Aluminum Case Dynamic Braking Resistors
 
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We have worked with many OEMs and built a broad knowledge of their product specifications for dynamic braking resistors. You can rely on the industry’s most innovative resistor manufacturer with over 30 years of industry experience.

Innovative manufacturing techniques are adapted in our braking resistors to get fail safe feature for VFD protection and offer superior performance. Per kilowatt capacity performance our resistor is equal to 2.4 KW capacity of conventional resistor. There is no need to over sizing resistor capacity for our resistor.

 
  What is Fail safe feature?
  • AC Drives do not offer protection for brake resistor failures. DBR fail safe feature will only protect VFD against resistive element turn to turn short or element to ground shot. (Resistive element experience in expansion when subjected to repeated heating and cooling which normally happens while DBR working)
  • If element turn to turn short will lead to resistor value reduce to below recommended minimum value resulting chopper module failure in VFD.
  • Similarly if element short to ground will lead to VFD DC Bus short, resulting whole VFD rectifier section failure.
  • Our unique construction adapted to resistive element encapsulation, arrest turn to turn short and also element short to ground.
  • This fail safe feature is only available with GR DBR. This feature is essentially required for the braking resistors use in VFD applications.
 
  Braking resistor – Short time Pulse Rating:
  • The VFD manufacturers recommend that the minimum requirement for braking resistor is to meet short time pulse rating of 10 times of continuous power for 12 sec. This means that the1KW capacity resistor to with stand short pulse rating of 10KW power for 12 sec. the unique design of encapsulated constructions allow our resistor to meet this short time pulse rating.
  • The best designed conventional wire wound resistor has a short time pulse rating is only 10 times power for 5 sec. To meet 10 times power for 12 sec, the conventional resistor capacity is to be over sized to 2.4 times.
 
  How to calculate DBR average power rating?
  • Average power will be calculated based on the duty cycle. Many VFD manufactures recommend 10% duty for horizontal load braking application and 40% duty for vertical loads i.e. Hoist application.
  • Braking torque will be normally 100% for light duty and for 150% for heavy duty application.
  • Formula for calculating DBR Average Power is Duty cycle X Braking Torque X VFD Power rating.(Use factor 0.1 for10% duty cycle and 0.4 for 40% duty cycle. Use factor 1.0 for 100% braking torque and 1.5 for 150% braking torque)
 
  Example & Cost Comparison:

Example for Braking resistor capacity calculation: VFD Capacity 10KW, Application is Heavy duty Braking 150% braking torque, Duty cycle is 10%

DBR power Rating: 0.1 X 0.15 X 10 = 1.5KW (Resistor should withstand 15KW for 12 Sec) For the same application, if conventional wire wound resistor selected, the resistor wattage rating is 1.5 x 2.4 = 3.6KW.

The cost of GR Alu case braking resistor of 1.5 KW for 10KW VFD will be Rs. 2000/ and the cost conventional resistor of 3.6KW for the same 10KW VFD will be in the range of Rs.2000 to 2200/. Our braking resistors are more economical with built-in more features. Table below shows the features comparison between GR Alu case resistor and conventional wire wound resistor.

 
  Features Comparison:
 
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  DBR Construction:

Alu case resistor consists of an alloy coil-type resistance element assembled into Aluminum Case enclosure and encapsulated with ceramic potting material and after hardening, the insulation is achieved through a high temperature process. Since the resistor is embedded in the heat-proof ceramic material, it is a flame proof and not affected by external mechanical force, dusty environment, and extreme duty.

It is durable, vibration proof, dissipates heat well, and has low temperature co-efficient. The encapsulation protects the winding and element any movement of winding due to expansion of wire when subjected to repetitive heating and cooling. Our DBR resistors have excellent heat dissipation for continuous braking with highest active surface area per KW in the industry, giving outstanding short time performance for repetitive braking duties. The encapsulated construction is the key to the efficient cooling ability of the unit and allows the element to come in direct contact the cool ambient. The unique design allows rapid, natural convection cooling of the resistive element.

 
  Repetitive Duty Cycle Ratings:

GR Braking resistors are designed for continuous 100% duty at the power rating which is on the label. All braking resistors are designed and tested the continuous duty is calculated to be 120seconds or longer at full power rating. Therefore if braking resistors are used for less than 120second in all operating periods then they can be used at a higher power rating.

The table below shows power multiplier based on 6 sec to 120 sec on time against duty cycle. Multiply the resistors ‘continuous’ power rating by the power multiplier, number to calculate power during the on time. A 120sec period with 10% duty cycle means 12sec on, & 108sec off

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Example: 12 sec on, in 120 sec period, is defined as a 10% duty cycle. A 10% duty cycle gives 10times power multiplier using the above table. A 1Kw resistor would be rated 10Kw (10 x 1Kw) for 12 sec in 120 sec period.

 
  Resistance and Element Temperature & Design Feature:
  • GR Resistors use high grade Ferritic alloy (FeCrAl) or Nical-Chromium resistor elements. The resistance value changes little over the temperature range of element. Element temperature will typically not exceed more than 600 degree centigrade.
  • The resistance increase will typically be less than that 4% at full rated power. This ensures full braking torque is always available to motor from even at hot condition.
 
  Brake-Resistor – Minimum Ohmic Valve:
  • The ohmic value sets the rate at which we put the energy in to the resistor – braking power. The lower the ohmic value, the higher the power, shorter the brake time.
  • The minimum ohms are set by the manufacturer, and will produce braking power at the peak rating of the drive (or its chopper module)
  • Higher ohmic values can be used; they will reduce the braking power proportionately, and hence increase the stop times of any given load.
  • The ohmic value is less than the value recommended by the drive manufacturer – result-in VFD failure (or its Chopper module). The correct ohmic value of the brake resistor should be equal or slightly higher by 10% of the value set by the drive manufacturer.
 
  Caution:
  • The resistor element surface should not get red hot under normal operating conditions. If the element surface get red hot or seems to run excessively hot, you probably need a higher wattage rated DBR unit.
  • No separate fuse in the braking circuit is required if the mains cable of VFD is protected with fuse.
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