Allen-Bradley 20A User manual

PowerFlex Dynamic Braking Resistor Calculator
Catalog Numbers 20A, 20B, 20F, 20G, 22A, 22B
Application Technique
Original Instructions

Important User Information
Read this document and the documents listed in the additional resources section about installation, configuration, and
operation of this equipment before you install, configure, operate, or maintain this product. Users are required to
familiarize themselves with installation and wiring instructions in addition to requirements of all applicable codes, laws,
and standards.
Activities including installation, adjustments, putting into service, use, assembly, disassembly, and maintenance are
required to be carried out by suitably trained personnel in accordance with applicable code of practice.
If this equipment is used in a manner not specified by the manufacturer, the protection provided by the equipment may
be impaired.
In no event will Rockwell Automation, Inc. be responsible or liable for indirect or consequential damages resulting from
the use or application of this equipment.
The examples and diagrams in this manual are included solely for illustrative purposes. Because of the many variables and
requirements associated with any particular installation, Rockwell Automation, Inc. cannot assume responsibility or
liability for actual use based on the examples and diagrams.
No patent liability is assumed by Rockwell Automation, Inc. with respect to use of information, circuits, equipment, or
software described in this manual.
Reproduction of the contents of this manual, in whole or in part, without written permission of Rockwell Automation,
Inc., is prohibited.
Throughout this manual, when necessary, we use notes to make you aware of safety considerations.
Labels may also be on or inside the equipment to provide specific precautions.
WARNING: Identifies information about practices or circumstances that can cause an explosion in a hazardous
environment, which may lead to personal injury or death, property damage, or economic loss.
ATTENTION: Identifies information about practices or circumstances that can lead to personal injury or death, property
damage, or economic loss. Attentions help you identify a hazard, avoid a hazard, and recognize the consequence.
IMPORTANT Identifies information that is critical for successful application and understanding of the product.
SHOCK HAZARD: Labels may be on or inside the equipment, for example, a drive or motor, to alert people that dangerous
voltage may be present.
BURN HAZARD: Labels may be on or inside the equipment, for example, a drive or motor, to alert people that surfaces may
reach dangerous temperatures.
ARC FLASH HAZARD: Labels may be on or inside the equipment, for example, a motor control center, to alert people to
potential Arc Flash. Arc Flash will cause severe injury or death.Wear proper Personal Protective Equipment (PPE). Follow ALL
Regulatory requirements for safe work practices and for Personal Protective Equipment (PPE).

Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017
Table of Contents
Preface
Waste Electrical and Electronic Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Product Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Personal Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Additional Resources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Summary of Changes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Chapter 1
UnderstandingHowDynamicBraking
Works
How Dynamic Braking Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Dynamic Brake Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Chapter 2
Determining Dynamic Brake
Requirements
How to Determine Dynamic Brake Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 11
Determine Values of Equation Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Example Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Chapter 3
Evaluating the PowerFlex 7-Class
Internal Resistor
Evaluating the Capability of the Internal Dynamic Brake Resistor. . . . . . . . . . . 23
PowerFlex 70 Power Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
PowerFlex 700 Power Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
How to Evaluate a PowerFlex 750-Series Internal Resistor. . . . . . . . . . . . . . . . . . 32
Chapter 4
Selecting An External Resistor for
PowerFlex 7-Class Drives
How to Select an External Dynamic Brake Resistor for
PowerFlex 7-Class Drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Appendix A
Minimum Dynamic Brake Resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

4Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017
Table of Contents
Notes:

Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017 5
Preface
Read the General Precautions
Waste Electrical and
Electronic Equipment
Product Safety
Personal Safety
Additional Resources You can view or download publications at
http://www.rockwellautomation.com/global/literature-library/overview.page.
To place an order for paper copies of technical documentation, contact your
local Allen-Bradley distributor or Rockwell Automation® sales representative.
At the end of its life, this equipment should be collected separately from any
unsorted municipal waste.
ATTENTION: This drive contains ESD (Electrostatic Discharge) sensitive parts
and assemblies. Static control precautions are required when you install,
test, service, or repair this assembly. Component damage can result if ESD
control procedures are not followed. If you are not familiar with static control
procedures, reference any applicable ESD protection handbook.
ATTENTION: To avoid an electric shock hazard, verify that the voltage on the
bus capacitors has discharged completely before servicing.
ATTENTION: Measure the DC bus voltage at the power terminal block by
measuring between the +DC and -DC terminals or between the +DC and -
DC test point sockets if equipped. Also measure between the +DC terminal
or test point and the chassis, and between the -DC terminal or test point and
the chassis.The voltage must be zero for all three measurements.
2
0V
0V
DC+ DC-
1L1 L2 L3
O
I

6Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017
Preface
Summary of Changes This manual contains new and updated information as indicated in the
following table.
Topic Page
Added 200…240V AC definitions to Appendix A 67

Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017 7
Chapter 1
Understanding How Dynamic Braking Works
How Dynamic Braking Works When an induction motor’s rotor turns slower than the synchronous speed set by
the drive’s output power, the motor is transforming electrical energy obtained
from the drive into mechanical energy available at the drive shaft of the motor.
This process is referred to as motoring. When the rotor is turning faster than the
synchronous speed set by the drive’s output power, the motor is transforming
mechanical energy available at the drive shaft of the motor into electrical energy
that can be transferred back to the drive. This process is referred to as
regeneration.
Most AC PWM drives convert AC power from the fixed frequency utility grid
into DC power by means of a diode rectifier bridge or controlled SCR bridge
before it is inverted into variable frequency AC power. Diode and SCR bridges
are cost-effective, but can only handle power in the motoring direction.
Therefore, if the motor is regenerating, the bridge cannot conduct the necessary
negative DC current, the DC bus voltage increases and causes an overvoltage
fault at the drive. More complex bridge configurations use SCRs or transistors
that can transform DC regenerative electrical power into fixed frequency utility
electrical energy. This process is known as line regeneration.
A more cost-effective solution can be provided by allowing the drive to feed the
regenerated electrical power to a resistor which transforms it into thermal energy.
This process is referred to as dynamic braking.

8Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017
Chapter 1 Understanding How Dynamic BrakingWorks
Dynamic Brake Components A Dynamic Brake consists of a Chopper (the chopper transistor and related
control components are built into PowerFlex® drives) and a Dynamic Brake
Resistor.
Figure 1 shows a simplified Dynamic Braking schematic.
Figure 1 - Simplified Dynamic Brake Schematic
Chopper
The Chopper is the Dynamic Braking circuitry that senses rising DC bus voltage
and shunts the excess energy to the Dynamic Brake Resistor. A Chopper contains
three significant power components:
The Chopper Transistor is an Isolated Gate Bipolar Transistor (IGBT). The
Chopper Transistor is either ON or OFF, connecting the Dynamic Brake
Resistor to the DC bus and dissipating power, or isolating the resistor from the
DC bus. The most important rating is the collector current rating of the Chopper
Transistor that helps to determine the minimum resistance value used for the
Dynamic Brake Resistor.
Signal
Common
Dynamic
Brake
Resistor
Chopper
Transistor
Chopper Transistor
Voltage Control
To
Voltage
Control
To
Voltage
Control
– DC Bus
+ DC Bus
To
Voltage Dividers
Voltage
Divider
Voltage
Divider
FWD
FWD

Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017 9
Understanding How Dynamic BrakingWorks Chapter 1
Chopper Transistor Voltage Control regulates the voltage of the DC bus during
regeneration. The average values of DC bus voltages are:
Voltage dividers reduce the DC bus voltage to a value that is usable in signal
circuit isolation and control. The DC bus feedback voltage from the voltage
dividers is compared to a reference voltage to actuate the Chopper Transistor.
The Freewheel Diode (FWD), in parallel with the Dynamic Brake Resistor,
allows any magnetic energy stored in the parasitic inductance of that circuit to be
safely dissipated during turn off of the Chopper Transistor.
Resistor
The Resistor dissipates the regenerated energy in the form of heat. The
PowerFlex Family of Drives can use either the internal dynamic brake resistor
option or an externally mounted dynamic brake resistor wired to the drive.
Wiring
Frames 0…4
Wire to the DB resistor should be no longer than 10 feet from the drive
terminals. Wire should be twisted to minimize inductance.
Frames 5…6
Wire to the DB resistor should be no longer than 100 feet from the drive
terminals.
Drive Input Voltage Transistor Turn-On
Voltage
Maximum Power Calculation
Voltage
208 375V DC 395V DC
240 375V DC 395V DC
400 750V DC 790V DC
480 750V DC 790V DC
575 937.5V DC 987V DC
600 937.5V DC 987V DC
600 (Frame 5 and 6) 1076V DC 1135V DC
690 1076V DC 1135V DC

