What is Dynamic Braking

By: Jeff Theisen, Rockwell Automation

Posted By: Michael Cromheecke, Rockwell Automation

When an induction motor’s rotor is turning 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 into the utility grid. This process is referred to as
‘regeneration’. On most AC PWM drives, the AC power available from the fixed frequency
utility grid is first converted into DC power by means of a diode rectifier bridge or controlled
SCR bridge, before being inverted into variable frequency AC power. These diode or SCR
bridges are very cost effective, but can handle power in only one direction, and that direction is
the motoring direction. If the motor is regenerating, the bridge is unable to conduct the necessary
negative DC current, and the DC bus voltage will increase until the drive trips off due to a Bus
Overvoltage trip. There are bridge configurations, using either SCRs or Transistors that have the
ability to transform DC regenerative electrical energy into fixed frequency utility electrical
energy but are expensive. A much more cost effective solution is to provide a Transistor Chopper
on the DC Bus of the AC PWM drive that feeds a power resistor which transforms the
regenerative electrical energy into thermal heat energy which is dissipated into the local
environment. This process is generally called ‘Dynamic Braking’, with the Chopper Transistor
and related control and components called the ‘Chopper Module’, and the power resistor called
the ‘Dynamic Brake Resistor’. The entire assembly of Chopper Module with Dynamic Brake
Resistor is sometime referred to as the ‘Dynamic Brake Module’.

Chopper Modules are designed to be applied in parallel if the current rating is insufficient for the
application. One Chopper Module is the designated ‘Master’ Chopper Module, while any other
Modules are the designated ‘Follower’ Modules. Two lights have been provided on the front of
the enclosure to indicate Chopper Module operation – the ‘DC Power’ light and the ‘Brake On’
light. The DC Power light will be lit when DC power has been applied to the Chopper Module.
The Brake On light will be lit when the Chopper Module is operating or ‘chopping’ and will be a
flickering type of indication.

How it works

There are two different types of control for dynamic braking, hysteretic control and PWM
control. Each used by themselves in a standard stand alone product has no advantage over the
other. The preferred control would be the PWM method when the application is common dc bus.
This advantage is decribed below.

Hysteretic Control
The hysteretic method of dynamic braking uses a voltage sensing circuit to monitor the dc bus.
As the dc bus volage increases to the Vdc_on level the brake IGBT is turned on and is left on
until the voltage drops to the Vdc_off level (which is not so desirable in common dc bus
applications - see below). Some of the Powerflex drives[1] allow the Vdc_off level, [DB
Threshold],to be adjusted if the application required it. Setting this level lower will make the
dynamic braking more responsive but could lead to excessive DB activation.

References

1. ↑ PF40, PF40P

PWM Control

This type of control to operate the brake IGBT is similar to the way output voltage to the motor
is controlled. As the dc bus voltage increases and hits some predetermined limit the brake IGBT
is turned on/off according to a control algorithm switch at 1khz. This type of control virtually
eliminates bus ripple. The big advantage is when this type of control is in a common bus
configuration.

Duty Cycle
Common DC Bus Applications

In a common bus configuration when a dynamic braking resistor is installed on each drive
sharing the dc bus, it’s possible that the brake IGBT in some drives may not turn on, giving the
impression that the drive is not functioning correctly or seeing one drive’s brake IGBT failing
consistently while the other drives are fine. Looking at the below diagram, it shows the dc bus
level for two drives on common bus. The delta between these voltages are exaggerated for
clarity. As the voltage increases, drive 1’s IGBT turns on and decreases the voltage level before
drive 2 sees voltage high enough to be told to turn on. This results in drive 1 doing all the “DB
work” and drive 2 taking a powder. Now this situation could be ok as long as the minimum
ohmic value for resistance is not violated and the regen event isn’t so great that a single resistor
can’t handle the power. Of course if there is a large regen event where the voltage continues to
rise after drive 1 has “turned on”, drive 2 will fire it’s IGBT when it reaches the voltage limit.

Here are two drives with PWM DB control on a common bus. Since one drive will turn on at a
certain duty cycle the bus voltage will likely continue to rise guaranteeing that the other drive’s
IGBT will turn on (at a different duty cycle).
How to Select A Chopper Module and Dynamic Brake Resistor

In general, the motor power rating, speed, torque, and details regarding the regenerative mode of
operation will be needed in order to estimate what Chopper Module rating and Dynamic Brake
Resistor value to use. A rule of thumb to use is that a Dynamic Brake Module can be specified
when regenerative energy is dissipated on an occasional or periodic basis. When a drive is
consistently operating in the regenerative mode of operation, serious consideration should be
given to equipment that will transform the electrical energy back to the fixed frequency utility.

The peak regenerative power of the drive must be calculated in order to determine the maximum
Ohmic value of the Dynamic Brake Resistor and to estimate the minimum current rating of the
Chopper Module. The Rating of the Chopper Module is chosen from the Brake Chopper Module
manual. Once the Chopper Module current rating is known, a minimum Dynamic Brake
Resistance value is also known. A range of allowable Dynamic Brake Ohmic values is now
known. These values exist from the minimum value set by the Chopper Transistor current rating
to a maximum value set by the peak regenerative power developed by the drive in order to
decelerate or satisfy other regenerative applications. If a Dynamic Brake Resistance value less
than the minimum imposed by the choice of the Chopper Module is made and applied, damage
can occur to the Chopper Transistor. If a Dynamic Brake Resistance value greater than the
maximum imposed by the choice of the peak regenerative drive power is made and applied, the
drive can trip off due to transient DC Bus overvoltage problems. Once the choice of the
approximate Ohmic value of the Dynamic Brake Resistor is made, the wattage rating of the
Dynamic Brake Resistor can be made.

