Bigger Isn’t Always Better
BY MICHAEL T. MYERS
For a lower cost, highly efficient solution for low speed, high horsepower, vertical pumping appli- cations, think smaller. Using planetary gearing and
standard, low-pole-count motors offer a very good alternative
to traditional high-pole-count motors. This article will examine
design criteria and advantages of vertical gearmotor technology
as well as applications that could benefit from this technology.
In today’s business environment, everyone is striving to
reduce costs and improve energy efficiency. High-flow, low-head vertical pumping applications such as cooling water,
wastewater, irrigation, flood control and desalination have
traditionally relied on low-speed pumping systems that use
high-pole-count synchronous or induction motors. By choosing newer vertical gearmotor technology, facilities can reduce
capital costs, cut maintenance, improve efficiencies, and increase system reliability.
High pole count motors designed to achieve the torque
required for low-speed applications require considerable material content, making them inherently very large and costly. A vertical gearmotor utilizes planetary gearing to increase
the torque and slow the speed of a more standard low-pole-count motor. The result is a much smaller pump drive package
weighing 30 to 60 percent less than the high-pole solution.
Not only is the integral vertical gearmotor package a compact
solution but it also eliminates alignment issues between the
motor and the gearing component. Along with cutting capital
expenditures for the pump driver, the smaller, lighter vertical
gearmotor package can result in sizable cost savings on foundation, support structures, enclosures, and other civil works
associated with pumping projects.
In addition, high-pole-count synchronous and induction
motors typically have longer lead times and higher maintenance costs than low-pole motors. Due to the size and complexity of these motors, the choices of repair facilities for large
high-pole-count motors is also limited.
The practice of using high-pole-count induction motors re-
sults in a drastic reduction in efficiency and power factor. Using
synchronous motors will provide high efficiencies and power
factors but are a significantly more expensive option than in-
duction motors in both capital and maintenance costs.
When single reduction planetary gearing is used to increase
the torque of a low-pole-count, high-speed motor it does so
while maintaining a very high efficiency. Typically, losses are
less than 1 percent. The power-dense design uses a sun gear
and three to four planet gears to divide the power into multiple contact points within a small envelope. All this contributes to a smaller, lighter, less costly package while maintaining
gear efficiencies of approximately 99 percent.
MORE CONTROL FOR OPTIMIZED PERFORMANCE
Impeller speed is an important factor in determining the operating point of the pump system. Pump performance and reliability is directly related to operation at the Best Efficiency Point (BEP)
for the pump. Centrifugal pumps should be selected to operate
at or near the manufacturer’s design-rated conditions of head
and flow, referred to as BEP. There is only one BEP for optimum
operation of a pump. Any pump operated at excess capacity will
surge and vibrate, creating bearing and shaft seal problems as
well as requiring excessive power. When operation is at reduced
capacity the radial thrust on the rotor will increase, causing higher shaft stresses, increased shaft deflection, bearing problems,
seal problems, vibration and axial shaft movement. Continued
operation in this mode will result in accelerated deterioration and
may result in pump failure. Therefore, it is important to select the
proper output speed to optimize pump system design.
When using a motor alone, the base operating speed is fixed
by the discrete number of poles in the motor and the voltage
frequency. However, with a vertical gearmotor, the output speed
is a function of the base motor speed and the gearing ratio. A
gearing ratio can be selected to fix the pump impeller speed to
the desired RPM at the low-pole-count motor’s base speed. This
improves system efficiency, pump performance, and reliability.
SOFT START FOR IMPROVED PERFORMANCE
In many high-flow pump applications, the pump system
A vertical gearmotor utilizes planetary gearing to increase the torque
and slow the speed of a more standard low-pole-count motor. The result
is a much smaller pump drive package
weighing 30 to 60 percent less than the
high pole solution.