Pumps & Systems, September 2007
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 complexity. 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. Still 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 this mode. A speed tolerance of 0.1 percent is attainable with encoder feedback, compared to around 2 percent to 5 percent without encoder feedback.
A reliable encoder signal minimizes speed variance and helps eliminate these unwanted effects. A rotary encoder is an optical or magnetic device that translates rotary motion into an electrical signal.
In the optical version, a light source passes through a slotted or etched disc to activate a sensor on the other side, resulting in an electronic pulse.
For the magnetic variety, a sensor reads north-to-south-pole changes along the circumference of a magnetized wheel, again resulting in an electronic pulse.
Most VFDs operate with an encoder input of 1024-PPRs (pulses per revolution) and provide superior motor performance. 1024 PPR is a good balance between encoder reliability and smooth speed control, and is ample resolution to control the motor smoothly, even at low speeds.
Making the Right Choice
Choosing an encoder for speed control applications typically involves several criteria: resolution (PPR), mounting method, electrical specification, and environmental sealing. Once resolution is determined (usually dictated by VFD requirements), mounting is typically the next selection criteria.
For motor control, common sense suggests the selection of a motor-mount encoder. A variety of motor-mount encoders exist, and the most popular are hollow-shaft units. The hollow-shaft encoder rides directly on the motor shaft (or stub-shaft), secured by an anti-rotation arm known as a tether. This eliminates the need for brackets and couplings that are prone to failure, and simplifies installation and service.
When selecting this type of encoder, electrical isolation from the shaft is an important feature. Electrical current that can build up on the rotor wants to find its way to ground, and without an isolated encoder bore it can pass through the encoder's bearings - not a desirable situation with optical encoders. This current can ultimately damage the bearings and cause premature failure. In addition, an isolated bore helps prevent unwanted heat conducted from the shaft to the encoder.
Electrical requirements, such as voltage and current specifications, and output signal type are the next major pieces of the puzzle. The type of cable, cable length, and drive requirements all play a role, but typically differential line driver outputs are the preferred choice because of their ability to support a wide voltage range and provide a clean, reliable signal over long cable runs and reject noise.
Electrical noise can be a major source of problems, especially when using VFDs and high horsepower motors in close proximity to the encoder. Differential line drivers and properly shielded twisted-pair cabling help to minimize these headaches.
Environmental issues are also a very important consideration when selecting the right encoder. Because industrial pumping systems are rarely used in a clean environment,
