Click on the advert above to visit the company web site

Product category: Industrial Drives/Controls
News Release from: Power Electronics International | Subject: Motor encoders
Edited by the Engineeringtalk Editorial Team on 14 July 2006

Why use encoders on no load brake hoists

Why Sensorless Vector drives are being misapplied in the crane industry.

Power Electronics International has over 35 years in the manufacturing, engineering and design of AC drives for overhead hoist and cranes Previous to this the President and founder of the company, Victor 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 ie, load drop.

Sensorless Vector drives are designed for systems that don't require an encoder.

By understanding the AC 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 - Extreme destruction, death of individuals, damage to crane structure can be the result.

Power Electronics International 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 as one of the industry's leading corporations involved in the design of AC 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 recognising 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 is the only company in the world, which designs hoist and crane electronic AC drives specifically for those applications.

All others are simply relabelled 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.

• Power Electronics International: contact details and other news
• Email this article to a colleague
• Register for the free Engineeringtalk email newsletter
• Engineeringtalk Home Page

Search the Pro-Talk network of sites

Put your news on Engineeringtalk  |   Advertise  |   Get the free Engineeringtalk newsletter  |   Engineeringtalk Home
About Engineeringtalk  |   Copyright © 2000-2008 Pro-Talk Ltd, UK