22 |
Oilfield Technology
August
2015
that are important for a normal line‑start motor that really do
not apply to a VFD specific motor. It is, essentially, a design
carryover.
For example, line‑start motors would have been designed
to operate very efficiently at the rated nameplate voltage,
frequency and load, but performance away from that
ideal operating point would likely result in indeterminate
performance, because performance away from that operating
point was not considered at design time. Additionally, line‑start
motors, as the name implies, are started from the main power
source ‘across the line’. The starting currents experienced as
an induction motor input goes from 0 V, 0 Hz to 480 V 60 Hz
are typically 6 ‑ 8 times full load current; this excessive current
results in excessive heat production from starting and significant
stresses on many parts of the machine; a motor started from
a VFD will not experience such harsh transients.
Ideally, a new motor design intended only for use on a VFD
must take into account the intended use of the motor over the
entire operating speed, frequency, voltage, and power range.
A vigilant motor designer will take the following things into
account at every step of the design process:
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The motor will only have the features required to perform
the job allowing the designer to eliminate any ‘typical’
features that will not ever be needed over the entire life of
the machine.
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Analysis of an electric machine used to require copious hand
calculations and estimates based on experience – today,
those calculations are carried out in seconds by computers
and finite‑element techniques have eliminated much of the
uncertainty and guess‑work from the design of induction
motors.
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By leveraging modern computing resources, numerous
competing designs can be evaluated quickly at numerous
operating points so that the several competing designs can
be fairly evaluated and optimised over the entire operating
envelope rather than at a single operating point.
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On a VFD, the power delivered to the motor is strictly
managed over the entire operating envelope. Starting
currents do not exist because the VFD applies power in a
controlled manner, smoothly accelerating the machine from
a dead stop to full load; advances in VFDs have even enabled
starting an induction motor at full torque.
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Even though an induction motor is typically >93% efficient,
it will generate significant amounts of waste heat when
loaded at nearly every operating speed. Since it is feasible
to operate a motor at a low speed but with high heat
production, shaft mounted fans are not practical and
continuous forced air cooling is required. One of the
most thermally sensitive parts of a motor is the electrical
insulation system; the better a designer can remove heat
from the heat producing parts of a machine, the more
powerful that machine can be.
Since the limits of an induction motor are largely
determined by temperature, thermodynamic principles must
be at the forefront during the preliminary design stages.
In any thermodynamic system and all other things being
equal, a temperature increase is a result of too much heat
production or inadequate heat removal. By leveraging an
intimate understanding of the underlying physics involved, the
motor designer can attack one or both of these antagonists
to temperature control by ensuring that cooling capacity is
delivered to the portions of the machine that need it most, and
implementing design changes in areas of the machine that are
close to temperature limits by improving cooling efficacy or
decreasing heat production.
In such cases, computer analysis tools can be used to
flush out these weaknesses early in the design to minimise
compromises later in the design. Often, simply specifying better
quality materials or more of the ‘working’ materials will be
sufficient to address a weakness and results in a comparatively
marginal impact on product cost.
The Ward Leonard WL12BB serves as an example of
these principles by including more, higher quality materials
in construction and employing cooling strategies such as
intra‑slot cooling. Combined, the result is a mass‑power dense,
volume‑power dense, VFD specific motor targeted at the
demanding top‑drive and rotary table rig applications.
Notes
1.
Equivalent SI units could be kW/kg and kW/m
3
.
2.
CCFL – cold cathode fluorescent lamp; LED – light emitting diode.
Figure 3.
WL13BB080 ‘long’ high power density drillingmotor delivers
up to 33%more torque and horsepower within the same standard
600 hp frame size.
Figure 2.
WL16BC080 ‘short’ high power density drillingmotor delivers
800 hp ina standard 600 hp frame and comes in vertical and horizontal
configurations.