10 Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017
Chapter 1 Understanding How Dynamic BrakingWorks
Notes:

Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017 11
Chapter 2
Determining Dynamic Brake Requirements
How to Determine Dynamic
Brake Requirements
When a drive is consistently operating in the regenerative mode of operation,
serious consideration should be given to equipment that transforms the electrical
energy back to the fixed frequency utility grid.
As a rule, Dynamic Braking can be used when the need to dissipate regenerative
energy is on an occasional or periodic basis. In general, the motor power rating,
speed, torque, and details regarding the regenerative mode of operation are
needed to estimate what Dynamic Brake Resistor value is needed.
The Peak Regenerative Power and Average Regenerative Power that is
required for the application must be calculated to determine the resistor that is
needed for the application. Once these values are determined, the resistors can be
chosen. If an internal resistor is chosen, the resistor must be capable of handling
the regenerated power or the drive will trip. If an external resistor is chosen, in
addition to the power capabilities, the resistance must also be less than the
application maximum and greater than the drive minimum or the drive will trip.
The power rating of the Dynamic Brake Resistor is estimated by applying what is
known about the drive’s motoring and regenerating modes of operation. The
Average Power Dissipation must be estimated and the power rating of the
Dynamic Brake Resistor that is chosen to be greater than that average. If the
Dynamic Brake Resistor has a large thermodynamic heat capacity, then the
resistor element will be able to absorb a large amount of energy without the
temperature of the resistor element exceeding the operational temperature rating.
Thermal time constants in the order of 50 seconds and higher satisfy the criteria
of large heat capacities for these applications. If a resistor has a small heat capacity
(defined as thermal time constants less than 5 seconds) the temperature of the
resistor element could exceed its maximum.
Peak Regenerative Power can be calculated as:
•Horsepower (English units)
•Watts (The International System of Units, SI)
•Per Unit System (pu) which is relative to a value
The final number must be in watts of power to estimate the resistance value of the
Dynamic Brake Resistor. The following calculations are demonstrated in SI units.
Gather the Following Information
•Power rating from motor nameplate in watts, kilowatts, or horsepower

12 Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017
Chapter 2 Determining Dynamic Brake Requirements
•Speed rating from motor nameplate in rpm or rps (radians per second)
•Required decel time (per Figure 2, t3– t2). This time is a process
requirement and must be within the capabilities of the drive programming.
•Motor inertia and load inertia in kg•m2or WK2in lb•ft2
•Gear ratio (GR) if a gear is present between the motor and load
•Motor shaft speed, torque, and power profile of the drive application
Figure 2 shows typical application profiles for speed, torque and power. The
examples are for cyclical application that is periodic over t4seconds. The
following variables are defined for Figure 2:
(t) = Motor shaft speed in radians per second (rps)
N= Motor shaft speed in Revolutions Per Minute (rpm)
T(t) = Motor shaft torque in Newton-meters
1.0 lb•ft = 1.355818 N•m
P(t) = Motor shaft power in watts
1.0 Hp = 746 watts
b= Rated angular rotational speed
o= Angular rotational speed less than b (can equal 0)
-Pb= Motor shaft peak regenerative power in watts
2N
60
----------
=
Rad
s
----------
Rad
s
----------

Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017 13
Determining Dynamic Brake Requirements Chapter 2
Figure 2 - Application Speed, Torque, and Power Profiles
0t1t2t3t4t1+ t4t
0t1t2t3t4t1+ t4t
0t1t2t3t4t1+ t4t
ω(t)
T(t)
P(t)
0t1t2t3t4t1+ t4t
-Pb
Prg
ωo
ωb
Speed
Torque
Power
Drive Rated
Regen Power

14 Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017
Chapter 2 Determining Dynamic Brake Requirements
Determine Values of
Equation Variables
Step 1 – Total Inertia
JT= Total inertia reflected to the motor shaft
(kg•m2or WK2in lb•ft2)
Jm= Motor inertia (kg•m2or WK2in lb•ft2)
GR = Gear ratio for any gear between motor and load (dimensionless)
If the gear ratio is 2:1 then
JL= Load inertia (kg•m2or WK2in lb•ft2)
1.0 lb•ft2= 0.04214011 kg•m2
Calculate Total Inertia:
Record Total Inertia:
JT=
J
TJmGR2JL


+=
GR Load Speed
Motor Speed
-----------------------------=
GR 1
2
-- 0.5==
J
Toooooooooooooooooooo oooooooooo+=

Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017 15
Determining Dynamic Brake Requirements Chapter 2
Step 2 – Peak Braking Power
Pb= Peak braking power (watts)
1.0 Hp = 746 watts
JT= Total inertia reflected to the motor shaft (kg•m2)
b= Rated angular rotational speed
o=Angularrotationalspeed,
less than rated speed down to zero
Nb= Rated motor speed (rpm)
t3– t2= Deceleration time from bto o(seconds)
Calculate Peak Braking Power:
Record Peak Braking Power:
Compare the peak braking power (Pb) to the drive rated regenerative power (Prg ).
If the peak braking power is greater than the drive rated regenerative power, the
decel time will have to be increased so that the drive does not enter current limit.
Drive rated regenerative power (Prg ) is determined by:
Prg = Drive rated regenerative power
V= DC bus regulation voltage from Appendix A
R= Minimum brake resistance from Appendix A
Pb=
P
b
JTbbo –
t3t2
–
-------------------------------------------
=
Rad
s
---------- 2Nb
60
-------------=
Rad
s
----------
Pb
ooooooooooooooooooooooooooo ooooooooo–
ooooooooo ooooooooo–
--------------------------------------------------------------------------------------------------------------------------------------------------------=
Prg
V2
R
------
=
Prg
ooooooooo
2
ooooooooo
------------------------------------
=

16 Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017
Chapter 2 Determining Dynamic Brake Requirements
Record Rated Regenerative Power:
For the purposes of this document, it is assumed that the motor used in the
application is capable of producing the required regenerative torque and power.
Step 3 – Minimum Power Requirements for the Dynamic Brake
Resistors
It is assumed that the application exhibits a periodic function of acceleration and
deceleration. If (t3– t2)equals the time in seconds necessary for deceleration from
rated speed to ospeed, and t4is the time in seconds before the process repeats
itself, then the average duty cycle is (t3– t2)/t4. The power as a function of time is a
linearly decreasing function from a value equal to the peak regenerative power to
some lesser value after (t3– t2)seconds have elapsed. The average power regenerated
over the interval of (t3– t2)seconds is:
Pav = Average dynamic brake resister dissipation (watts)
t3– t2= Deceleration time from bto o(seconds)
t4= Total cycle time or period of process (seconds)
Pb= Peak braking power (watts)
b= Rated angular rotational speed
o=Angularrotationalspeed,
less than rated speed down to zero
Prg=
IMPORTANT If application cycle time exceeds 900 seconds, for calculation purposes
t4will be equal to 900 plus the decel time. This is because the resistor
will have cooled completely after 15 minutes.
If t4> 900 then t4= 900 + (t3- t2).
Pb
2
------ bo
+
b
------------------------
Rad
s
----------
Rad
s
----------

Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017 17
Determining Dynamic Brake Requirements Chapter 2
The Average Power in watts regenerated over the period t4is:
Calculate Average Power in watts regenerated over the period t4:
Record Average Power in watts regenerated over the period t4:
Step 4 – Percent Average Load of the Internal Dynamic Brake Resistor
Skip this calculation if an external dynamic brake resistor will be used.
AL = Average load in percent of dynamic brake resistor.
Pav = Average dynamic brake resistor dissipation calculated in Step 3
– Minimum Power Requirements for the Dynamic Brake
Resistors on page 16 (watts)
Pdb = Steady state power dissipation capacity of dynamic brake
resistors obtained from Appendix A (watts)
Calculate Percent Average Load of the dynamic brake resistor:
Record Percent Average Load of the dynamic brake resistor:
The calculation of AL is the Dynamic Brake Resistor load expressed as a percent. Pdb
is the sum of the Dynamic Brake dissipation capacity and is obtained from the
Pav=
IMPORTANT The value of AL should not exceed 100%.
AL=
Pav
t3t2 –
t4
------------------ Pb
2
------ bo
+
b
------------------------=
Pav
oooooo oooooo–
oooooo
--------------------------------------------------oooooo
2
------------------------oooooo oooooo+
oooooo
--------------------------------------------------=
AL Pav
Pdb
---------100=
AL oooooooooo
oooooooooo
--------------------------------------100=

