Transcript No Slide Title

Visual ModelQ Training
Voltage regeneration
in servo systems
This unit discusses
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© 2002
QxDesign, Inc.
What voltage regeneration is.
How to model a servo system during
regeneration
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.
© 2002
QxDesign, Inc.
**This unit can be completed with a free (unregistered) copy of Visual ModelQ
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.
© 2002
QxDesign, Inc.
**This unit can be completed with a free (unregistered) copy of Visual ModelQ
Install Visual ModelQ
To run Visual ModelQ the first time:
© 2002
QxDesign, Inc.
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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
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The “default model”
should appear
**This unit can be completed with a free (unregistered) copy of Visual ModelQ
Load the model “Regen Voltage”
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Visit www.QxDesign.com/VisualModelQ#Training
Download “Regen Voltage.mqd”
Review the model
• The next several slides will review the model for regenerative
voltage section-by-section:
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QxDesign, Inc.
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.
Review the model
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QxDesign, Inc.
The top portion is a standard servo control system. Torque and
Velocity “extenders” carry those signals to the regen circuit.
Review the model
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QxDesign, Inc.
Mechanical power is formed as Velocity x Torque
Multiply by -1 to calculate power from the motor to the bus
Review the model
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QxDesign, Inc.
Add a constant power loss as a simple approximation
In practical systems, the power loss formula is more complex
Review the model
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QxDesign, Inc.
Divide the output power by the bus voltage; this forms current
This step converts mechanical power to electrical power
Review the model
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QxDesign, Inc.
Subtract the current flowing through the regen resistors
We will discuss to how this current is calculated later
Review the model
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QxDesign, Inc.
Divide by capacitance to calculate the integration rate in
the bus capacitor
Review the model
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QxDesign, Inc.
Integrate the cap voltage using a clamped integrator with a lower
limit of 300V, the DC bus. The cap will not discharge below this.
Review the model
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QxDesign, Inc.
Monitor the bus voltage. If it goes above the upper limit (400V)
show a message indicating a fault.
Review the model
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Use a hysteresis block to control the regen transistor. Turn on
when bus > 360 and off when bus < 330.
Review the model
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Use an analog switch as a simple model for a transistor. When
hysteresis controller is on, connect bus to regen resistors.
Review the model
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Calculate current from internal regen resistor (80 ohm).
Review the model
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QxDesign, Inc.
Calculate current from external regen resistor (10 ohm).
Review the model
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QxDesign, Inc.
Sum regen resistor currents and subtract from capacitor current.
Review the model
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QxDesign, Inc.
Monitor RMS regen current and RMS regen voltage. Note the
RMS voltage across the resistor is not equal to the bus voltage.
Review the model
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QxDesign, Inc.
Calculate and monitor average power
Review the model
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QxDesign, Inc.
Also, monitor average voltage on a dial meter.
Review the model
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QxDesign, Inc.
Finally, monitor cap voltage (above) and against velocity. Trigger
with Vc (left of scope) to sync with upper scope.
Review the results
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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
© 2002
QxDesign, Inc.
Review the results
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© 2002
QxDesign, Inc.
In this snapshot, VBus = 343V, regen power = 489W, RMS
external regen current = 15.6A, & RMS resistor voltage = 156V.
Visit www.QxDesign.com 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
2000
© 2002
QxDesign, Inc.
Click here for
information on Visual
ModelQ
Click here for information on
Observers in Control Systems,
published by Academic Press in
2002