reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

AUXILIARY HOIST A.

Suar,

I HRu r FUSES

' FAST MOTOR 'CREEP MOTOR

SINGLE

PHASING

PROTECTION

SINGLE

PHASING

PRO TEC CON

EARTH BAP

REvE R ,;■ NG

CON TAG Top

Cranes

Creep South/slow South/

fast South.

(6)Cross-traverse control Creep West/slow West/

(9)Anti-collision system Key-operated. override

(10)Warning hooter on/off switch.

The above controls operate directly into the contactor control circuits, through a control station selection located adjacent to the 'local' controls on the

bridge-mounted protection panel.

3.4.2 Radio control

Radio control overcomes the shortcomings of cab control and offers the following advantages:

•The crane operator is at operating floor level and unrestricted, thereby enhancing crane operating efficiency.

•Parking of the crane is not governed by personnel access requirements, except for maintenance purposes.

•The full span of the bridge can he utilised for traversing and hoisting operations.

The system is not without disadvantages, however, and these are summarised below:

•Radio control is susceptible to interference and elaborate safeguards must be included in the system to prevent malfunction.

•The control system is more complex, since it requires a transmitter, receiver and interface relay equipment.

•The portable transmitter/control units are susceptible to accidental damage and abuse if their usage and storage is not strictly controlled.

An outline of the radio-control system is given below. The control functions provided by a body-worn transmitter/control unit are the same as those described in the previous section. A typical layout is shown in

Fig 10.5,

Radio control systems are VHF, UHF or, more usually, LF, and comprise a transmitter/control unit, which is carried by the crane operator on a waist/ shoulder harness, plus a receiver and interface relay equipment, the last two items being located on the crane bridge. Contacts on the interface relays are connected through sequence relay equipment to operate the crane control contactors.

A typical LF radio control system uses coded, con- tinuously-transmitted multi-frequency signals to safeguard against malfunction of the crane due to external interference. For each crane control to function, a

minimum of three radio frequency signals and a master security-frequency signal must be transmitted simultaneously. Unique control frequencies are allocated to each crane to safeguard against the incorrect matching of transmitter and crane. As a further safeguard, the transmitter range is limited to approximately 60 m.

3.4.3 Pendant contra!

The simplest and most economic means of crane control is by pendant control station. Applications are necessarily limited to lower-capacity cranes, such as

CRANE PROTECTIVE PANEL

DOWNSHOP

COLLECTORS

CONTACTOR PANEL

CROSS TRAVERSE AND

LONG TRAVEL RESISTOR PANEL

END CARRIAGE

EASTSIDE

LONG TRAVEL CREEPSPEED

CONTROL UNIT

Figures 10.2 and 10.3 show typical distribution and 0, itching arrangements.

415/110 V transformers are provided to supply ,ontrol circuits and crane auxiliary circuits, such as

illk way lighting and anti-condensation heaters.

3.5.3 Limit switches

Limit switches are provided in hoist, long travel and , ross-traverse motion circuits. They are metalciad and mechanically operated, normally having a self-resetting action in both directions, and are positioned so that :hey are accessible from the crane bridge walkways for maintenance purposes.

On hoist motions, limit switches are connected into the motion contactor control circuit to prevent overlowering and over-hoisting. Over-hoisting is more dangerous, and to safeguard against contactor or limit

switch malfunction, a back-up system is provided consisting of two limit switches connected directly into two phases of the 415 V, three-phase supply to the hoist motor. These limit switches are manually reset and no override facility is provided.

On long travel and cross-traverse motions, four limit switches are installed in each direction of motion to prevent overtravel and safeguard against limit switch or contactor failure. The first limit switch trips the motion contactor, the second limit switch trips the main supply contactor and the third and fourth limit switches, which share the same position, trip motion and main supply contactors, respectively. Limit switches are positioned as follows:

The first limit switch stops the crane when travelling at fast speed before it reaches the second limit switch.

