View Full Version : Soft start semi hermetic compressors

23-01-2016, 06:52 PM
Hi. Is it possible to soft start a 3 phase semi-hermetic compressor? Or does the load (gas pressure) make it impossible?

23-01-2016, 07:52 PM
There is always pressure on the discharge side and there are no problems with soft start, set it not more than 700ms.

Carlo Gavazzi are making good soft starters for refrigeration compressors at set rump up time, i use them mainly on scrolls over 7.5 HP.

23-01-2016, 08:30 PM
Also good for recip?
What about those with part winding, like some bitzers?

26-03-2016, 03:20 AM
copeland has a designed VFD for their 3 phase discus line. Never seen one on a recip but i'd use it.Part wind starting is eliminated as your VFD or soft start will lower your starting amps.

29-03-2016, 04:39 PM
Shouldn't be any problem at all.

Just make sure to size the soft starter to suit your application and take into account the higher start current after standstill. *EDIT* (spoke to my electrician friend and he said choose a soft starter that is the next size up from the one that fits the motor Kw to save yourself a lot of headache, and also the specify that you need one designed for a "heavy start") *EDIT*
Altivar and Danfoss makes good soft starters in my opinion

On one system I have been on, the customer hired an electrician to replace the star/delta starter with a soft starter, and he had measured the amps the motor was pulling while it was running at normal operating pressures and choose a soft starter to fit that power consumption.

After installing it he got it running with the help of the on site engineer, but after that it kept stalling (before the holding contactor could take over) after every defrost (cold room at +3C) when the suction pressure would rise.

It took a whole day of playing with ramping times and start current settings, while running defrost cycles and testing what the highest suction pressure it could manage was.

If you are considering a soft starter, you should also look into the cost of getting a VFD instead, prices don't differ that much and a VFD gives you more options for compressor regulation.