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# How to calculate the delay time for a soft starter ?

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#### munzir

##### Full Member level 6
How to calculate the delay time for soft starter for converting star into delta ?

What factors and things should be taken into account ?

Does the calculation of delay time for normal star delta circuit same as for Soft Starters ?

thanks

Re: How to calculate the delay time for soft starter ?

the object of a soft start for a motor, is to reduce the inrush current to the motor and to reduce mechanical strain on mechanical systems. So the maximum time is the time the motor acceleration on the first step is zero, i.e. the motor has reached its maximum speed on the first step. This time can be shortened but only by allowing the motor to take excessive current. i.e. inrush current on switch on star fed = 50A, after some time it falls to 30A, then if switching the motor to delta the inrush is again 50A, falling to 30 amps. I would think this is the optimum time period. It also depends on how many stop/start cycles the motor is going through as the problem is that the motor will over heat if run at 50 A for more then say, 10 seconds. So if in the morning the motor is switched on and off again at evening time then its OK. if its switching on and off continuously , like a a machine tool, then the motor might slowly overheat and burn out.
Frank

Re: How to calculate the delay time for soft starter ?

the object of a soft start for a motor, is to reduce the inrush current to the motor and to reduce mechanical strain on mechanical systems. So the maximum time is the time the motor acceleration on the first step is zero, i.e. the motor has reached its maximum speed on the first step. This time can be shortened but only by allowing the motor to take excessive current. i.e. inrush current on switch on star fed = 50A, after some time it falls to 30A, then if switching the motor to delta the inrush is again 50A, falling to 30 amps. I would think this is the optimum time period. It also depends on how many stop/start cycles the motor is going through as the problem is that the motor will over heat if run at 50 A for more then say, 10 seconds. So if in the morning the motor is switched on and off again at evening time then its OK. if its switching on and off continuously , like a a machine tool, then the motor might slowly overheat and burn out.
Frank

the motor is of 700KW and it takes almost 4000 amps when it runs on star and the acceleration time is about 80 seconds and then it converts into the delta .....

Does the calculation of delay time (acceleration time i.e. the time that motor takes from zero speed to full speed) for normal star delta circuit same as for Soft Starters ?

Re: How to calculate the delay time for soft starter ?

Soft starter application seems only reasonable when a slower motor start than with star/delta is wanted and feasible. The large acceleration time of 80 sec in star/delta makes me doubt that a soft starter with sufficient start time will be avaibale at all. A 700 kW soft starter is an expensive part (10k $range) and you can expect detailed manufacturer guidance for projecting it. Re: How to calculate the delay time for soft starter ? Soft starter application seems only reasonable when a slower motor start than with star/delta is wanted and feasible. The large acceleration time of 80 sec in star/delta makes me doubt that a soft starter with sufficient start time will be avaibale at all. A 700 kW soft starter is an expensive part (10k$ range) and you can expect detailed manufacturer guidance for projecting it.

Means , soft starter application is only for Slower Motor Start ? What do you mean by that ?

What does it mean by that , "when a slower motor start than with star/delta is wanted and feasible." ?

Re: How to calculate the delay time for soft starter ?

Means , soft starter application is only for Slower Motor Start ?
Yes, you'll use a soft starter to start a motor "slower" (with less torque and motor current) than achieved with direct or star/delta start.

What's your intention to use a soft starter?

Re: How to calculate the delay time for soft starter ?

"s of 700KW and it takes almost 4000 amps " is just a motor with a lot of zeros attached to it. How much mechanical load does it drive?. If you switch it off how long does it take for the system to coast down to zero speed. Is it a high stiction load? Why are you querying the 80 second start up time, do you think its too long. Is 4000 amps overloading the motor?. So many questions which you should be able to answer then work out what to do.
Frank

munzir and FvM

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### munzir

Points: 2
Re: How to calculate the delay time for soft starter ?

"s of 700KW and it takes almost 4000 amps " is just a motor with a lot of zeros attached to it. How much mechanical load does it drive?. If you switch it off how long does it take for the system to coast down to zero speed. Is it a high stiction load? Why are you querying the 80 second start up time, do you think its too long. Is 4000 amps overloading the motor?. So many questions which you should be able to answer then work out what to do.
Frank

basically there's a damper attached to this motor and this motor is used to **** up the dust and fumes produce during melting process of metal scrap by induction furnace (oven) ........

it takes 80 seconds to reach its max. speed and takes more than 80 seconds to stop fully

i just want to know how the engineer set up its acceleration or ramp time to 80 seconds , how he calculated it ?

Is it a trial and tested method ?

thanks

Re: How to calculate the delay time for soft starter ?

i just want to know how the engineer set up its acceleration or ramp time to 80 seconds , how he calculated it ?

Is it a trial and tested method ?
Yes, and the timing usually isn't critical. An experienced engineer will set the parameters within a few minutes.

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