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Electronics for EOT Cranes and Hoists

Whitepapers and Press Articles

Why use Encoders?Dam Fine JobLifting PowerCrane Duty

Why use encoders on no load brake hoists?
Why “Sensorless Vector” drives are being misapplied in the crane industry?

Power Electronics International, Inc. has over 40 years in the manufacturing, engineering and
design of a.c. drives for overhead hoist and cranes. Previous to this the President and
founder of the company, Victor J. Habisohn, was project manager of General Time
Corporation with the responsibility of the design of the Timing System used in the Apollo
program. Equipment supplied was to be of the highest reliability since it triggered all other
functions in the spacecraft – quality, reliability and safety were extremely important. This
philosophy has continued in all PE’s products lines. Safety and Reliability is of the utmost
importance.
Given the above, it is understandable that PE would take great pains to understand all the
safety concerns inherent in overhead hoist and crane electronic control. And indeed the
ramifications of a “failure” of any part of the electrical system become more and more
obvious as one understands the system “from the inside out”. Small electronic components,
some no larger than a small coin can, in many cases, are the only safety link available
between a safely running hoist or crane and one “out of control”. Redundant safety circuitry
must be applied so that if one fails the other “takes over”.
In most, if not all, “general purpose” variable speed drives, safety is designed into the drive
for “standard” applications such as conveyor use, simple speed control of pumps and fans
etc. Failure of a component could simply stop the equipment and not allow it to run properly
or to run erratically. With a hoist drive, a failure in certain circuits becomes catastrophic in
the sense that even though the equipment may not function properly and may not become
damaged, the hoist “holding brake” could be told to “open” with insufficient torque or energy
to “hold” the load i.e., load drop.
Sensorless Vector drives are designed for systems that don’t require an encoder. By
understanding the a.c. motor and creating a “model” of its electrical characteristics,
the sensorless drive can “estimate” motor speed and position. Under various
conditions the estimates and internal sensors “fail” and the hoist can be told to
operate within a range or parameters that are unsafe. The questions then arises of
the frequency of failure and what kinds of failures can the user of the equipment
accept. On a conveyor or CNC equipment, the loss of a product or erratic control of
the conveyor is not catastrophic, and may even not be very noticeable. On a hoist,
failure or “imprecision” because of an “estimate” can become catastrophic—i.e., load
drop etc. Extreme destruction, death of individuals, damage to crane structure can
be the result.
Power Electronics International, Inc. in studying all the ramifications and safety concerns
decided that so-called “Sensorless” Vector technology should not be used on hoists without
mechanical load brakes. Sensorless Vector technology is attractive to some because of the
perception that it makes a physical encoder unnecessary on the hoist or hoist motor thus
decreasing system cost. Nothing could be further from the truth, an encoder is necessary to
eliminate the “guesswork” and have 100% assurance of hoist speed and position (not a
guess, relying on “motor models”). Safety is the prime reason to use the encoder; a
mathematical somewhat blind “guess” is not sufficient in hoisting and heavy equipment,
heavy industrial environment where a “failure” can mean a disaster.
Power Electronics International, Inc. as one of the industry’s leading corporations
involved in the design of a.c. drives for hoists and does not feel that Sensorless
Vector drives have any place in no load brake hoist control. (A “load brake” being a
device which, separate from the “holding brake” will physically stop the hoist from
falling by means of a mechanical “internal” ratcheting or other type of resistance
which must be “overcome” in the down direction).
Sensorless Vector drives can be misapplied very easily because of the extensive “marketing”
of the technology to other non-crane industries. Even some “engineers” have made the
serious mistake of not recognizing the inherent problems, mostly due to overzealous drive
salesmen who “oversell” their products features and have little to no real knowledge of the
safety aspects which need to be addressed. Often even a group of engineers who approve the
drives for “hoist” use have never looked closely at the areas of failure (and aging failure
modes), which can occur with a hoist “fighting gravity”, and at “Zero-speed”. Zero and very
low speeds are particularly dangerous points which sensorless vector drives have accuracy
problems. On a conveyor or other horizontal types of equipment zero and very low speeds
are not usually problematic. But on a hoist it is another story. A hoist’s brake opens at zero
speed and gravity takes over! Not an application for Sensorless systems. Zero tolerance of a
brake opening at the wrong time must be the standard in the hoist-crane industry. A motor
encoder is necessary to help with this safety need. The possibility of failure of internal drive
circuitry is also reason enough for using an external encoder as a check on the drive function.
It must also be understood that there are many other safety concerns, besides the use of an
encoder, which must also be addressed in an electronic hoist drive – these are not covered in
this paper,
In addition, use of multiple motors either in tandem or switching between one or another is
not possible with sensorless drives since the exact motor parameters are required for the
internal “motor model” which is for one motor only. Motor parameters also change with
temperature and other variables. An encoder solves many safety and reliability problems by
giving the drive direct and absolute feedback so it can react to any changes.
Power Electronics International, Inc. is the only company in the world, which designs hoist
and crane electronic a.c. drives specifically for those applications. All others are simply relabeled
or re-programmed general purpose drives. All drives are simply NOT the same. If
it’s not a PE drive it is not crane-hoist designed. Ask for PE equipment from your crane dealer
or crane service company – they are readily available.
This short note is not meant to be a thorough scientific analysis, but is a simple to help point
out why encoders are important on an a.c. overhead “no-load brake hoist” using electronic
speed control. The above information is meant to be a short beginners explanation. For more
specific information call us at 1-847-428-9494.

