Application Notes for
High
Power Wire Wound Resistors
A. Choosing Power Wire Wound
Resistors and parameters determination :
1.
resistor power is
calculated by Power W=I2R
where :
W = Resistor
Power I = maximum
loading current
R = rated
resistance value or maximum resistance of Power Resistor
2.
Never over
load the resistor beyond the specified voltage, rated power
and loading current.
3. We
recommend choosing a resistor with a rated power of at least
1.2 to 4 times higher than the actual required loading power if
your application requires the resistor to run continuously
for 100% duty cycle. This can extend the resistor's service
life and lower it's surface temperature.
4.
If the maximum
or surge power larger than the rated resistor
power, please tell the actual working condition like
peak / surge voltage, resistance value, duty cycle,
loading time and repetition rate.
5.
For Surge
Voltage application, if loading Voltage is larger than the rated
Voltage according to resistor rated Power and Resistance
value, please tell us the Peak to Peak Voltage range,
Duty Cycle, Repetition Rate per unit time or Frequency
and loading time.
6. Our
resistor can withstand 5-10 times of the rated power for 5
seconds, depending on the current pulse width, resistor
series and cooling system.
7.
For very low
ohmic Power Resistor, it is better to tell the
working Voltage, Loading Time and Duty Cycle of your application. As
different Voltage can induce different resistor current. In
turn different raw materials and production process is needed
to withstand the generated high current and temperature.
8.
Different Resistance values for a
given rated resistor power can have very different load
current capacities, in turn different resistance material and
even production processing will be needed.
For example, load Current for 1 ohm and 1k ohm for a 1kW
Power resistor is 31A and 1A respectively.
Please feel free to tell the best suitable resistance for
your application.
9.
Maximum
working voltage for a Resistor is limited by Ohm's Law,
rated resistance, power, resistor dielectric
and insulation.
10. We
recommend choosing Low-Inductive Resistors for high
frequency application.
11.
We can make Power Resistors according to
customer's application need like Resistance,
Rated Power, Resistor Size, Mounting Fixture and
Inductive / Low Inductive, Pulse Voltage condition
etc...
12.
Do not touch
the Resistor when it is connected to a power
source due to high resistor surface temperature and
chance of ELECTRIC SHOCK.
13.
Salty, dusty and
corrosive environment can affect the wire wound resistors
performance.
B. Other Application note :
1. The
surface temperature of resistors can reach as high as 150°C
to 500°C while at full load, depending on resistor series,
resistor rated power, resistance value, working
conditions, ambient temperature and cooling system design. Keeping the temperature below 150°C to 200°C
will extend it's service life.
2. Adding
a cooling system such as external forced cooling fans can
lower resistors' surface temperature.
3. We
recommend keeping all temperature sensitive components away
from the resistor.
4.
Do not touch
the Resistor when it is connected to a power
source due to high resistor surface temperature and
chance of ELECTRIC SHOCK.
5.
Below is
one of the Derating
Curves for Power Resistors in general, for an individual
resistor's derating curve, please contact us.
6.
We recommend cleaning
the
Resistor Tab Terminals before use. Do not clean Resistor
surface
with organic solvents.
7.
Do not
scratch surface of Resistor with any hard or pointed
object.
8.
DDR-F and DQR-F
series Power Resistors are coated with UL 94V-0
class silicon coating. The Resistors should be installed
away from any flammable materials.
9. Silicon
coated resistors might emit smoke during initial power
loading. It is a normal phenomenon. No smoke will be emitted
after loading it 100% for 1-3 hours.
C.
All our Load banks RB3A, RLB3A, RB, DB,
RBA, DSR-WB, DSR3-WB, FVRB and RBC series, should be
Ground connected below
connecting the Load Source and loading the load bank.
D. Adjustable Wire Wound
Resistors DSR-F / Rheostats
FVR / Rheostat Boxes
FVRB
and DSR-WB series application note :
Rheostat and Adjustable Wire Wound Resistor is a kind of
wire wound resistor.
It’s performance is bounded by both
Ohm’s Law and resistance wire Current
carrying capacity.
Di. Parameters determination
:
1.
Rheostat rated Power =
(Rheostat maximum load Current)2 x rated Resistance
2. The
maximum load current is determined by the current of an
existing application before the adjustable Power Resistor or
Rheostat is inserted. This consideration
is for Circuit Current adjustment.
