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DYNAMIC BRAKING RESISTORS |
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Dynamic
braking is the simplest and very widely used method of
braking a load, driven through a d.c. motor as it
involved no wear and rear unlike with mechanical brakes.
For dynamic braking, the motor armature is closed on a
suitably designed
Dynamic Braking Resistor (DBR), while
the motor field remains energised. The kinetic energy
stored in motor and load gets dissipated in the DBR and
the system comes to a hair which a few seconds. |
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Calculations for Selection of Dynamic Braking Resistance |
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When
the motor is braked, it is desired that it comes to a
hair in minimum possible time. Hence the ohmic value of
DBR is selected such that it causes maximum braking
current at the instant of starting the braking. This
maximum braking current depends on the overload capacity
of the motor. |
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Ohmic Value of DBT is R =
Vr / Ip |
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Where Vr
is motor armature rated voltage and Ip is
maximum permissible current.
Generally Ip can be considered as twice the
value of motor armature rated current. Dynamic braking
starts with maximum current and falls exponentially with
time, while the field current is maintained constant.
The armature voltage and motor speed also decay
exponentially. Refer figure 1. The time constant r of
this decay is given by Fig.1
The motor comes to a hair in a time period equal to
approximately twice this time constant since the
friction also aids the braking process. In practice it
is found that for most of the applications r is less
than 5 seconds and hence dynamic braking period is less
that 10 seconds. The DBR must be designed such that it
can carry the braking current till the motor comes to a
hair. |
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Design
of DBR |
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The foregoing information
helps you in calculating the parameters required for the
design of DBR. After the above calculations, the
following parameters should be specified. |
1. R - ohmic value
of the resistance required
2. Ip - Maximum current through DBR
3. Dynamic Braking period - when this is less
than 10 seconds (as is normally the case), it need not
to be specified.
4. Vr - Motor armature rated voltage -
this is required for designing the insulation
resistance. If Vr is less than 500 volts, it
need not be specified.
The DBR is designed based on the above data. It is built
from our standard sun-assemblies to suit individual
requirements as detailed below. |
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Construction of DBR |
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The DBR is constructed out
of stainless steel grid
resistors (for medium and high
ratings) or wirewound resistors (for low ratings). |
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- Grid Resistors:
These are made our of stainless steel wire of suitable
diameter bent to form a grid (Figure 2) of standard
dimension. The required number of grids are put in
series to give desired resistance. This assembly is
supported through a mica insulated steel rod with mica
washers, which separate the grids. Refer Figure3. This
assembly, known as stack, has two steel endframes for
mounting it on the panel or the steel rod is bolted
directly to the two sides. of a box. The terminals are
brought out suitably for external connection this entire
assembly has the advantage of lightness. Compactness and
high mechanical strength. The high resilience of the
material makes it suitable for applications where the
equipment is subjected to vibration. |
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Wirewound Resistors:
These are our standard
resistors made out of
continuous wire of suitable gauge. Wound on a grooved
ceramic base. Refer figure 4.
The number of grids to be put in series or the number or
wire wound resistor tubes to be put in series to have
the desired resistance can be determined from the table
of Figures 5 and 6. The dimensions of the resistance
stack depend on the number of grids as shown in figure
3. For higher peak currents the stacks can be
paralleled. |
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DBR
with Enclosure |
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DBR can be supplied loose
as described above or in a separate enclosure. There are
different sizes of enclosures depending on the number of
grids per stack and number of stacks. These are shown in
fig. along with dimensions. The normal enclosure
protection class is IP21. We can also supply protection
class IP31 and IP41. A removable bottom plate is
provided for making holes for fixing glands for cable
entry. The recommended cable rating is 25% of the value
of Ip. Bigger size enclosures are offered
against enquiry. For wire wound
resistors, enclosure is
supplied as per requirement. |
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Variation from standard design: |
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1. The current
ratings given in Fig. 5 and 6 hold true for dynamic
braking time upto 10 seconds. For higher dynamic braking
period, Ip permitted for each resistor
would be less. |
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2. It is presumed
that dynamic braking is not applied very frequently i.e.
the time gap between 2 successive dynamic braking
operation is sufficient that DBR cools down to ambient
temperature. This time gap depends on DBR rating but
generally lies within a range of 5 to 20 minutes. In
case an application required dynamic braking at shorter
intervals, the interval and no. of successive dynamic
braking operations at such intervals should be
specified. Sometimes the repeated operations are
required at lower speeds (e.g. inching), then the speed
(in % of full speed) should also be specified. Dynamic
braking at lower speeds is less severe than at full
speed.
If your applications required the above variations or
any other feature, the same can be offered against
enquiry. |
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