable speed before Delta can be accomodated in most cases. If the load is cycling, you might need to reach Delta quicker and this might be the hurdle to Overcome: Here, a clutch/Flywheel system could be useful.

In Any case, the OEM of the crusher would have designed the system with some conditions attached regarding the power source. The need to modify the system will arrise if the power source is not up to the recommendation.

Can you reduce the load until the motor is up to speed?

eg if it is a hammer mill lift the hammers off their cams during start up?

I have handled similar problem of same 132kw(180hp) siemens motor.It starts,but when about to switch to delta,the 400amp breaker installed will trip.

The breaker by my calculation would have been a little higher than 400amps,but not up to 630amps which would be over sizing.But 400amp was within range and should be able to start the motor.

I had to check the timer setting from star to delta,i found out that the timing was a little longer and and already causing some heating before changing to delta,which normally trip the breaker.I had to reset the timer by reducing it and it started well in star and then to delta and since then it has been working till date from year 2007.Another one happened,but this one was 110kw,this time i had to change the fuse from 315amps to 400amps and increased the timer setting for delta and it has been working fine since year 2008 till date.

Did you use, closed transition Y-delta starter?

Why don't you give more details like rating of CB,thermal/magnetic range,settings,OL setting,starting duration in star,current in star,current in delta etc. Did you measure the current with machine only but without load,whether it is tripping after adding load?.

This maybe a strange solution, but in my case it worked. On a compressor with a 11 kW motor and a supply 400V 20A 3 phase, it worked well for more than 10 year already. The motor was started using star / delta, but the breaker tripped in most cases. So we installed a 3 phase capacitor parallel on the motor. The capacitor changed the power factor of the motor to unity and decreased the current. I don't remember exactly but I thought it was about 6 ampères on a +/- 22 ampères full load current that we won….

Assuming that the fuses are correctly rated, the reason of having high inrush current is that, during the transition there is a spinning rotor within the stator and has a magnetic field. Due to the low impedance of the rotor circuit, the time constant is quite long and the action of the spinning rotor field within the stator is that of the generator which generates voltage at a frequency determined by the speed of the rotor. When the motor is reconnected to the supply, it is reclosing on an unsynchronised generator and this result in a very high current and torque transient. These current and torque can be much higher than the DOL current and torque.

Therefore, try DOL starting since the requirement is high starting torque andyour problem will be solved.

hi,

I do not have enough knowledge about motor starting connection but in our plant we also 250 KW motor that drives our Horizontal Impact Crusher grinding hard limestone, i cant be sure if it uses star delta connection but what I am sure of is that we used a sort of grid resistor arrays that controls the starting current and by that we do not have any problem about high starting current.

I hope this will help..

by the way our plant was commissioned last 1966 and our Impact Crusher was manufactured by Kobe steel....

There are two problems that can result from the use of a star/delta starter and give you the high current you describe.

1. The motor has a pretty flat speed current torque curve until it gets to above 90% speed. If you transition from star to delta at anything less than 95% speed, you will get a start current close to the locked rotor current of the motor until it reaches full speed.

2. As the standard star/delta starter is an open transition starter, there will be a very high inrush current following the transition that ay last for a few cycles.

For more information, see http://www.lmphotonics.com/m_start.htm#StarDelta

I would recommend a soft starter for this application, have done plenty.

Best regards,
Mark
http://www.power-factor.co.nz