9.2 Bulk Capacitance

Having appropriate local bulk capacitance is an important factor in motor drive system design. It is generally beneficial to have more bulk capacitance, while the disadvantages are increased cost and physical size.

The amount of local capacitance needed depends on a variety of factors, including the:

• Highest current required by the motor system
• Power supply’s capacitance and ability to source or sink current
• Amount of parasitic inductance between the power supply and motor system
• Acceptable voltage ripple
• Type of motor used (brushed DC, brushless DC, stepper)
• Motor braking method

The inductance between the power supply and motor drive system will limit the rate that current can change from the power supply. If the local bulk capacitance is too small, the system will respond to excessive current demands or dumps from the motor with a change in voltage. When adequate bulk capacitance is used, the motor voltage remains stable and high current can be quickly supplied.

The data sheet generally provides a recommended value, but system-level testing is required to determine the appropriate-sized bulk capacitor.

Figure 25. Example Setup of Motor Drive System With External Power Supply

The voltage rating for bulk capacitors should be higher than the operating voltage, to provide margin for cases when the motor transfers energy to the supply.