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Benefits
of the Written-Pole® Single Phase Motor |
The following summary
provides an overview of benefits of Written-Pole ®
single-phase motors. Based on Written-Pole® technology,
a patented concept developed by Precise Power Corporation, these
innovative single-phase motors offer many advantages over
conventional single or three-phase electric motors.
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* Low Starting Current Requirements |
* Low Temperature Rise Upon Starting |
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* Energy Efficient Operation |
* High Inertia Starting Capability |
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* Unity Power Factory Option |
* Instantaneous Restart Capability |
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* Simple, Robust Construction |
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Low Starting Current Requirements |
The most significant
characteristic of single-phase motors is their low inherent
starting current. Written-Pole® single-phase motors
exhibit starting current requirements ranging from 2 times or less
full rated current, unlike conventional induction motors (single
or three-phase) that have typical starting currents ranging from 6
to 9 times their rated current. A typical Written-Pole®
single-phase motor has a starting current, comparable to a
conventional single-phase motor one-fourth its rating,
dramatically increasing the maximum rating that may be started on
rural single-phase distribution systems.
The low starting current of
Written-Pole® single-phase motors is a by-product of
their unique design that combines the starting characteristics of
a permanent magnet hysteresis motor (the surface of the rotor is
covered with a continuous layer of ferrite magnet material) with
those of a high slip, high power factor, NEMA Design D induction
motor (the rotor contains a high resistance induction cage that
uses steel rotor bars and end-rings to increase the electrical
resistance of the rotor under starting). Upon reaching 75 to 80%
of its rated speed, an excitation coil in the stator winding is
energized (using AC power at line frequency), creating a strong
alternating magnetic field that tailors the magnetic pole
configuration of the ferrite layers for optimum starting torque
and acceleration. The excitation coil is de-energized when rated
speed is attained allowing the motor to operate as an energy
efficient, synchronous permanent magnet motor.
Since a Written-Pole®
single-phase motor's low starting current is attained without
reduced voltage starting, the design does not sacrifice starting
torque as is common with three-phase motors started using reduced
voltage devices designed to limit starting current. Not only does
this simplify the design and construction of the motor, but it
also enhances the usability of Written-Pole®
single-phase motors under a variety of applications. Back
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Energy Efficient Operation |
The
synchronous mode of operation under which Written-Pole®
single-phase motors operate allows for very high operating
efficiencies, comparable to those of premium efficiency three-phase
motors with similar ratings. The resulting reductions in transformer
loading and operating costs can represent a significant advantage
over comparable installations using three-[phase motors and phase
converters.
The high operating efficiency
of a single-phase motor is made possible by the fact that its
synchronous mode of operation eliminated losses commonly
associated with the slip in conventional induction motors. By
eliminating the resulting rotor losses, Written-Pole®
single-phase motors are able to substantially reduced total
losses, improving operating efficiency. Back
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Unity Power Factory Option |
The Synchronous mode
of operation also results a .99 to unity power factor operation
at rated load (slightly leading at part load) further reducing
transformer and distribution line loading. The following table
illustrates the comparable operating efficiency and power factor
of a Written-Pole® single-phase motor compared to a
conventional single or three-phase induction motors. Back
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Written-Pole®
Single-Phase |
Conventional
Three-Phase |
| Rating (hp) |
30 |
30 |
| Voltage |
230 |
208 |
| Speed (rpm) |
1800 |
1765 |
| Full Load Amps |
104 |
70 |
| Starting Amps |
197 |
445 |
| Efficiency (rated load) |
93.6% |
93.6% |
| Power factor (rated
load) |
Unity |
0.85 |
| Input Power (rated load) |
23.9 kW |
23.9 kW |
| Input kVA (rated load) |
23.9 kVA |
28.1 kVA |
| Input kVA (starting) |
45.3 kVA |
160.3 kVA |
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Simple, Robust Construction |
The
basic construction of a Written-Pole® single-phase motor
is similar to that of a conventional single or three-phase induction
motor. The motor frame is constructed using ductile iron or aluminum
castings for maximum durability. stator laminations and windings are
constructed using high grade electrical steel laminations and
inverter-grade copper wire and a slot insulation materials for
long-term reliability and surge resistance (long single-phase
distribution lines are a prime target for lightning strikes). The
basic layout of the stator windings is similar to a conventional
capacitor start/run single-phase induction motor with the addition
of a concentrated excitation winding occupying four slots in the
stator.
The rotor construction is also
similar to that of a conventional cage induction motor with the
addition of a continuous ferrite magnet layer on the surface of
the rotor. The center portion of the rotor is manufactured using
high-grade electrical steel lamination and a high resistance
induction cage constructed of carbon steel rotor bars and
end-rings. The ferrite layer is constructed using specially
developed magnets, which enable the excitation coil to alter the
magnetic pole configuration as needed during operation and is
bonded to the rotor using a patented system developed by Precise
Power Corporation of Bradenton, Florida. Back
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Low Temperature Rise Under Starting Conditions |
Since Written-Pole® motors are generally much more
efficient under starting than conventional induction motors,
current requirements and corresponding electrical losses are
reduced dramatically. Since electrical losses in the stator
windings of an electric motor are proportional to the square of
current, a typical Written-Pole ® motor generates 9
times less heat under starting than a conventional induction
motor. The resulting reduction in heat provides for improved
reliability, increased starting duration and a higher frequency of
start cycles. Back
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High Inertia Starting Capability |
Since Written-Pole® motors are capable of sustaining
starting conditions for much greater duration than conventional
motors without overheating, they are able to start very high
inertia loads for a given rating. This capability is further
enhanced by the ability to synchronize under load allowing
synchronous operation to be attained even though the motor is
under load (a by-product of the ability to tailor the rotor's
magnetic pole configuration for optimal starting characteristics).
Back
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Instantaneous Restart Capability |
The low starting current and flexible starting characteristics of
Written-Pole® single-phase motors allow them to
undertake an instantaneous re-start after a momentary power
interruption without inducing the high transient torque pulsations
common to conventional single or three-phase induction motors when
re-closed out of phase with the utility supply. This capability
allows Written-Pole® single-phase motors to be
configured for instantaneous restarts without fear of damaging the
motor or driven equipment. Back
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