Big Motors, Big Trouble: RO Hard Start
By Matthew Wirth
A tsunami, a rogue wave, a tidal wave, a storm surge brought on by a hurricane—the enormous force created by water is undeniable. Watch any news story related to a natural disaster involving water and the proof is evident in the images of unimaginable destruction. Water can be an unstoppable force; it is dense and described as being only slightly compressible. Anyone who ever executed a belly flop or crashed while being towed behind a boat would question the definition of ‘slightly’. Water has a density of 1,000kg/m3; air has a density of 1.275kg/m3, making water 784 times denser than air. Therefore, falling onto an air mattress is a softer landing than onto a waterbed. For our discussion, air for example, easily compresses and water does not. Face it; there are few forces in nature that can withstand the power created by moving water.
With this in mind, it is important to consider the power of water and the damage hydraulic forces have on pieces and parts installed in a system subjected to moving H2O. Any system controlling the flow of water is subject to hydraulic forces and hydraulic shock conditions: water hammer. When water starts and stops, momentum shifts back and forth. According to Newton’s laws of motion, “Third law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction to that of the first body.” Simply put, for every action, there is an equal and opposite reaction. Moving water has momentum created by its mass and velocity. When the momentum stops abruptly, (i.e., a ball valve is abruptly closed) the inertia or power (kinetic energy) created by the moving water does not just vanish. Kinetic energy, as it relates to water, is the energy which it possesses due to its motion or flow. It is defined as the work needed to accelerate a given mass of water from rest to its stated velocity. Having gained this energy during its acceleration, water maintains this kinetic energy unless its speed changes. The kinetic energy, the work created by the moving water, must go somewhere. If the energy is not dissipated or cannot escape, then the water system receives a hydraulic shock caused by the inertia related to the change in momentum: the stopping and/or restriction of the water flow. Depending on the material, the system’s design and the forces at play, pieces and parts can break. All this hydraulic theory leads into the true purpose of this article: large RO pump motors and how to correctly bring them online. Simply put, knowing that water is a destructive force makes it critical to control how fast one releases water into a system.
Large RO and NF units employ high-pressure pumps to overcome the trans-membrane pressures required to force water through the membranes and into the permeate side of the elements. In addition, the water velocity created by high-pressure pumps keeps cross flow consistent and allows RO and NF elements to remain functional. Large motors drive these high-pressure pump heads. Some small to medium RO systems use single-phase motors utilizing 110 to 220 VAC, while larger industrial systems likely utilize higher voltages and three-phase 240 VAC or 480 VAC service or higher. Note: There are other variations in voltage. The type of power servicing the motor is important to know because it determines how the motor starts turning.
Before discussing motor starting and the effects it can have on water hammer, here is another important bit of information from Sir Isaac Newton:
- An object that is at rest will stay at rest unless an external force acts upon it.
- An object that is in motion will not change its velocity unless an external force acts upon it.
Think of a bicycle on an even, level path. When one starts peddling from a dead stop, it takes more strength to get the bike moving than it does to peddle the bike down the path. Rule #1: Your leg muscles are the external force. Rule #2: If the desire is to go faster, one must peddle harder.
One may ask: “Why does this matter?” When an electric motor is not running, its rotor is at rest—it’s not moving. To make a motor using single-phase power start turning, the starting mechanism uses a capacitor to give the motor a jolt of extra power to begin rotation. This kick-start happens over just milliseconds of the switch being turned to the ON position. Once the motor is turning, the capacitor is out of the electric service loop and line voltage keeps the motor running. Most single-phase motors are five horsepower or less. Single-phase AC motors above 10 HP (7.5 kW) are uncommon. Pumps over one to 1.5 horsepower use contacts to handle the amps required to operate larger single phase motors. Note: The use of a standard pressure switch to start and stop an RO motor can become problematic if the switch constantly receives maximum amp loads. Most standard-pressure switch contacts are rated for 1.5 horsepower motors, maximum. Experienced operators like to see larger contacts on motors over one horsepower to minimize electrical issues.
Large motors greater than five horsepower and even some smaller motors less than five horsepower operate on three-phase power. Again, single-phase AC motors above 10 HP (7.5 kW) are uncommon. There are distinct reasons for operating on three phase. Among them are:
- In three-phase, there is always a magnetic field around the winding to pull the rotor; therefore, it does not use a capacitor to start.
- 120/208 V three-phase service has 1.5 times the power capacity of equally rated 120/240 V single-phase.
- A three-phase motor will be more compact and less costly than a single-phase motor of the same voltage class and rating.
- Three-phase motors will also vibrate less and hence, last longer than single-phase motors of the same power used under the same conditions.
- Most industrial sites have three-phase power available.
Starting three-phase motors
Big electric AC motors are powerful machines. When they are running, they operate effectively under extreme workloads. Because they are so powerful, it is very important that one consider how to best bring them to full power, how best to turn them on. Sir Isaac Newton reminds one to be careful and remember for every action, there are consequences. The key question here is: “Where does the work done by the motor go and does the apparatus being run by the motor use the work immediately or is there a delay?” If the work is not immediately used, it has to go somewhere.
