CRANES pose unique challenges for motors and drives engineering. The range of movement required of them varies greatly - from slow, delicate and powerful as they pick up or position their load, through to rapid and responsive as they speed along beams or jibs to their next task.
Cranes have to maintain safe, precise control of their suspended loads at all times, a task that makes great calls not only on the skills of their operators, but also on their designers who are faced with hugely varied tasks in manufacturing, mining, maintenance, construction and other major industrial sectors.
Traditionally, the crane or hoist has a requirement for two speeds (one low speed and one high speed) largely achieved by the use of two-speed motors. This method is cumbersome, however, and has a number of disadvantages:
1. Limited and reduced starting torques available with two-speed motors.
2. Higher shock loads generated when changing from high to low speed or stopping.
3. Expensive and inefficient 415V contactors required to change speed.
4. "Pendulation" (or swinging of the load) on speed change, or starting and stopping.
5. A requirement for "special" motors, insofar as two-speed brake motors are nowhere near as readily available as single-speed geared motors (and often they need to be fitted with flywheels in an attempt to control pendulation of loads).
6. High levels of maintenance required on key items - such as air gap adjustment and friction-facing wear on brake discs
This complexity leads to a myriad of problems and issues many engineers can relate to, having seen these arise in service.
Fortunately, technology moves on, as always, and provides us with new ways to solve old problems.
One technology highly relevant to cranes is the new generation of electronic drive systems, represented by the Bonfiglioli range by the ACT series.
This new series of selectable vector drives is designed to offer outstanding flexibility for many variable speed industrial process applications, extending from simple variable speed, right through to complex servo-style operation.
Such applications include not only cranes, but also conveyors, pumps, mixers, thickeners, high speed doors, metering systems, packaging lines and other equipment that benefits from process control far finer than that attainable by conventional mechanical means.
To achieve their flexibility, the new Vectron ACT 201 and 401 models (in both 200V and 400V versions, with power ratings to 55kW) combine award-winning hardware design with an unparalleled flexibility of macro configuration-based software control.
What makes them so attractive for cranes is that the ACT series offers extremely precise power and speed control, while being simple easy to use, with advanced features and outstanding advantages in their scalability and compact size.
They also provide an "out of the box" standard solution to all the crane control issues we have raised. It does this very quickly, efficiently and simply, in the following ways through:
1. Predictable and consistent starting torque values including 150% overload for 60 seconds (with 200% for 1 second). This capability is NOT a function of the motor windings or slot design, so is easily replicated.
2. Adjustable starting and stopping methods to allow crane manufacturers to optimise starting and stopping behaviour to avoid shock loading to the system
3. Allowing the use of low voltage (24V) control relays to start/stop and change speeds, giving both commercial and safety benefits to the manufacturer.
4. Fully adjustable acceleration and deceleration times (six in total) to prevent pendulation of loads during starting/stopping and speed change.
5. Allowing the use of standard single speed IEC brake motors - or optional single speed compact brake motors. This provides cost-efficient engineering with reduced gear motor size and far greater local stock access.
6. Allowing the use of the mechanical failsafe brake to only be used as a parking/emergency brake, thus saving wear and tear and costly maintenance.
ACT inverter technology has been further adapted to crane and hoist operation through a number of features that offer further benefits to the materials handling engineers.
These include a 240V relay output to control power supply to the mechanical brake. This technology is interlocked to operate before acceleration begins and after deceleration stops and is pre-programmed to operate Bonfiglioli FD brakes with no further control wiring.
Another useful feature is the dc voltage controller. This allows an automatic reduction in deceleration time if (as sometimes occurs) the motor acts like a generator because of high inertia on stopping.
It also permits controlled deceleration in the event of a power failure - and it allows the fitting of optional brake resistors to dissipate excess electrical energy as heat.
Because the dc voltage controller is standard in the drive, fitting of the resistors is extremely cost-effective as an option on the ACT equipment.
An additional plus of inverter technology is that up to eight independent preset speeds of a fixed value can be configured into the drive to enhance flexibility for the crane manufacture and user - or the system can use what's termed as a MotorPOT or digital pot to give an infinitely variable high speed controlled by the operator in order to maintain highly efficient and safe movement of carried loads.
Alternatively, many of the above features and benefits can also be realised via the use of new LMD (localised motion device) systems.
The appeal of this versatile technology is further enhanced through its availability with Bonfiglioli V.V.V.F Electronic Drive systems that integrate, within rugged cast aluminium enclosures, the latest electronic control systems with the company's motors and geared motor lines.
The time-saving and money-saving modular drive concept is specified to the international IP66 standard for high levels of water and foreign body protection, mechanical strength for reliability and innovative features such as epoxy encapsulation of the electrical components for humidity and vibration protection.
The new packaged gearbox, motor and drive technology applications, known as LMS or Localised Motion Systems, can be applied to the host of industrial processes mentioned earlier, including cranes.
This modular, localised drive concept saves time and money on electrical installation by reducing or even removing cubicle installations as well as negating the requirement for shielded cable normally used between the motor and electronic drives in conventional setups.
It offers quick and easy setup, because the inverter is already tuned for use with its motors, thus delivering the shortest possible commissioning time with optimum performance values.
The Bonfiglioli Vectron LMD series (Localised Motion Device), which lays at the core of the LMS concept, offers state-of-the-art control of industrial electric motors up to 3kW, complementing Bonfiglioli Electronic Automation Drive (EAD) equipment in styles and configurations up to 800kW.
* Sean Richardson is Bonfiglioli's National Manager, Electronic Automated Division (EAD).