Reliable control technology is extremely important for tower cranes and passenger hoist because of the huge height, accessibility to many areas and project dependency. In tower cranes In tower cranes, most of the buyers know about PLC and Variable Frequency Drive (VFD) controls, etc and they specifically ask for it. PLC controls are excellent in accurate operation without any logic failure. The identification of faults is very easy and quick through PLC input/output indicators are an additional benefit.

VFD is mainly used for smooth start and stop as well as to save about 35 per cent power consumption. But there are two important practical difficulties with the VFD mechanisms.

If the VFD is using for hoisting mechanism, then a feedback encoder must be used to avoid free fall of load. Many manufacturers do not know or use this feedback encoder and some times the hook falls freely resulting in fatal accidents. Because of various reasons, sometimes the hoist motor does not rotate (non-availability of power from VFD, motor burned, open winding, open cable connection, cable cut, etc). In this case, while operating, the brake will open up and motor will not be rotating. And the hanging load will fall freely without any chance to control it.

If the VFD is used for the slewing (rotate the tower crane) mechanism, then the rotation and control of tower crane will be very difficult in monsoon/heavy wind conditions. If the operator needs to rotate against this heavy wind, then VFD mechanism won?t be powerful enough to rotate the crane. It has been seen in many sites that the operation of tower crane will be suspended during heavy wind especially in seashore area like Mumbai, Chennai, etc.Therefore, it is strongly recommended to use RCV (Regulated Control Voltage/ Rotation Controlled by Voltage) mechanism for slewing operation. RCV mechanism is very powerful during heavy winds as well as smooth start and stop operations. Also, the operator gets better control.

In passenger hoists
In passenger hoist, the best control technology is the VFD mechanism. Since it handles people to vertical heights, it is very important that a VFD control gives a very smooth start/stop operation as well as saves power up to 35 per cent.

The safety systems are the vital part of passenger hoists in terms of reliability and durability. After reading this article, If site in-charge or machinery in-charge physically check the functioning of safety switches in their passenger hoist, you can see that some of the safety switches will be bypassed and are running in danger. This has been done because of the pressure from the project schedule, frequent failure of parts, improper knowledge on their own safety, etc. So it has been recommended to conduct a safety check also along with the weekly scheduled maintenance. Most of the site management, operation and safety department people do not know about the safety checks and replacement schedule of parts. While looking only on the cost of the equipment, the purchaser must know the proven durability and reliability of the equipment along with the details of parts and mechanism used.

We have witnessed many passenger hoists are running under heavy vibrations even with VFD controls. The smooth running not only depends on VFD, but also on the quality of rack, pinions, alignment and the accuracy of structural manufacturing.

The author is Managing Director, Everest Engineering Equipment.