13th International Conference on DEVELOPMENT AND APPLICATION SYSTEMS, Suceava, Romania, May 19-21, 2016

Study Solution of Induction Motor Dynamic Braking

Mihai Rata1,2, Gabriela Rata1,2

1
Faculty of Electrical Engineering and Computer Science, Stefan cel Mare University of Suceava, Romania
2
Integrated Center for Research, Development and Innovation in Advanced Materials, Nanotechnologies and Distributed Systems
for Fabrication and Control (MANSiD), Stefan cel Mare University, Suceava, Romania
mihair@eed.usv.ro

Abstract—The three-phase squirrel-cage induction motors are + D C

mostly used in industrial drivers since they are rugged, reliable + D C
and economical. The DC bus voltage of the AC drives increases
when the motor regenerates. The regenerating energy is usually L 1 L 1
released in the form of heat. A short mathematical description L 2 E n e rg y L 2 E n e rg y
and the experimental results for different cases of dynamic brake L 3 L 3

regime are presented in our paper. This solution is recommended

for student use and laboratory work to improve understanding of -D C

dynamic brake working. -D C

a) b)
Keywords—Variable speed drives, Induction motors, Snubbers,
Electrical engineering education Fig. 1. Different rectifier topologies used in variable frequency drive.

In most cases (pumps, fans, etc.) where the kinetic energy

I. INTRODUCTION
in the load is small or the braking time is not an important
The most widely used motor in industry for variable speed parameter and can be increased, the regenerating energy is
applications has been the DC brushed motor because of its smaller than the power losses in the driver and in the motor [4,
simplicity of controlling speed [1]. But the drawbacks of this 5]. In applications (i.e. centrifuges, cranes, some conveyors,
motor are high maintenance and low life-span for high drives that require a very fast speed reversing, etc.) where it is
intensity uses. necessary to brake the motor fast, power losses in the driver
The three-phase induction motors with short-circuited rotor and in the motor are not enough for regenerating energy. There
(squirrel cage) are mostly used in industrial applications that are three ways to handle it as follows:
require fixed speed or variable speed [2, 3]. The induction  Recovering of regenerative energy in the supply system.
motors work in two different regimes depending on the In this case, the drive must have the ability to change
direction of energy flow as follows: the DC bus energy into fixed frequency utility power
 “motoring” regime, when the motor rotor turns slower through a “regenerative bridge converter” (illustrated in
than prescribed speed. In this case, the electrical energy Fig.1.b) or a “regenerative brake”. It can use a
is transformed into mechanical energy at the motor supplementary braking module with braking resistor if
shaft; it is necessary to control the deceleration of the
induction motor in power failure case.
 “generating” regime, when the motor rotor turns faster
than synchronous speed set by a drive output. In this  Using of regenerative energy from one motor/drive that
case, the mechanical energy from the motor shaft is works in regenerating regime by another motor/drive
transformed in electrical energy. connected to the same DC bus line that works in
motoring regime.
In the majority of cases the induction motors are fed by
PWM frequency converters that convert the power first into  Converting of regenerative energy in the form of heat
DC by a diode rectifier bridge and then in AC by an IGBT by placing a braking module and a braking resistor,
three-phase inverter [1]. These types of converters allow the often called “Chopper” or “Dynamic Brake” across the
energy to handle only in “motoring” direction (as shown in drive DC bus.
Fig.1.a.), and have a very low cost. During the motor is in The major difference between the chopper and the dynamic
regenerative condition, the energy from the motor flows brake lies in the construction [6]. A dynamic brake has contains
backward through the inverter bridge diodes and the DC bus the controller (regulator circuit), the transistor (switching
voltage increases. According to the Laws of Physics, energy is device) and the brake resistor, in same unit. It is used for small
never lost or gained; this energy needs a place to transformed. power and up to 20% duty cycle of dynamic brake rating (the
ratio between the brake time and motor cycle time). A chopper

