Illustrations for a Dynamic Braking Chopper
Solution in a Medium Voltage AFD (rev 2.0)
Stan R. Simms
Senior Product Design Engineer
EATON
Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
Abstract
As an induction machine’s rotor is
accelerated to a desired process
speed the electromagnetic torque
is positive. Therefore, real power is supplied to the machine and
it operates in the ‘motoring’ mode. However, when the rotor is
slowed, yet still positive in speed, the difference in load and
inertia can result in negative electromagnetic torque. This
‘generating’ mode machine power can be absorbed in
adjustable frequency drives using a dynamic braking chopper.
• Learn how the chopper is sized for a full speed ramp stop.
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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Process Load: Negative Torque
In Centrifugal Variable Torque, or VT, ‘fluid’ loads the power
and torque vary as the cube & square of speed respectively
[affinity laws].
However in other process
loads a machine can be
ramped to full speed,
dwelled, and then decelerated.
At light loads a process like this
has the potential for negative sum torque on deceleration. Thus
in adjustable frequency applications with drives, power is no
longer supplied to the machine and must be handled accordingly.
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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Inertia & Torque
Inertia is an object’s amount of resistance to change in
velocity. In rotation the angular momentum of a rigid body is
unchanged, unless an external torque is applied.
So in our lightly loaded process load we have created a flywheel
with a significant amount of stored energy after being ramped to
full speed. Thus to decelerate this machine we can expect the
following energy and applied torque given the reflected inertia;
[1] EK.E.(Joules or Watt-Seconds) = ½ *
s
2
[2] t
[3] P
t
s
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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Load to Motor Inertia, Applications
The ratio between the load inertia and motor can vary widely
depending upon the application (compressor, pump, fan,
shredder, etc.). Some examples from prior Motor Starting
Studies are as following;
18,000 hp: 4.16 kV cracking motor, 4:1
5,000 hp: 4.16 kV pipeline pump, 1.4:1
2,500 hp: 4.16 kV gas compressor, 1.8:1
85 hp: 480 V sewage pump, 2.2:1
Knowing this ratio without assumption really aides in
designing a braking/acceleration solution.
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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MV AFD Scheme & Interconnect Diagram
MULTI-PULSE SINGLE
WAY DIODE RECTIFIER
& DUAL CAPACITOR
3-LEVEL NEUTRAL
POINT CLAMPED
INVERTER.
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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PWM Voltage Output & Motor Current
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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BRAKING Means
There are multiple ways to brake the forward motion of the rotor during
ramp down, here is an abbreviated list of ways;
1.) Voltage Source Inverter, DC Bus Braking Chopper
2.) Mechanical Friction Brake
3.) Flux Braking (Over-excitation, loss inducing)
4.) DC Current Injection (High Slip, Low Speeds)
5.) Double Frequency (Cogging 5th sequence component)
6.) Active Converters, Regenerative Front End
7.) Eddy Current Brake
8.) Plugging, Reversing Two of Three Phases
9.) High Slip Control Techniques (loss inducing)
10.) Coast Stop (Windage & Friction)
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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DC Bus Chopper in VSI
The previously shown 3-Level NPC inverter is capable of positive and
negative power flow. Where as the single way rectifier constrains the
power flow direction. Therefore a DC Bus Chopper is employed to
prevent the capacitor voltage rise with regards to tripping thresholds;
[4] R1 = UDC2 / P
[5] ERB = D1 * P * TimeCHOP
[6] D1 = Q1-On / Q1-Off
[7] TimeDWELL= Cool Down
equation [5] ≈ [1]
It is not expected that [6] will remain
constant for the entire chopping
duration if a full ramp is performed.
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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Machine Characteristics and
Power Electronics’ Constraints
In this example we are deriving the DC bus chopper for a three phase 2400
Volt 2500 horsepower 6 pole 60 Hz machine. The FLA is 434 Amps and
magnetizing current is 152 Amps. The reflected load and shaft inertia is
20,003 lb-ft^2 (1:2). The peak power of the shaft is thus 1.865 MegaWatts
with synchronous speed of 125.67 rad/sec at 14.84 kN-m torque.
