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Concerning current reverse transfer from inter­me­diate circuit capac­i­tors

Inter­me­diate circuit capac­i­tors cause current reverse transfer – this is a thesis that users of EC tech­nology have recently discussed. The discus­sion focussed partic­u­larly on large instal­la­tions with many EC fans in evap­o­ra­tors or control cabinet cooling systems. The scenario assumes that the inter­me­diate circuit capac­i­tors remains active even if the motor stands still. A proposed solu­tion is to discon­nect the devices from the power system when there is a stand­still. However, a more accu­rate consid­er­a­tion of the func­tion and design of modern EC motors contra­dicts this thesis.

Phase voltage L1-N and phase current L1 in an idle 3 kW Green­Tech EC motor.

An EC motor is driven by control and power elec­tronics using DC voltage. For mains-powered systems, this DC voltage is gained from AC voltage. This so-called DC-link voltage is smoothed by the inter­me­diate circuit capac­itor. If no current is drawn from the inter­me­diate circuit, the capac­itor will remain charged. Due to the effect of the inter­me­diate circuit capac­itor, in this oper­ating state there can be no phase shift between voltage and current, no idle power, no current harmonics and there­fore no current reverse transfer, since no charging current flows through a charged capac­itor. There­fore it is not neces­sary to sepa­rate the motor from the power system when the speed is zero.

However, if current is accepted from the running motor, the nonlinear charge current of the inter­me­diate circuit capac­itor loads the power system through current harmonics. In addi­tion, the switching oper­a­tions of the power elec­tronics cause higher frequency faults. In state-of-the-art elec­tronics, however, both phenomena can be reduced to a stan­dards-compat­ible degree.

The corre­sponding current harmonics spec­trum of phase current L1 for an idle 3 kW Green­Tech EC motor.

To counter current harmonics in our Green­Tech EC motors we use current harmonic filters; we use mains filters to reduce higher frequency inter­fer­ence emis­sions at the line input of the elec­tronics. These filters are composed of capac­i­tors and chokes and lead to capac­i­tive idle currents at the line input which cause an idle power input for as long as the elec­tronics remain connected to the power system. However, the effec­tive power in standby mode, for example, is only approx­i­mately one-thou­sandth of the rated output for a 3 kW EC motor. This output is buffered by the inter­me­diate circuit capac­itor, but in return, the charge current is so low that, compared to the idle current of the mains filter, it is of no conse­quence. Thus the idle current output at rest has nothing to do with the inter­me­diate circuit capac­itor, but instead is to be traced back to the use of the mains filter.

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