Calculation of Braking Resistor Power in Hoisting Frequency Conversion Speed ​​Regulation System

1. Introduction The speed regulation technology of cranes has a long history of development, from DC speed regulation to AC speed regulation, from AC stator speed regulation technology to DC thyristor speed regulation device, and then to the widely used rotor series resistance speed regulation technology today. However, these technologies have many disadvantages such as easy damage of components, inconvenient maintenance, large equipment impact, and small speed regulation range. Since the 1990s , variable frequency speed regulation technology has become more and more mature, and has begun to be widely used in the field of lifting with its advantages of large speed regulation range, simple structure, convenient maintenance, reduced noise, and power saving. In the operation of the lifting variable frequency speed regulation system, when stopping or descending, the potential load generated by the heavy object makes the motor in a power generation state, and the energy is fed back to the power supply side. Since most frequency converters do not have an electric energy feedback device, this part of the energy must be released by the brake resistor in the form of heat energy through the brake unit, so the brake unit and brake resistor play a very important role in the lifting variable frequency speed regulation system. This article focuses on how to correctly select the brake unit and brake resistor. 2. Working principle of inverter energy consumption braking In the same electric traction system , when the motor speed is higher than the synchronous speed corresponding to the inverter output frequency , the inertial energy of the motor and the load in the power generation state will be fed back to the inverter ( this situation usually occurs when the motor is towed , such as the crane weight falling ) . However, most general inverters are not designed to feed back the regenerative energy to the three-phase power supply, so all the energy absorbed by the inverter from the motor will be stored in the electrolytic capacitor, which will eventually cause the DC bus voltage in the inverter to increase due to capacitor charging. If handled improperly, the inverter will alarm and shut down. (See Figure 1 )

The method usually adopted for general frequency converters is to equip the frequency converter with a braking unit and a braking resistor. The braking unit determines whether the DC bus voltage Ud exceeds the specified limit (such as 660V or 710V ) through level detection. If it is overvoltage, the resistor can be turned on for a short time to consume the electrical energy in the form of heat energy. Therefore, it is crucial to accurately calculate parameters such as braking power, braking resistor value and power capacity for the normal operation of the frequency converter. 3. Simple calculation of the braking power of the lifting frequency converter The calculation of braking power usually adopts the method of calculating the braking torque, but this method is not very applicable and the calculation is too complicated for the calculation of the braking power of the lifting frequency converter. Frequency converter manufacturers at home and abroad have not given a convenient calculation method for the braking power of the lifting frequency converter. If it is selected only according to its selection manual according to the general parking condition, it usually cannot be used normally. The braking power capacity required for the parking condition of the lifting frequency converter is small, while the braking power capacity required when the heavy object is lowered is large. Therefore, the selection should meet the requirements of the maximum descending weight, the maximum descending stroke and the fastest descending speed. When the crane is lowering a heavy load, the motor acts as a generator to generate electrical energy, and the drive of the motor comes from the potential energy of the weight. According to the law of conservation of energy, the generated electrical energy should be equal to the release of the potential energy of the weight, and equal to the heat energy consumption of the resistor (without considering power loss). Therefore, you only need to calculate the power generated by the potential energy of the weight to get the required braking power. It is easy to calculate the power generated by the potential energy of the descending object. PE = NM ╳ VM PW = PE ╳ ( 1-η ) PE Power generated by the descending potential energy Unit: Watt PW Braking power unit: Watt NM Maximum descending weight unit: Newton VM Fastest descending speed unit: m / s η The internal friction power coefficient of the motor and inverter is generally 20% After calculating the braking power PW , configure the corresponding braking unit and braking resistor according to the manual provided by the manufacturer. Be sure to configure the braking unit strictly according to the manual data, otherwise the braking unit may burn out due to excessive current due to improper configuration. 4. Practical application examples a . 75/20 ton overhead crane in the main plant of a power plant in Luoyang (only taking the main hoist as an example) The main hoist motor power is 37KW and the operating speed is 0.2











-2 meters / minute, the maximum lifting weight is 100 tons. The inverter adopts Puchuan PI8100 series 45KW inverter. Calculate the braking power: NM = 100 tons ╳ 1000 ╳ 9.8 VM = 2 /

60 meters / sec PW = PE ╳ ( 1-η ) =NM ╳ VM╳ ( 1-η )

= 100╳1000╳9.8╳2╳ ( 1-0.2 ) /60
= 26.13 kW Therefore, the braking power of the main hoisting mechanism should not be less than 26.13 kW. According to the selection manual of the Puchuan brake unit, each unit can provide a braking power of 9.6KW , and 3 units can be selected for parallel use. We used 2 units in the equipment commissioning, but the acceptance test could not be completed due to overheating of the resistor. 5 Conclusion For the hoisting variable frequency speed regulation system, the selection of the brake unit and the brake resistor is very important. Improper selection will cause the resistor to overheat and fail to work, and even the inverter will explode. The brake resistor of the large and small car travel mechanism can be selected according to the selection sample provided by the manufacturer, but for the brake resistor of the hoisting mechanism, it must be selected after calculation. The calculation method provided in this article is relatively simple and suitable for long-term descending conditions. Practice has proved that the braking power calculated according to the above formula fully meets the actual needs.