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Crane speed regulation technology (1)

crane speed regulation technology 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 today's widely used rotor series resistance speed regulation technology. However, these technologies have many shortcomings, such as fragile components, inconvenient maintenance, large equipment impact, small speed regulation range, which affect cotton production and so on. Since the 1990s, the variable frequency speed regulation technology has become more and more mature. With its advantages of large speed regulation range, simple structure, convenient maintenance, noise reduction, power saving and so on, it has been widely used in the lifting field

in the operation of the hoisting variable frequency speed regulation system, when the crane stops or descends, the potential load generated by the heavy object makes the motor in the power generation state, and the energy feeds back to the power side. Because most frequency converters have no electric energy feedback device, at this time, this part of energy must be released by the braking resistance in the form of heat energy through the braking unit, so the braking unit and braking resistance play a very important role in the hoisting variable frequency speed regulation system. This paper focuses on how to correctly match and calculate the braking resistance

so far, many versions of matching calculation methods have been found, which are summarized as follows

method 1. Calculation of resistance and power of braking resistance

1.1 braking utilization rate ed%

braking utilization rate ed%, that is, braking utilization rate ed% in delta instructions. The braking utilization rate ed% is defined as the deceleration time T1 divided by the deceleration cycle T2. The braking utilization rate is mainly to allow sufficient time for the braking unit and braking resistance to dissipate the heat generated by braking; When the braking resistance is heated, the resistance value will increase with the rise of temperature, and the braking torque will also decrease. Braking utilization rate ed% = braking time/braking cycle =t1/t2*100%. (Figure 1)

Figure 1 definition of braking utilization ed%

now use an example to illustrate the concept of braking utilization: 10% braking frequency can be understood in this way. If the braking resistance can consume 100% power in 10 seconds, then the braking resistance needs at least 90 seconds to dissipate the generated heat

1.2 brake unit action voltage level

when the DC bus voltage is greater than or equal to the brake voltage level (discrimination threshold), the brake unit action consumes energy. Delta brake voltage level is shown in Table 1

Click to see the original drawing

1.3 braking resistance design

(1) engineering design. Practice has proved that when the discharge current is equal to half of the rated current of the motor, the braking torque that is the same as the rated torque of the motor can be obtained. Therefore, the rough calculation of the braking resistance is:

among them:

braking voltage level

rated current of the motor

in order to ensure that the frequency converter is not damaged, it is mandatory to limit the resistance when the current flowing through the braking resistance is the rated current. The value of the test host is the minimum value of the braking resistance. When selecting the resistance value of braking resistance, it cannot be less than this resistance value from the perspective of production technology

according to the above, the selection range of the resistance value of the braking resistor is:

power consumption of the braking resistor. When the braking resistor works in the circuit with a DC voltage of,

meaning of power consumption: if the power of the resistor is selected according to this value, the resistor can be connected to the circuit for a long time

the resistance power used in the field mainly depends on the brake to better do the work of the experimental machine for customers, and the support utilization rate is ed%. Because the braking time of the system is relatively short, the temperature rise of the braking resistance is not enough to achieve a stable temperature rise in a short time. Therefore, the principle to determine the capacity of braking resistance is to minimize the capacity on the premise that the temperature rise of braking resistance does not exceed its allowable value (i.e. rated temperature rise). The rough algorithm is as follows:

(2) design example. According to the above formula, we can roughly calculate the resistance and power of the braking resistance we need. Take the 7.5kW motor driven by delta vfd075f43a inverter as an example. If the rated current of the 7.5kW motor is 18a and the input voltage is ac460, there are:

therefore, the resistance value range of the braking resistance:

it is appropriate to select the model and power segment that can be bought in the market, and the resistance value is 75 Ω

according to the actual situation, the calculated numerical power can be appropriately expanded

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