Inverter braking resistor design calculation method one (simple calculation)
1. First, determine the power of the frequency converter based on the size of the motor;
2. The power of the braking unit is generally selected to be (1 to 2) times the power of the frequency converter;
3. Formula for selecting braking resistor value: 700 / motor power (kW) (When multiple braking units are connected in parallel, the resistor value configured for each braking unit shall not be less than 700 / motor power (kW); the minimum resistance value shall be found in the relevant configuration table).
4. The power rating of the braking resistor is greater than the motor power in KW/2. (According to the formula Pb=8Q*v*η)
5. The rough calculation of the number of braking resistor boxes is: motor power (KW) / 11.2 (rounded to the upper limit of the integer).
Method 2 for Designing and Calculating the Braking Resistor of a Frequency Converter
Selection of braking unit and braking resistor
1. First, estimate the braking torque.
Under normal circumstances, there is a certain loss inside the motor when braking, which is about 18%-22% of the rated torque. Therefore, if the calculated result is less than this range, there is no need to connect the braking device.
2. Next, calculate the resistance value of the braking resistor.
During the operation of the braking unit, the rise and fall of the DC bus voltage depends on the constant RC, where R is the resistance of the braking resistor and C is the capacitance of the electrolytic capacitor inside the frequency converter. The braking unit's operating voltage is typically 710V.
3. Then select the braking unit.
When selecting a braking unit, its maximum operating current is the sole criterion.
4. Finally, calculate the nominal power of the braking resistor.
Since the braking resistor operates on a short-time duty cycle, based on its characteristics and technical specifications, we know that its nominal power rating will be less than the power consumed when energized. This can generally be calculated using the following formula:
Braking resistor nominal power = Braking resistor derating factor × Average power consumption during braking × Braking utilization rate %
5. Braking characteristics
The advantage of energy-efficient braking (resistive braking) is its simple structure, but its disadvantage is reduced operating efficiency, especially during frequent braking, which will consume a lot of energy and increase the capacity of the braking resistor.
Braking torque calculation
Sufficient braking torque is required to produce the desired braking effect; if the braking torque is too small, the frequency converter will still trip due to overvoltage.
The greater the braking torque, the stronger the braking ability and the better the braking performance. However, the greater the required braking torque, the greater the equipment investment will be.
Precise calculation of braking torque is difficult; estimation is usually sufficient to meet the requirements.
Designed with 100% braking torque, it can meet more than 90% of the load.
For elevators, hoists, and cranes, use 100%.
Unwinding and winding equipment, calculated at 120%.
Centrifuge 100%
Large inertial loads requiring rapid stopping may need 120% braking torque.
80% of normal inertial load
In extreme cases, the braking torque can be designed to be 150%. In this case, the braking unit and braking resistor must be carefully calculated because the equipment may be operating at its limit. Calculation errors may damage the inverter itself.
Exceeding 150% of the torque is unnecessary, because once this value is exceeded, the frequency converter itself reaches its limit and there is no room for further increase.
Calculation of braking current for the resistor braking unit (based on 100% braking torque)
Braking current refers to the direct current flowing through the braking unit and the braking resistor.
380V standard AC motor:
P — Motor power P (kW)
k — Mechanical energy conversion efficiency during feedback, generally k = 0.7 (applicable in most cases).
V — DC operating point of the braking unit (680V-710V, generally 700V)
I — Braking current, in amperes
Calculation benchmark: The regenerated electrical energy of the motor must be completely absorbed by the resistance.
Regenerative electrical energy of motor (watts) = 1000 × P × k = Power absorbed by resistor (V × I)
Calculation yields I=P... Braking current in amperes = motor kilowatts
That is, a braking current of 1 ampere is required for every kilowatt of motor to achieve 100% braking torque.
Braking resistor calculation and selection (based on 100% braking torque)
The resistance value indirectly determines the magnitude of the system's braking torque. If the braking torque is too small, the frequency converter will still trip due to overvoltage.
The selection of resistor power is based on the resistor's ability to work safely for a long time. If the power is not selected properly, it will overheat and be damaged.
380V standard AC motor:
P — Motor power P (kW)
k — Mechanical energy conversion efficiency during feedback, generally k = 0.7 (applicable in most cases).
V — DC operating point of the braking unit (680V-710V, generally 700V)
I — Braking current, in amperes
R — Equivalent resistance of the braking resistor, in ohms.
Q — Rated power dissipation of the braking resistor, in kW
s — Safety factor for braking resistor power consumption, s = 1.4
Kc — Braking frequency, referring to the proportion of the regenerative process in the entire motor operation process. This is an estimated value and needs to be determined based on the load characteristics.
Kc typically takes the following values:
Elevator Kc = 10~15%
Kc = 10~20% for oilfield pumping units
For unwinding and winding, Kc should ideally be calculated based on system design specifications, ideally at 50-60%.
Centrifuge Kc = 5~20%
For cranes lowered to a height exceeding 100m, Kc = 20~40%.
The load Kc for accidental braking is 5%.
Other Kc = 10%
Resistance calculation standard: The regenerative electrical energy of the motor must be completely absorbed by the resistor.
Regenerative electrical energy of motor (watts) = 1000 × P × k = Power absorbed by resistor (V × V/R)
The calculation yields: Braking resistance R = 700/P (Braking resistance value = 700 / motor kW).
Resistance power calculation benchmark:
The regenerative electrical energy of the motor must be completely absorbed by the resistor and converted into heat energy for release.
Q=P×k×Kc×s＝P×0.7×Kc×1.4
Approximately Q = P × Kc
Therefore, we get:
Resistor power Q = Motor power P × Braking frequency Kc
Braking unit safety limits:
The current flowing through the braking unit is 700/R.
