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Transforming a Common Induction Motor into a Low Cost, High Efficiency Variable Speed System with the Resonant Field Exciter™
Gary Box CTO Digital Motor Holdings LLC
Abstract: Variable speed drives can improve system efficiency by as much as 50% and have been available
for almost 30 years. However, by one industry leader's estimate, variable speed has penetrated
only 10% of possible applications. Clearly the cost and complexity of switching to variable speed
drives has been a barrier, until now.
Join us as we walk through the transformation of an ordinary commercial induction motor into a
Resonant Field Exciter™ fed Wound Field Motor. See how applying the patented Resonant Field
Exciter™ technology (US Pat. No. 9,525,376) provides reliable variable speed, high efficiency (up
to 95%), at a lower cost than systems with magnets or inverters. This disruptive technology
utilizes the basic components and manufacturing processes common to motor manufacturing
worldwide.
Background & motivation
The potential for saving energy through the variable speed operation of electric motors is well
known. Variable speed operation takes advantage of the nonlinear affinity curve present in
pumping, air handling and compressing for energy savings of up to 50%.
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On a global scale, the cumulative energy savings of switching to variable speed electric motors
would be significant.
“Fitting energy-efficient electric motors on all pumps and fans with devices to regulate their
speed would save 3,338 terawatt hours (3.3 million gigawatt hours), roughly equivalent to the
amount of electrical energy produced in the European Union in 2013.” - Energy efficiency – the
fast track to a sustainable energy future, Ulrich Spiesshofer, president and chief executive of ABB
Corporation. ABB Ltd Corporate Communications at COP21, Paris, 2015
The technology to accomplish these savings has been available for 30 years, yet market
penetration to date is only around 10%. Converting existing applications in the field is expensive,
costing as much as 3x more than replacing a conventional single speed motor. Incorporating
custom variable speed at the OEM level often involves embracing new technology and launching
long and expensive development cycles.
The most common approach is to incorporate conventional inverter based control of either an
induction motor or a permanent magnet motor.
VFD/IM block diagram
At first glance this appears relatively simple. However, as always, the devil is in the details. The
practical inverter based variable speed system is much more complicated.
Conventional inverter based variable speed relies on chopping all the power to the motor stator
at audible or super audible frequencies which introduces a variety of unwanted side effects
which then must be mitigated.
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For the motor, the new presence of high frequency electric and magnetic fields requires changes
in magnetic materials, insulation, manufacturing and testing resulting in a more expensive
“inverter rated motor”.
For the electronics, high frequency chopping of the motor power imposes strict design
requirements to control and contain voltage, current and magnetic fields and transients. Design,
component selection, printed circuit board layouts and manufacturing are all critical. In addition,
switching 1 to 15 kilowatts of energy at up to 20,000 times a second generates additional heat
loss, which must be managed to prevent component overheating.
The Wound Field Motor and the Resonant Field Exciter (RFE™)
All motors operate on the interaction between the stator magnetic field and the rotor magnetic
field. In the case of the permanent magnet motor, the rotor field is fixed. In the case of the
induction motor, the rotor field is induced by transformer action of the stator on the rotor. In
both cases, speed and torque control must be achieved through the power to the stator.
Another, older motor, the wound field motor, is the only motor configuration that allows
independent control of both stator and rotor magnetic fields. The universal motor, the DC motor
and the grid powered wound field synchronous motor are examples of wound field motors. Only
in the wound field motor can speed and torque control be achieved through control of the rotor
magnetic field only.
Traditionally, the rotor field of the wound field motor has been powered through sliprings and
brushes. More recently, rotary transformers, either operating at grid frequency or as nonlinear
power converter transformers have been used to power wound field motor rotors. These
techniques successfully provided power to the rotor, but neither provided the linear, wide
bandwidth, high dynamic range of control required for motor control through the rotor only.
Control of speed and torque still required the use of conventional inverters on the motor stator.
The patented Resonant Field Exciter provides this linear, wide bandwidth, high dynamic range
control through the rotor of the wound field motor only. No inverter or modulation of the stator
power is required, eliminating the negative side effects of switching losses and poor power
quality by simply avoiding them.
RFE™/WFM Block diagram
The Resonant Field Exciter™ fed Wound Field Motor (RFE™/WFM) achieves high efficiency
variable speed without the negative side effects of processing all the power to the motor. The
motor itself is a conventional wound field motor. The stator, frame and shaft can be identical to
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an induction motor. The stator winding would be scaled to the motor power level, but the
winding process is identical.
Only the rotor is different from an induction motor. However, the wound field rotor is like that
of a universal motor and easily fabricated with conventional techniques. The world already
makes all the parts of the 1 to 10 HP wound field motor. Full power motor torque and speed can
be controlled by controlling the much lower power required by the rotor.
The Resonant Field Exciter™ (RFE™) is the key element that makes the wound field motor
practical at under 10 HP. The RFE™ provides power to the rotor field through a rotating
transformer operating at resonance where the frequency and amplitude are adjustable
independently, much like an AM radio.
The frequency is controlled to always be in resonance, regardless of mechanical gap width,
maximizing power transfer. Independently, the amplitude is then adjusted to control the rotor
field current maintaining a motor back EMF close to the stator supply voltage, eliminating the
need to process the power to the stator. Rotor power is around 3% of the mechanical shaft
power.
