Minimum Start Current

The minimum start current is a function of the trhee major components in the system, the driven load, the motor and the starter.

The Driven Load requires a miimum amount of torque to start and accelerate it to full working speed. This is a function of the driven load and can not be changed by the motor or the starter. Changes to the driven load can alter the minimum starting torque. For example closing the valve on a centrugal pump, pressure equalising a screw compresor, lifting the valves on a reciprocating compresor.

The Motor converts amps into newton meters, or current into torque. Some motors are better than others in this function. In effect, the starting efficiency of motors varies tremendously from motor to motor. This is a function of the rotor design in the motor and is independant for the starter and the load provided that standard componentary is used. Starters that introduce high even harmonic contents or negative sequence currents will reduce the output torque from the motor for a given input current.
The motor has two characteristics that indicate it's ability to convert amps into newton meters. These are the Locked Rotor Current (LRC) and the Locked Rotor Torque (LRT). The ideal motor has a low LRC and a high LRT. Motor comparisons can be made by taking the LRT/LRC with both expressed as a percentage. A higher value equates to a better starting result.

The Starter controls the voltage applied to the motor and this in turn controls the start current and torque applied to the load. If the load requires a high torque, the starter can not cause the motor to develop that torque without providing the required current.

The load dictates the torque, the motor then dictates the current and the starter controls the voltage to deliver that current.

The Electrical Calculations software provides a means to estimate the minimum start current required based on load characteristics and motor characteristics.





Select your load, or enter an estimate of the value for the maximum load torque during start.
Enter your motor details, either by starting efficiency or Locked Rotor characteristics. This will give you an indicative start current requirement.