Click here to view the PDF article.

Dam fine job

Two firms teamed up for the upgrade of historic cranes at the Bagnell Dam, in the picturesque Lake of the Ozarks region, Missouri

Located about three-and-a-half hours southwest of St. Louis, the Bagnell Dam was built over a two-year period between 1929 and 1930 by Union Electric Light &Power Company (now known as AmerenUE). When completed, it formed the beautiful area known as the Lake of the Ozarks region, Missouri.

As part of the original plant equipment, Whiting Corporation was contracted to

supply all of the plant's overhead gantry cranes. These cranes, including two 7M capacity head gate cranes, as well as the 150t capacity power house crane, are primarily used to perform maintenance and removal/replacement of major plant equipment.

The two head gate cranes service 24 spill gates that assist in controlling lake levels. Periodically, each of these gates must be raised for various maintenance activities.

Although the crane controls were still in excellent condition, upon review of the new and more stringent Federal Emergency Management Agency (FEMA) requirements under the National Dam Safety Program, AmerenUE elected to upgrade the existing controls. In August 2007, Whiting Corporation was favored with the contract for this work, with

Whiting Services, Inc. and Power Electronics International, Inc. performing on site work and project management.

A key FEMA requirement of the new control system specified

precise torque limiting capability of the hoist motion to help prevent damage to any of the spill gates as they are raised. These gates can remain in place for some time and it is not uncommon for them to "stick" in place. Therefore, uncontrolled lifting torque during hoisting can (and had at another hydro plant) literally snap the gate pivot bearing and distort the gate structure.

Another key FEMA requirement called for an onboard, propane-supplied, 40kW back-up generator capable of transferring power to either head gate crane in the event of loss of plant power. Installing the generatar in the crane's control room was no small task. The plant and cranes are registered with the Historical Landmarks Society, which meant removing and replacing two large sections of original casement style windows to place the unit out of sight so that the appearance of the cranes was not altered.

To meet these control requirements, Whiting Services selected Power Electronics International Inc. (PE) as its supplier for the new motor and control system. The hoist motions were converted to PE's Micro-Speed Multi-Vector drives and the bridge and trolley motions employed Micro-Speed open loop drives. Together, these drives provided infinitely variable speed and lifting control of the load while also achieving all of the torque limiting requirements to meet the plant's expectations.

Other safety and control system upgrades included: radio/cab modes that allow either single or dual motion functions; an anemometer (wind gage) alarm system; anticollision,

hard-wired festoons; various safety status lights and horns; ground level emergency stop stations; cab lighting and heating; and new 1,000 watt metal halide area lights for

night time work activities. Because of the success of the head gate crane upgrade, in June 2008 Whiting and Power Electronics International, Inc. were awarded with a contract to perform a similar upgrade to the 15M power house crane. This upgrade also included an engineering study to allow for an engineered lift of approximately 16M to replace a turbine assembly.

Whiting Corporation and Whiting Services were able to review the original circa 1930s engineering drawings and calculations to provide viable, effective, long-term solutions.

Other enhancements included: eliminating the original single motor bridge drive with over 80 feet of line shafts; bevel reducers; and support bearings in favour of compact, twin drive motor reducers located at the wheels. These enhancements increase the effective life of the bronze bushings and shafts to further reduce long-term maintenance costs.

PE controls are used at Bruce Power

Lifting power
Written by Richard Howes, OCH Magazine

With 85 cranes and over 750 hoists on site, the crane guys at North America’s largest nuclear power site have their work cut out.

