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Oilfield Technology
August
2015
capabilities unmatched by existing technology. API standards
place constraints on how strong certain load bearing and torque
path components must be, which limits the design flexibility
and material choices; therefore, wanting to remove metal from
a structure that is intended to support hundreds of tonnes is a
difficult proposition to consider. However, if other components
of the apparatus could get lighter or take up less space without
sacrificing performance, a distinct and quantifiable advantage
would be achieved.
One strategy that simultaneously reduces weight while
increasing profits would be to employ more powerful AC motors
in top‑drives and rotary tables without making them larger or
heavier – an advancement now made available by Ward Leonard’s
line of high power density AC motors (HPDMs) targeted specifically
at the demands of the oil and gas markets.
Defininghighpowerdensity
In physics, mass density refers to the overall mass (weight) of an
object relative to that objects’ volume. For example, a container
full of water definitely weighs less than the same container full
of sand even though the size (volume) has remained unchanged.
From this one can conclude that sand has a higher density
than water. In the same way, one may define any useful parameter
relative to the mass of an object, and in some cases, it may also
be useful to define some parameter in reference to the size of the
object.
As it relates to electric motors, power density can be defined
as the ratio that describes the total rated power of the machine
relative to its weight. A higher power density would imply that
a motor could deliver more power with comparable weight or
similarly, comparable power, at a reduced weight. A motor with a
higher power density would enable an end user to ultimately reduce
transportation costs compared to heavier motors, or increase
productivity (through higher power output) without increasing
transportation costs. In either case, overall profitability increases.
Another form of power density is defined as the ratio of total
rated power of the motor relative to the volumetric footprint
required. One can imagine a scenario where a 600 hp motor
might occupy 12 ft
3
of space, and the specifications for such a
motor would be suitable for either top‑drive or rotary table duty.
Rotary tables and top‑drives tend to be more space constrained
than other electric motor applications in the oilfield, making
it desirable to keep the size (or volumetric foot print) as small
as possible. If it were possible to achieve that same level of
performance by consuming only 10.8 ft
3
of space, an additional
1.2 ft
3
that was previously unavailable for use now becomes
available. In tightly confined top‑drive assemblies, extra space
makes removal/installation and maintenance of the motor and
other components much easier.
With the above definitions, it is possible to describe
mass‑power density and volume‑power density expressed in any
convenient set of units.
1
For example, it may be convenient to
express mass‑power density in horsepower per tonne (hp/t) and
volumetric‑power density in horsepower per cubic foot (hp/ft
3
).
The Ward Leonard WL12BB060 high power density motor
(HPDM) has identical external dimensions and mounting
requirements as standard 400 hp ‘square style’ top‑drive motors,
yet it is capable of delivering 150% of the torque at the same
rated speed. A standard 400 hp top‑drive motor is approximately
19.6 in. x 19.8 in. x 47.9 in. and operates at 40 Hz, 575 V, 1170 rpm,
and provides around 1795 lbf‑ft of torque. The WL12BB060 has
the same volumetric envelope (19.6 in. x 19.8 in. x 47.9 in.) but
achieves 150% of the torque at the same frequency, voltage,
and speed. Because the volume and weight have not changed
appreciably, the mass‑power density has increased from around
286 hp/t to close to 428 hp/t. The volume‑power density has
likewise increased from 37.2 hp/ft
3
to 55.8 hp/ft
3
.
Anapplicationexample
A popular style of top‑drive incorporates twin 400 hp motors
connected mechanically in parallel to a common drive train via a
Table 1. Comparison between standard 400 hp and high power density
600 hp motor
Specification
‘Standard’ 400 hp motor HPDM 600 hp motor
Dimensions (W x D x H;
inches)
19.6 x 19.8 x 47.9
Weight (lb)
2800
Rated power (hp)
400
600
Torque (lb‑ft)
1795
2685
Current (Arms)
350
470
Speed (rpm)
1172
1163
Voltage (Vrms)
575
Frequency (Hz)
40
Volume‑power density
(hp/ft
3
)
37.2
55.8
Mass‑power density
(hp/t)
286
428
Figure 1.
Punch the drill string harder with theWL12BB060 high power
density drillingmotor that provides up to 600 hp ina standard 400 hp
‘square style’ motor frame.
