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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.
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.
© Copyright 2005 Power Electronics® International, Inc.