4.1.2 Choosing a power supply
In any application with servo drives, the power consumption and regenerated power must be taken into
account. An unstable power line causes harmful harmonics and could damage the system. In most
applications, there is more than one drive which requires an accurate sizing of the DC bus. A system with
minimum failure rates and little power consumption can be realized by properly sizing the bus capacitors,
braking choppers, power supplies, cabling, earthing and using the right connectors. It’s recommended to use a
protected extra-low voltage supply (PELV) instead of a safety extra-low voltage supply (SELV). If a SELV is used,
it’s possible that the isolation from the earth can be violated through the servo drive (e.g. through heatsink or
mechanical mountings) and the supply becomes PELV.
4.1.2.1 Regenerated voltage
Electrical motors are reversible machines that can act as a motor or generator. When a voltage is applied to
the motor, a torque is generated that causes acceleration. But the motor will generate power during
deceleration. Regenerative energy can be a serious issue when high inertia, low friction axes undergo fast
deceleration. If no counter-measures (particularly adding a braking chopper to the circuit) are taken, the
voltage in the DC bus will rise beyond the overvoltage threshold of the servo drives which will cause the drives
to switch off and thus the system will become uncontrollable. The problem will be exceeded in multi-axis
applications with heavy loads that are affected by gravity. For details, please see section Regenerative energy
4.1.2.2 Bus capacitance
The bus capacitance consists of all capacitances on a DC link:
DC bus capacitance = power supply capacitance + drives capacitances + other capacitances
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