BRAKING RESISTORS FOR VARIABLE FREQUENCY DRIVES • Double insulated for 1,000 Volts • Resistor wired to terminal block with high temperature wire • Thermal overload available (N.C. or N.O. contacts) • Well ventillated enclosure with durable ANS70 Gray Powder coated finish • Built-in junction box with multiple conduit knockouts for easy wiring • Wall or roof mounted • Front mounted nameplate with complete ratings.

The Braking current (Bi) is the actual current that will flow through the drive’s braking transistor and the DB resistor for the duty cycle time period. Caution: Check the drive manufacturer’s specification for “Braking Current” or “Peak Current” for the braking module. This value should not exceed the calculated braking current! An Example of Deceleration Braking: Customer information: • Motor Horse Power: 75 HP • Drive Input Voltage: 460Vac • Braking Torque: 150% • Duty Cycle • On Time: 3 Seconds • Off Time: 15 Seconds • Minimum DB Ohmic Value (Per Manufacturer’s Specifications): 5 Ohms • Maximum Braking Current Value (Per Manufacturer’s Specifications): 130 Amps • Regeneration Type: Deceleration Braking 1. Motor Wattage (MW) = 75 HP x 746 = 55950 Watts 2. Peak Wattage (PW) = 55950 Watts x 150% Brake Torque (Use 1.5) = 83925 Watts 3. Resistance = (750Vdc)^2 / 83925 Watts = 6.7 Ohms 4. Duty Cycle (DC) = 3 Seconds on / 15 Second off = .2 or 20% 5. DB Resistor Wattage (DBrw) = (83925 Watts x .2 Duty Cycle)/2 = 8392.5 Watts 6. DB Resistor Current (DBi)= SquareRoot (8392.5 Watts / 6.7 Ohms) = 35.4 Amps 7. Braking Current (Bi) = SquareRoot (83925 Watts / 6.7 Ohms) = 112 Amps The same calculation for an overhauling load would be as follows: 1. Motor Wattage (MW) = 75 HP x 746 = 55950 Watts 2. Peak Wattage (PW) = 55950 Watts x 150% Brake Torque (Use 1.5) = 83925 Watts 3. Resistance = (750Vdc)^2 / 83925 Watts = 6.7 Ohms 4. Duty Cycle (DC) = 3 Seconds on / 15 Second off = .2 or 20% 5. DB Resistor Wattage (DBrw) = 83925 Watts x .2 Duty Cycle = 16785 Watts 6. DB Resistor Current (DBi)= SquareRoot (16785 Watts / 6.7 Ohms) = 50.0 Amps 7. Braking Current (Bi) = SquareRoot (83925 Watts / 6.7 Ohms) = 112 Amps Note that the braking current (119.5) is less than the maximum of 130 Amps as specified by the drive manufacturer. The lower resistance causing the higher current, enables a braking torque about 6% higher than requested by the customer. If this is a problem, a third resistor would be added in series to increase the resistance (at an increased cost as well). Duty Cycle 50% or Higher/Deceleration Braking If a Deceleration Braking load is specified with a duty cycle higher than 50%, the overhauling load form of the calculation is used. This is because the braking waveform no longer looks like a right triangle but begins to look more like the rectangular pattern of the overhauling load. TERMS TO KNOW: Variable Frequency Drive (AKA Drive or VFD): An electronic unit that takes a constant voltage / frequency input and delivers a variable voltage / frequency output to a motor. This output will turn the motor at a RPM less than its natural synchronous speed. Overhauling Load: A condition that exists when a motor is being dragged by the load coupled to its output faster than the speed set by the drive. This condition causes voltage to be regenerated back into the drive and is different from that of the braking cycle because the energy being dissipated by the DB resistors is essentially constant during this period of time. This means that the DB resistors must absorb approximately twice the energy during an overhauling load period versus the same braking time period. DC Bus: The “DC bus” is the DC voltage section of the drive that is after the input rectifiers and before the output transistor section. The output transistors take the DC bus voltage and “chop” it to an approximated three phase sine wave (with a variable voltage and frequency) to drive the output motor. Maximum DC bus voltage for various voltage drives are as follows: • 230 Vac drive = 350 to 400Vdc • 460 Vac drive = 750 to 800Vdc • 575 Vac drive = 925 to 975Vdc If the DC bus voltage rises above the allowable level, a transistor in the drive turns on and dumps the excess voltage into the DB resistor.

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