10 Motor chokes
10.1 General
Shielded motor cables
For reasons of electromagnetic compatibility, the motor must be supplied with power via a
shielded cable. The structure of a cable with 100% shielding and the capacity equivalent cir-
cuit diagram (to earth) are shown below.
Why use motor chokes?
l To compensate high capacitive charge/discharge currents typical of shielded motor
cables approximately 25 m and longer.
l To reduce current alternation noise in the motor.
l To reduce current ripple in the motor.
The digital servo amplifiers’ high switching frequencies and steep switching edges give rise
to the transfer of capacitive currents to the shield by the three phases (U, V, W). These cur-
rents flow from the shield to earth. Depending on the cable length and cable capacity (determ-
ined by design), this can lead to the generation of shield currents with peak values of up to
20A.
These shield currents place a load on the servo amplifiers and motor and, on large systems,
lead to shifts in potential which can damage other components.
This effect is evident in particular on systems with multiple amplifiers operating in parallel on
the same mains filter.
The motor chokes slow down the rate of rise of the motor current (reduce edge steepness),
thereby reducing the current transferred to the shield.
Why is the cross-section of the motor cable important?
Motor cables longer than 50 m with a small cross-section (e.g. 4 x 1.0 mm²) and therefore a
higher equivalent resistance are able to reduce the oscillation tendency of the LCR oscil-
lating circuit (amplifier/choke/cable/motor). This cross-section can also be advantageous for
cable lengths shorter than 50 m if the cable capacity and motor inductance are very high.
However, the current loading of the cable must always be within the limits specified by
EN60204.
Accessories Europe | 10 Motor chokes
Kollmorgen | December 2014 41
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