Overvoltage and undervoltage faults

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Drive faulting due to voltage fluctuations in HV DC bus are commonly experienced with servo systems. These faults occur when drive measures a HV DC bus supply voltage that is not within the range defined by FUV and FOV parameters. The deviation of voltage may be impossible to notice with multimeter as length of these voltage surges can be in millisecond range.

Overvoltage faults

Servo drive attached to a motor can act two ways: energy supply and energy consumer. The energy consumer behavior occurs during decelerations and this causes current flow from motor to drive power supply capacitors. If that generated energy is not absorbed anywhere, the voltage of capacitors will rise above overvoltage threshold and trigger an software clearable overvoltage fault. Overvoltages faults that are caused by returned energy from motor, can be dealt with a regenerative resistor and with optional extra capacitance in HV DC bus.

Scenarios where returned energy is causing the rise of HV DC bus voltage:

  • Deceleration of motor speed when there is significant amount of energy stored in mechanical motion (rotating inertia or moving mass). Typical reason in spindles and linear axes.
  • Sudden reversal of torque setpoint. This can generate voltage spike even when motor is standing still. Typical reason in high bandwidth torque control applications (such as racing simulators). These spikes are very short and an added capacitor to HV DC bus might provide a solution.

Sizing regenerative resistor

The needed regenerative resistor value can be calculated by equation:

R=nominal_supply_voltage/peak_motor_current [Ohms]

I.e. if supply voltage is 48VDC and peak current is 10A, then a resistor of 4.8 ohms may be needed to consume all current that is returning from the motor. However, in most practical cases the regenerative current is less than the motor peak current, which allows using higher resistance thus reducing risk of overloading the MOSFET switch operating the resistor. It is recommended to experiment with higher resistor values first, and gradually move to lower resistances if problem persists.

Undervoltage faults

Undervoltages come from drop of power supply voltage during surges. Best solution is to set undervoltage paramater [FUV] to a lower value and use power supply that doesn't shut down or drop to near zero under power surges. For very short current surges (millisecond range), an added capacitor to HV DC bus might provide a solution.

Using additional capacitor in HV DC bus

For short current surges/spikes, a capacitor added to HV DC bus might provide a solution for filtering out the spikes. Capacitor can be sized by equation:

C=peak_motor_current*surge_duration/max_voltage_change [Farads]

I.e. if current is 20A, surge duration 0.01 seconds and maximum allowed voltage change during that surge/spike is 10 VDC, then capacitance becomes C=20A*0.01s/10V=0.02F which equals 20000µF.

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