Posted tagged ‘variation’

INDUCTIVE CHARGING

August 23, 2011

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In the future all electronic devices will be wirelessly powered. Small, battery-powered gadgets make powerful computing portable.

The battery charger should be capable of charging the most common battery types found in portable  devices today.  In addition, the charging  should be  controlled from the base station and a bidirectional communication system between  the pickups  and base  station  should be developed.


Inductive Power Systems:

Inductive Power Transfer (IPT)  refers to the concept of transferring electrical power between two isolated circuits across an air gap.  While based on the work and concepts developed by pioneers such as  Faraday and Ampere, it  is  only recently that IPT has been developed into working systems.

Essentially, an IPT system can be divided into two parts;

  • Primary and
  • Secondary.

The primary side of the system is made up of a resonant power supply and a coil. This power supply produces a high frequency sinusoidal current in the coil.  The secondary side (or ‘pickup’) has a smaller coil, and a converter to produce a DC voltage.

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Working of Inductive Power Transfer:

In this system communications signals are encoded onto the waveform that provides power to the air gap. Communication from the primary side to the secondary is implemented by switching the power signal at the output of the resonant converter between its normal level  and a lower level which is detectable by the pickup but still provides enough power to control the pickup microcontroller. This process is called Amplitude Shift Keying (ASK). This is achieved by varying the output voltage of the buck converter which provides an input DC voltage to the resonant converter.

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Communication from the secondary to the primary is achieved by a process called Load Shift Keying (LSK).  This involves varying the loading on the pickup.   Any load on the pickup will reflect a voltage on the primary circuit proportional to the load.  Therefore a variation in the load on the pickup can be detected by the charging station.

The communications system must provide two discrete levels of voltage reflected onto the primary side,  to represent the on and off states for digital communications. The difference must be easily detected on the primary side to provide a robust communications channel. Signals are decoded by simple filters and comparators which feed a  digital signal to the microcontrollers.

Advantages:

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IPT has a number of advantages over other power transfer methods  – it is unaffected by dirt, dust, water, or chemicals.  In situations such as coal mining IPT prevents sparks and other hazards.  As the coupling is magnetic, there is no risk of electrocution even when used in high power systems.  This makes IPT very suitable for  transport  systems where vehicles follow a fixed track,  such as  in factory materials handling.

MAGNETIC BEARING TECHNOLOGY

August 22, 2011

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Magnetic bearings have been utilized by a variety of industries for over a decade with benefits that include non-contact rotor support, no lubrication and no friction.

Conventional mechanical bearings, the kind that physically interface with the shaft and require some form of lubrication, can be replaced by a technology that suspends a rotor in a magnetic field, which eliminates friction losses.

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There are two types of magnetic bearing technologies in use today – passive and active.  Passive bearings are similar to mechanical bearings in that no active control is necessary for operation. In active systems, non-contact position sensors continually monitor shaft position and feed this information to a control system.  This in turn, based on the response commanded by the system, flows to the actuator via current amplifiers.  These currents are converted to magnetic forces by the actuator and act on the rotor to adjust position and provide damping.

Additional benefits of magnetic bearings include:

  • No friction
  • No lubrication
  • No oil contamination
  • Low energy consumption
  • Capacity to operate within a wide temperature range
  • No need for pumps, seals, filters, piping, coolers or tanks
  • Environmentally friendly workplace
  • Impressive cost savings

In practice, these attractions are balanced in order to maintain a gap between the shaft (rotor) and static parts (stator). The function of the magnetic bearing is to locate the shaft’s rotation axis in the center, reacting to any load variation (external disturbance forces),


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Floating rotors could boost compressor efficiencies

Traditional centrifugal compressors are based on low-speed drives, mechanical gears and oil-film bearings, resulting in high running costs because of their high losses, wear, and need for maintenance.

This new compressor drive (above) uses a permanent magnet motor, operating at an efficiency of around 97%, to drive a rotor “floating” on magnetic bearings, which spins the compressor impeller at speeds of around 60,000 rpm. These drives experience almost no friction or wear, and need little maintenance. They also minimize the risk of oil contamination, and result in compressors that are about half the size of traditional designs.


How they work

 

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Magnetic bearings are basically a system of bearings which provide non-contact operation, virtually eliminating friction from rotating mechanical systems. Magnetic bearing systems have several components. The mechanical components consist of the electromagnets, position sensors and the rotor. The electronics consist of a set of power amplifiers that supply current to electromagnets. A controller works with the position sensors which provide feedback to control the position of the rotor within the gap.

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The position sensor registers a change in position of the shaft (rotor). This change in position is communicated back to the processor where the signal is processed and the controller decides what the necessary response should be, then initiates a response to the amplifier. This response should then increase the magnetic force in the corresponding electromagnet in order to bring the shaft back to center. In a typical system, the radial clearance can range from 0.5 to 1 mm.

This process repeats itself over and over again. For most applications, the sample rate is 10,000 times per second, or 10 kHz. The sample rate is high because the loop is inherently unstable. As the rotor gets closer to the magnet, the force increases. The system needs to continuously adjust the magnetic strength coming from the electromagnets in order to hold the rotor in the desired position.