Engineering of Water Systems

December 28, 2018 Thad Plumley Pumps and Water Systems, The Water Works 0

Part 15(a)—Understanding Pump Drivers, Part 1

 By Ed Butts, PE

 We completed our discussion on the various types of pumping equipment used for deep and shallow well and booster applications in the last installment of The Water Works (October 2018). The next two columns will outline the various kinds of drivers used to power these pumps including electric motors, engines, generators, and gear drives.

Figure 1. Electric motor rotating field on three-phase power.

As the two columns are intended to address the various types of drivers used for pumping applications, the design methods for cable sizes, voltage drop, and short circuit/overload protection will not be included but detailed in future editions of The Water Works.

An electric or engine-driven well pump assembly must generally include three components to operate:

  1. The driver: This can consist of an electric motor or internal combustion engine to directly drive a pump or a generator set that can be used to develop the electrical power to drive an electric motor.
  2. The power transmission mean: This is used to transmit power from the driver to the driven equipment. For a vertical turbine pump, a lineshaft is employed to drive the pump from a vertical electric motor or right-angle gear drive. A driveline is often used to transmit the power from the horizontal axis of an engine to the vertical axis of the well pump, generally through a belt or gear drive. A submersible pump motor uses insulated copper or aluminum cable to transfer electric current from the wellhead to the motor.
  3. The well pump: This generally consists of a vertical turbine or submersible pump.

Electric Motors

Figure 2. 3600 RPM (two-pole) submersible motor.

By far, electric motors comprise the majority of powered drivers on pumps. By comparison, the second most common method of pump driver, an internal combustion engine, occupies a smaller percentage of the total. Electric motors for most water works and water well applications are available in both common AC and DC voltages, ranging from 115 up to 4160 volts in single- and three-phase power supplies and in horsepower from ½ HP to more than 2000 HP.

As opposed to conventional internal combustion engines, electric motors are also versatile in operational orientation with capability in both horizontal and vertical configurations and in working environments with abilities to operate in open, dirty, explosive, or wet exposures or even submerged under hundreds of feet of water. As such, electric motors are the principal type of pump driver outlined in this column.

Conventional Electric Motor Theory

An AC squirrel-cage motor has two basic electrical components: a stator and a rotor. The term “squirrel cage” is used because a rotating motor often has the same appearance as that of a squirrel running around a closed cage.

The stator is the stationary electrical component; it consists of a group of individual electro-magnets aligned in such a manner they form a hollow cylinder, with one pole of each magnet facing toward the center of the group. The term “stator” is derived from the word stationary; thus, the stator is the stationary or fixed part of the motor.

The rotor is the rotating electrical component; it also consists of a group of electro-magnets aligned around a cylinder with the poles facing toward the stator poles. The rotor is located within the stator and is mounted to the motor’s shaft, which is connected to the pump shaft. The term “rotor” is derived from the word rotating; thus, the rotor is the rotating part of the motor.

The objective of these motor components in concert is to cause the rotor to spin, which in turn rotates the motor and pump shaft. This rotation will occur because of the magnetic phenomenon: unlike (+/–) magnetic poles attract; like magnetic poles (+/+ or –/–) repel.

If we progressively change the polarity of the stator poles in such a way their combined magnetic field rotates around the circumference of the cylinder, the rotor will follow and rotate with the magnetic field of the stator.

Although there are various types of electric motors in use today—split phase, shaded pole, and synchronous motors—the most common type of motors used in water well and water works is the induction motor. There are basically two types of induction motors: the single-phase induction motor and three-phase induction motor. A single-phase induction motor is not a self-starting motor since it requires an external or separate means of starting and acceleration. A three-phase induction motor, however, is inherently a self-starting motor.

In a three-phase system are three single-phase lines, each with a 120° phase difference. Therefore, the rotating magnetic field has the same degree of phase difference, which will cause the rotor to turn.

Figure 3. Vertical hollowshaft (VHS) motor.

If we consider three alternating current (AC) phases (A, B, C as shown in Figure 1) at time 1, the current flow (amperage) in the phase A poles is positive and pole A-1 is N (North). The current flow in the phase C poles is negative, making C-2 an N-pole and C-1 is S (South). There is no current flow in Phase B, so these poles are not magnetized.

At time 2, the phases have now shifted 60°, making poles C-2 and B-1 both N and C-1 and B-2 both S. Thus, as the phases shift their current flow, the resultant N and S poles move clockwise around the stator, producing a rotating magnetic field. The rotor acts like a bar magnet, being pulled along by the rotating magnetic field.

