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Application Note for Power Wire Wound Resistors
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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 overload a power resistor beyond the specified Voltage, rated Power and Current.

3. We recommend choosing a resistor with a rated power of at least 1.3 to 4 times higher than the actual loading power if your application requires the resistor to run continuously at full Power. Extra safe margin Power/Current can extend the resistor’s service life and lower its 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. If the surge/peak voltage is larger than the rated resistor Voltage SQR(P*R), 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 the rated power for 5 seconds, depending on the current pulse width, resistor series, installation and cooling system.

7. There is no standard resistance value for Power Resistor. It is better to tell the working Voltage, Loading Time and Duty Cycle of your applications for the Low Ohmic Power Resistors. As different Voltage can induce a very different resistor current. Different raw materials and production processes might need to withstand the high Current and Temperature.
For example, load Current for 1 ohm and 5 ohm 10kW Power resistor is 100A and 44A, respectively.

8. The resistor’s maximum working voltage must obey Ohm’s Law SQR(P*R)

9. We recommend choosing Low-Inductive Resistors for high-frequency applications.

10. Most of our Power Resistors can manufacture according to customers’ applications like Resistance, Rated Power, Resistor Size, Mounting Fixture and Inductive / Low Inductive, Pulse Voltage condition etc…

11. Do not touch the Resistor when connected to a power source due to the high resistor surface temperature and the chance of ELECTRIC SHOCK.

12. The salty, dusty and corrosive environment can affect the wire wound resistors performance.

B. Other Application notes:

1. The surface temperature of resistors can reach as high as 100C to 500C 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 150C to 250C will extend the resistor service life.

2. Use guards and warning labels where necessary for the hot resistors.

3. Adding a cooling system such as external forced cooling fans can lower resistors’ surface temperature and do not cover the resistors.

4. We recommend keeping all temperature-sensitive components away from the resistor.

5. Do not touch the Resistor after connecting to a power source due to high resistor surface temperature and the chance of ELECTRIC SHOCK.

6. Below is one of the Derating Curves for Power Resistors in general. Please contact us for an individual resistor’s derating curve.

       

7. Always clean the Resistor Tab Terminals before use. Do not clean Resistor surface with organic solvents.

8. Do not scratch the resistor surface with any hard or pointed object.

9. DDR-F and DQR-F series Power Resistors coat with UL 94V-0 silicone coating. The Resistors should be installed away from any flammable materials.

10. Silicon coated resistors might emit smoke during initial power loading. It is a normal phenomenon. After loading it 100% for 1-2 hours, the smoke emitting will stop.

11. The ASZAHR and HER Resistor external metal enclosure can be a source of interference for most sensitive circuits. Grounding the resistor metal housing can solve the concern.
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.

.

C. Adjustable Wire Wound Resistors DSR-F / Rheostats FVR / Rheostat Boxes FVRB and DSR-WB series application notes :

1. Rheostat and Adjustable Wire Wound Resistor is a kind of wire wound resistor.

2. Rheostats’ performance is bounded by both Ohm’s Law and resistance wire Current carrying capacity.
3.
The function of a rheostat is to adjust the circuit current between the maximum current at the minimum resistance and the minimum current at the rated resistance.

Ci. Rheostat 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 – like a Rheostat in series with a fixed resistor ( the equivalent circuit).

3. From a material point of view, the allowable Current is limited by the Ohm’s Law and the resistance wire’s Current carrying capacity – whenever the lower one. Loading Current beyond this can damage the adjustable resistor / rheostat.

4. The maximum current for two Rheostats with the same rated Power can be very different.

For example, load Current for 1 ohm and 5 ohms 10kW Power Rheostat is 100A and 44A, respectively.
There is no standard resistance value for Power Resistor.

5. Rheostat minimum resistance value can be calculated with the maximum current and voltage.

6. Rheostat maximum resistance value can be calculated with the minimum acceptable 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

Cii. Other Rheostat Application Notes :

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 in order to keep the load current below the rated Current.

5. A fixed power resistor can be connected in series with the rheostat to protect it from overcurrent damage.
The Rheostat rated resistance = Rheostat Power / (maximum load current
)2  

    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.

    The Load Power / Current adjustment by preseting steps – discrete resistance values.
    The Load Current is achieved via circuit breakers or power switches at the control panel.
    Each switch can control a preset resistance value ON/OFF.
    With different ON/OFF combinations, a 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
   Our Load Bank supports precision resolutions 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.

Ciii. Other Rheostat Application Notes :

1. The resistance adjustment is achieved by sliding the metal brusher across the metal resistance material.
There is a chance of flashover between two metal parts when the resistance is being adjusted, especially at high voltage, current and/or power conditions.
It is better to power OFF the load source across the Rheostat before adjusting the resistance values.

2. Do not touch the Adjustable Resistor / Rheostat surface when it is connected to the power source due to High Surface Temperature and avoid ELECTRIC SHOCK.

3. We recommend choosing a rheostat with a rated power of at least 1.2-4 times higher than the circuit maximum power if any application requires the rheostat to run continuously at the full Power. Extra safe margin Power/Current can extend the rheostat’s service life and lower its surface temperature.

4. Due to high power application and Rheostat consists of metal movable parts within the Rheostat.
    We suggest installing the rheostat on a fixed and level bench to avoid vibration.

5. The 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.

Cvi. 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

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