Part 18(c)—Electrical Systems and Control, Part 3
By Ed Butts, PE, CPI
As a continuation of this series on electrical theory and application in The Water Works, this month’s edition will outline the various methods of motor starting and control for three- and single-phase motors.
Let’s dive right in.
Step 1: Determine Required Voltage and Phase (1a) and/or System Load (1b)
This first step may seem confusing. However, whether 1a or 1b is the first actual procedure taken depends on several factors. Among these is whether the installation is a retrofit or replacement installation with the same or more horsepower on an existing power supply, or a new installation involving a new electrical service.
When applying a replacement or new submersible motor on an existing service, then Step 1a becomes the most logical first step. If a new electrical service or generator operation is required, then Step 1b is generally the first step to allow sizing of a new service by the utility or a generator, if used.
Step 1a: Voltage and Phase Selection
Since our example situation which was highlighted in the January 2021 installment of The Water Works already details the system voltage, phase, and available current—much of the work needed for a new installation is already complete. The information on the electrical power supply is critical for the proper design and application of the various electrical components, particularly the submersible motor, drop cable, and controls used in the system.
Although having an advance notice of the technical data regarding the electrical service and installation is important knowledge for all alternating current (AC) motors, there is probably no other type of motor where this is more important than with a submersible motor.
Since most submersible motors are designed for a single voltage and phase application, the most obvious initial design element needed to know is the available voltage and phase.
For our design example, the electrical service consists of a 200-ampere, 480-volt, AC, three-phase service with 160 amps of available full load current capacity. In theory, this level of power can operate up to a single 150 HP motor or a group of three 40 HP motors (156 amps).
For an installation using a submersible motor, not much can proceed until the designer is satisfied he fully recognizes and understands the voltage and phase that either the serving utility or a generator will provide to the site. Although it may seem contrary to logic for the voltage and phase to be the first criteria, in many parts of the country—particularly remote rural areas—the availability of higher voltages or polyphase power may be limited or not possible at all. Therefore, verifying these design factors with the utility or generator supplier is generally the first step I take.
Although the United States power systems are designed for 60 hertz (frequency) operation, throughout the rest of the world there appears to be an endless array of possible voltage and frequency combinations. In 50 hertz regions, such as Canada and many international countries, virtually all power systems are of the four-wire, grounded wye type, so a typical arrangement would be the use of a 220/380-volt AC power system.
In this case, as in the case of a typical 120/208-volt, wye, 60 hertz system in the United States, the lower 220-volt source is only available for single phase and the 380-volt source is available as single- or three-phase power.
As a result of the voltage described as 220/380, specifications often require that three-phase motors be wound for a dual voltage of either 220 or 380V. Although feasible, it is usually unnecessary to do because the 50 Hz three-phase motors can only be operated on 380-volt, three-phase power. Since many submersible motors are designed to operate on either 50 or 60 hertz power, these motors are rated for 380- or 460-volt operation from either source.
Many submersible motor manufacturers also supply motors capable of operating on much higher voltages than the lower voltage systems below 600 volts. The application of these motors is their use with medium voltage sources between 1,000 to 69,000 volts, with the most common voltage for
three-phase motors either 2400 or 4160 volts.
For those charged with the responsibility of voltage selection or consulting, it is generally the best and most economical decision to select a reasonable compromise between the system voltage, phase, and the current based on the connected load (horsepower) and total cost. In other words, since current (amperage) often has the higher impact on component and system cost, selecting the higher voltage applicable with three-phase to lower the corresponding current is often the best overall choice.
This must be tempered, though, with the knowledge that a higher voltage or three-phase system may not be technically or economically available within a given region. You sometimes have no choice but to take what they will give you.
You may also have noticed the nominal voltage value for many three-phase systems is generally higher than the applicable operating (nameplate) voltage for a submersible motor, up to 5% for a 240- or 480-volt system operating a 230- or 460-volt motor.
Although certainly not universal, where this discrepancy is available, it provides a built-in added safety factor by providing a beginning voltage higher than what is required by the motor. I do not recommend as a standard design course to use these few percentage points of difference as anything but a compensatory fudge factor of voltage that will provide adequate starting and running voltage to the motor under all normal and abnormal conditions.
If the maximum 5% allowance for the drop cable loss is observed on a 480-volt system, then the remaining voltage at the motor terminals should equal around 456 volts, only 4 volts lower than the nameplate 460 voltage, or 2 volts lower than a nameplate 230-volt value. This will help ensure the motor is operating within the allowable range and extend the life of the motor to the maximum possible.
Lastly, the type of power can also be critical to the performance and life of the motor and installation. The use of a 120/208- or 277/480-volt, four-wire wye electrical service over a 240- or 480-volt delta or open delta power supply will almost always be preferred since the wye service tends to be more balanced.
If available, requesting a wye service, even if it comes with a higher cost, will pay long-term dividends to the customer and prevent a potential series of premature motor failures. Even though this type of power is usually preferred, there are methods available to lessen a harmful impact to a submersible motor from a delta power supply. We will discuss these methods in Part 5 of this series later this year.
