The eletrical industry produces switchboards and controls for applications in various industries. A special electrotechnical application is the rotating converter.
Rotating converters are a combination of an electrical motor and a generator with their shafts tightly coupled. Both machines run with a speed of 600 rpm; the motor is a 10 pole machine, the generator a 12 pole machine (or vice versa). The converter is aiming to change the net frequency from 50 to 60 Hz (or vice versa). The converter can for instance be used to provide power supply to foreign vessels in a harbour. For the start calculation the moment of inertia of both machines must be taken into account. The start time of rotating converter normally ranges between app. 10 - 20 seconds. They can be started nearly without load, because the generator will only be loaded after the start; only a very low counter torque due to friction has to be considered which greatly reduces the start current. Normal start current for converters (Ia/In) are in the range of app. 1 - 1,5 times the motor rated current (In). Below you will find a characteristic example for a calculation.
|motor rated power||Pn||2.000||kW|
|motor rated current||In||250||A|
|motor start current DOL||Ia||5||× In|
|motor rated speed (synchronous)||nn||600||rpm|
|moments of inertia|
|start voltage||Us||0,47||× Un|
The starter starts the motor with reduced voltage, 47 % of the rated voltage, in this example (Us/Un = 0,47). The torque speed curve of the motor will be reduced by the factor Ts/TDOL = (Us/Un × F)2.
Ts/TDOL = (0,47 × 0,9)2 = 0,18 (F is a factor depending on the motor). The torque of the motor must always be higher than the counter torque. The start voltage Us/Un has been determined accordingly. The difference between the reduced motor torque Ts and the counter torque is the acceleration torque Ta.
The start time ts is calculated from the acceleration torque Ta and the total moment of inertia J = JM + JL = 450 + 500 kgm2 = 950 kgm2.
The motor current is IMot = Us/Un × IDOL × F = 0,47 × 5 × 0,9 = 2,1 × In
Mains start current:
A transformer has the same power at primary and secondary, which leads to the fact that the products of current and voltage are the same.
Un × Is = Us/Un × Imot => Is = Us/Un × IMot / Un
Is= 0,47 × 2,1 = 1 × In
The mains start current is therefore much lower than the motor start current.
For other start methods (electronic soft starter, starting reactor etc.) the following is valid: The mains current equals the motor current:
Is = IMot = 2,1 × In
For that reason these starting methods create a higher mains voltage drop.
Machines driven by electric motors exist in a variety of different designs and types. For the most common ones we have prepared detailled information for the start procedure. Needless to say, our starter can also be used for further types of machines. We will gladly provide you with an offer and technical support.