Voltage regeneration in servo systems

1. Visual ModelQ Training Voltage regeneration in servo systems • This unit discusses • What voltage regeneration is. • How to model a servo system during regeneration

2. What is voltage regeneration? • In servo systems, when a motor decelerates under power, the motor operates in “generator” mode, feeding energy back to the drive. • Standard servo drives use diodes to feed line power into the bus, and those diodes block the energy from returning to the AC line. The result is that the mechanical (kinetic) energy feeds into the bus capacitor and is converted to potential energy, raising the bus voltage. • If this process continues uncontrolled, the voltage can elevate to levels high enough to cause catastrophic failure. **This unit can be completed with a free (unregistered) copy of Visual ModelQ

3. What is voltage regeneration (cont.)? • The most common solution is to use a transistor to temporarily connect a high-power resistor across the line to burn the energy as heat (I2R) losses. • Many servo drives include a regen resistor that can dissipate the regen power for most applications. • For systems with larger amounts of regenerative energy, an external resistor is added in parallel to the internal resistor. The external resistor is usually larger and dissipates a greater amount of power. • Most drives monitor the bus voltage and shut down the drive when this voltage reaches an upper limit. This protects the drive when the regen circuit cannot hold the bus down. **This unit can be completed with a free (unregistered) copy of Visual ModelQ

4. Install Visual ModelQ To run Visual ModelQ the first time: • Click here to visit www.QxDesign.com • Download Visual ModelQ** • Run Visual ModelQ installation • Launch Visual ModelQ using the Windows start button or clicking on the icon • The “default model” should appear **This unit can be completed with a free (unregistered) copy of Visual ModelQ

5. Load the model “Regen Voltage” • Visit www.QxDesign.com/VisualModelQ#Training • Download “Regen Voltage.mqd”

6. Review the model • The next several slides will review the model for regenerative voltage section-by-section: • Start with a standard servo system. • Calculate the mechanical power. • Deduct a constant power drain to represent system losses. • Divide the power entering the bus capacitor by the bus voltage to calculate current feeding the bus capacitor. • Deduct the current flowing through the regen resistors from the current feeding the bus capacitor. • Integrate the current in the bus capacitor to derive the bus voltage. The integrator should never fall below the line voltage; the line voltage acts like a lower limit on the integrator, holding the bus voltage from falling below that value. • Monitor the voltage for a fault condition (> 390V). • Use a hysteresis block to control the regen transistor (330V to 360V). • Connect the bus voltage across the regen resistors when the hysteresis block is on. • Calculate the current flowing through the regen resistors due to the bus voltage using Ohm’s law. • Monitor bus voltage, regen current, and regen power.

7. Review the model • The top portion is a standard servo control system. Torque and Velocity “extenders” carry those signals to the regen circuit.

8. Review the model • Mechanical power is formed as Velocity x Torque • Multiply by -1 to calculate power from the motor to the bus

9. Review the model • Add a constant power loss as a simple approximation • In practical systems, the power loss formula is more complex

10. Review the model • Divide the output power by the bus voltage; this forms current • This step converts mechanical power to electrical power

11. Review the model • Subtract the current flowing through the regen resistors • We will discuss to how this current is calculated later

12. Review the model • Divide by capacitance to calculate the integration rate in • the bus capacitor

13. Review the model • Integrate the cap voltage using a clamped integrator with a lower limit of 300V, the DC bus. The cap will not discharge below this.

14. Review the model • Monitor the bus voltage. If it goes above the upper limit (400V) show a message indicating a fault.

15. Review the model • Use a hysteresis block to control the regen transistor. Turn on when bus > 360 and off when bus < 330.

16. Review the model • Use an analog switch as a simple model for a transistor. When hysteresis controller is on, connect bus to regen resistors.

17. Review the model • Calculate current from internal regen resistor (80 ohm).

18. Review the model • Calculate current from external regen resistor (10 ohm).

19. Review the model • Sum regen resistor currents and subtract from capacitor current.

20. Review the model • Monitor RMS regen current and RMS regen voltage. Note the RMS voltage across the resistor is not equal to the bus voltage.

21. Review the model • Calculate and monitor average power

22. Review the model • Also, monitor average voltage on a dial meter.

23. Review the model • Finally, monitor cap voltage (above) and against velocity. Trigger with Vc (left of scope) to sync with upper scope.

24. Review the results • During decel, the bus voltage (blue) grows until it reaches 360V; then the regen circuit turns on until bus voltage falls below 330V. 360V 330V 300V

25. Review the results • In this snapshot, VBus = 343V, regen power = 489W, RMS external regen current = 15.6A, & RMS resistor voltage = 156V.

26. Visit www.QxDesign.com for information about software and practical books on controls. Click here for information on Visual ModelQ Click here for information on Observers in Control Systems, published by Academic Press in 2002 Click here for information on Control System Design Guide (2nd Ed.), published by Academic Press in 2000