Why is it called a “squirrel cage rotor?”: Most fractional horsepower (FHP) AC induction motors utilize a die cast rotor design that consists of a skewed lamination stack with die cast rotor bars and end rings. The common term for this rotor design is “squirrel cage”, because if you remove the steel laminations after the rotor was die cast, you’ll end up with a skeleton that looks much like a hamster wheel. Our guess is that back in the day when this term was coined, more people had squirrels as pets than hamsters. The die cast squirrel cage rotor design is inexpensive and relatively easy to manufacture if you produce them in volume.
In an AC induction motor, the stator winding sets up a magnetic field which reacts with the current-carrying conductors of the rotor to produce rotational torque. The rotor currents are induced in the rotor conductors by the stator’s changing magnetic field, rather than by means of a commutator and brushes (like in a PMDC motor). This induction action is the central operating principle of AC induction motors. The essential operating characteristics of AC induction motors will vary according to: 1) winding types (split-phase, shaded-pole, three-phase, etc.), and 2) the number of phases, the frequency and the voltage of the power source.
The rotor of a typical induction motor is constructed from a series of steel laminations, each punched with slots or holes along its periphery. When laminations are stacked together and riveted, these holes form channels which are filled with a conductive material (usually copper or aluminum) and short-circuited to each other by means of conducting end rings. The conductors are typically formed by die-casting. In open frame AC induction motors, the die cast rotor usually includes integral fan blades which allow for efficient and cost effective motor cooling. The common term for this type of rotor is “squirrel cage” (because of resemblance to the runway of an old-fashioned squirrel cage). It is an inexpensive and common form of AC induction rotor.
As the rotating field sweeps past the bars in the rotor, an induced current is developed. Since the flow of current in a conductor sets up a magnetic field with a corresponding polarity, an attraction will result between the rotating magnetic field of the stator and the induced field in the rotor. Rotation results from the rotor’s attempt to keep up with the rotating magnetic (stator) field. The rate of change at which the lines of flux cut the rotor determines the voltage induced. When the rotor is stationary, this voltage is at its maximum. As rotor speed increases, the current and corresponding torque decreases. At the point of synchronous speed (speed of the rotating field), the induced current and developed torque both equal zero.
The rotor in a non-synchronous AC induction motor will always operate at some speed less than synchronous unless it is aided by some supplementary driving device. This lag of the rotor behind the rotating magnetic field is called “slip”, and is expressed as a percentage of synchronous speed:
In designing rotors for induction motors, the shape and dimensions of the slots have a demonstrable effect on the performance characteristics of the motor. The slot (bar) shape can be seen in the lamination examples in the photo. Another design factor common to most squirrel cage induction rotors is the deliberate “skewing” of the slots (positioning the slots at a slight angle to the shaft) to avoid cogging action and wide variations in starting torque which may result when bars are placed parallel to the stator slots. It is also important to note that single-phase AC motors require an auxiliary starting scheme. See the Bodine Gearmotor Handbook for more information on design principles and various motor designs.
(Below photo shows two different rotor laminations on the left, the finished AC induction motor rotor with bearings pressed on the shaft, and the squirrel cage after the lamination steel was dissolved with nitric acid.)
Copyright Bodine Electric Company © 06/2016. All rights reserved.
Worm gearing is a proven and economical solution for applications that require high speed reductions in limited space, and with very smooth and quiet operation. Worm gears have inherent self-locking ability depending on design and ratio. Lubrication is very important for the life of the worm gear set and efficiency can be improved by using enveloping worm sets.
A worm gear assembly resembles a single threaded screw that turns a modified spur gear with slightly angled and curved teeth. Worm gears can be fitted with either a right-, left-hand, or hollow output (drive) shaft. This right angle gearing type is used when a large speed reduction or a large torque increase is required in a limited amount of space. Figure 1 shows a single thread (or single start) worm and a forty tooth worm gear resulting in a 40:1 ratio. The ratio is equal to the number of gear teeth divided by the number of starts/threads on the worm. A comparable spur gear set with a ratio of 40:1 would require at least two stages of gearing. Worm gears can achieve ratios of more than 300:1.
