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 (h=I2xR). I was new to crane maintenance in 1990 and having an electrical/electronic background I believed new technology existed. Several of the newer devices needed alternating current input. These SCRs (silicon-controlled rectifiers), VFDs (variable frequency drives) and PWMs (pulse-width modulators) were becoming common acronyms in newer plants. The possibility of upgrading our pre-existing 250V DC 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.
The plant
This facility is an integrated steel mill. It dates back to 1917 when Henry Ford built it to supply his Ford Motor Company auto manufacturing enterprise. It was operated as Ford Steel Division until 1989, when it was sold and the name changed to Rouge Steel. In 2004 the Russian-owned steel firm Severstal purchased the facility from bankrupt Rouge Steel.
The market price for steel was flat in the early part of the new millennium, forcing departments to look for cost savings. One improvement was the installation of a new type of digital electronic control panels built by EC&M, a subsidiary of Schneider Electric, with technology from Cableform. When we bought it in 2003, it was the first DC electronic crane control in 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, running the crane at maximum limits and higher production rates. This has caused premature equipment failures and production downtime. With three aging cranes in this bay, the maintenance costs were rising to new heights. Those of us involved in maintenance are finding that distributors and manufacturers are downsizing or have gone out of business, making replacement parts costly or obsolete. The market drivers of today are forcing the change to newer technologies.
The controls
We asked the crane panel manufacturers for a new approach to design, and they responded. They added some of the newest technology to the oldest methods of crane controls. The results were high-current transistor switching with a 250V DC input. The well thought-out design integrated the original motors, limits, switches and wiring. The speeds were controlled by sending the motors only enough current to safely lift and lower the load. The motors are soft stopped (reversed plugged) before the brakes close. This saves wear on components, reducing cost and increasing productivity. Without the need for reduction resistors there is no energy wasted, maximizing the energy savings.
The newest part of the control circuit is an insulated gate bipolar transistor (IGBT). These transistors make this panel work with 250V 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 consumption and high switching). IGBTs are semiconductors that combine a high voltage and high current bipolar junction transistor (BJT) with the low power and fast switching of a metal oxide semiconductor field-effect transistor (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.
The architecture of the circuit is not unique. The 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 pulse width modulated frequency allowing 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. 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.
The difference to the old DC circuit is especially marked when the IGBT is off. In this instance, the motor acts as a generator, producing circulating currents through the flyback diode and maintaining self-induced motor currents. This effect not only reduces ripple but also provides current which 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.
Heat stroke
Up to 90% 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°C (221°F), it will be permanently destroyed.
The IGBT temperature directly affects the maximum usable load current and maximum allowable ambient temperature.
When electrical current cut-back 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 cut-back) of the load current that can be handled.
It is necessary to provide an effective means of removing heat from the IGBT. Heat sinking, including consideration of air temperature and air flow, is essential to the proper operation of any solid state relay. Care must be taken when mounting solid state relays (SSRs) in a confined area. SSRs should be mounted on individual heat sinks whenever possible. They should never be operated without proper heat sinking or in free air as they will thermally self-destruct under load. A simple way for monitoring temperature is to slip a thermocouple under a mounting screw. If the base temperature does not exceed 45°C, the SSR is operating at optimal.
The heat sink removes the heat from the SSR and transfers that heat to the air in the electrical enclosure. To cool the enclosure, this air must transfer heat to the external environment. Providing vents and forced ventilation are good ways to accomplish this.
SSRs are also susceptible to overloads. Devices such as circuit breakers and slow blow fuses cannot react quickly enough to protect the SSR in a shorted condition and are not recommended. Semiconductor fuses are the only reliable way to protect SSR’s. 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, providing a means of fully protecting SSRs against high current overloads. An I2T fuse rating (ampere squared time/seconds) is useful in aiding in the proper design of SSR fusing. This rating is the bench mark for a SSR’s ability to handle a shorted output condition. Every SSR has an I2T rating, the idea is to select a fuse matching the capability of the solid state relay for the same duration.
Motor switching and dynamic loads produced by elements such as motors and solenoids can create special problems for SSRs. They draw a high initial surge current because their start-up impedance is usually very low. As a motor rotates, it develops a counter-electromotive force (CEMF) that resists the flow of current. This CEMF can also add to the applied line voltage and create over-voltage conditions during turn-off and regenerative cycles.
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 high energy conduction. Voltage dump resistors may be needed in extreme cases, and should be engineered to meet your systems demand. It is therefore important that SSRs are chosen to withstand the highest expected sustained voltage.
The panel installation of SY-4 crane was completed in 2003 and to date is still running. The results are smoother movements with little energy loss (heat).
We installed the panels on the deck of the trolley, rather than on the bridge. This position aids in troubleshooting and reduces excessive wiring mainly at the weak point of the cable powertrack. This allowed us the time and ability to perform all setup work during mini downturns without disabling the original hoist. We also left the original panel in place as a backup, as failures could not be predicted. To date we have not needed to use the fail-safe mode.
We pre-wired the panels and
pre-tested functions prior to crane installation. When the transfer day came, we needed only to reroute the master switch, motors and limit leads to the new system. On-the-job tuning and monitoring for the first couple of days were vital. It was important to have crane operators involved for that “personal feel” and to obtain their buy-in to the project, increasing awareness and productivity. No-load and full-load current tests were run with great results.
An added benefit to this control is the electrical current savings. Without reduction resistors for speed points and the added benefit of regenerative power produced when lowering, the crane saves over $25,000 in electricity annually. This can be a very important consideration if substation power is near critical usage level. The demand this system imposes is much less than a similar contactor system. With energy costs on the rise, this is of more concern for every project considered.
After one year we calculated the actual savings. The plant saved 62% on maintenance and labour savings of 21%, leading to a complete payback in 6.2 months. Cost savings and efficiency gains were greater than expected. These gains have led the way to the next drive conversion that is scheduled for 2006, again with EC&M panels and Cableform drives. With cooperation from sales, manufacturers, engineers and end users, we have improved our ability to compete successfully.
