Leakage currents

When operating servo drives, operationally related leakage currents occur in various frequency ranges (capacitive): In addition, it is possible for a smooth DC residual current (ohmic) to be produced after the rectifier. These currents would prevent a residual current circuit breaker (RCCB or RCD) of the type A or AC from tripping. In the event of a fault, therefore, it would be possible for dangerous voltages to be present on the housing parts. For 3-phase applications the statutory regulations in different countries (please check whether your country is affected) require the use of AC/DC-sensitive RCDs. These should have a rated residual current of ≤ 300 mA. In order to be able to meet these requirements it is necessary to know or calculate the expected leakage currents.

Formulas

The leakage current level depends on the fixed leakage currents, the motor cable length and the supply voltage. The following formulas were determined empirically.

Leakage currents 1:

Calculation basis

The values for the leakage current calculated with the equations are valid only if:

  • original Beckhoff motor cables are used and
  • shielding and grounding concepts are adhered to
  • In addition it should be noted that the calculated leakage current value is not exact but merely reflects the maximum expected value, with associated dispersion.
Leakage currents 2:

Composition of the max. total leakage current

The max. total leakage current is composed of:

  • a device-dependent fixed part with 50 Hz (single-phase feed) or 150 Hz (three-phase feed)
  • plus a variable part that depends on the motor cable length and clock frequency. If no other specifications are applied, the clock frequency is around 8 kHz.

Leakage currents for individual devices

ILCdevice = ILCfix + ILCvar

AX5000 up to 12 A – single-phase connection, leakage current in [mA]:

Leakage currents 3:

AX5000 up to 12 A – three-phase connection, leakage current in [mA]:

Leakage currents 4:

AX5118 - three-phase connection, leakage current in [mA]:

Leakage currents 5:

AX5125 - three-phase connection, leakage current in [mA]:

Leakage currents 6:

AX5140 - three-phase connection, leakage current in [mA]:

Leakage currents 7:

The total leakage current is composed of the sum of the individual device leakage currents:

ILCtotal = ILCdevice1 + ILCdevice2 +...+ ILCdevicen

Leakage currents in a DC link

If several devices are connected via a DC link, only the fixed leakage currents for 50 Hz or 150 Hz are present, as long as no axis is enabled. As soon as an axis is released, the complete fixed leakage currents (50 Hz or 150 Hz) are present and additionally a fixed portion of 8 kHz with a motor cable length of 0 m. The following diagrams illustrate the individual leakage current components:

Sample

Leakage currents 8:

1 x AX5000 (enabled) without DC link

 

ILCtotal = ILCvar + ILCfix

Leakage currents 9:

2 x AX5000 (not enabled) in DC link

 

IABtotal = IABfix_1 + IABfix_2

Leakage currents 10:

1 x AX5000 (enabled) + 1 x AX5000 (not enabled) in DC link

 

IABtotal = IABvar_1 + IABfix_1 + IABfix_2

 

If the AX5000_2 is also enabled the equation is as follows:

IABtotal = IABvar_1 + IABvar_2 + IABfix_1 + IABfix_2

Influence of the motor chokes

Motor chokes are used in order to protect the power semiconductors and the motors through lower voltage edges and therefore reduced peak values of the commutation or leakage currents. However, the reduction in voltage edges has no influence on the RMS value of the leakage currents. Since this is precisely what an RCD invariably assesses, motor chokes have no positive influence here.