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June 2008
Who needs Brushless??
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Printed Armature Motor Technology
Known in the trade by a multitude of names the Printed Armature or Pancake electric motor is still as relevant a technology today as it was at its conception in the late 60’s. Originally designed as a solution to smoothly wind magnetic tape for the budding computer industry, the Pancake technology has found its way into a whole host of new applications with the number of advantages the technology has over conventional electric motor technologies.
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Internal Construction
The Flat Armature design is essentially an ironless disc with coils punched from sheet copper and formed onto non-magnetic disc insulators. The brushes short across two or more points to create a path through the copper thereby creating a magnetic field which interacts with the permanent magnets to create motion. With no magnetic material present in the armature undesirable ‘cogging’ is completely eliminated, providing smooth operation and continuous torque down to zero RPM, with minimal torque ripple. Torque is proportional to current and not limited by saturation, whilst speed is directly proportional to voltage.
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Long Brush Life
The ironless design of the printed armature means that there is virtually zero inductance. The result of this feature is that when the magnetic field collapses energy does not discharge by arcing to the brushes. The effect of this is twofold; firstly the printed armature exhibits low EMC properties, essential for most of today’s applications. Secondly this means the wear rate of the brush is minimal, as it is arcing not friction that is the major contributing factor when calculating brush life. In certain applications PML have achieved over 10,000 hours working life expectancy on a high volume product.
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Perfectly Smooth
In conventional servo motors the iron laminations create preferential positions when the iron lamination and permanent magnets align, this is known as cogging. This causes “torque ripple” where by the torque applied to a load peaks and troughs relative to the position of the rotor. These disturbances can cause serious issues in critical applications. Because the Printed armature motor does not contain any iron it does not have preferred positions and eliminates cogging giving perfectly smooth rotation.
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High Speed, Instant Torque
In conventional permanent magnet DC motors internal losses increase with speed. These losses are associated with the inductance, and therefore arcing, which means that the motor must use some of the torque developed in order to keep itself rotating, thereby reducing the torque available in the application. This also can cause the motor to behave irregularly at high speed. The minimal inductance if the Printed armature motor means that these losses are small and the motor can deliver torque much more effectively at high speeds. Also because the inductance has a negative effect on the initial development of torque the printed armature motor can reach maximum torque in a fraction of the time, compared with conventional motors.
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Slim Profile
A printed armature motor with Neodymium magnets can produce up to 3Nm (without a gearbox) however the clever part is that the motor length, from the mounting face, is only 4.5cm! This means that the internal space of a machine can be optimised and designers don’t have to cope with filling space created by the long conventional permanent magnet motors.
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Extra Torque on Call
For rapid acceleration and deceleration, higher than normal torque is usually required. To produce this temporary peak torque, a peak current is applied to the motor. In an iron-core design, the magnetic field of the armature can interact with and demagnetize the permanent magnets. Because of this effect, peak current is generally limited to 2 or 3 times the continuous current rating. With the non-magnetic Printed armature and axial magnetic field, this problem is virtually eliminated. Most Printed Armature motors are rated for peak current of 10 times the continuous rating.
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Magnets
Printed armature motors are available with standard Ferrite, AlNiCo and NeFeB (Rare Earth) magnet types. Each type offer different performance advantages with regard to speed, torque and temperature stability.
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Design Options
Most printed armature manufacturers are able to offer specials shafts, Dual armatures configurations, integrated Tacho, encoder mounting adaptors or motors complete with encoders, variety of armature configurations and diameters, gearboxes, pulleys and we as environmental and EMC protection.
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Recommended Applications:
- Cooling Fans
- Process Industry
- Automotive
- Robotics/Automation
- Tensile Testing
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- Generators
- Instrumentation
- Film Processing
- Biomedical
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Tel: +44 (0) 1420 594140, Fax: +44 (0) 1420 83930, enq07@pmlflightlink.com
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