5. How is the capacitor calculated in a Steinmetz circuit?
Formula for the operating capacitor in the Steinmetz circuit
For the Steinmetz circuit the motor circuit, star or delta connection, must be made to match the existing mains voltage, which in Europe is usually 230 V. A metal paper capacitor in accordance with DIN EN 60252-1 (VDE 0560-8:2011-10) is permitted as the operating capacitor, which is connected to the third free connection of the motor and, depending on the desired direction of rotation, to the phase conductor or the neutral conductor. If the capacitor is connected to the phase conductor, the motor rotates to the right; if the capacitor is connected to the neutral conductor, it rotates to the left. In various technical books and forums, with reference to DIN 48501, which was withdrawn years ago, a guide value of approx. 70 µF per 1 kW motor power is recommended for an operating voltage of 230 V. The general formula for calculating the operating capacitor with the capacitance C and a power P of the electric motor at the rated voltage U is:
with 
In the Steinmetz circuit, the motor can be operated either in delta or star configuration, depending on the coil voltage. The delta connection is preferred.
Delta connection
Delta connection - direction reversal
Star connection
Half star connection
An auxiliary phase is generated by the capacitor. However, instead of 120°, this only has a phase shift of less than 90°. This means that only an elliptical rotating field is generated, but this is sufficient to give the motor a direction of rotation so that it can start independently. However, this gives the motor elliptical operating characteristics. With the Steinmetz circuit, the motor can be operated either in delta or in star, depending on the coil voltage. The delta circuit is preferred. The capacitor and the coil of the motor together form a series resonant circuit. During operation, a peak voltage of up to 330 V is generated on the capacitor at 230 V mains voltage. To prevent it from being destroyed, it must be rated for the highest voltage that occurs. As the metal paper capacitors used age over the years, a safety margin of 70 V to 80 V is added. For a mains voltage of 230 V, a capacitor with a dielectric strength of at least 400 V is therefore used. Due to the capacitor, the currents in the individual strings are different. The starting current of the motor depends on the required torque and is a multiple of the rated current. As the operation of larger motors results in increased single-phase loads, the operation of motors with Steinmetz switching is limited to 1.5 kW to a maximum of 2 kW, depending on the supplier.