How would you like to reduce unscheduled downtime associated with your conductor bar system? You can, if you follow some simple rules when choosing the conductor bar system for your crane application. This article will show you what to pay particular attention to and lead you through selecting the correct conductor for your situation.
Buyers need to know: the length of the system, number of conductors, power feed location, voltage, current requirements of each crane, temperature, duty cycle, application, environmental conditions and mounting considerations.
Many of these factors are interdependent. The conductor required for a particular length will be greatly affected by the voltage, current requirements, temperature and duty cycle of the crane(s). Below is a look at each factor and the effect of each on the choice of conductor.
Length
Length determines not only how much conductor you will need, but what size is required, especially on longer runs. As length increases, the effect of voltage drop becomes more pronounced (see box, to right).
Increasing the number of feed points and placing them in the optimal locations can save money by allowing a long run to utilize a lower amperage conductor. A decision must be made as to whether the savings in conductor bar offsets the cost of the extra material and labour to run conduit and cabling to the two power feed locations.
The effects of expansion and contraction become more critical as the length of the run increases. For shorter systems, the conductor can be anchored in the centre. As the temperature of the conductor rises, the expansion simply pushes the conductor outward. The maximum system length that can successfully be anchored only in the centre depends on the friction of the hanger and the rigidity of the conductor. When the cumulative friction through the hangers becomes too great, the conductor will take the path of least resistance and bow to the side. This is referred to as snaking and can be easily observed by sighting down the runway. The conductor will bow alternately left and right between hangers. This puts additional strain on the collectors and hangers. Eventually, the snaking may get to the point where the collectors can no longer track and collector disengagement will occur.
When the distance from anchor point to the end of the run becomes too great for the system to effectively dissipate conductor movement, expansion sections need to be installed. In order for expansion sections to work properly, the conductor needs to be anchored properly between each expansion, and between the last expansion and the end of the run. The expansion slider compensates for the expansion and contraction of the conductor located between anchors. As temperature increases and the conductor expands, the slider closes. As temperature decreases and the conductor contracts, the slider opens. The gap in the slider is bridged by a jumper cable to maintain electrical continuity and the slider maintains a consistent running surface for proper collector tracking. The distance between anchors, or the length of run an expansion section can manage, is determined by the temperature variance, the material of the conductor and the length of the expansion slider.
Keep in mind that the friction created by the hangers increases in dirty environments over time. Dirt accumulates on the conductor and makes it harder for the conductor to slide through the hangers. This needs to be taken into consideration in planning the number of expansions in the system. A conductor system may begin showing signs of snaking after a year or two in operation. If precautions are taken at the time of installation, costly repairs can be avoided later.
The number of conductors
Recent changes to the US NEC (National Electrical Code) now require a dedicated ground for all overhead cranes. Per Article 610.61 of the NEC, “The trolley frame and bridge frame shall not be considered as electrically grounded through the bridge and trolley wheels and its respective tracks. A separate bonding conductor shall be provided.” This applies to all cranes built from 2005 and thereafter.
Power feed location
We have already seen how placement of the power feed affects voltage drop. On high-demand systems, power feed location can also affect the performance of the collectors. All conductor bar manufacturers have higher amperage conductors than collectors. For example, they may manufacture a 1000 amp or higher conductor but the highest-rated collector may only be 600 amps or less.
For most applications, the size of the conductor is determined by voltage drop considerations for long runs, or where there are multiple cranes on a single runway. The conductor must be sized to handle all cranes but the collectors are sized only for the crane they service.
If the crane draws more than a single set of collectors is rated for, additional collectors must be added. Care must be taken to allow adequate collector capacity when using multiple sets of collectors. Usually, the collector closest to the power feed point will carry a larger load than collectors farther down the line.
Voltage
System voltage has a direct effect on allowable voltage drop. Since CMAA recommends a maximum voltage drop of 3% on runways and 2% on bridges, the drop in terms of volts will vary according to voltage available. For example, if the voltage at the power feed is 480 volts, a 3% voltage drop is 14.4 volts. However, if the system voltage is 115 volts, a 3% maximum voltage drop is only 3.45 volts.
In the case of higher voltage, adequate insulation is required. Most conductor bar manufacturers have 600 volt rated covers. Any voltage above 600 volts requires insulators rated for the appropriate voltage. In the case of medium voltage, for example 4160 volts, other considerations such as fault force may need to be taken into consideration. Qualified engineers should be consulted to design a conductor system to withstand a given fault force requirement.
Crane current requirements
NEC Article 610-14 requires the minimum allowable conductor size be rated for 100% of the current for all of the largest motors for any single movement plus 50% of the next largest motor. It is important to remember that multiple motors may be involved with a single movement. One should include the auxiliary hoist motor if it will work in conjunction with the main hoist on any lift. One should also include 100% of magnets, lighting, air conditioning, etc., that will be energized when the largest motors are engaged. For multiple cranes on one runway, the above amperage requirement is figured for each crane and all cranes are added together. A diversity factor is then applied to the total (see NEC, Table 610-14(e)). For example, for two cranes multiply the total by .95. The diversity factor recognizes that all cranes will not be pulling maximum load all the time and usually not at the same time. (N.B.: if two cranes are working in tandem, each crane’s largest motor must be figured at 100% since they are included in “all motors for any single movement.” No diversity factor should be applied.)
Installation of a new conductor system is a good time to plan for any expansion that may be foreseen. If additional cranes, or larger cranes, are planned or may be an option in the future, now is the time to include them in the sizing of the conductor bar. A small investment now may pay huge dividends in the future.