978-1-5090-1993-9/16/$31.00 ©2016 IEEE

has only the controller and the switching device, in same unit. P o w e r P o w e r
The braking resistors are treated as separate components that
allow choosing an accurate size for the specific application.
Furthermore, it permits placing the chopper in a protected case t t
and the resistors at a distance of up to 30 m. The choppers tB tB
represent a more “heavy duty” solution than the dynamic brake tC tC
and, therefore, are more suited for a dynamic brake duty cycle a) b)
greater than 20% [6].
Fig. 2. Differences between deceleration braking and overhauling load.
In this paper the authors propose an efficient solution to
study the dynamic brake regime at induction motor powered by The duty cycle of the dynamic brake obtained by using (4),
a variable frequency drive. This solution can be used in is necessary to calculate the power resistor brake. If D=1, the
laboratory work by the students and it permits them to study resistor dissipates energy continuously.
how the equipment works when different parameters are
changed (brake time, kinetic energy of load, resistor brake, and
motor speed reduction) D
tB
 100 (4)
tC
II. LIST OF THE USED SYMBOLS
TB required braking torque in (Nm) The power of brake resistor can be calculated as in (5),
which represents the continuous dissipated power (average
Tm rated motor torque in (Nm)
power).
TB% required percent of braking torque in (%)
Jtot total moment of inertia in (kgm2) D
PRb  Ppeak  , for deceleration braking regime
aB deceleration during braking in (rad/s2) 2 (5)
 motor angular speed reduction in (rad/s) PRb  Ppeak  D, for overhauling load regime
n motor speed reduction in (RPM)
tB motor deceleration time, or brake time in (s) The power of resistor brake is influenced also by brake time
tC motor cycle time in (s) and by how the motor is used: in deceleration braking
D duty cycle of the dynamic brake in (%) (ventilation systems), as shown in Fig.2.a, or in overloading
RB brake resistor in () load (conveyors, cranes, elevators), as illustrated in Fig.2.b. We
Ppeak peak of brake power in (W) can notice that the braking energy (hatched areas in Fig.2) for
PRb power of brake resistor in (W) overhauling load is twice bigger than energy of deceleration
braking.
VDC DC bus Voltage in (V)
The majority of resistor manufacturers recommend the
III. DYNAMIC BRAKE calculation of resistor power like for overhauling load regime,
in the case where the motor is used in deceleration braking
When transforming the regenerative energy into heat forms regime with the braking time higher than 60s.
while the induction motor reduces its speed, it is very important
to choose an optimum brake resistor value [6-8]. For this 2
reason, first, it is necessary to determine the required braking RB 
VDC
(6)
torque (TB) using (1). Ppeak

 2  n The value of brake resistor can be calculated using (6). The

TB  J  a B  J  J (1)
tB 60  t B used brake resistors must be induction-free.
IV. PROPOSED SOLUTION
The maximum braking power can be calculated using (2).
In this paper, the authors propose a solution for studying
2  n2 dynamic braking of induction motor that can be used by
Ppeak  TB    TB  (2) students in laboratory work. The motor is powered through a
60 variable frequency drive (VFD). The proposed dynamic brake
is connected to DC terminals. The block diagram is illustrated
The necessity of using dynamic braking is determined by in Fig3.b. The solution adopted for monitoring the DC link
(3). If TB% is smaller than 20%, dynamic brake is not necessary voltage (VDC) uses a sensor with galvanic isolation (LEM
due to natural dissipation of regenerative energy in the losses in LV25P), an IGBT transistor (model SKM5050GAL 123D from
both variable frequency drive and motor. If TB% ranges between Semikron), and a driver SKHI10, also from Semikron [9].
20% and 150%, a dynamic brake is required.
The output voltage from the LEM sensor (VDC_1) is
T compared in two hysteresis comparators with two different
TB %  B  100 (3) threshold voltages Vref_1 and Vref_2. If VDC_1 > Vref_1, the first
Tm comparator makes the brake IGBT transistor to turn ON.
 + D C
L 1 U Brake
L 2 V Variable Resistors
Frequency
L 3 W Drive
-D C
V a ria b le F re q u e n c y D riv e
a)

+ D C Chopper
R B
+ D C + H T
V D C _ 1
V D C
L E M M A G a te D riv e r
L V 2 5 p V S K H I1 0
r e f_ 1
-D C -H T
-D C
V D C _ 1
E x t_ T rip
V r e f_ 2
a)
D y n a m ic B ra k e

b)

Fig. 3. Block diagram for dynamic brake.