Even though a three-phase motor will slowly rotate with only two of the three phases, it will not ramp up to full speed. This is due to the loss of the third phase, requiring the motor to skip over this lost phase while starting. This is a condition known as single phasing, which must be protected from to avoid possible motor and electrical system damage.

As far as single-phase is concerned, we know that a 1-phase AC power supply consists of a sinusoidal wave that produces a pulsating magnetic field in the uniformly distributed stator winding. Since we can assume the pulsating magnetic field as two opposing rotating magnetic fields, the motor is stalled and there will be no resultant torque produced during the starting of the motor. Therefore, the motor will not start or run. After energizing the motor, if the rotor is made to rotate in either direction by an adequate external force, then the motor will start to run.

This problem can be alleviated by converting the stator winding into two separate windings; one is the main or run winding and the other is the auxiliary or start winding by connecting a capacitor in series with the run winding.

The capacitor will make a phase difference when current flows through both coils. When there is a phase difference, the rotor will generate the needed starting torque and the motor will start to rotate. We can practically see the motor does not rotate when the capacitor becomes disconnected from the motor. However, if the rotor is rotated by hand, it will start rotating. That is why we use a capacitor with a single-phase induction motor.

For a general three-phase induction motor, when a supply of voltage is applied to the stator winding, a magnetic flux is produced in the stator due to the flow of a start current in the winding. The rotor windings are so arranged that each winding becomes short-circuited. The flux from the stator cuts a line of force (induction) across the short-circuited coil in the rotor.

As the rotor coils are short-circuited, in accordance with Faraday’s Law of Electromagnetic Induction, a current will start flowing through the winding of the rotor. When the current through the rotor winding flows, another magnetic flux is generated in the rotor. Now, there are two fluxes generated within the motor; one is a stator flux and the other a rotor flux. The rotor flux will slightly lag in respect to the stator flux as if trying to catch up.

Because of that, the rotor will experience a resultant torque or turning force. This will cause the rotor to rotate in the direction of the rotating magnetic field.

The rotational speed of the rotor, in revolutions per minute (RPM), varies according to the power frequency and the number of poles in the motor. At a 60 hertz power supply, a two-pole motor has a synchronous speed of 3600 RPM, but due to internal losses the actual speed will be around 4% less or 3450 RPM. A four-pole motor has a synchronous speed of 1800 RPM (1725 RPM actual) and a 6-pole motor has a synchronous speed of 1200 RPM (1150 RPM actual).

The difference in these two speeds is called the motor’s slip. The space between the rotor magnets (bars) and stator magnets (bars) is called the air gap. The air gap is a universal, albeit small, annular space to permit the magnetic lines of force to transmit back and forth between the rotor and stator. The air gap is maintained on both ends of the rotor by an inboard bearing and outboard bearing. This is the working principle of an induction motor for either type, a single-phase and three-phase motor.

Submersible Electric Motors

Submersible motors (Figure 2) used to drive deep well pumps are cylindrical in design and construction, built to fit within conventional well casing diameters and bolt to standard (NEMA class) well pumps, which are also built in a cylindrical shape and primarily using stainless steel components for the outer shell and shaft.

As with conventional electric motors, they are available from ½ HP to more than 1000 HP. Motor diameters range from 4 inches to 16 inches and voltages from 115 volts single-phase to 4160 volts three-phase or more at 50 or 60 hertz.

Because they are diameter-limited by application and owing to the need to fit inside of 6-inch and smaller well sizes for a comparably sized multistage pump to deliver high head performance, most submersible pump motors are designed to be longer than comparable HP conventional motors and for two-pole (3600 RPM) operation. However, many larger diameter (more than 8 inches) motors can operate at four-pole (1800 RPM) or six-pole (1200 RPM) speeds, which allows for a direct comparison to selection and performance for most vertical turbine pump bowls. Submersible well pump motors are generally built to comply with a UL Class F (155°C, 311°F) rated insulation.

Due to their operating environment under hundreds of feet of water in many cases, submersible motors must be built to withstand and prevent a potential partial or complete (dead) shorting of the motor windings to ground.

This is generally facilitated by embedding the motor windings inside an epoxy-poured or canned encasement. The epoxy effectively isolates the windings and electrical components from the impacts the surrounding water would have with electricity, while allowing the water to travel throughout the motor for cooling and lubrication purposes and across the air gap that exists between the stator and rotor in which the necessary magnetic forces are transferred to propel the motor.

With this type of design, water is permitted to enter the motor through a filtered check valve located on the motor’s outer shell. The external head generated in the well is counterbalanced internally in the motor by using a pressure-compensating spring and diaphragm.

