How to select DC link capacitor for motor driving with H bridge configuration. And why a capacitor places across motor?
Size of the DC link capacitor is influenced by the DC voltage ripple specification for the design as well as the lifetime of the capacitor.
Usually, for electrolytic capacitors lifetime is specified between 20000 to 40000 hrs, depending on designers choice.
DC voltage ripple is usually specified around 7%. This again varies based on designers choice and application. Higher the ripple, it will be difficult to control the motor's speed and torque response.
Circuit simulation to identify the Fourier spectrum of the capacitor currents is first step. DC ripple at full load should not exceed specification. And lifetime of the electrolytic capacitor can be computed by tools provided by some manufacturers, based on the frequency spectrum of capacitor currents
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The capacitor might be required to avoid any noise from motor affecting the main power path and hence the circuitry operation on it.
The DC Link capacitor helps to,
1. Reduce Ripple. That is what he meant by 7% ripple in the above reply.
The selection of capacitor depends on,
1. Obviously ripple voltage, generally specified as ripple to ripple
2. Switching frequency
3. Load current on the supply
4. Duty cycle applied
The formula is, C = (Maximum output current * 1000 * Duty cycle * (1 - Duty cycle))/(Switching frequency * peak-to-peak ripple)
It is preferred to use capacitance above this calculated value.
The only problem is hard to say exact ripple to ripple voltage in the beginning and choose a capacitor. So, we have to calculate a capacitor based on the assumed ripple voltage and then reassess the capacitor if required by actual ripple voltage on the board.
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One of the reference for this topic:
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