3. The
maximum loading current is limited by the resistance wire's
current carrying capacity and Ohm's Law. Loading Current
beyond this can damage the adjustable
resistor / rheostat.
4. In
terms of Rated Power, a smaller Rated Resistance value means
the rheostat can have a larger Rated maximum Current capacity.
It is not necessary to buy the standard resistance value.
Most of the
times, ordering Customized Resistance rheostat will not
affect the cost and lead time.
5.
Rheostat minimum resistance value can be calculated with the
maximum current and voltage.
6.
Rheostat maximum resistance value can be calculated with an
acceptable minimum current and the voltage.
7.
The
workable Power will decrease as the resistance is adjusted
to a smaller value.
The workable power at the adjusted
resistance is about the ratio of (adjusted
resistance) to (the rated maximum resistance of that rheostat)
x ( rated rheostat power) or
i.e. in material point of view : Power per Unit Resistance
Dii. Other
Application Note :
1.
Load Current at the adjusted resistance value
=< Rheostat
rated Current
2.
Load Power at the adjusted resistance value =< Rheostat
rated Power
3.
Rated
resistance value is not the same as an adjusted resistance
value.
4. The
voltage across Rheostat might need to decrease to avoid over
loading the rheostat when reducing the resistance value.
5. A fixed
power
resistor can be connected in series with the rheostat to
protect it from over current damage.
The resistor resistance value is determined
by the rheostat maximum load current.
The Resistor power = (maximum load Current)2
x rated Resistance.
6. The
main role of Adjustable Power Wire Wound Resistor DSR-F,
Rheostat FVR, Rheostat Box FVRB and DSR-WB is to decrease,
not increase, the electrical
current in the circuit.
7.
The above is for Continuous
Load Current
- Continuous
Resistance
range
design.
8.
For some situations, we will suggest RBA series Adjustable
Load Bank.
This Load Power / Current adjustment by
preseting steps - discrete resistance values.
Load current is achieved via circuit breakers or power
switches at control panel.
Each switch can control a preset resistance value ON/OFF.
With different ON/OFF combination, different load current
can be achieved.
For example : 250Vdc with max. current 150A.
The setting can be :
10A, 20A x 2, 50A x 2 = 150A in 5 steps with resolution 10A
or
1A, 2A x 2, 5A, 10A x 2, 20A, 50A x 2 = 150A in 9 steps with
resolution 1A or
If necessary, we can support some other precision resolution
like 0.5A, 0.2A and 0.1A.
The load Current is achieved by load Step adjustment.
e.g. 97A is needed, switch : 2A, 5A, 10A, 10A, 50A is ON
This load bank rated power is 250V x 150A = 37.5kW only.
When compare with rheostat, RBA series load banks, this can
be a cost saving option.
Diii. Other application note :
1. For
Rheostats, the resistance adjustment is achieved by sliding
the metal brusher across the metal resistance material.
In high current, voltage and power condition, there is chance to have
flashover among the
movable
metal parts and resistive material.
We suggest power OFF the load source across the Rheostat before
adjusting the resistance value for High Voltage or
High Current situation.
2. Do not
touch the Adjustable Resistor / Rheostat surface when it is
connected to the power source due to
High Resistor
Surface Temperature and avoid
ELECTRIC
SHOCK.
3.
We
recommend choosing a resistor with a rated power of at least
1.2 to 4 times higher than the actual required loading power if
your application requires the resistor to run continuously
for 100% duty cycle. This can extend the resistor's service
life and lower it's surface temperature.
4.
There are metal
movable parts within the Rheostat.
Due to high power application, we suggest the rheostat has
to install in a fixed and level bench to avoid vibration.
5. Salty,
dusty, humid, high temperature, vibration
and corrosive environment can affect the Rheostat
performance.
6. Both above
Section A and B
application notes are also true for adjustable wire wound
resistors.
Dvi.
Rheostat Bank FVRB /
Adjustable Load Bank DSR-WB
options :
1.
Meter
: Ammeter, Voltmeter, Wattmeter,
Ohm meter and Temperature meter
2. OverCurrent
protection
3.
Thermal
protection
4.
Cooling Fans
system