Thrust, developed by the pump but transferred to the motor in most cases, is resisted through use of a Kingsbury-type thrust bearing, usually situated in the bottom end bell of the motor. The thrust developed by the pump during operation is transferred to this fluid-film bearing, which rides on a various number of pads to absorb the load.

Depending on the motor diameter and thrust rating, fluid-film thrust bearings contain a number of sector-shaped pads, arranged in a circle around the motor shaft which are free to pivot and realign. These create wedge-shaped regions of a thin lubricating film inside the bearing between the pads and a rotating disk, which support the applied thrust and eliminate metal-on-metal contact.

The information shown in Table 1 reflects typical submersible pump motor data for most of the manufacturers listed. Minimum velocities, shown as recommended velocity in feet per second (FPS), are intended to provide an adequate flow past each motor diameter in order to maintain a motor shell temperature within the allowable range.

A recommended flow velocity for the group of 10 to 16 inches nominal diameter submersible pump motors of 0.80 FPS is higher than the minimum of 0.50 FPS listed by some motor manufacturers. However, from my experience, the higher value is needed to provide the needed safety factor to adequately remove the expelled heat from the higher horsepower motors and to provide a reasonable span value for the average flow rate of variable flow installations using variable frequency drives or control valves, common to many larger pump installations.

In addition to water-lubricated submersible motors, certain manufacturers use an oil-filled design with an internal self-contained force-feed oil circulation system that maintains continuous lubrication and provides excellent insulation and corrosion resistance. This type of motor construction is often reserved for larger motors in deep-water or oil well applications.

Oil-lubricated and cooled submersible motors should be used with caution on potable water applications with the designer verifying the motor and oil is NSF-approved for potable water service before using.

Although oil-cooled and lubricated submersible motors are more tolerant to overheating, generally the oils used in these motors can only reliably function up to 90°C before the oil begins to carbonize and degenerate (the oil turns black and develops a pungent burnt smell when the motor is opened).

Due to the expansive nature of petroleum products, oil-cooled motors have to be designed to allow the oil to expand as it heats up from the operating temperature. Typically, the motor’s internal oil will expand by at least 10% and the pressure-compensating bellows have to expand to accommodate this as the motor heats up and contract when the motor cools down.

Water does not expand to the same degree when it heats up, so it is easier to design the pressure-compensating means to allow for this inevitable cycling of expansion and contraction.

Internal water escapes from the motor and external water will inter-exchange and enter the motor eventually. However, this should not be a problem for the water-cooled motor unless sand or foreign matter manages to enter the motor or block off the filter/check valve, as this scenario will likely lead to increased bearing wear and eventual failure.

Canned and other water-filled motors tend to be more reliable than oil-filled motors because of the complexity and equipment involved with ensuring the oil can safely expand and contract without escaping from the motor. As a general rule of thumb, most standard submersible motors, except for specific high temperature models, are designed to operate up to maximum service factor horsepower in water up to 86°F (30°C), along with adequate cooling flow and velocity past the motor.

Motor Selection Data

Tables 1 and 2 are intended to aid the system designer with the available types, sizes, and nomenclature of the various electric motors used for driving submersible and vertical turbine well pumps, respectively.

Typical thrust ratings are provided for estimating the capacity of a motor against a selected pump’s downthrust, and the motor frames and sizes are provided to verify the motor’s fit onto a vertical turbine pump’s discharge head or submersible pump end.

The specific technical data associated with the above general data can generally be provided by the pump and motor supplier or manufacturer during the evaluation and selection process.

Figure 3 lists common features of a vertical hollow-shaft (VHS) pump electric motor for U.S. standard voltage, alternating current (AC), 60 hertz electric power supplies.

Note that corrections for speed, voltage, and horsepower (kilowatts) are required for 50 hertz or international power supplies or applications.

Nameplate Data

The nameplate data for a specific type of motor varies with the motor’s application, type, construction features, manufacturer’s guidelines, and NEMA Standard MG 1-10.40 (National Electrical Manufacturers Association for U.S. motors) or IEC-IP (International Electrotechnical Commission for international motors) guidelines and standards. Typically, nameplate data includes all or some of the following information.

