One of the main direction in improving the efficiency of crane operation is reducing of energy consumption. Another direction is increasing the reliability of the crane’s units/parts. Both directions allow more efficient use of lifting equipment.
Use of a variable speed drive system enables effective solution of these problems. Currently as the driving element in most cases an AC current induction motor is used. The adjustable asynchronous electric drive or variable frequency drive consists of induction motors and inverter (frequency transformer), which acts as a speed regulator of induction motor.
The inverter converts the input supply voltage to the output impulse voltage by means of pulse width modulation, which generates simple harmonic current with frequency from 0 Hz to 400 Hz in motor windings. Change of the frequency and amplitude of the output voltage according to specified algorithms can enable step less speed control of rotor spinning.
Advantages of variable speed drive system compared to other electric motor control circuits:
- Reliability, simplicity and relative low cost;
- High speed control range;
- High utilization of electric motor capacity (coefficient of efficiency close to 1);
- No need to use filter compensating device;
- Higher electric reliability;
- Tolerance of higher loss in voltage in the supply mains.
Disadvantages of the classical control circuit of electric drives using controllers:
- large electric power losses due to low coefficient of efficiency;
- impact change in engine torque, and as a consequence, dynamic overloads of the crane units due to stepped extraction of resistance from the electric motor’s rotor circuit;
- electric motor breakdown due to the distortions in the rotor circuitry caused by loss of contacts in resistance circuits or in accelerating contactors.
Acceleration and deceleration of electric drives using frequency transformer ensures gradual growth of torques and efforts in all units and gears of the crane. The electric drive does controlled deceleration, considerably reducing the load’s swinging after stopping.
The use of inverter solves a huge number of special tasks - smooth start and stoppage of the motor, constant speed control, overload protection, reverse etc. Using analog and digital inputs signals and setting the work program you can have a variety of system modes - automatic start and stop, speed change depending on the time or environment.
And above all - the use of an inverter, and as a result, the optimization of drive mods, enables:
- energy savings from 30% to 80% and prolonging life of electric motors. Installing of the variable speed drive system almost always pay its way less than 1 year!
- prolong the life of metal structures by 25-50% due to smoothness of movement, the lack of shocks, reducing of dynamic loads on gears.