Worms can be made with multiple threads/starts as shown in Figure 2. The pitch of the thread remains constant while the lead of the thread increases. In these examples, the ratios relate to 40:1, 20:1, and 13.333:1 respectively.
Worm gear sets can be self-locking: the worm can drive the gear, but due to the inherent friction the gear cannot turn (back-drive) the worm. Typically only in ratios above 30:1. This self-locking action is reduced with wear, and should never be used as the primary braking mechanism of the application.
The worm gear is usually bronze and the worm is steel, or hardened steel. The bronze component is designed to wear out before the worm because it is easier to replace.
Proper lubrication is particularly important with a worm gear set. While turning, the worm pushes against the load imposed on the worm gear. This results in sliding friction as compared to spur gearing that creates mostly rolling friction. The best way to minimize friction and metal-to-metal wear between the worm and worm gear is to use a viscous, high temperature compound gear lubricant (ISO 400 to 1000) with additives. While they prolong life and enhance performance, no lubricant additive can indefinitely prevent or overcome sliding wear.
Enveloping Worm Gears
An enveloping worm gear set should be considered for applications that require very accurate positioning, high efficiency, and minimal backlash. In the enveloping worm gear assembly, the contour of the gear teeth, worm threads, or both are modified to increase its surface contact. Enveloping worm gear sets are less common and more expensive to manufacture.
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Copyright Bodine Electric Company © 04/2016. All rights reserved.
Large distribution warehouses operated by wholesalers, retailers, or large manufacturers have turned to Automated Guided Vehicles (AGVs) to keep up with ever-increasing demand for faster and more economical deliveries. A major original equipment manufacturer (OEM) for AGVs contacted Bodine Electric Company to help develop two new custom gearmotors for their latest AGV. Bodine has since built, tested and shipped over 50,000 gearmotors to this customer.
The OEM’s specifications required two gearmotors, one of them had to lift up to 1,000 lbs (with substantial peak loads). The AGV’s chassis had already been finalized, and left only limited space for the new gearmotors. The gearmotors were required to operate almost continuously for five years, under worst-case environmental conditions, and would be subjected to extreme vibration and shock.
Almost every part of these new gearmotors was engineered to match the customer’s extensive list of requirements.
- Low-voltage brushless DC motors paired with all-new, highly efficient gearboxes prolong the AGV battery life and minimize downtime
- Gearbox designed to simplify the assembly process of the AGV
- Custom output shaft assembly designed for extremely heavy loads
- Feedback device tracked the position of the drive shaft
- 1024 PPR encoder provided servo feedback to control the gearmotors
- “Military Style” plug and screw connections
- Temperature sensors monitor gearmotor performance and to prevent overloads
- Manual over-ride in the event of a power failure
With Bodine’s help, these sophisticated robotic vehicles have been performing flawlessly all over the world. Bodine not only developed two entirely new gearmotors for the application, they also helped navigate the difficult third-party approval process. Bodine engineers extensive experience in motion control made them an ideal partner in the development of this new product, which in turn is making warehouse jobs easier, more productive, and cost-efficient.
Bodine Electric engineers bring over 110 years of application engineering and problem solving experience to a wide range of applications in industries as diverse as medical, packaging, industrial automation, and solar powered outdoors equipment. We look forward to working with you on your next FHP gearmotor design challenge.
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Copyright Bodine Electric Company © 03/2016. All rights reserved.
Posted in Application Tips, Company News, Custom Solutions, Engineering Talk | Tags: AGVs, Automated Guided Vehicles, Autonomous Robotic Vehicle, Bodine Electric Company, Brushless DC Gearmotor, Factory Automation, Fractional Horsepower, Gearmotors, Industrial Automation, Low-Voltage BLDC Motor, Self-driving Factory Robots, Self-driving Vehicles, Variable Speed Gearmotors, Warehousing Robotics