Engineering20 M-Max Series Adjustable Frequency Drive MN04020003E—October 2013 www.eaton.comVoltage BalanceBecause of the uneven loading on the conductor, and withthe direct connection of greater power ratings, deviationsfrom the ideal voltage form and asymmetrical voltages canbe caused in three-phase AC power networks. Theseasymmetric divergences in the input voltage can lead todifferent loading of the diodes in input rectifiers withthree-phase supplied frequency inverters, and as a result,an advance failure of this diode.In the project planning for the connection of three-phasesupplied frequency inverters (MMX32, MMX34, MMX35),consider only AC power networks that handle permittedasymmetric divergences in the input voltage +3%.If this condition is not fulfilled, or symmetry at the connectionlocation is not known, the use of an assigned main choke isrecommended.Total Harmonic Distortion (THD)The THD (Total Harmonic Distortion) is a measurement forthe occurring harmonic distortion of the sinusoidal oscillation(input power side) input variables with the frequencyinverter. It is given in percent of the total value.U1 = fundamental componentTHD k = 0.1 K = 10% ~ –20 dB (THD suppression)With M-Max series frequency inverters, the permitted valuefor the total harmonic distortion THD is >120%.Idle Power Compensation DevicesCompensation on the power supply side is not required forM-Max series frequency inverters. From the AC powersupply network, they take on very little reactive power of thefundamental harmonics (cos ~ 0.98).In the AC power networks with non-choked idle currentcompensation devices, current deviations can enable parallelresonance and undefinable circumstances.In the project planning for the connection of frequencyinverters to AC power networks with undefinedcircumstances, consider using main chokes.Input ReactorsA input reactor (also called commutation inductor) increasesthe inductance of the power supply line. This extends thecurrent flow period and dampens input deviations.On frequency inverters, a input reactor limits the inputfeedback to permissible values. The harmonic currentemissions that are fed back into the input network (“inputfeedback”) are reduced. This reduces the input-side apparentcurrent to about 30%.Toward the frequency inverter, the input reactors dampenthe interference from the supply network. This increases thewithstand voltage of the frequency inverter and lengthensthe lifespan (diodes of the input power rectifier, intermediatecircuit capacitors).For the operation of the M-Max frequency inverter, theapplication of main chokes is not necessary.We do recommend, however, that an upstream main chokeis used because the network quality is not known in mostcases.While planning the project, consider that a input reactor isonly assigned to a single frequency inverter for isolation.Using a large input reactor for multiple small frequencyinverters should therefore be avoided if at all possible.When using an adapting transformer (assigned to a singlefrequency inverter), a main choke is not necessary.Input reactors are designed based on the input current (ILN )of the frequency inverter. Input chokes and the assignmentto M-Max frequency inverters are explained in the appendix.K U 22 U+ 32 U 42 ... U n+ + 2+U 12 U+ 22 U 32 U 42 ... U n+ + 2+ +-----------------------------------------------------------------------------------------------------------= • 100%THDU22 U+ 32 U42 ... Un+ + 2+U1----------------------------------------------------------------------------------------------------=
