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PAGE 16 Digital Drive Crane Hoist Conversion While working on the bridge of a crane, I remember feeling the intense heat of the speed reduction Resistors. I looked over the prints and tried to figure out how to reduce this energy loss. As I understood, heat is the product of energy lost (). I was new to crane maintenance in 1990 and, having an electrical/electronic background, Crane panel manufacturers desired a novel method of crane control that combines new technology with some of the oldest. The new crane panel resulted in lower costs, increased productivity and reduced wear on components, as well as energy savings.I believed new technology existed. Several of the newer devices needed alternating current input. SCRs, VFDs and PMWs were becoming common acronyms in newer plants. The possibility of upgrading our pre-existing 250 VDC distribution was cost-prohibitive, Various transistors could run DC, but not at the ampere demands we needed. With crane panel replacement under consideration, we challenged our panel suppliers to develop new crane control technology Digital Hoist Conversion Severstal North America Inc. is an integrated steel mill dating back to 1917, when Henry Ford built it to supply his Ford Motor Co. auto manufacturing enterprise. It was operated as Ford Steel Division urti11982, when it became Rouge Steel Co. In 2004, OAt Severstal Steel purchased the assets of Rouge Industries and Rouge Steel. Figure1.A digital drive controller was installed Figure2. Preliminary setup of DDC hoist panel on this 135-ton-capacity slab-handling crane The market price for steel was flat in the early part of the new millennium, forcing departmental groups to look for cost-saving improvements. One improvement was the installation of a new type of digital electronic control panel in 2003. This panel represented the introduction of DC electronic crane control to Rouge Steel and the largest duplex crane hoist controller (dual 200-hp) of its type in North America. The original panels were built on a P&H 135-ton slab-handling crane having standard DC hoisting contactor controls. They were industrial and functional, designed to handle the loads of this crane in 1972. The loads are greater now with heavier slabs, runing the crane at maximum limits and higher production rates. This caused premature equipment failures and production down-time. With three aging cranes in this bay, maintenance costs were rising to new highs. Those involved in maintenance were finding that distributors and manufactures were downsizing or had gone out of business, making replacement parts costly or obsolete. The market drivers of today are forcing the change to newer technologies Figure3. Digital panel installed on crane trolley deck Figure4.Prewired resistors reduced start-up time A novel design approach was asked of the crane panel manufactures. They replied with a proposed partnership and an effort to add some of the newest technology to the oldest methods of crane controls. The result was high-current transistor switching with a 250 VDC input. The design was well-thought-out and included integrating the original motors, limits, switches and wiring. Now speeds are controlled by sending the motors only enough current to safely lift and lower the load. The motors are soft stopped (reverse plugged) before the brakes close. This saves wear on components, reduces costs and increases productivity. Without the need for reduction resistors, there is no energy wasted, maximizing the energy savings. The panel installation of the SY-4 crane was completed in 2003 and is still running. The results are smoother movements with little energy loss (heat). The new panels were designed for installation on the trolley deck, as opposed to the bridge deck This aids in troubleshooting and reduces excessive wiring mainly at the weak point of the cable powertrack This allowed the time and ability to perform all setup work during mini-downturns without disabling the original hoist. The original panel was left in place as a backup, as failures could not be predicted. To date, the fail-safe panel has not been required. The panels were pre-wired and pre-tested prior to crane installation, further reducing crane downtime. When the transfer day came, only the master switch, motors and limit leads needed to be rerouted to the new system. On-the-job tuning and monitoring were vital for the first couple of days. It was important to have crane operators involved for that personal feel and to obtain their buy-in to the project, to increase awareness and productivity. No-load and full-load current tests were run with great results. An aded benefit to this control is the electrical current savings. Without reductin resistors for speed points, and with the added benefit of power produced when regenerative lowering, this single crane installation saves more than 25000 in electricity annually. This can be a very important consideration if substation power is near critical usage levels. The demand this system imposes is much less than a similar contactor system. With energy costs on the rise, this is a concern for every project considered. Figure 5 indicates an example of electrical current savings potential by comparing contactor panel loads(top) to digital drive loads(bottom). How it works in the circuit is not unique. The insulated gate bipolar transistor(IGBT) takes the place of the contactors and acceleration resistors. As the master switch is selected for greater speed, the circuitry triggers the transistor at a frequency(pulse width modulated) that allows current to flow through the IGBT. The current circulates in the standard series armature and series field along with the series brake. The longer the input is turned on, the higher the output average voltage (Figure 6). The higher the voltage, the higher the horsepower produced. This system can provide high torque with low currents(heat) as the result of motor regenerative properties. High speed with no load can also be accomplished. Much of this could not be achieved with the original panels. Figure5.Example of energy saved during lowering sequence. The difference is noticed when the IGBT is in its off cycle(Figure 7). In this instance, the motor acts as a generator, producing circulating currents through the flyback diode and maintaining self-induced motor currents. This effect reduces ripple and provides current that was not provided by the original power source. The reduction of current loads on system feeders and hardware further adds to the total savings package. The following items are important considerations when determining if this system will work with an application. IGBTs are the newest part of the design