9.1 Bulk Capacitance

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

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

• The highest current required by the motor or load
• The capacitance of the power supply and ability to source current
• The amount of parasitic inductance between the power supply and motor system
• The acceptable voltage ripple of the system
• The motor braking method (if applicable)

The inductance between the power supply and motor drive system limits how the rate current can change from the power supply. If the local bulk capacitance is too small, the system responds 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 minimum value, but system level testing is required to determine the appropriately sized bulk capacitor.

Figure 36. System Supply Parasitics Example