Start current calculator

Which is the lowest start current possible?

The start current is depending on various technical data of the electric motor and the driven machine. Concerning the motor the torque speed curve  (M/Mn - n/nn) and the DOL start current (Ia/In) of the motor will be considered. The driven machine (pump, compressor etc.) provides the counter torque (M/Mn - n/nn) during the start. Technical experienced users, who already have these data on hand or can reasaonably estimate them, can use this on-line start current calculator at their convenience. Clients who are not disposed to acquaint themselves with the start topic, are kindly asked to contact us. We gladly give advice and prepare an individual calculation for them.


Ma / Mn
limited to 0,4 - 1
Mk / Mn
limited to 1,6 - 2,5
Mg / Mn
limited to 0,2 - 0,8
Ia / In
limited to 3 - 8



Start current


How to use the start current calculator:

Motor data:

In order to generate the DOL torque speed curve (M/Mn - n/nn) of the motor enter the start torque in relation to the motor rated torque (Ma/Mn). Values Ma/Mn between 0,4 and 1 can be used. The start torque is the minimum value of the torque at speed n/nn = 0. Also enter the breakdown torque in relation to the motor rated torque (Mk/Mn). Values Mk/Mn between 1,6 and 2,5 can be chosen. The breakdown torque is the highest value of the torque at the speed n/nn = 0,95. Entering these values will adjust the motor curve in the diagram "torque" accordingly. Now the DOL start current of the motor (Ia/In) has to be determined. Values Ia/In between 3 and 8 can be used. The input will adjust the curve DOL in the diagram "start current".  


Data of the driven machine:

In order to generate the torque speed curve (M/Mn - n/nn) of the driven machine enter the counter torque in relation to the motor rated torque (Mg/Mn). The counter torque is the highest torque requirement of your machine at the speed n/nn = 1. Values Mg/Mn between 0,2 and 0,8 can be used. Entering this value will adjust the counter torque curve in the diagram "counter torque". Please pay attention to the fact that most of the driven machines can be started with reduced load (f.ex. a pump can be started with closed discharge valve). This results in a lower counter torque which reduces the start current further. We recommend to always use the counter torque curve with the lowest possible torque (maximum load reduction).


The curve start current (mains) in the diagram "start current" shows the current (I/In) of your motor/machine combination during the start with our starter. The curve motor depicts the current at the motor terminals. For other starting methods (electronic soft starter, start reactor, etc.) the following is valid: the mains current during the start equals the motor current. Due to the fact that our starter is applying a transformer, the mains current is lower than the motor current during start (mains voltage > motor voltage, mains power = motor power). The lower start current reduces the mains voltage drop in comparison with other starting methods.

How to proceed:

We gladly will prepare an individual calculation on the basis of your motor/machine data. In addition to the start current we will determine the start time of your application. If you can send us the data of your MV grid, we will also calculate the start voltage drop of your application .

request calculation


This calculator is only a preliminary estimation to indicate the lowest start current which can be achieved with our starter. It cannot substitute the individual calculation, which we will glady provide on the basis of your motor and machine data. In our online calculation typical shapes of the torque curves are being assumed. The real curves can deviate. Values which you enter are limited to certain ranges; your real values may exceed these ranges. Conditions of the electrical power supply and its voltage drop are not taken into consideration.