that makes this panel work with 250 volts DC. They combine the advantages of the bipolar transistor(high voltage and current) with the advantages of the metal oxide semiconductor field effect transistor(MOSFET)(low power consumtion and high switching). IGBTs are semiconductors that combine a high voltage and high current bipolar junction transistor(BJT) with a low-power and fast-switching MOSFET. Consequently, IGBTs provide faster speeds and better drive and output characteristics than power transistors and offer higher current capabilities than equivalent high-powered transistors. Figure6.Hoist current flow when the IGBT is on Figure7.Hoist continuing motor current flow when the IGBT is off. Heat sinking, including consideration of air temperature and air flow, is essential to the proper operation of any solid-state reply. It is necessary to rovide an effective means of removing heat from the IGBT. The importance of using a proper heat sink cannot be overstressed, since it directly affects the maximum usable load current and maximum allowable ambient temperature. Up to 90 percent of the problems with transistors are directly related to heat. Lack of attention to this detail can result in improper switching(lockup) or even total destruction of the IGBT. If the device ever reaches an internal temperature of 105, it will be permanently destroyed. One of the problems encountered at Severstal NA was program temperature cutbacks due to excessive heating. When electrical current cutback does not control the drive, it will stop on software limits. Transistors develop heat as a result of a forward voltage drop through the junction of the IGBT. Beyond this point, heat will cause a reduction(software cutback) of the load current that can be handled. If the demand is too great, the program is designed to shut down. Care must be taken when mounting solid-state relays(SSRs) in a confined area. SSRs should be mounted on individual heat sinks whenever possible. SSRs should never be operated without proper heat sinking or in free air, as they will thermally self-destruct under load. A simple way to monitor temperature is to slip a thermocouple under a mounting screw. If the base temperature does not exceed 45, the SSR is operating at its optimal level. Remember that the heatsink removes the heat from the SSR and transfers that heat to the air in the electrical enclosure. In turn, this air must circulate and transfer its heat to the outside ambient. Vents and forced ventilation are good ways to accomplish this. Semiconductor fuses are the only reliable way to protect SSRs. They are also referred to as current-limiting fuses, providing extremely fast opening while restricting let-through current far below the fault current that could destroy the semiconductor. This type of fuse tends to be expensive, but cheap by comparison to the damage that could occur, providing a means of fully protecting SSRs against high current overloads. An fuse rating is useful in aiding in the proper design of SSR fusing. This rating is the benchmark for an SSRs ability to handle a shorted output condition. Devices such as circuit breakers and slow blowfuses cannot react quickly enough to protect the SSR in a shorted condition and are not recommended. Every SSR has an rating. The idea is to select a fuse matching the capability of the solid-state relay for the same duration. Figure8.IGBT and components mounted on heat Figure9.External mounted fans removed IGBT heat Figure10. Panel fans removed internal heat buildup Figure11. Semiconductor fuses provide the best protection for solid-state relays Motor switching and dynamic loads, such as motors and solenoids, can create special problems for SSPs. High initial surge current is drawn because its star t-up impedance is usually very low. As a motor rotates, it develops a counter electromotive force (CEMF) that resists the flow of currenL This CEMF can also add to the applied line voltage and create over-voltage conditions during turn-off and regenerative times. It should be noted that over-voltage caused by inductive voltage doubling or CEMF from the motor cannot be effectively dealt with by adding voltage-transient suppressors. Suppressors such as metal oxide varistors (MOVs) are typically designed for brief high-voltage spikes and may be destroyed by sustained hlgh-energy conduction. Voltage dump resistors may be needed in extreme cases and should be engineered to meet a systems demand, It is therefore important that SSRa are chosen to withstand the highest expected sustained voltage. Problems encountered while running the200-hp dual drive were few hut worth mentioning. The program allows for setdng many variables (i.e., speed, currents, brake-open voltage). Most of these are detrimental to the drive or motor if set incorrectly. Although staying within the drive specifications is safe, this may not produce the desired actions. Ambient temperatures must also be considered, since most useful application are near higher-temperature areas. Following are several problems(and solutions) observed during installation and trials: Problem 1:The first problem presented itself when applying excessive brake-open curent. The direction contactors were flashed and pitted. Also, the emergency brake contactor appears bluish from high heat. Reducing brake-open current and power-on time to a shorter duration solved the problem. Problem 2:Hall effect transistors and IGBT were thought to be faulty parts and/or wiring, but this could not be duplicated. Many suspect parts were replaced, but it was determined that internal panel ambient temperature was the problem. This was solved with cooling fans on the doors and on the IGBT cooling fins. Problem 3:Temperature cutbacks usually led to errors. It was found that a new crane operator did not like operating the hoist at full speed. Longer run time and higher IGBT cycles caused unnecessary heating in panels. Reducing field current settings eased this problem. This increased the lowering speeds but greatly reduced the IGBT voltage drop, in turn reducing its heat dissipation. Cooling fans eliminated the problem. Problem 4:All power must be disconnected from the line because all lines feed from a common bus capacitive filtering system. This means that the typical way of pulling motor disconnect and running the controls only to troubleshoot does not work. The panel diagnostics and troubleshooting information provided is very helpful. Problem 5:Lack of electronic knowledge by the electricians is a concern. When production downtime is critical, the time to troubleshoot is a high-priced commodity. This ultimately puts pressure on the electricians, causing frustration. The solutions was to ensure that the crew is involved Mth project desig