If VDC_1 rises above the threshold value Vref_2 the VFD will
trip because the output of the second comparator is connected
to External Trip Digital Input of VFD. It is recommended that
our solution for dynamic brake be studied in lab work than in
industrial environment since students can easily understand,
using a scope, how works this equipment. More than that, the
electrical circuit is designed to allow students to adjust the
threshold voltage values (Vref_1, Vref_2) and to observe the b)
changes of the dynamic brake working. An industrial dynamic
brake cannot offer these features. The manufacturers do not
offer the schematic diagram, the values for threshold voltages
are fixed, and it is difficult for the students to measure the
signals with the scope because the PCB is very compact. It is
very important for the students to understand the practical lab
works and to get them closer to real world applications. The
experimental arrangement is illustrated in Fig. 4. c) d) e)
This solution enables the laboratory to be more practical in Fig. 4. Experimental setting.
order to provide experience closer to the real world
applications to students. For this reason, we use an industrial These disks can be removed independently from the shaft
VFD (AEG-MICROVERTER D 10.5/380), and the connected and fixed with screws to the supports, as presented in Fig. 4.c,
dynamic brake that we propose in this paper. The equipment d and e. The students can measure the maximum diameter (D),
allows students to study the dynamic brake regime of induction minimum diameter (d), the weight (m), and they can calculate
motor in the following different conditions: the inertia for each disk using (7).
 different brake time,
 different motor speed reduction, D2  d 2
J  m (7)
 different total moment of inertia, 8

 different value for brake resistor, The different values of brake resistor can be very easily
obtained using two resistors of the same value (in laboratory,
 different threshold voltage values.
we choose two resistors of 47 /600 W). The students should
The different brake time and motor speed reduction can be use them as follows: resistors in parallel connection (23.5 ),
obtained through VFD parameterization. The proposed solution resistors in series connection (94), or only one resistor (47).
for changing the load moment inertia is to use a load realized
by different weight disks, as illustrated in Fig.4.b.
 V. EXPERIMENTAL RESULTS
Fig. 5 illustrates the experimental results for a braking
regime without connecting the dynamic brake. The signals
illustrated represent the following:
 Ch1 – the DC link voltage of VFD,
 Ch2 – the induction motor speed (measured with a
tachogenerator mechanically coupled at the induction
motor shaft and that has the measure constant 6V for
1000RPM),
 Ch3 – the gate voltage of brake IGBT transistor, a)

 Ch4 – the current through brake resistor.

In this case the DC voltage is increasing during the brake
regime (when the induction motor speed is decreased). If the
braking time is an important parameter that must be reduced,
the DC bus voltage will continue to rise until the drive
eventually tripped on a bus overvoltage fault.
A solution of removing that energy from the DC link is to
use dynamic brake. Fig. 6 illustrates some different cases for
dynamic braking regime. In all cases the dynamic brake limits
the overvoltage in DC link. Analyzing the figures in a specific
b)
order, we can see the influence of some parameters on the
working equipment which we studied, as follows:
 Results for different inertia moment of load are
illustrated in Fig.6.a (low inertia moment) and Fig.6.b
(high inertia moment). When the inertia moment
increases, the braking time is increasing also.
 Results for different brake resistor values are illustrated
in Fig.6.b (47 ), Fig.6.c (94 ) and Fig.6.d (23.5 ).
If the braking resistor increases, the braking current is
decreasing.
 Results for different brake times are illustrated in c)
Fig.6.b (tB = 500 ms) and Fig.6.e (tB = 2000 ms). If the
brake time decreases, the duty cycle of braking current
is increasing.

d)

Fig. 5. Experimental results without dynamic brake connected.

VI. CONLUSIONS
The solution proposed by authors can be used by the
students in laboratory work for understanding the working of a
dynamic brake connected at a variable frequency drive that
control an induction motor. e)

Fig. 6. Experimental results with dynamic brake connected.

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