For many decades, motors and drives have been utilised in electric hoists, reels and winches. Andy Pye explains how they are incorporated into these systems.
Drive applications can be categorised as single-quadrant, two-quadrant, or four-quadrant:
High-performance applications involving four-quadrant loads (Quadrants I to IV) where the speed and torque can be in any direction, include hoists, elevators, and hilly conveyors.
It is often thought that DC technology equates to old technology, with many engineers adopting the simplistic view that the DC motor is complicated and requires a lot of maintenance, which makes it expensive to run. The AC motor, they argue, is simple and sturdy, does not need much maintenance, is therefore less expensive, and possesses a higher degree of protection into the bargain. Of course, it is true that for simple, high volume applications such as the control of fans, pumps and compressors, AC variable speed drives are excellent and energy efficient.
Meanwhile, at the other end of the scale, AC drives offer far greater bandwidth due to their higher carrier modulating frequency and the forced commutation of the IGBTs.
But in reality, modern DC drives are also at the forefront of variable speed drive technology. Easy to use configuration and diagnostic software tools make setting up today’s DC drives quick and simple, and a range of networking and communications options are available. Digital microprocessor-controlled power converter technology has now reached a level of technical sophistication which enables almost any drive job to be handled both with DC and AC drives. It often comes down to economics:
In cranes and hoists, as with many market sectors, there is a trend to employ AC motors with flux vector control. However, due to the very large legacy population of DC motors currently in use, especially of the wound field type, AC motors still represent only a small part of this industrial application. Wound field DC motors (series, shunt or compound) are used for coil feeding reels in milling/metalforming, crane hoists, motor-driven cable reels and elevator hoist motors. Permanent magnet and explosion proof DC motors are all also used.
In many applications with overhauling loads, such as cranes and hoists, where the motor’s ability to hold full load at zero speed means mechanical brakes may not be required for control purposes, DC is often the most cost-effective and safely-controllable option. The relatively small size of a DC drive, compared to an inverter drive, may also weigh in its favour.
A standard 4-quadrant DC drive is able to motor and brake in both directions of rotation, with the energy generated under braking returned into the mains. Unlike the AC drive, this regenerative braking is achieved without the need for intermediate storage, resistive dumping or an additional power bridge.
Tough industrial applications, such as rolling steel and turning a cement kiln, are other areas where DC drives may be favoured. DC drives are also very effective for non-motor applications, such as electromagnets, charging batteries and electrolysis.
Modern DC drives inherit the system integration flexibility more typical of AC drives.
When dealing with a legacy system based on DC drives, there is the question of whether it is worthwhile modernising an existing DC drive or less expensive to replace it entirely with an AC drive. Hence the choice between AC and DC technology is a difficult one and various options need to be assessed:
The following main criteria are important:
A good example of a retrofit where both AC and DC technologies are used in tandem is for four floating grab cranes in Amsterdam IGMA Bulk Terminal.
All four cranes have the four-rope grab system and are mainly used for ship to quay bulk handling. Two of the cranes are rated at 16t and have been retrofitted with AC variable speed drives, while the two larger 25t cranes are using DC drives.
The AC-motors for the hoist and close movement of the grab, as well as the motors for the luffing and slewing movement, are all equipped with AC drives.
A notable feature of this installation is control of the slewing movement: on many conventional cranes, the slewing movement is undertaken with slip-ring motors. The slip-ring motor provides good motor torque control for acceleration and deceleration and it is possible to coast when the controller is moved to zero. However, this method of control is very poor at low speeds, with sudden steps in torque between resistor steps, wasting a lot of energy, and the system requires very regular and intensive maintenance.
To counter this effect, software has been developed that gives the crane driver optimal control over the swaying load, without the need for a PLC. The slewing control system provides the driver with control over both the speed and the motor torque.