Motors basics

While not glamorous, motors are the workhorses that keep inventory moving in automated materials handling systems.

By Bob Trebilcock, Editor at Large -- Modern Materials Handling, 3/1/2007

Without question, motors are the power behind the scenes when it comes to automation. It's true whether you're talking conveyors, automated storage systems, palletizers or stretchwrappers.

Often, motors are specified by the original equipment manufacturer making the equipment. But end users also have a stake in choosing the motors that run their solutions. Some have specialized needs in their industry. Elsewhere, some companies standardize on a particular type of motor across a network of facilities to minimize maintenance and spare parts. And sometimes, cost considerations play a part in motor selection by the end user.

In materials handling applications, five motors are most commonly used:

• DC permanent magnet and AC induction motors drive equipment that runs all the time, like traditional conveyor;

• Servo motors provide the precision positioning required for palletizing and automated storage and retrieval systems;

• Linear induction motors provide the speed necessary for high-speed sortation;

• And motorized roller conveyors combine a small motor and gear box inside a sealed roller to power conveyors that don't need to run all the time.

Warehouse workhorse

The workhorse of powered materials handling equipment remains the DC permanent magnet motor and, more commonly today, the AC induction motor. While DC motors are prevalent in legacy systems, AC induction motors are the motor of choice for most new facilities.

Why induction motors?

“An AC induction motor is one of the simplest and least expensive motor technologies and hence the most applied,” explains Tim Albers, director of marketing for the industrial motors division of Emerson Motor Company (888-637-7333, www.emersonmotors.com).”

When choosing an AC induction motor, two factors come into play—efficiency rating and type of enclosure.

1. Efficiency rating. While the government has established minimum efficiency ratings for basic motors, manufacturers also build motors to a higher standard, known as the National Electrical Manufacturers Association (NEMA) premium standard.

   While traditional motors are 75 to 85% efficient, high efficiency motors are 85 to 95% efficient, according to John Mazurkiewiz, product marketing manager of Baldor Electric (479-646-4711, www.baldor.com).
   

   AC induction motors are the most common motor for materials handling applications.

   “High efficiency motors cost more up front,” says Mazurkiewicz, “But if you're operating in an environment where the motor is running constantly, you will see a payback very quickly.” (For more information on NEMA motor efficiency programs visit www.nema.org/gov/energy/efficiency/premium).

2. Type of enclosure. This is also important to consider when choosing an induction motor. Five types of enclosures are common.

   • ODP – An open drip proof motor is the most common one used in industrial settings. This motor doesn't have a fan and has openings which can be penetrated by dirt and water, so it is ideal for indoor use.

   • TEFC – A totally enclosed fan cooled motor has a fan mounted to blow air across the motor frame. Because the fan is sealed, no external elements can get in. TEFC fans are often used with conveyors.

   • TENV – A totally enclosed non-ventilated motor is also used in materials handling applications where another source of ventilation to cool the motor is available.

   • TEBC – A totally enclosed blower-cooled motor includes a fan that is controlled apart from the motor. These are typical in applications calling for larger horsepower, like a crane and hoist, or in variable speed applications where the motor may operate near zero revolutions per minute.

   • EPFC – An explosion proof fan cooled motor is used in environments where combustible elements like gasoline, oil, ammonia, coal or combustible dust are present.

What's more, all motors are more flexible thanks to variable frequency drives (VFD). “Traditional induction motors run at a fixed speed,” says Mazurkiewicz. “Variable frequency drives allow an end user to vary the speed at which a motor, and a system, operates.”

In a materials handling system, variable speed drives let a manager run a system at a high speed during peak periods and drop down to a slower speed at other times.

Servos

Servo motors feature feedback devices that allow for greater control over positioning and speed than AC induction motors.

While AC induction motors are the most common motor used, servo motors are used in niche applications that require precise control over position and speed.