The hard start
When a simple contactor is used to start a three-phase motor, the voltages are applied to the motor simultaneously and in full. Because the motor’s rotor is at rest, the initial current is five to seven times the full-load current. This abrupt application of power is called a hard start and results in the motor going from zero rpm to full operating speed in minimum time. This type of start uses a magnetic starter. A magnetic starter has a contactor and an overload relay, which will open the control voltage to the starter coil if it detects an overload on a motor. When the start button on a magnetic starter is pressed, the motor is not powered directly; rather, the electromagnet in the contactor is energized. The magnetic switch in the contactor then engages,
simultaneously switching current to the motor and providing self-sustaining current to maintain its own state. Thus, when the start button is released, the magnetic switch remains engaged and the motor remains running. There are numerous issues related to hard starting a large motor. One might expect that an electrical surge five to seven times normal could cause problems and it often does. Such a surge may cause fuses to blow and breakers to trip and can produce an excessive voltage drop on power lines. This voltage drop can cause:
- Difficulty in starting the motor
- Lights to dim
- Other motors to stall
- Sensitive equipment, such as computers and PLCs, to malfunction
In addition, hard starts result in mechanical and thermal stresses within the motor, as well as mechanical stresses to the drive train and the RO pump. Particularly if the drive train has any looseness or play, repeated hard starts can result in excessive stress and wear. When hard starting a motor on an industrial or any large RO, the problem becomes magnified by hydraulic shock. The immediate energy created by the motor starting faster than the work is used results in excess energy. This energy hammers the internal parts of the RO system and can do irreversible damage. In Figure A, a membrane element spacer was crushed by a combination of water and membrane element mass added to the forces exerted by the motor hard starting. In Figure B, the anti-telescoping end cap is physically separated from the fiberglass outer shell of the membrane element. Systems using magnetic starters on large RO systems with multiple membrane elements can become problematic, especially if there is any play in the membrane array. If the membrane elements are free to move inside the array, sudden hydraulic forces can turn the membrane elements into hammers, crush internal parts and destroy O-rings. Shimming the membranes on the inlet side of the array helps take out the play between the elements and keeps the elements from shifting in the array at start-up. Hard starting becomes a real issue if the membranes load with biofilm or scale with mineral salts. The pressure differential increase through them causes the system to be sluggish at start-up and compounds the problem of too much energy in the system. The best practice is to start the motor slowly and let the RO system start flowing before all the energy from the motor releases into the system.
Soft starts and VFDs
A motor soft starter is a device used with AC electric motors. It temporarily reduces the load and torque in the power train and electrical current surge of the motor during startup. It works like a dimmer switch on a light fixture. In addition to reducing the mechanical stress on the motor and electrical stresses on the attached power supply, it avoids hydraulic overload to the RO system. Soft starters can be set up to stop an RO pump from creating pressure surges. While a motor soft starter can be mechanical, RO systems are likely to utilize electrical soft starters. They can control the motor speed by temporarily reducing the voltage or current input, or temporarily altering how the motor is connected in the electric circuit. A softer start allows the RO system to absorb the work energy by the pump system in a manner that avoids hydraulic shock to the system and its parts. In addition, it reduces the stress on the motor, drive train and pump. Less stress means fewer problems and less wear on the system, as well as a longer service life for the RO machine.
A variable frequency drive (VFD) is another way to control the speed of an electric motor. It adjusts the electrical frequency, the Hertz (Hz) delivered to the motor. A motor running at 30 Hz runs slower than when the frequency is 60 Hz. It operates like the gas pedal in your car: the harder you push it, the faster it goes. When you let up on the gas pedal, the engine runs slower while retaining the engine’s torque. The most accurate way to control the work of an electric motor is to control its speed. VFD devices control constant pressure systems and multiple applications requiring variable motor speeds. On an RO system motor, a VFD can not only control the start procedure, it can regulate the pump output and pressure. A VFD would sound like the best option for an RO system motor as it will maintain the motor’s torque, where a soft start reduces both the speed and the torque, which can be detrimental. While they have many advantages over hard starters and soft starters, they are also more expensive and require an advanced knowledge to operate.
While the joke is, if you can’t make it work then get a bigger hammer, this is only a joke. Water hammer is no joke when it exists in any water treatment device. Not only does it cause service issues, it is also quite dangerous. Just Google water hammer accidents—there are plenty of examples. If motor hard starts are causing issues with an RO system, then by all means, consider having a soft starter or VFD installed. If water hammer is causing problems with other water treatment devices, consider installing a water hammer arrestor. They come in many sizes and designs but they are all designed to take the unused kinetic energy out of your system.
Special thanks to Cliff Fasnacht, President of Pacific Purification and Dougherty Pump and Drilling (Board of Directors PWQA) Salinas, CA and Gary Mickelson, VP of Jerry and Dons Yager Pump (President CGWA) Petaluma, CA for their assistance and expertise with pump motors.
Figure A & Figure B
- Energy and Minerals Section–P.E.I Department of Economics and Tour- ism. (1997, February). When Does Motor “Soft Starting” Make Sense? Prince Edward Island, Canada printing. Vol. 3, Issue 3.
About the author
Matthew Wirth, Technical Advisor and Trainer for Pargreen Water Technologies (www.pargreen. com), is a second-generation water professional with over three decades in the industry. He received engineering training at the South Dakota School of Mines and Technology, Rapid City, SD and also earned a BA Degree in organizational management and communications from Concordia University, St. Paul, MN. In addition, Wirth holds a Water Conditioning Masters License in the State of Minnesota. A contributing author to WC&P, he is also a member of its Technical Review Committee. Wirth can be contacted via email, firstname.lastname@example.org or phone, (630) 443-7760.