3-Level NPC VSI Thresholds
Nominal Voltage:
3394
Under Full Load:
3240
No Load, High Line:
3733
Chopper Hysteresis:
3750/3600
Trip, DC Bus O.V.:
> 4000
Half Bus Capacitance:
Bleeding Resistors:
Ramp Rate:
The Machine Model
L1 = 766 uHy
R1 = 0.01752 Ohms
L2 = 1.14 mHy R2 = 0.0132 Ohms
slip = 0.00492
LM = 24 mHy
J = 1:2 = 1320
Unloaded torque speed, tL= 0.07*ω2
6975 uF
(2) 8.5kOhms
20 Seconds
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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Sizing the Resistor
The equations have been shown for sizing the resistor for a full ramp
stop however in this example their will only be a 5 second decel
followed by a dwell period (i.e. 100% to 75% speed in 5 seconds).
[1] E = ½ * 1320 kg-m^2 * (125.67 rad/sec [1-.75])^2 = 4.56 Mega Joules
[2] t = 1320 kg-m^2 * 0.25 (125.67 rad/sec) / 5 seconds = 8.3 kN-m
[3] P = 8.3 kN-m * 125.67 rad/sec = 1.04 Mega Watt (peak, instantaneous)
[4a] RB = (3750V)2 / 1.04 Mega Watt = 13.5 Ohms (55% p.u. torque)
[4b] RB = (3750V)2 / 1.865 Mega Watt = 7.5 Ohms (100% p.u. torque)
Two series 4.25 Ohm resistors were selected for the 3LI NPC inverter to
allow for inertia estimate errors and maximizing the allowable
torque producing current in the inverter & machine.
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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The Results
Under these conditions the machine was controlled from 100% speed to
75% in 5 seconds as independent half bus DC bus Hysteresis chopper
controllers turned on and off at 1875V and 1800V respectively.
The resulting pulse trains
of current through the chopping
transistors is as shown to the
right during the 5 second
ramp down. In our application
this pattern would be repeated
every 2-1/2 minutes given our
customer’s process profile.
[5] ERB = 0.55*441A^2 *8.5 Ohms
* 5sec = 4.55 MJoules
[5] ≈ [1], [6] & [7] available on right
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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TORQUE (5 kN-m/div.)
SPEED (500 rpm/div.)
Torque and Speed Plots
TIME (10 Seconds / div.)
At time equal 27.4 Seconds the shaft speed is near 900 rpm (at the end
of the DECEL ramp) & the electromagnetic torque is negative 7.4 kN-m
[with windage & friction modelled], no S-Curve employed in simulation.
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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Other Considerations
Beyond the steel resistor bank, transistors are required along with a
controller. In this example for the 2400 Volt NPC Inverter, two single
3300 Volt transistors in the 800 Amp package are selected. An additional
fast recovery dual diode in 3300 Volt & 400 Amps is used for the freewheeling diode (given self inductance in the resistor bank and cabling).
During the chopping duration the estimated conduction losses per transistor
is 772 Watts. An additional 222 Watts is estimated for switching losses. If the
baseplate of the transistor is maintained at 85 degrees Celsius, the junction
operates around 107 degrees.
For simplicity, we modeled in simulation the bus controller with a 556 dual
timer SRFF. The chopper switching rate is controlled with the amplitudes of
the ON and OFF voltages.
A 5 Mega Joule 8.5 Ohm resistor bank is approximately 3’x4’x2’
A low inductance DC bus & local capacitor and/or snubber is also considered.
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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Coast to Stop Example
A four pole 6000 horsepower load testing dynamometer consisting of
two 4160 Volt three phase AC induction motors was accelerated to full
speed at 60Hz and then coast stopped. With a watch the machine
required 14:38 for a complete stop (friction & windage). Each motor has
an inertia of 4126 lb-ft^2 (or 173 kg-m^2) for rated K.E. of 6.16 MJoules.
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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Ramp Stop, Less Chopper Example
The same machine(s) from the prior slide was then decelerated with an
aggressive ramp setting using a 3LI NPC MV AFD (less a DB Chopper).
Given the losses in the machine and the AFD’s inverter plus discharging
resistors it was brought to a halt in 1:27 [and 1:12 with additional Flux
Braking].
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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Questions?
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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Thank you!
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Illustrations for a Dynamic Braking Chopper Solution in a MV AFD, Rev. 2u
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