Step 1b: Determine the System Load: Motor Horsepower and Diameter
Once it has been determined the proper voltage and phase is available for the project, the next step is to move into the determination of the system load—specifically the motor’s required horsepower and diameter. Obviously, since the wet end (pump) has already been selected, the motor must match the mounting and horsepower criteria required from the pump.
In order to determine the motor’s required horsepower, we must first repeat the step we previously used for the well pump:
Primary COS: Brake horsepower (BHP) =
Alternate COS: BHP =
14.77 BHP at 2950 RPM (red. speed, 85.5%)
(1) In this case, we have entered the exact efficiencies from our selected pump at both duty points.
The calculated horsepower (41.5 BHP) at the primary condition of service (COS) could technically be provided from a 40 HP motor by using the 15% service factor (FLC × 1.15) available in most motors. However, to extend the life of the motor to the maximum, plus provide for any possible power or system anomalies, I recommend not using any of the allowable service factor for design of most municipal, commercial, or industrial applications. Therefore, for our example, a maximum input load of 41.5 BHP will require a 50 HP, 460 VAC motor.
Selecting the motor diameter is largely a choice between what is available for the horsepower and the size and mounting configuration of the pump. For example, a 50 HP submersible motor has four choices that would generally be available. Use of either a 6-inch or 8-inch-diameter motor for 3600 RPM (2-pole) applications, or an 8-inch or 10-inch-diameter motor for 1800 RPM (4-pole) applications.
Our selected pump is a 7-inch-diameter × 3600 RPM bowl assembly, and information from the manufacturer has informed us that either a 6-inch or 8-inch-diameter motor would be acceptable for this pump. As a personal preference, I would most likely opt to use a 6-inch-diameter motor for this application. Besides being a much cheaper motor and easier to get from most suppliers, I usually prefer not to use a motor that is larger than the pump I am connecting it to.
The requirement for cooling at the VFD-reduced flow of 156 GPM is met by either motor in the 12-inch well casing, so that factor is not a concern. Using the smaller motor will also provide more annular space between the well casing and motor as well as prevent any implied moment from misalignment or vibrational forces to the smaller pump due to the torque or possible misalignment between the larger motor and smaller pump.
In addition, it is also critical to verify any larger motor considered for attachment to a smaller submersible pump will not apply any excessive weight to either the mounting bracket or bowl assembly of the pump and that the shaft interconnection between the two components is the same.
It is important to note that my decision for our example is based on using a 6-inch-diameter motor with a single stator and rotor and not a tandem unit. If the needed 6-inch-diameter motor had to be constructed by using two 25 HP motors in series (a tandem unit), a common practice years ago, I would have definitely opted for the 8-inch-diameter unit as I have personally observed many problems with tandem motors from electrical inconsistencies and imbalance between the two motors.
In summary, although the horsepower and often the voltage and phase is usually a requirement of the application, a choice can often be made in the pump and motor’s speed and diameter. This decision can only be made after evaluating all salient factors of the design, including the installation parameters, annular well clearance, pump to motor mounting options (NEMA fit or not), motor starting and running characteristics, availability of new and replacement motors, and cost.
Step 2: Determine Motor’s NEC Full Load Amperage
The next step is used to determine the NEC amperage for the selected HP. Typically, by code for a single motor application, the NEC current values from NEC Table 430.248 (Single-Phase Motors) and 430.250 (Three-Phase Motors) are used to design the service, short-circuit, motor starter, and conductor loads and sizes, while the actual motor amperages for submersible and VTP motors are used for sizing the overload protection. The tabular motor amp values in Table 1 are reprinted from NFPA 70 National Electrical Code 2020.
Step 3: Determine Motor Starting Requirements
When considering operating an electric motor, it is not only important how well you can run a motor, but how easy it is to start one. The limiting factor with any single or polyphase electric motor is very often the ability to draw sufficient power from the line over the few seconds needed to generate the torque and magnetizing current required to overcome the inertia and accelerate the pump and motor.
With a submersible motor this is often an unforgiving element since the long lengths that often exist between the power supply and the motor can create enough of a voltage drop to prevent full voltage starting. In addition, the high current inrush, up to six to seven times the full load current, associated with the starting of any motor can also cause enough voltage drop to result in a local condition known as flicker.
Flicker is the momentary drop in voltage caused by a large inrush of current that can result in a brief loss of light intensity from fluorescent and incandescent fixtures. This condition is avoided by either providing adequate power to accommodate the high inrush current with a minimal of voltage drop, or by lowering the starting current through a process called reduced voltage starting.
From our example, we know there is adequate power available to startup to a 50 HP motor without using reduced voltage starting. Therefore, we could have used a direct-on-line (DOL) starting method, which is also known as full voltage or across-the-line starting methods. The motor starts using full or 100% voltage under locked rotor amperage and accelerates up to full speed within a few seconds.
Full voltage starting remains the most common method of motor starting and accounts for more than 95% of the motor-starting applications. Although this would have been a feasible and the lowest cost method of motor starting in this case, we designed for a primary and secondary design condition which required using either a control valve or variable frequency drive (VFD) for flow and pressure control.