The wattage rating of the Dynamic Brake Resistor is estimated by applying the knowledge of the
drive motoring and regenerating modes of operation. The average power dissipation of the
regenerative mode must be estimated and the wattage of the Dynamic Brake Resistor chosen to
be slightly greater than the average power dissipation of the drive. 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, the
temperature of the resistor element could exceed the maximum temperature limits during the
application of pulse power to the element and could exceed the safe temperature limits of the
resistor.

The peak regenerative power can be calculated in English units (Horsepower), in The
International System of Units (SI) (Watts), or in the per unit system (pu) which is dimensionless
for the most part. In any event, the final number must in Watts of power to estimate Dynamic
Brake Ohmic value. Calculations in this page will be demonstrated in SI units.

Speed, Torque, Power Profile

The following figure is a typical dynamic braking application. The top trace represents speed and
is desigated by the omega symbol. In the profile the motor is accelerated to some speed, it holds
that speed for a period of time and is then decelerated. This deceleration is not necessarily to zero
speed. The cycle is then repeated.

The middle trace represents motor torque. Torque starts out high as the motor is accelerated then
drops down to maintain the commanded speed. Then the torque turns negative as the motor is
decelerated. The cycle is then repeated.

The bottom trace represents motor power. Power increases as the motor speed increases. Power
decreases some to maintain the commanded speed then goes negative when deceleration starts.
(this point called -Pb is the first value that needs to be calculated). The cycle is then repeated.
Dynamic Braking Module
Figure 1 shows a simplified schematic of a Chopper Module with Dynamic Brake Resistor. The
Chopper Module is shown connected to the positive and negative DC Bus conductors of an AC
PWM Drive. The two series connected Bus Caps are part of the DC Bus filter of the AC Drive.
The significant power components of the Chopper Module are the protective fusing, the Crowbar
SCR, the Chopper Transistor (an IGBT), the Chopper Transistor Voltage Control (hysteretic
voltage comparator), and a freewheel diode for the Dynamic Brake Resistor.

The protective fuse is sized to work in conjunction with the Crowbar SCR. Sensing circuitry
within the Chopper Transistor Voltage Control determines if abnormal conditions exist within
the Chopper Module. One of these abnormal conditions is a shorted Chopper Transistor. If this
condition is sensed, the Chopper Transistor Voltage Control will fire the Crowbar SCR, shorting
the DC Bus, and melting the fuse links. This action isolates the Chopper Module from the DC
Bus until the problem can be resolved.

The Chopper Transistor is an Isolated Gate Bipolar Transistor (IGBT). There are several
transistor ratings that are used in the various Chopper Module ratings. The most important rating
is the collector current rating of the Chopper Transistor that helps to determine the minimum
Ohmic value used for the Dynamic Brake Resistor. 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 Chopper Transistor Voltage Control regulates the voltage of the DC Bus during
regeneration. The average value of DC Bus voltage is 375 Volts dc (for 230 Vac input), 750
Volts dc (for 460 Vac input), and 937.5 Vdc (for 575 Vac input). The voltage dividers reduce the
DC Bus voltage to a low enough value that is useable 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.

Figure 1

Sizing the Dynamic Brake Module

Gather the following information:

2. The nameplate speed rating of the motor in rpm, or rps.

3. The motor inertia and load inertia in kilogram-meters2, or lb-ft2.

4. The gear ratio, if a gear is present between the motor and load, GR.

5. Review the Speed, Torque Power profile of the application.

Equations used for calculating Dynamic Braking values will use the following variables.

ω(t) = The motor shaft speed in Radians/second, or

N(t) = The motor shaft speed in Revolutions Per Minute, or RPM

T(t) = The motor shaft torque in Newton-meters, 1.01 lbft - 1.355818Nm

P(t) = The motor shaft power in Watts, 1.0HP = 746 Watts

-Pb = The motor shaft peak regenerative power in Watts

Step 1 – Determine the Total Inertia

JT = Jm + GR2 X JL

JT = Total interia reflected to the motor shaft, kilogram-meters2, kg-m2, or pound-feet2, lb-ft2

Jm = Motor inertia, kilogram-meters2, kg-m2, or pound-feet2, lb-ft2

GR = The gear ratio for any gear between motor and load, dimentionless

JL = Load inertia, kilogram-meters2, kg-m2, or pound-feet2, lb-ft2 -- 1 lb-ft2 = 0.04214011 kg-m2

Step 2 – Calculate the Peak Braking Power

JT = Total inertia reflected to the motor shaft, kg-m2

ω = rated angular rotational speed,

N = Rated motor speed, RPM

Pb = peak braking power, watts ( 1.0 HP = 746 Watts)

Compare the peak braking power to that of the rated motor power, if the peak braking power is
greater that 1.5 times that of the motor, the deceleration time, (t3-t2), needs to be increased so
that the drive does not go into current limit. Use 1.5 times because the drive can handle 150%
current maximum for 3 seconds.

Peak power can be reduced by the losses of the motor and inverter.

Step 3 – Calculating the Maximum Dynamic Brake Resistance Value

Vd = The value of DC Bus voltage that the chopper module regulates at and will equal 375Vdc,
750Vdc, or 937.5Vdc

Pb = The peak braking power calculated in step 2

Rdb1 = The maximum allowable value for the dynamic brake resistor