Presentation on theme: "Voltage regeneration in servo systems Visual ModelQ Training"— Presentation transcript:
1Voltage regeneration in servo systems Visual ModelQ Training This unit discussesWhat voltage regeneration is.How to model a servo system during regeneration
2What 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
3What 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
4To run Visual ModelQ the first time: Install Visual ModelQTo run Visual ModelQ the first time:Click here to visitDownload Visual ModelQ**Run Visual ModelQ installationLaunch Visual ModelQ using the Windows start button or clicking on the iconThe “default model” should appear**This unit can be completed with a free (unregistered) copy of Visual ModelQ
5Load the model “Regen Voltage” VisitDownload “Regen Voltage.mqd”
6Review the modelThe 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.
7Review the modelThe top portion is a standard servo control system. Torque and Velocity “extenders” carry those signals to the regen circuit.
8Review the model Mechanical power is formed as Velocity x Torque Multiply by -1 to calculate power from the motor to the bus
9Review the model Add a constant power loss as a simple approximation In practical systems, the power loss formula is more complex
10Review the modelDivide the output power by the bus voltage; this forms currentThis step converts mechanical power to electrical power
11Review the modelSubtract the current flowing through the regen resistorsWe will discuss to how this current is calculated later
12Review the modelDivide by capacitance to calculate the integration rate inthe bus capacitor
13Review the modelIntegrate the cap voltage using a clamped integrator with a lower limit of 300V, the DC bus. The cap will not discharge below this.
14Review the modelMonitor the bus voltage. If it goes above the upper limit (400V) show a message indicating a fault.
15Review the modelUse a hysteresis block to control the regen transistor. Turn on when bus > 360 and off when bus < 330.
16Review the modelUse an analog switch as a simple model for a transistor. When hysteresis controller is on, connect bus to regen resistors.
17Review the modelCalculate current from internal regen resistor (80 ohm).
18Review the modelCalculate current from external regen resistor (10 ohm).
19Review the modelSum regen resistor currents and subtract from capacitor current.
20Review the modelMonitor RMS regen current and RMS regen voltage. Note the RMS voltage across the resistor is not equal to the bus voltage.
21Review the modelCalculate and monitor average power
22Review the modelAlso, monitor average voltage on a dial meter.
23Review the modelFinally, monitor cap voltage (above) and against velocity. Trigger with Vc (left of scope) to sync with upper scope.
24Review the resultsDuring decel, the bus voltage (blue) grows until it reaches 360V; then the regen circuit turns on until bus voltage falls below 330V.360V330V300V
25Review the resultsIn this snapshot, VBus = 343V, regen power = 489W, RMS external regen current = 15.6A, & RMS resistor voltage = 156V.
26Click here for information on Visual ModelQ Visit for information about software and practical books on controls.Click here for information on Control System Design Guide (2nd Ed.), published by Academic Press in 2000Click here for information on Visual ModelQClick here for information on Observers in Control Systems, published by Academic Press in 2002