1. Selection of Frequency Converter
(1) Selection of frequency converter for walking mechanism
The traveling mechanism of a crane is divided into a trolley mechanism (longitudinal) and a gantry mechanism (lateral), both of which generally employ a multi-motor drive scheme. Due to the large transmission inertia of the crane's traveling mechanism, a large acceleration torque is required to meet the starting needs of the motors. Therefore, the required motor shaft output power PM of the crane's traveling mechanism should consist of the load power Pj and the acceleration power Pa, i.e.
PM≥Pj+Pa
The traveling mechanism can be equipped with one frequency converter for each motor, or all traveling motors can share a single frequency converter. The frequency converter can be a general-purpose basic U/f control frequency converter with open-loop control. When using a single frequency converter, the frequency converter capacity selection should satisfy the following formula.
ICN≥knIM
In the formula, k is the current waveform correction coefficient, which is taken as 1.05 ~ 1.1 ;
ICN - Rated output current of the frequency converter (A);
Rated current (A) of a single motor under IM-power frequency power supply;
n - The number of motors driven by a frequency converter.
When the frequency converter adopts "one-to-many" control, the electronic thermal relay protection function provided by the frequency converter cannot achieve overload protection for a single motor. Therefore, a low-voltage circuit breaker with thermal overload protection function must be connected in series in the circuit of each motor to achieve overload protection for a single motor. The motor fault signal is taken from the auxiliary contact of the low-voltage circuit breaker.
(2) Selection of frequency converter
The inverter's capacity must be greater than the output required by the load, that is...
In the formula, K is the overload factor, which is generally taken as 1.33 ;
PM - Motor shaft output power (kW) required by the load;
η - Motor efficiency;
cosφ - the power factor of the electric motor.
The hoisting mechanism requires a starting torque of 1.3 to 1.6 times its rated torque. Considering the need for a 125% overload capability, its maximum torque needs to be 1.6 to 2 times its rated torque to ensure safe operation. For frequency converters driving motors of equivalent power, they can provide an overload capacity of up to 60 seconds and 150% of the rated torque. Therefore, the overload factor k = 2/ 1.5 = 1.33 .
After selecting the inverter capacity, a current verification should also be performed, i.e.
ICN≥kIM
In the formula, k is the current waveform correction coefficient, which is taken as 1.05 ~ 1.1 ;
ICN - Rated output current of the frequency converter (A);
Rated motor current (A) when using IM-power frequency power supply.
To improve the dynamic characteristics and high torque output capability of the frequency converter at low speeds, a vector control frequency converter should be selected, and a speed closed-loop control system composed of a pulse encoder should be adopted.
In large-tonnage cranes, the same hook has two independently driven hoisting mechanisms. Each hoisting mechanism is driven synchronously by two motors, each rotating its own wire rope drum. The hook is then lifted via a multi-stage reduction gearbox via a moving pulley system. The variable frequency speed control transmission scheme for the hoisting mechanism adopts a "one-to-one" configuration, with one frequency converter driving one motor. A master-slave control scheme with power balancing and speed synchronization functions is used between the two frequency converters in each hoisting mechanism. These control schemes enable precise torque balance distribution between the two motors and speed synchronization between the two hoisting mechanisms.
2. Electric motor braking
The inverter's lifting motor operates under both load and load conditions. When lifting heavy objects, the motor outputs energy, operating under load conditions; when the load decreases, the motor becomes a generator, operating under load conditions. The travel motor, when rapidly decelerating or running with the wind, also operates in regenerative braking mode. In this mode, the motor converts the mechanical energy of the transmission mechanism into electrical energy, feeding it back to the inverter. This feedback current is rectified by the inverter's six freewheeling diodes, charging the filter capacitors on the inverter's DC bus and increasing the voltage on the DC bus. When the DC bus voltage rises to a certain level, energy release is required; otherwise, if the voltage reaches the protection limit, the inverter will trip due to overvoltage.
Common methods for releasing electrical energy include braking with a braking resistor or regenerative braking. Since the travel motor operates relatively infrequently and generates relatively little regenerative energy, braking with a braking resistor is suitable. However, the hoisting motor, due to its frequent braking and large regenerative energy output, requires regenerative braking.
To prevent the hook from slipping when the crane motor stops, a braking device must be installed on the motor. The braking device is released when the motor is running, and when the motor stops, the braking device clamps the motor rotor to prevent it from rotating.
(1) Calculation of braking resistor
The resistance value RBO of the braking resistor can be calculated using the following formula.
In the formula, Uc is the voltage on the DC circuit bus of the frequency converter (approximately 700V).
TB - Braking torque (kg·m);
TM - Rated torque of the motor (kg·m);
n1 - The speed (r/min) of the motor when it begins to decelerate.
The discharge circuit of the braking resistor consists of a braking unit and a braking resistor. Its maximum current is limited by the maximum allowable current Ic of the braking transistor. The minimum allowable value of the braking resistor Rmin = Uc/Ic. Therefore, when selecting the braking resistor, its actual value RB should meet the following conditions:
Rmin < RB < RBO
(2) Empirical formula for braking resistor
The selection of braking resistors has a certain range. In practical engineering, if precise calculation data is difficult to obtain, empirical formulas can be used. When the current through the braking resistor is 50% of the motor's rated current, the braking torque received by the motor is the motor's rated torque. If the braking torque is calculated as equal to the rated torque, then...
In the formula, RBO is the braking resistor (Ω).
IB - Braking resistor current (A);
Uc - DC bus voltage (V);
IM - Rated current of the motor (A).
The power PBO of the braking resistor is
When the unit of Uc is V and the unit of RBO is Ω, the unit of PBO is W.
