Such applications include portal/whirly and quay cranes, wood yard cranes, turbine room and reactor room cranes, scrap yard and refuse cranes, machine shop cranes and mill duty cranes of every sort.
AC static stepless controls use wound-rotor motors, solid state components, and stepless induction master switches to provide infinitely variable speed and torque control. Creep-speeds of 10:1 are possible.
It is called ‘stepless’ because it varies the motor speed by changing the inductance of a wound rotor without the use of speed step magnetic contactors and resistors. The rate that the crane accelerates or decelerates is completely under the operator’s control, only limited by available torque and how quickly the operator advances the master switch.
Plugging control is integral on most traverse systems. However, performance not withstanding, AC static stepless controls are not without their problems: they are expensive; use a multitude of components such as saturable reactors, transformers,
and/or thyristors; and the poor output waveform and fixed secondary resistor result in poor efficiency at reduced speed which may produce excessive motor heating, especially during prolonged low speed operation.
Furthermore, bridge and trolley systems have a tendency to accelerate to full speed when lightly loaded and accurate speed regulation can be problematic. Systems with saturable reactors require a large amount of space and add considerable weight to a crane.
Torque requirements vary greatly between cranes and crane motions, depending upon the particular type of crane, the job to be done, and/or performance desired. Magnetek’s Impulse VG+ flux vector control, for example, with its built-in digital microprocessor and unique software, provides such flexibility, allowing it to be programmed to meet just about every production need.
The Impulse VG+ drive can be programmed to provide ramped acceleration and deceleration which reduces load swing, provides controlled dynamic braking, and permits a less skilled operator to safely control the crane. Additionally, the crane operator can be given more control of speeds, acceleration, deceleration and braking, similar to that used in traditional AC static stepless control.
Monitoring speed and direction
Working in a closed-loop, a flux vector drive utilises an incremental encoder to monitor the speed and direction of the motor shaft. When the drive is told to go to a higher speed, it attempts to achieve the speed set point, limited only by the torque setting.
Experienced crane operators are known for their skill in placing delicate loads on a dime and controlling load swing. They frequently ‘plug’, for example, rapidly reverse bridge and trolley motors to develop a counter-torque that slows or even stops a crane. A drift point is often used on the motion master to release the electric brake, thus permitting the bridge or trolley to coast until retarded by friction or the foot brake.
A unique concept for controlling
a cab operated crane (US Patent 7,190,146) was developed by Magnetek and embedded in the software of Impulse VG+. This feature, called static stepless simulation, allows the operator to use a footbrake to either augment or completely control the deceleration and/or stopping of the crane. It provides improved reverse-plugging response, eliminates motor current spikes, and reduces open circuit motor decay.
With traditional AC static stepless controls, current and voltage spikes can occur when a reversed polarity voltage is applied (reverse-plugging). These transient spikes can even occur when the operator makes a speed adjustment while coasting in the same direction.
With the use of Flux Vector crane control, it is not necessary to reverse the polarity of the motor. Instead, the motor is retarded by the application of an adjustable amount of retarding torque, determined through the drive’s software, and the position of the master switch in the direction opposite to the crane’s motion. This helps to ensure a smooth transition from coasting to slow-down and even allows reversing the direction of travel of the bridge or trolley.
What is unique with static stepless simulation is that the flux vector drive sends a frequency to the motor substantially equal to the frequency at which the coasting motor is rotating, thus creating a speed match that reduces spikes and virtually eliminates open motor decay. Response time is thereby improved since no time is lost waiting for the magnetic field to decay in the rotor.
Included with static stepless simulation is a feature called ‘brake stand prevention’. When an operator applies pressure to the brake foot pedal, a signal is sent to the VFD from either a micro switch or hydraulic pressure switch that braking is applied.
The drive software in turn prevents the motor from driving into the brakes, thus saving wear and tear on the controls, brakes and crane itself. Hydraulic brake systems tend to leak and create maintenance and environmental issues.
However, Magnetek’s brake-by-wire Braketronic package, with its foot pedal operated AC thruster brakes, addresses these issues and provides operators with the same feel they had with hydraulic brakes. This brake-by-wire package integrates seamlessly with static stepless simulation software enhancing performance and reducing maintenance costs.
Exceptional speed regulation has been the hallmark of both traditional AC static stepless and AC flux vector crane controls. Wherever precise spotting or extremely slow speeds are required, such as in the aerospace industry, or nuclear power plants, these controls have been the performance leaders. However, AC flux vector control routinely provides creep speeds of 1000:1, compared to only 10:1 for AC satic stepless, and does so without incurring the motor heat-buildup that normally accompanies prolonged slow speed operation. Impulse VG+ with static stepless simulation can be programmed to operate in several modes.
Linear torque – slight speed limiting
In this configuration, some speed control at lower input levels, but does not limit the speed at the upper end. Output torque is the same as in traditional AC static stepless control.
High torque – linear speed
In this configuration, the output torque is very high, but the output speed will increase at a constant
rate as the master switch is moved through its range. Incremental steps can be independently programmed to suit any desired operator preference or performance criteria. Creep speeds are predictable and can be made even more functional when micro-positioning is enabled. Various other operating modes are available with two-, three-, or five-step inputs.
Conclusion
Static stepless simulation eliminates current spikes and excess mechanical torque/stress on the drive train; allows quick but smooth starting or changing of direction; reduces maintenance costs; and easily interfaces with existing induction masters, footbrakes and motors.
Static stepless simulation provides the best of all worlds, by combining the intellect, judgment, dexterity and other positive human traits of the crane operator with the latest in crane control technology and safety. It is the ideal control for high performance cab controlled overhead cranes.
About the author
Aaron S. Kureck is the controls products and development engineering manager at Magnetek, Inc., and has been working in the material handling industry for more than 13 years.
