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Digital Motor Holdings, LLC © 2017 1

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%.


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.


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


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.


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


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


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.


System Tests


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.


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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RFE VOLTAGE AND CURRENT

VOLTAGE CURRENT (2A/ DIV)

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1TEST MOTOR BEMF


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.


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%.


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


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.

transforming a common induction motor into a low cost ... · converting existing applications in...

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