Soft-start

[bob]

Hi,

 

We have actually in our plant one 250 k W electric motor driving an ammonia compressor for a chiller plant. The motor is statrted by an open transition star/delta starter. I am looking forward to replace this starter by a soft starter. Is a solid-state reduced voltage starter suitable for such an application ?

 

Bob

[marke]

Hi Bob

 

Yes, a soft starter is very suitable for this application, I have done this many times.

You need to ensure that you size the soft starter to supply sufficient start current to get the machine up to full speed.

The start current is dependent on the start torque requirements of the compressor and the ability of the motor to develop that torque.

 

For screw compressors with the slides fully closed (off load start), you will commonly require 40 - 50% Full Load Torque to start the compressor. You can determine the start current from the motor characteristics by taking the full voltage start current and start torque curves.

As you reduce the start current, you reduce the start torque by the start current reduction squared. i.e. half current gives you a quarter torque.

For motors of this size, it is common for the start current to be in the order of 400 - 450% FLC on screw compressors.

High efficiency motors require a higher start current to develop torque than standard efficiency motors.

 

I would recommend that you select a soft starter capable of at least 450% start current for this application.

 

Best regards,

[kens]

Hi Bob, I wonder if it is not worth investigating putting a VSD on this application. Is the system over capacity at all? There is the potential for significant energy savings on screw compressors that are not running at 100%. Basically you will leave the slide fully open after start and vary the speed. This means that the compressor will run at its most mechanically efficient. If you are going to replace the satarting method any way it may be worth a look.

If the machine runs fully loaded all the time then dont worry about a drive just stick with the soft starter, however i would recommend that you bypass it once the start sequence is over to avoid SCR losses.

Both these method are very common on large ammonia plants here.

 

Ken

[marke]

Hi Ken

 

Yes, that is a good point.

You must do the sums though, as the drive has losses that reduce the potential savings and the drive also has a limited life compared to the soft starter.

I have seen many installations of drives sold on the grounds of energy saving, but running at a constant speed, very close to rated speed, where the losses in the drive are probably of the same magnitude as the savings and there was no commercial advantage at all, plus a high capital cost and limited life. The soft starter would have been cheaper to buy and would last considerably longer.

 

I believe that some drives sales personnel forget that the drive only reduces the mechanical losses when the machine can operate for a period of time at reduced speed.

 

Best regards,

[AB2005]

The motor is started by an open transition star/delta starter.

 

Hello Marke

 

I know about the star delta starter which has three contactors and a timer. Then what is the meaning of term "Open transition"? Please explain.

 

Thanks a lot.

 

[marke]

Hello AB2005

 

Have a look at http://www.lmphotonics.com/m_start.htm and http://www.lmphotonics.com/star_delta.htm

This should explain things for you.

 

Best regards,

[bob]

Thanks all, Marke. Is 450% start current a standard feature for Soft starter ?

 

Regards.

 

Bob

[marke]

Hello Bob

 

No, there is no real standard start current that everybody uses. Some starters are rated at 300% current, some at 400% current and some at 500% current.

Some starters have different ratings for different start currents so that you can rerate to 450%, but some just can not be set that high. Hence th reason that I mentioned it, because if you buy on price, you may end up with a light duty starter that just is too small for the job. - It often happens!!

 

The other variable is start time, i.e. 300% for 5 seconds is very different from 300% for 30 seconds.

I would target 450% for at least 15 seconds with an appropriate number of starts per hour.

 

Best regards,

[kens]

 

"You must do the sums though,"

 

"I believe that some drives sales personnel....."

 

Best regards,

 

 

Hi Marke, all I do now is the former, used to do the latter

 

Many screw compressors are oversized for the load as like most engineers the fridgys hate to have an undercapacity machine at the end of commisioning

I feel that with a 250kW machine it is well worth looking at, but like you say, spend the time doing the analisys. Most bean counters I know would demand a pretty sound arguement when the drive will probably cost 3 x the SS. If possible try to look into the future as often demand changes and the oversized machine today may end up flat out tomorrow as the plant managers try to get the most out of underutilised plant.

 

Ken