5. How is the capacitor calculated in a Steinmetz circuit?

Category: Electric Motors - General

Formula for the operating capacitor in the Steinmetz circuit

For the Steinmetz circuit, the motor must be connected in a star or delta connection to match the existing mains voltage, which in Europe is usually 230 V. The operating capacitor is a metal paper capacitor according to DIN EN 60252-1 (VDE 0560-8:2011-10). Permissible, which is connected to the third free connection of the motor and, depending on the desired direction of rotation, to the external conductor or the neutral conductor. If the capacitor is connected to the external conductor, the motor rotates to the right, if the capacitor is connected to the neutral conductor, it rotates to the left. In various specialist books and forums, with reference to DIN 48501, which was withdrawn years ago, a guideline value of approx. 70 µF per 1 kW of motor power is recommended for an operating voltage of 230 V. The general formula for calculating the operating capacitor with the capacity C and a power P of the electric motor at the nominal voltage U is:

$C = \frac {2 P} {\sqrt{3} \, \omega \, U^2}$

with $\omega$ the circular frequency

With the Steinmetz circuit, the motor can be operated in either delta or star, depending on the coil voltage. The delta connection is preferred.

Delta connection

Delta connection – reversal of direction

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°. So only an elliptical rotating field is generated, which is sufficient to give the motor a direction of rotation so that it can start independently. However, this gives the engine elliptical operating characteristics. With the Steinmetz circuit, the motor can be operated in either delta or star, depending on the coil voltage. The delta connection is preferred. The capacitor and the coil of the motor together form a series resonant circuit. During operation, the capacitor produces a peak voltage of up to 330 V at 230 V mains voltage. To ensure that it is not destroyed, it must be dimensioned for the highest voltage that occurs. Since the metal paper capacitors used age over the years, a safety margin of 70 V to 80 V is added. With a mains voltage of 230 V, a capacitor with a dielectric strength of at least 400 V is used. Due to the capacitor, the currents in the individual strands are different. The starting current of the motor depends on the required torque and is several times the rated current. Since the operation of larger motors results in increased single-phase loads, the operation of motors with a Steinmetz circuit is limited to 1.5 kW to a maximum of 2 kW, depending on the supplier.