Pete Richards, multi-trade team leader, crane crew, and Fred Wolsey, system engineer, cranes and hoists, are responsible for the safe operation and upkeep of all the lifting equipment at Bruce Power, Canada’s first private nuclear generating company and the source of more than 20% of Ontario’s electricity. The facility is located approximately 250km northwest of Toronto.

On a day in mid-May this year, Bruce Power’s Unit 5 was in a ‘shutdown’ state for regular maintenance. The unit which usually generates about 890 megawatts of electrical power had the focus of numerous workgroups. Replacing the energy supplied by such a unit is of substantial value to the company and its stakeholders.

The crane crew, which consists of a team of six electrical and eight mechanical technicians, was working elsewhere on site at the spent fuel bay at the Bruce ‘A’ station where a newly installed Canadian Overhead Handling Inc. 100 ton crane, with 80ft span, was being inspected.

This crane has a technical positioning system which allows movement of the load (highly radioactive spent fuel) only within confined coordinates. The spent fuel is encased in a shielding flask which protects the workers and environment from any external dose. It weighs close to the full capacity of the 100 ton crane.

Having completed work at the spent fuel bay, the crew found themselves in transit to the ‘B’ station, where a crane in the reactor vault had ceased to function in one direction during the ‘shutdown’ phase. The cranes at Unit 4 of the ‘B’ station have been in operation since commissioning almost 30 years ago. In the hot, highly radioactive environment, insulating material and plastics deteriorate at a higher rate than in conventional systems.

The electrical technicians of the crane crew have acquired experience and skills to enable them to expeditiously locate the “gremlins,” as the crane guys put it, which in this case invaded the control circuits of the vault crane. With this problem solved, detailed work reports and system weaknesses identified, the crew returned to the day’s original activities.

Having regular inspections by qualified and knowledgeable staff has ensured safe operation of these cranes over many years. It has become apparent, however, that in order to maximize efficiency and minimize downtime and radiation dose exposure to the crane crews, the cranes in each of the Bruce Power ‘B’ plant units will need to be replaced. Planning is now underway to begin replacement of the first unit’s cranes in the Spring 2009.

The commitment to replace the eight vault cranes (two cranes per unit), with 10.5 ton capacity Yale units with 60ft spans, has been accepted by the crane crew who will help design, build, install and commission the cranes. Ultimately, the crew will have intimate knowledge of the working of the new cranes which will help provide safe, reliable operation, crucial to the activities associated with shutdown maintenance.

The crane maintenance program at Bruce Power is recognized as an Industry Best Practice by the Electric Power Research Institute (EPRI).

CMAA Crane Duty Classifications
CMAA Class	Description	Further Explaination
Class A	Standby or Infrequent Service	This class normally includes installation and maintenance cranes. This equipment usually operates at slow speeds for precise handling with long idle periods between lifts. Maximum capacity lifts are sometimes required.
Class B	Light Service	This class of crane is used in repair shops, light assembly operations, service buildings, light warehousing, etc. Service requirement is light and speeds are slow. Loads vary from none to occasional full capacity. Lifts per hour would range from 2 to 5, and average 10 feet per lift.
Class C	Moderate Service	In terms of numbers, most cranes are built to meet Class C service requirements. This service covers cranes that may be used in machine shops or papermill machine rooms. In this type of service, the crane will handle loads that average 50% of the rated capacity with 5 to 10 lifts per hour averaging 15 feet. Not over 50% of the loads at rated capacity.
Class D	Heavy Service	This service covers cranes which may be used in heavy machine shops, foundries, fabricating plants, steel warehouses, container yards, lumbermills, etc., and standard duty bucket and magnet operations where heavy duty production is required. In this type of service, loads approaching 50 percent of the rated capacity will be handled constantly during the working period. High speeds are desirable for this type of service with 10 to 20 lifts per hour averaging 15 feet, not over 65 percent of the lifts at rated capacity.
Class E	Severe Service	This type of service is reserved for top riding bridge and gantry type multiple girder electric overhead traveling cranes and requires a crane capable of handling loads approaching rated capacity throughout its life. Applications may include magnet, bucket, magnet/bucket combination cranes for scrap yards, cement mills, lumber mills, fertilizer plants, container handling, etc., with 20 or more lifts per hour at or near the rated capacity.
Class F	Continuous Severe Service	This type of service is reserved for top riding bridge and gantry type multiple girder electric overhead traveling cranes and requires a crane capable of handling loads approaching rated capacity throughout its life. Applications may include custom designed specialty cranes essential to performing the critical work tasks affecting the total production facility. These cranes must provide the highest reliability, with special attention to ease of maintenance features.

Crane Manufacturers Association of America, Inc.

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Electronics for EOT Cranes and Hoists

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