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
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
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.