  1. Voltage: voltage the motor is designed to operate on; it is usually 115V, 208/230V, 460V, or 575V in the United States.
  1. Frequency: the frequency, in cycles per second or hertz of the supply voltage; typically 50 hertz for international power supplies or 60 hertz in the United States.
  1. Phase: number of AC power lines or phases of the power supply; indicated in single (1ϕ) or three (3ϕ) phase.
  1. Current: current draw of the motor, generally shown as the full load current (FLC) or amperage (FLA) in amps. The listed FLC is used to select the motor starter size, motor conductors, and overload protection.
  1. Manufacturer data: the manufacturer’s name, motor type, model, and serial number.
  1. Power factor: the motor’s power factor, usually shown as a decimal value at full load (0.80 = 80%). It is sometimes indicated or shown on a nameplate as P.F. or the Cos ȹ angle (cosine).
  1. kW or horsepower: the rated output power of the motor in kW or HP (1 HP = 0.746 kW, 1 kW = 1.34 HP).
  1. Full-load speed: the rated or synchronous speed: 3600/1800 RPM rated = 3450/1760 RPM at full load RPM.
  1. Efficiency: motor efficiency at full load (FL). Efficiency is generally indicated in a decimal form: 0.90 = 90%.
  1. Duty: this parameter defines the length of time in which the motor can deliver its nameplate rating safely. In many cases, the motor can provide it continuously, which is indicated by “continuous duty” on the nameplate.
  1. Insulation class: an expression of the standard classification (A, B, F, H) of the thermal tolerance of the motor winding and is a single letter designation such as “B” or “F”. This depends on the winding’s ability to withstand and survive a given operating temperature over a predicted lifespan. The farther down the alphabet, the better the insulation’s performance. For instance, a Class F insulation has a longer nominal life at a given operating temperature than a class B rating. See Table 3 for insulation class definitions related to the insulation temperature rating below.
  1. Temperature rise: the maximum temperature rise is the allowable increase (in degrees) in which the motor is permitted to rise or elevate above the ambient temperature during operation. For example, a motor’s temperature rise rating of 60°C (140°F) over an ambient temperature rating of 40°C (104°F) means the motor can safely operate at a surface temperature of 40°C + 60°C = 100°C or 104°F + 140°F = 244°F, higher than the boiling temperature of water. This is why so many 60°C rated motors run so hot.
  1. Maximum ambient temperature: the maximum ambient (environmental) temperature at which a motor is designed to safely operate. The maximum rating is generally 40°C (104°F) or 60°C (140°F) for motors. The motor can run and still be within the tolerance of the insulation class at the maximum-rated ambient plus rise temperature.
  1. Altitude: this indicates the maximum height above sea level at which the motor will remain within its design temperature rise while complying with all other nameplate data. When the altitude is not indicated on the nameplate, the maximum elevation above sea level is 1000 meters (3300 feet).
  1. Enclosure: this classifies a motor as to its degree of protection from its environment and its method of cooling. Typically, most AC motors are equipped with an ODP (open drip-proof), WP-1 (weather-proof), TEFC (totally enclosed, fan cooled), or EXP (explosion proof) rating. NEMA enclosure ratings are similar to IEC ratings but begin with an IP designation, as a NEMA-open drip proof (ODP) motor corresponds to an IP22, a NEMA-totally enclosed motor corresponds to an IP54, and a NEMA-weatherproof (WP-1) motor to an IP45.
  1. Frame: this determines the mounting dimensions such as the foot-hole mounting pattern and the shaft height. Most U.S. motors follow NEMA standards for submersible motors and 200-300-400-500-6000 frames for vertical motors.
  1. Bearings: this information allows advanced ordering and/or stocking of replacement bearings. The information is usually given for both the driven-end (inboard) bearing and the bearing opposite the driven-end (outboard).
  1. NEMA, code letter: the code letter defines the locked rotor current in a kVA per horsepower (KVA/HP) basis. The letter code consists of letters from A to V. The farther away from the letter code A, the higher the inrush current will be per horsepower. See Table 4 for code letter values.
  1. the different categories. Most motors are either design A or B motors. Design A motors possess normal starting torques with high starting inrush current (most premium efficient motors are design A). Design B motors (the most common) possess normal starting torque with low starting inrush current. Design C motors possess high starting torque with low starting inrush current. Design D motors possess high starting torque with low starting inrush current, but with a resultant greater slip (loss of speed from nominal). When replacing a motor for a specific application, it is important to verify the design, as some manufacturers assign their products with letters not considered industry standard. The incorrect design may lead to starting problems.
  1. NEMA service factor: a motor designed to operate at current no greater than its nameplate power rating is said to have a service factor or overload ability of 1. This means the motor can operate at 100% of its rated power. Most three-phase motors are provided with a 1.15 (115%) service factor (S.F. or SFA).

