DC Link Capacitor
It is the large-value capacitor placed at the driver's supply input, as close as possible to the power switches. It supplies the discontinuous current pulse at the moment of switching locally; this largely takes the inductance of the supply cable out of the circuit.
The power stage does not draw a smooth current from the supply; it draws it in pulses chopped by PWM. If you try to take these pulses from a battery metres away, the voltage at the driver input collapses and recovers at every pulse because of the cable inductance. The DC link capacitor supplies the pulse from centimetres away and suppresses this oscillation.
Sizing rests on two criteria: the acceptable voltage ripple and the ripple current the capacitor can carry. The second is usually decisive; the capacitor's heating is determined by its internal resistance (ESR) and the square of the ripple current.
The capacitor has a second job as well: it is the first refuge of the energy coming back from the motor during braking. If the energy is large the voltage rises; for this reason, in four-quadrant operation the DC link capacitor alone is not enough and a brake resistor may be required.
On the installation side the practical rule is this: connect the driver to the supply source with as short and as thick a cable as possible. A long supply cable makes the job of the on-board capacitor harder.
The context in which this term is used: DC motor and driver fault diagnosis matrix