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Reducing Start Current 70%


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Just slipped into this forum when searching for soft starter manufactuers in google. Seems to be people here with great knowledge why I try to post a question.


I work as a development engineer for a small swedish heat pump manufactuer and due to a german regulation we've encountered a big problem regarding starting current for our compressors. We have a potential customer in Germany for a bigger heat pump with a compressor with locked rotor current 101A. The mentioned regulation says that maximum starting current should not exceed 30A.


With the soft starters we use today we cannot reduce starting current more than 40-45%, i.e starting current 55-60A.


We've therefore looked to AC-drives where we can reduce the start current to about 1,5 times the rated current, in this case 14A. This solution is though very expensive and we cannot use all the benefits with an AC-drive as the compressor design doesn't allow AC controlling. I.e. we buy a expensive AC-drive just for soft starting the compressor and after that just pass the current through it.


Does anyone know any other soft starting technology which can reduce the start current in a better way?



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Hello Henrik


Welcome to the forum.


You really have two options, variable voltage (VVCF), or variable voltage and variable frequency (VVVF).


If you use a variable voltage starter, then during the start, you are operating the motor at a high slip.

During this time, the current is reduced by the voltage reduction, and the torque is reduced by the voltage reduction squared.


In order to start a machine at at reduced current, you need to ensure that the start torque that can be developed by the motor at that current exceeds the required load torque of the machine.

There are things that you can do as a manufacturer to reduce the torque requirement of the compressor during start, and also to increase the torque output of the motor during start.

Fore example, if you can pressure equalize the compressor during start, you will need less start torque, and if you use a motor with a high locked rotor torque and a low locked rotor current, you will get much more torque at reduced current.

The motor characteristics can be changed by altering the design of the rotor. If you use a rotor with thin bars that penetrate well into the rotor, you will increase the start torque and reduce the start current.


Best regards,


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  • 4 months later...

Hi Henrik,


Although your post was last year, it has only just reached my attention...


You didn't define if your application was three or single phase. If it was single phase, we produce a softstarter capable of starting single phase motors in approximately the same time as a DOL start but at just 30% of the current.


It is for this reason that our product is used in single phase heat pump applications - it allows compliance with the limits on motor starting currents.


These are distributed by Emerson in Europe, however you may contact us directly for any engineering support.




Based in Australia, feel free to contact me, john@hypereng.com


John Reay

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