Pumping With
Encoder Feedback
A Reliability-Centered Decision
Allen Chasey, Dynapar
Most VFD systems use some form of closed-loop feedback to avoid
surging, excessive noise, and in extreme cases, vibration. A reliable
encoder signal is one method that minimizes speed variance and
helps eliminate these unwanted effects. Here’s how it works.
Whether pumping water, slurry, or viscous fluids
through a pipeline, one thing is certain: there’s a
motor on the other end of the pump doing the
work. The motor runs wide open and the pump does its job.
But what happens when variable flow is desired? Throw a valve
on and you’re in business.
Although simple, this methodology of variable flow
pumping has some considerable drawbacks. The first is the
pump cavitation that results when dead-heading into a valve
and the heat buildup created inside the pump volute is considerable. So considerable, in fact, that in slurry applications,
pumps have been known to burst when the water inside vaporizes, pressurizing and ultimately bursting the pump.
As any maintenance specialist knows, bringing a motor
and pump offline due to failure is no small task. Downtime
and cost can be significant, sometimes taking a full 24 hours
to repair with the required labor for new piping, seals, and
motor couplings. A plant can spend between 60 percent to
80 percent of the purchase price of a new pump to repair an
existing one.
A second – and equally important – drawback is in energy
costs. Running an AC motor at full speed requires high current
draw, regardless of the pump flow required. Because energy
now comes at a premium, it seems archaic to be wasting energy
on a process that does not require that power 100 percent of
the time.
So why has this design become so prolific? Lower initial cost is the primary motivator. Fewer components usually
mean fewer dollars spent. Another reason is lack of complex-
ity. A basic system doesn’t require a lot of automation controls
behind it.
The question now becomes “is there a better reliability
solution?”
The Encoder’s Role
An emerging trend pairs AC motors with a variable frequency
drive (VFD) to modulate pump output. Although the VFD
technology has existed for many years, recent VFDs have
reduced complexity and initial cost. While more expensive
than a non-VFD system, the advantages of lower energy and
maintenance costs can mean a quick return on investment.
Depending upon application and duty cycle, a 20 percent
to 50 percent increase in efficiency over non-VFD applications
is possible, resulting in significant annual cost savings through
lower pump wear and energy savings. Maintenance costs can
be hard to define, but eliminating just one day of downtime
due to a damaged pump can be very beneficial to maintenance
budgets and continuous operation of a facility.
In most speed-controlled systems, some form of closed-loop feedback is required to make the system run properly.
Tight speed control is critical to avoid surging, excessive noise,
and in extreme cases, unwanted vibration.
This is sometimes accomplished with pressure transducers telling the drive when to speed up the motor. Although
there are some new drives out there that operate “sensorless,”
the vast majority of installed systems utilize encoder or tach
feedback to control speed. It is possible to run systems open
loop (or without feedback), but speed tolerance suffers in