Voltage Balance and Motor Derating Protocol

When the line voltages applied to a poly (three)-phase induction motor are not equal, unbalanced currents in the stator windings will result. This small percentage of voltage unbalance will result in a much larger percentage of current unbalance.

Consequently, the temperature rise of the motor operating at a particular load and related percentage of voltage unbalance will be greater than for the same motor operating under the identical conditions with balanced voltages.

Maintaining the proper balance of voltage between phases on a three-phase system is crucial to ensure proper performance and optimum life of the motor. Most motor manufacturers and designers recognize the potential impact unbalanced currents can have on a motor’s performance and life, but many do not readily understand that voltage unbalance plays the primary role in unbalanced current.

In addition to the motor, three-phase distribution systems often serve other diversified single-phase loads. An imbalance caused from system impedance, harmonics, or load distribution across the three phases can contribute to imbalance across all three of the phases.

Potential faults may occur in the branch circuit cable to the motor, terminations at the starter or motor, and potentially within the windings themselves. This imbalance can lead to stresses in each of the phase circuits in a three-phase power system.

In other situations, the use of an open-delta or two-transformer systems for a three-phase power supply can also cause severe voltage unbalance between the phases, resulting in current unbalance. This can present a problem with any three-phase motor, especially submersible types.

In a system with unbalanced power the current unbalance between phases results in a negative sequence voltage within the motor windings. This negative voltage has the effect of causing a counter-voltage in the motor that resists the normal current and can lead to a current unbalance of more than 10%.

At the simplest level, all three phases of voltage should always have the same magnitude of voltage. However, there are cases in which this is just not possible, particularly where true or full three-phase power (three primaries) is not available, so an open-delta power supply is the only viable option. In these circumstances, derating a load to counteract the negative sequence voltage will often reduce the voltage unbalance to an acceptable level.

The values in Table 5 can be used as a guide for motor load derating:

As an example, apply a submersible motor to a 29.5 BHP, 460 VAC, 3ϕ, load with the best combination of voltages:

Phase 1 to Phase 2: 457 volts

Phase 1 to Phase 3: 461 volts

Phase 2 to Phase 3: 483 volts

Voltage Unbalance (%): 1.

Find average voltage = 457V (1-2) + 461V (1-3) + 483V (2-3) = 1402/3 = 467.3V

Voltage Unbalance (%): 2.

Subtract the greatest variation of readings from the average: 483V – 467.3V = 15.7V

Voltage Unbalance (%): 3.

Divide the difference by the average voltage: 15.7V/467.3V = 0.0336 × 100 = 3.36%

Since this exceeds a 2% voltage unbalance, refer to Table 5. By using interpolation, a 3.36% voltage unbalance will necessitate a motor derate to ~0.86 (86%) of full load.

Required motor HP: 29.5 BHP (pump HP)/0.86 = 34.30 HP—Use 40 HP motor

The same procedure can be used to determine the current unbalance. Using a different example:

After rolling motor leads three times:

1) Ph. 1: 65 amps, Ph. 2: 73 amps, Ph. 3: 66 amps-Avg. = 68 amps

2) Ph. 1: 64 amps, Ph. 2: 75 amps, Ph. 3: 65 amps-Avg. = 68 amps

3) Ph. 1: 62 amps, Ph. 2: 76 amps, Ph. 3: 59 amps-Avg.= 65.6 amps

Current unbalance for combination 1): 73A – 68A = 5A/68A = 0.0735 × 100 = 7.35%

Current unbalance for combination 2): 75A – 68A = 7A/68A = 0.1029 × 100 = 10.29%

Current unbalance for combination 3): 76A – 65.6A = 10.4A/65.6A = 0.1585 ×100 = 15.85%

Use combination 1 as 7.35% is the lowest value of current unbalance and less than the maximum value of 10%, although it is still considerably higher than the recommended limit of 5%.

In this case, note the highest current value remains on the same leg (Phase 2) each time the motor legs are rotated. This tends to indicate the power supply may be the culprit, requiring motor derate (as shown above) or working with the utility to improve the power quality and supply to the site.

If the high current followed the leg or moved with the same leg as they were rotated, this would tend to indicate the problem is within the motor or drop cable (possible leakage). Typically, a voltage unbalance not exceeding 2% or current unbalance not exceeding 5% is recommended for most motors.

_____________________

This wraps up Part 1 of this two-part series on a basic understanding and applying the many types of drivers used for pumping plant applications with an emphasis on electric motors. The next installment of The Water Works will continue this discussion with an expanded overview on the various engines, gear drives, and generators used for this purpose.

Until then, keep them pumping!

Ed Butts, PE, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.