The second limit switch stops the crane when travelling at fast speed before it reaches the third and fourth limit switches.

The third/fourth limit switches both stop the crane when travelling at slow speed before it reaches the rail buffer stops.

Travel limit switches do not reset until the crane operating arm reverses past them.

The travel limit override control, referred to in Section 3.4.1 of this chapter, allows the crane to be driven over the first and second Limit switches at slow speed and up to the third/fourth limit switches. A second override control allows the crane to be driven over the third/fourth limit switches at slow speed up to the rail buffer stops.

When travel motions have single-speed motors, three li mit switches only are installed, the first to trip the motion contactor and the second and third, which share the same position, trip the motion and main supply contactors, respectively. The limit switches are positioned as follows:

The first limit switch stops the crane when travelling at fast speed before it reaches the second and third li mit switches.

The second/third limit switches stop the crane when travelling at fast speed before it reaches the rail buffer stops.

A travel limit override enables the crane to be driven at slow speed past the first and second/third limit switches up to the rail buffer stops.

Travel limit override controls spring-return to 'off' to prevent pre-selection and to ensure that overriding requires a deliberate action.

3.6 Anti-collision system

Sometimes two cranes share the same rails and are, therefore, provided with an anti-collision system to prevent them being driven into each other. When the cranes are required to be used in tandem for large lifts, a key-operated anti-collision override switch is provided at the control station (see Section 3.4.1 of this chapter) to enable them to be driven up to each other for coupling purposes.

Anti-collision systems are of the optical, radar or potentiometer-wire type. The last system has been used in a number of CEGB power stations and is briefly outlined below.

A potentiometer wire system consists basically of a low resistance register wire and a potentiometer wire running parallel with each other and with the downshop conductor systems of the two cranes. The poten-

tiometer wire is connected across the secondary winding of a supply transformer which causes a voltage d rop along the wire of the order of 24 V. Electronic units on each crane are connected to the potentiometer wi re by collectors and interconnected via collectors and th e register wire. The potential difference along the length of potentiometer wire between the collectors of the tw o cranes is monitored by the electronic units, the control loop being completed through the register wire. As th e cranes approach each other, the potential differenc e between the pick-up points on the potentiometer wire decreases until the minimum approach distance of the two cranes is reached. At this point, the electronic units trip the crane motion contactors and stop the cranes .

3.7 Travel motion supply systems

All systems are rated for a 31 MVA 415 V fault level,

3.7.1 Long travel

The 415 V three-phase 50 Hz supply and earth continuity connections to the crane comprise four fully. shrouded downshop conductors located immediatel y below the crane rails. These conductors are made of hard-drawn copper, copper/steel laminate or phosphor bronze, as dictated by current rating and volt-drop requirements. Protection against accidental contact with metal objects is provided by insulating shrouds.

Connections between the downshop conductors and the crane are made by short-boom type collectors, which are accessible from the crane bridge for maintenance.

The 415 V three-phase 50 Hz supply from the Station Switchboard is connected into a quick-break, threeposition (service, isolated and off/earth) supply isolating switch positioned at operating floor level and, ideally, half way along the length of the downshop conductors to minimise volt-drop on the system. The 415 V supply connections from the isolating switch to the downshop conductors are made in armoured cable.

At each end of and, in some instances, at regular intervals along the downshop conductors, warning lamps are fitted to indicate to personnel in the crane operating area that the conductors are energised. At each location, three red-coloured indicating lamps are connected in star formation to the three phases of the downshop conductors through HBC fuses.

Fully-shrouded downshop conductors are used for the majority of overhead travelling cranes in a power station. For goliath cranes, travelling cable systems or totally -enclosed rigid conductor/collector systems, mounted just above floor level, are used to satisfy personnel safety and weatherproofing requirements. - For overhead cranes with limited long-travel require ments, a travelling cable chain system may be considered instead of a fully-shrouded rigid conductor system.