Temperature/duty cycle
We have already seen the affect of temperature on expansion and contraction. Figures 1-4 (see p. 25) show that temperature has an effect on voltage drop. The higher the conductor bar temperature, the larger the voltage drop. Temperature also affects the conductor cover requirements. But how does one determine the correct operating temperature? Ambient temperature is one of the major indicators of operating temperature, but there are a few more factors to look at to truly plan for the effects of temperature.
Most conductors are rated based on temperature rise under a given load in ambient temperatures up to 120°F (49°C). For example, a 500-amp conductor can carry 500 amps without exceeding the temperature rating of its cover. However, the duty cycle that this rating applies to may change from manufacturer to manufacturer, or even conductor to conductor. For example, one manufacturer may rate conductors under intermittent duty while another may rate conductors under continuous duty.
Continuous duty is just that. In the above example, the conductor is put under a continuous load of 500 amps at a given ambient temperature, usually 86°F (30°C). The temperature is monitored until it stabilizes. The maximum temperature of the conductor cannot exceed the temperature rating of the cover. For most PVC covers, this is around 158°F (70°C). This is a standard rating of 40°C rise over 30°C ambient.
Intermittent duty ratings are applied in the same manner except the current is not continuous. The current is energized for a period of time and then de-energized for a period of time – a duty cycle. An intermittent duty rating may be based on any duty cycle but a 50% duty cycle is most common. This is usually based on one minute on and one minute off. An intermittent rating based on a 50% duty cycle may be sufficient for the majority of cranes in use today. Since a crane cannot lift continuously, the current is not at maximum for long periods of time. This allows the conductor to dissipate the heat generated under maximum current flow. Most cranes do not operate above a 40% duty cycle. However, cranes that see heavy duty may exceed intermittent ratings, especially Class D and E cranes. It is important to know if the rating of the conductor is continuous or intermittent, and if intermittent, at what duty cycle it is rated.
These ratings are under “normal” ambient temperature conditions, usually 30°C. The normal operating ambient temperature for a particular crane may be much higher. It is, therefore, necessary to add the conductor bar temperature due to current flow to the highest ambient temperature under operating conditions to get the maximum bar temperature. Take, for example, a Class D crane working in an operating ambient temperature of 120°F. The temperature rise of the conductor bar due to current flow may add another 50°F. The conductor has PVC cover effective to 160°F (70°C). Even though the ambient is well below the rated cover temperature, the combined ambient and bar temperature is now more than the cover can withstand. The result will be cover deformation or even melting, which can cause interference with collector tracking and interruption of power to the crane.
For expansion and contraction calculations, ambient plus bar temperature would be the high end of the temperature range. The low end would be the lowest ambient temperature the conductor could see, say, during a shutdown in December. This is the temperature range that must be taken into consideration to properly account for maximum expansion and contraction.
There is also a large difference in the effects of ambient versus radiant heat. Ambient heat is easy to measure and the effects are consistent with measured values. Radiant heat can be difficult to measure and its effects can be hard to anticipate. Any heat source needs to be considered and evaluated for its effect on the conductor bar and cover. Typical heat sources that may affect conductor bar components are furnaces, billets, slag, etc. Radiant heat will affect cover directly but has a more pronounced affect on metal components. The cover itself may withstand the radiant and ambient heat but the metal hangers may heat up considerably more and melt the cover. Heat shields are usually an effective way of minimizing the effects of radiant heat. If heat shields are not practical, higher temperature rated covers are required.
Application
The primary application consideration is whether the system will draw maximum current while moving or while stationary. Most crane applications are considered moving applications. The collectors are drawing current as the crane moves up and down the runway. This allows the heat generated during current flow to be dissipated over a wider area of the conductor bar. Stationary applications may include welding or testing equipment. The collectors are moved to one spot where they may draw maximum current for hours at a time. Heat generation is confined to a small area so heat dissipation is minimized. This consideration may be applicable to cranes where repeated lifting is done in the same location for long periods of time. For stationary situations such as these, the collectors are derated and/or the bar rating is verified to be able to handle the stationary current load.
Environmental conditions
Other elements in the operating environment may affect the conductor, such as chemicals, water, dust or radiation that may be present. Certain cutting oils have a negative effect on polycarbonates. Acid or base fumes may require the use of stainless steel hardware and components. Radiation requires the use of non-PVC and non-galvanized or plated materials. Water and some dusts require the use of insulated hangers to ensure adequate insulation between the conductors and ground. Any element present in the operating environment of the conductor system that is not present in an office should be communicated to the conductor bar provider for consideration.
Mounting considerations
Sometimes the space available to mount the conductor bar system may be limited. A typical bottom-entry mounting configuration may not be practical. Alternative mounting methods are usually available, including laterally mounted conductor or staggering collectors to minimize conductor separation. These need to be discussed with the conductor bar provider. Alternate mounting methods may present problems with regards to environmental conditions. For example, a laterally mounting conductor operating in outdoor, wet conditions is not advisable. Lastly, the user needs to take into account that the crane will move relative to the conductor bar. Most collector arms are designed to accommodate a certain amount of movement. But it the movement is excessive the collector could disengage from the bar.
When searching for a conductor bar supplier, solicit multiple bids and compare them. Pay particular attention to conductor bar size, number of expansions and cover material.
Although not a factor in selecting the best conductor bar system for your application, installation is the most important factor in determining how well the system will perform. It is imperative to strictly follow the manufacturer’s installation instructions in order to optimize the system’s performance.
