Input Capacitors

Ripple Voltage

Ripple Voltage is the difference in voltage between “peak” and “valley” from our DC power supply.

When the motor is at rest and there is no power being drawn, it has a resting voltage. When you load the DC supply, it sags in terms of the voltage, and this is going to be measured right at the input side - ripple voltage. The BLDC motor controller delivers the power to the motor at very high frequencies. What happens is that you get some power that is delivered from the battery pack to the motor controller, and it then turns a coil within the motor on and off in a split second. This process repeats itself numerous times within a one second time span. During this time, the battery is being loaded and unloaded at a very high rate. This is where we see ripple voltage occurring.

The maximum ripple voltage that we have within our controller is 10% of the nominal voltage.

When the motor pulls current, it causes the DC Bus Voltage to drop.

Large amounts of current flowing back into the power supply can cause voltage spikes at the output terminals of the motor systems as well as on the power supply. The excess current can be produced by the motor acting as a generator or by the stored energy on the motor flowing back to the supply during fast decay. This occurs because the kinetic energy that the motor possesses when it is spinning has to dissipate - which occurs by energy being transferred back into the circuit. This voltage that is output will cause noise to the input. The goal is to minimize the voltage spikes by controlling and absorbing all of the excess energy.

Ripple Current

Equivalent Series Resistance (ESR)

The Equivalent Series Resistance is the internal resistance that appears in series with the capacitance of the device.  Almost all capacitors exhibit this property at varying degrees depending on the construction, dielectric materials, quality, and reliability of the capacitor. ESR is resistance from a combination of energy loss mechanisms under specific operating conditions.

Some energy losses within a capacitor can be attributed to the conductors while others involve the dielectric material. These losses vary mainly depending on voltage and temperature. The most common energy loss mechanisms include dielectric losses, ferroelectric losses, dielectric conduction losses, interfacial polarization, partial discharge losses, ohmic resistance losses, sparking between conductors, electromechanical losses, and eddy current losses.

Together with its capacitance value, ESR defines a time constant for charging and discharging of the capacitor and thus how quickly the capacitor react on voltage/current changes/ripple. In practical smoothing applications capacitor technologies are combined in parallel, where high capacitance parts are taking care of bulk filtering (aluminum or tantalum capacitors) and small MLCC capacitors with low ESR are taking care of fast, high frequency spikes.