Written-Pole Technology

By : Rahul SaxenaShashank Arya(EN-VI Sem)

Introduction

Traditional UPS and power quality solutions such as batteries and diesel generators pose costs and operational limitations. Written-pole technology may be an economical alternative that offers reliable ride-through power as well as protection from transient power disturbances around the clock, protecting customers from costly losses.

With an innovative design that adds unique flexibility to a motor's magnetic characteristics, the technology drastically reduces the need for a high starting current and increases reliability. Thus it is particularly valuable for remote and rural applications, where single-phase power is the norm. Written-Pole technology is also being deployed in motor-generator sets that can serve as uninterruptible power supplies, offering high-tech power customers with sensitive loads an alternative to maintenance-intensive battery systems.

Limitations of conventional technology

1.Dependence of speed on no. of poles of rotor.

speed = (120 * frequency)/(no. of poles)

2.The machine must have even no. of poles and deviating from this condition will cause unstability at its rated speed.

An innovative patented technology

Precise Power Corp. of Bradenton, Florida is the industry leader in patented written-pole™ technology and with the support of EPRI, NRECA and utility partners. In early 1980s Precise Power set out to develop and commercialize a range of single-phase written-pole motors up to 75 hp in size.

Written-Pole Technology

The patented Written-Pole® Technology allows the magnetic pole pattern on the magnet layer to be changed to a desired pattern while the machine is rotating and therefore breaks the relationship between rotor speed and output frequency.

Construction

The design of a written-pole motor is straight forward and elegant. Scientists have known for nearly two centuries that moving a wire relative to a magnetic field will generate current, a phenomenon that remains the basis for all electric motors.

A continuous layer of ferrite magnets on the surface of the rotor, along with a high-density exciter winding contained within the stator winding .

The basic platform consists of a conventional steel shaft inserted into a lamination stack containing a high resistance rotor cage.

The high resistance cage is a key factor in limiting the starting current of Written-Pole motors.

The stator lamination stack is constructed using low loss electrical steel laminations.

The concentrated excitation windings are contained within the stator structure and are located between the main statorWindings.

Operation

In contrast to a standard motor, in which the north and south poles of a permanent magnet are fixed in one position, special coils inside a written-pole motor allow the magnetic poles to be written and erased in different locations 60 times per second.

When the motor starts, the alternating magnetic poles work to pull the rotor forward at a steady rate much as a high current would, increasing the rotor's velocity until the motor reaches its full design speed. Once full speed is attained, the poles remain fixed and the machine operates like a conventional motor.

Starting Characteristics Of Written-Pole motor

The written-pole motor is a synchronous motor that inherently develops starting torque. The rotor magnetics are designed to allow the stator winding starting current to influence the magnet layer and provide hysteresis torque. A squirrel-cage is added under the magnet layer in the rotor for additional starting torque.

The high resistance rotor cage is designed for 20% slip or more and with the inherent hysteresis torque brings the rotor to partial speed before the motor exciter coil winding is energized. The exciter coil is energized with alternating current at line frequency writing a pole pattern in the rotor magnetic surface that is magnetically synchronous with the stator rotating field.

The phase relationship of the written-poles and the rotating magnetic field of the stator causes the rotor to accelerate with maximum torque to synchronous speed. At this point, the exciter can be turned off and the motor will continue to operate as a permanent magnet synchronous motor.

This starting method results in a starting current of 1/3 or less of that required by an equivalent induction motor. This low starting current makes the written-pole motor particularly attractive forsingle-phase applications.

Operational Benefits

Features Conventional Written Pole

Speed 3560 rpm 3600 rpm

Efficiency 85% 93%

Power Factor 90.5% 100%

Full-load Power 53 kw 40 kw

Full-load Torque 73.5 ft-lb 73 ft-lb

Starting Torque 88 ft-lb 82 ft-lb

Full-load Current 219 amp 173 amp

Locked rotor Current 1450 amp 312 amp

Cost (p.u) 1.0 1.2

Advantages

1. Low starting current requirements

2. Energy efficient operation

3. Unity power factor operation

4. Simple construction

5. Low temperature rise

6. High inertia starting

7. Instantaneous restart capability

8. Ride through characteristics

9. Three-phase applications

Weather Conditions

The Written-Pole motor is designed to operate in ambient temperatures ranging from 32ºF to 104ºF.

However, test results show that Written-Pole motors have �reliably operated in temperatures ranging from 42ºF to 122ºF.

They may be operated safely in the open air if there is adequate �protection from direct rain. The motor base must be mounted on a raised concrete pad or steel frame to avoid exposure to standing water.

A Written Pole ride through motor used in a farm

Applications

Regulation of output voltage and frequency in electric generators, without relying on external regulators or power electronics.

In a family of single and three-phase motive power and power �protection systems used by many customers including the US Weather Service, NASA, Federal Aviation Authority, US Air Force and many commercial and industrial customers.

To create a simple, rugged power protection unit combining �Written-Pole motors and generators with an integral flywheel.

Award winning technology

A recipient of the R&D 100 Award in 1994, Written-Pole technology was recognized by R&D Magazine as one of that year‘s most technologically significant innovations.�The technology has also been recognized by numerous technical organizations including the Professional Society ofEngineers in the United States.

Conclusion

The written-pole motor, developed by Precise Power Corporation, is a motor design that allows for the operation of very large single-phase motors.

This unique product has applications virtually anywhere that three-�phase power is either not available, or is not economical to provide.

This motor technology has the additional benefit of being able to �ride through brief service interruptions on single-phase power lines.

Queries…

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