“A servo motor has been designed specifically to have the rotor diameter reduced while maintaining the same torque,” says Baldor's Mazurkiewicz. “The smaller rotor means lower inertia. That translates into faster acceleration and faster positioning.”

Servo motors also include closed-loop devices, such as a tachometer, resolver and encoder. The feedback from these devices allows for greater control over positioning and speed.

Servo motors are used for applications that require a great deal of precision, like robotic and traditional palletizing. They are also well suited to applications that require synchronization, like positioning a storage/retrieval machine for putaway in an automated storage and retrieval system.

Linear induction motors

Linear induction motors combine very fast speeds with pinpoint accuracy for applications that require a high degree of synchronization, like sliding shoe sortation.

Linear induction motors are the newest entry into materials handling applications. These offer the advantage of much faster speeds coupled with very high repeatability and exceptional accuracy, says Mazurkiewicz.

A linear motor uses the same basic theory to produce torque that produces force in a rotary motor. But instead of turning a shaft, a linear motor produces a magnetic field that moves a plate in a linear motion across the motor.

Most often, the plate is attached to the object that is being moved by the magnetic field. That means there are no parts to wear out from use. In a sortation system, for instance, the plate could be affixed to the sliding shoe of the sorter.

Linear motors are accurate to within 0.0004 inch per foot, which provides for very precise operations. A human hair, for comparison, measures approximately 0.0035 inch. Finally, with speeds of up to 200 inches per second, linear motors operate with very short cycle times.

Motor-driven rollers

Servos aren't the only innovation in motor technology. Motor driven rollers (MDR) are making inroads in conveyor applications lately.

Powered conveyor rollers feature a miniaturized motor and gearbox inserted into a roller tube.

“Two years ago, only a few vendors at ProMat were showing powered roller conveyors,” says Ken Bobick, global product manager for roller drive product at Interroll (910-202-2304, www.interroll.com). “This past January, nearly every conveyor vendor was demonstrating a powered roller.”

First adopted by the Postal Service, a powered conveyor roller features a miniaturized 24-volt DC motor and gearbox inserted into a roller tube. In a typical configuration, one powered roller can drive nine rollers.

Powered rollers are primarily used in applications that require accumulation or zones. “If you're just conveying product, outboard motors are more economical to run,” says Rich Kosick, senior vice president of Itoh Denki USA (888-310-8811, www.itohdenki.com). “But if you're doing accumulation, or if you want to maintain a separation between products, MDR is very efficient.”

There are a number of advantages to motor driven rollers. One is that they are inherently quieter than a traditional line shaft conveyor system.

Energy savings is another. Not only does the conveyor only run when it's being used, motorized rollers are more efficient than traditional motors.

In addition, the control systems provide more sophisticated control. “We have units in the field capable of operating at 300 feet per minute,” says Kosik. “But if you have a light day, you can easily change the control on the fly to operate at 100 feet per minute so you don't wear out your rollers.”

Finally, powered rollers are virtually maintenance free. “Since the motor is only running when it has to run, you can extend the life of these motors by years,” says Pete Shumway, marketing manager of Holjeron, (503-682-1324, www.holjeron.com). “If one dies, you simply pop the roller out and replace it with another without shutting the conveyor down.”

Motors and motor-driven roller suppliers

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• I left there because of the Office Manager BJ Bailey. She is a bear to work with and a horrible person!!!!!!! Blaming you for things she had done in the office! Yells at Richard Kiosk! She screams, \"RICHARD!\" from across the office. Catering to her friend that works the front desk. Sincerely, Former Employee of Itoh Denki

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  I left there because of the Office Manager BJ Bailey. She is a bear to work with and a horrible person!!!!!!!
  Blaming you for things she had done in the office!
  Yells at Richard Kiosk! She screams, "RICHARD!" from across the office.
  Catering to her friend that works the front desk.
  
  Sincerely,
  Former Employee of Itoh Denki

  
  

  
  Anonymous - 2009-26-6 14:56:00 EDT
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