Typically, the rotor power in a conventional wound field motor is provided through sliprings and
brushes. To keep brush current low, the rotor has many turns and high inductance, limiting the
response time of the rotor field.
Prior art does contain many technologies for providing rotor power without brushes, but these
are all focused on providing constant rotor power, not on wide bandwidth rotor control over a
wide dynamic range. With the Resonant Field Exciter™, the rotor power passes linearly through
a transformer, accommodating a wide range of rotor impedance and providing wide bandwidth,
wide dynamic range control of the rotor magnetic field.
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Mechanically, the RFE™ consists of two assemblies; a stationary assembly and a rotating
assembly with an infinite variety of configurations possible, one of which is shown below.
The motor
stator, rotor, shaft and housing are all conventional. Only the Resonant Field Exciter™
assemblies are unique.
The RFE/WFM Test System
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At Digital Motor Holdings (DMH) we have constructed a test system to characterize the
operation of the Resonant Field Exciter™ fed Wound Field Motor. To illustrate the feasibility of
building the system from conventional components we fabricated the motor as a 36 slot stator
and a 4 pole rotor. As mentioned, many other configurations are possible.
Both were then hand wound using conventional techniques
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For the test system, the rotary transformer of the Resonant Field Exciter was fabricated from a
ferrite pot core. Other shapes are possible.
Rotating RFE™ Assembly Stationary RFE™ Assembly
The Resonant Field Exciter™ was then attached to the motor. In practice the RFE would be inside
the motor housing.
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System Tests
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The tests system uses a Texas Instruments TMS320F22027 microcontroller to manage both the
RFE™ and motor commutation. For simplicity, hall sensors detecting a small magnetic collar
between the motor and the RFE™ are used to detect rotor position. The system is run with
trapezoidal commutation.
Results
The microcontroller detects the zero crossings of the RFE™ current and adjusts the RFE™ driving
frequency to maintain resonance. Thus, switching losses are minimal and losses are dominated
by the Rdson of the MOSFET switches. Resonant frequency of the test system RFE is about 120
KHz.
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Once the rotor field is energized. The motor resembles a permanent magnet brushless motor.
Motor Ke and Kt can be measured by simply spinning the motor shaft and measuring the
unconnected phase to phase voltage.
Note there is a small ripple from the stator teeth.
Rotor power, measured at the RFE™ power supply, was 16V 4 A or 64 watts. Back driven speed is 531 RPM and peak voltage is 12.6 V for a Ke of 23.73 V/KRPM. Kt is .167 A/ftlb.
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0.00E+00 2.00E-04 4.00E-04 6.00E-04 8.00E-04 1.00E-03 1.20E-03
RFE VOLTAGE AND CURRENT
VOLTAGE CURRENT (2A/ DIV)
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1TEST MOTOR BEMF
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Just like a permanent magnet BLDC motor, Ke and Kt are independent of speed or stator field
strength. If we use shaft position to commutate the stator windings and apply a fixed voltage to
the commutator, the motor will reach equilibrium when the back EMF is high enough to allow
only the current necessary to generate the torque to balance the load. Unlike the permanent
magnet BLDC motor, Ke/ Kt can be changed at will by changing rotor current. Thus speed or
torque of the RFE/ WFM can be adjusted without changing the voltage to the stator.
The resulting voltage and current in each motor phase have little high frequency energy,
reducing the EMI footprint of the system.
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Another effect of the RFE™/ WFM system is that motor current and power supply current are
the same. Since we know the Kt of the motor, we can calculate the motor torque. For our test
system running at a Kt of .167 ftlb/A, and a stator current of 5.5A, torque is .919 ftlb.
Conclusions and Road Map
At 3600 RPM the efficiency of the test wound field motor and commutator is 96%. The efficiency
of the complete system, grid to shaft, including the RFE is 85%, which is equal to a premium
efficiency induction motor operating directly off the grid and stacks up favorably to the other
variable speed technologies, without magnets or the complexity and power quality problems of
inverters.
GRID TO SHAFT EFFICIENCY SINGLE SPEED PREMIUM EFF INDUCTION MOTOR (GRID POWERED) 84
INVERTER FED INDUCTION MOTOR 78
INVERTER FED FERRITE BLDC MOTOR 81
RFE FED WOUND FIELD MOTOR (as tested) 85
Our roadmap is to improve the efficiency of our test system by incorporating synchronous
rectification in both the rotor and the input grid rectifier. For a constant rotor field and constant
speed, system efficiency gets better with increasing load. Synchronous rectification increases
grid to shaft efficiency up to 95%.
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At the system level the plan is to implement Software Defined Power Factor Correction to take
advantage of the dynamic range and impedance matching properties of the RFE™ to drive low
inductance rotor windings in sync with the grid. This will provide software based power factor
correction without any hardware changes.
TEST POINT ROADMAP PROJECTED EFFICIENCY
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At Digital Motor Holdings LLC we are developing the designs, tools, software, and supply chain
to build application specific prototypes for our OEM partners.
DMH is committed to bringing the Resonant Field Exciter™ fed Wound Field Motor technology
to market to accelerate the penetration of variable speed systems.