18 Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017
Chapter 2 Determining Dynamic Brake Requirements
minimum dynamic brake resistance table in Appendix A. This will give a data
point for a line to be drawn on one the curves provided in Chapter 3.
Step 5 – Percent Peak Load of the Internal Dynamic Brake Resistor
Skip this calculation if an external dynamic brake resistor will be used.
PL = Peak load in percent of dynamic brake resistor
Pav = Peak braking power calculated in Step 2 – Peak Braking Power
on page 15 (watts)
Pdb = Steady state power dissipation capacity of dynamic brake
resistors obtained from Appendix A (watts)
Calculate Percent Peak Load of the dynamic brake resistor:
Record Percent Average Load of the dynamic brake resistor:
The calculation of PL in percent gives the percentage of the instantaneous power
dissipated by the Dynamic Brake Resistors relative to the steady state power
dissipation capacity of the resistors. This will give a data point to be drawn on one
of the curves provided in Chapter 3.
PL=
PL Pb
Pdb
---------100=
PL oooooooooo
oooooooooo
--------------------------------------100=

Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017 19
Determining Dynamic Brake Requirements Chapter 2
Example Calculation A 10 Hp, 4 Pole, 480 Volt motor and drive is accelerating and decelerating as
depicted in Figure 2.
•Cycle period t4 is 40 seconds
•Rated speed is 1785 rpm and is to be decelerated to 0 speed in 15.0
seconds
•Motor load can be considered purely as inertia, and all power expended or
absorbed by the motor is absorbed by the motor and load inertia
•Load inertia is 4.0 lb•ft2and is directly coupled to the motor
•Motor rotor inertia is 2.2 lb•ft
•A PowerFlex® 70 drive, 10 Hp 480V Normal Duty rating is chosen.
Calculate the necessary values to choose an acceptable Dynamic Brake.
This information was given and must be known before the calculation process
begins. This can be given in Hp, but must be converted to watts before it can be
used in the equations.
This information was given and must be known before the calculation process
begins. This can be given in rpm, but must be converted to radians per second
before it can be used in the equations.
This value can be in lb•ft2or Wk2, but must be converted into kg•m2before it
can be used in the equations.
This was known because the drive is rated at 480 Volts rms. If the drive were rated
230 Volts rms, then Vd= 395 Volts.
All of the preceding data and calculations were made from knowledge of the
application under consideration. The total inertia was given and did not need
further calculations as outlined in Step 1 – Total Inertia .
Rated Power 10 HP=746 watts 7.46 kW=
Rated Speed b1785=rpm 2==
1785
60
---------186.98 Rad
s
---------------------=
Lower Speed o0=rpm 20
60
----0 Rad
s
----------===
Total Inertia JT6.2 12.4=lb•ft20.261 kg•m2 ==
Deceleration Time t3t2 –15 seconds==
Period of Cycle t440 seconds==
Vd790 Volts=

20 Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017
Chapter 2 Determining Dynamic Brake Requirements
Note that this is 8.1% of rated power and is less than the maximum drive limit of
150% current limit. This calculation is the result of Step 2 – Peak Braking Power
on page 15 and determines the peak power that must be dissipated by the
Dynamic Brake Resistor.
This is the result of calculating the average power dissipation as outlined in
Step 4. Verify that the sum of the power ratings of the Dynamic Brake Resistors
chosen in Step 3 is greater than the value calculated in Step 4.
For an internal resistor, refer to Appendix A to determine the continuous power
rating of the resistor in the given drive you are using. Skip this calculation if an
external dynamic brake resistor will be used.
In this case, a 10 Hp PowerFlex 70 drive has an internal resistor rated for 40
continuous watts. Because Pav = 114.1 watts,and is greater than the resistor’s
continuous watts rating, the drive will eventually trip on a Resistor Over Heated
fault. Calculate the minimum cycle time (in seconds) using the formula in
Chapter 3, Step 2B.
Recalculate the average power dissipation.
If the cycle cannot be adjusted, the decel time must be extended or the system
inertia lowered to reduce the average load on the resistor. Another option is to
use an external resistor.
Calculate the Percent Average Load. You will need this number to calculate the
Percent Peak Load.
Peak Braking Power Pb
JTbbo –
t3t2 –
-------------------------------------------
==
Pb
0.261 186.92 186.92 0–
15
------------------------------------------------------ 607.9 watts==
Average Braking Power Pav
t3t2 –
t4
------------------ Pb
2
------ bo
+
b
------------------------==
Pav
15
40
----


607.9
2
----------


186.92 0+
186.92
---------------------


113.9 watts==
607.9
2
----------15


40
---------------------------113.9 seconds=
Pav
15
114.1
----------


607.9
2
----------


186.92 0+
186.92
---------------------


40 watts==