Since a VFD was selected to provide variable speed operation as a motor control, this same unit also provides inherent reduced voltage starting and stopping, generally referred to as soft starting and overload protection.
Although the use of a VFD has become popular, there are still alternate ways to accommodate reduced voltage starting for submersible motors. Regardless of the method of motor starting selected, the proper knowledge of the motor’s locked rotor amperage (LRA) is generally required for the utility to determine the allowable inrush current for the service.
As an element of every motor’s nameplate and technical data, information regarding the motor’s kVA Code is available (Table 2). Code letters from A to R are shown with the accompanying kVA/HP range and the typical mid-range value. To determine the approximate locked rotor current requires investigation or confirmation of the specific code letter. This value is usually available from either the motor nameplate or design data.
As a shortcut, the following equations can be used to approximate the locked rotor amperage (LRA) for a three-phase motor:
200-volt LRA = code letter value (use mid-range value) × motor HP × 2.9
230-volt LRA = code letter value (use mid-range value) × motor HP × 2.5
460-volt LRA = code letter value (use mid-range value) × motor HP × 1.25
575/600-volt LRA = code letter value (use mid-range value) × motor HP × 1.00
For our design example with a 50 HP, 460-volt motor, the approximate 460 volt LRA is:
(from manufacturer data) code letter H. Use the mid-range
value = 6.7 × 50 HP × 1.25 = 418.75 amps.
Also please note that the above value is an approximation only and may vary +/- 2% from the actual value. Where available, always research and use the actual LRA value from the motor nameplate or manufacturer.
Typically, for low-voltage applications a combination motor starter includes three components:
- A disconnecting means, short circuit, and overload protection for the circuit referred to as branch circuit protection. This is generally in the form of fuses and a disconnect switch or a switched circuit breaker
- A magnetic contactor to electrically engage and disengage the motor
- An overload assembly installed immediately downstream from the contactor that connects directly to the motor branch circuit conductors.
In some cases, the short circuit and disconnecting means is provided from a separate external switch. In this instance, the remaining components are simply called a motor starter. Motor starters are generally classified and sized according to either NEMA or IEC standards.
NEMA full-voltage motor starter sizes begin at a 00 rating and continue upwards to a NEMA Size 9 rating. IEC starters are not rated or sized in the same manner but are rated according to their specific operating environment (utilization category) and maximum motor current (amperage). Utilization Categories AC3 and AC4 are the two most often applied to submersible motors.
Since IEC starters are designed to operate at a higher temperature than comparable NEMA-rated starters, in my judgement, they are therefore not as robust as NEMA-rated starters, HP for HP. In order to avoid possible problems, when IEC-rated components are used, I recommend using one or two sizes larger in current (amperage) rating for a submersible motor application.
Refer to Table 3 for NEMA sizing of motor starters and IEC protocol. An overload can either be a solid-state (electronic) or a thermal- (current) sensing device and is designed to sense motor current on both load circuits for single-phase motors and all three load circuits for three-phase motors from either two or three separate thermal heaters or an adjustable relay that senses current in all two or three lines.
In either case, proper sizing of the device is critical to ensure the overload cannot be adjusted above the safe current level for the motor HP. All thermal overloads used for submersible motors must be an ambient-compensated, quick-trip type so motor protection is provided throughout any potential change in air temperature.
When used with a submersible motor, the overload must provide no less than Class 10 protection, which means the overload protection will trip and open the motor starter circuit within 10 seconds under locked rotor conditions to protect the motor windings.
Regardless of the type and manufacturer of the starter itself, the appropriate short-circuit protection must also be provided from a properly sized and type of circuit breaker or fuses and overload protection from thermal heaters, an adjustable overload block, or a device inherent to and built into the starter such as a VFD or soft starter. Motor starters for medium-voltage (2300 and 4160) motors are much more application and site driven than most low-voltage applications. Each potential submersible motor application with medium voltage should be evaluated on a case by case basis, with the style of motor starter and protection selected only after consultation with the serving utility and motor manufacturer.
In addition to the typical methods of reduced voltage starting, submersible motors possess another unique characteristic not common to most other motor installations. Since the distance from the incoming power supply to the motor can be thousands of feet, an inherent voltage drop exists in the cable run due to the resistance in the cable. If the total distance of cable provides the maximum 5% allowed value of voltage drop, this can result in a corresponding lowering of the starting current by 15% to 20%.
This degree of voltage drop is sufficient in many cases to provide adequate motor starting without incurring a huge inrush of current. It is important to note the selected motor-starting mechanism is only one component of the entire motor controller. In addition to the starter, the electrical system must also include a method of disconnect and short-circuit protection.
Step 4: Select Switchgear
The next step in this process is the selection of the switchgear, which is also known as the motor controls. National and international standards define the manner and procedure in which electrical circuits of low voltage (less than 600 VAC) installations must be conducted along with the capabilities and limitations of the various switching devices which are collectively referred to as switchgear. The main functions of switchgear are:
- Electrical protection of the circuit from overload and short-circuit and personnel from short-circuit
- Electrical isolation or disconnection of specific elements of an installation
- Local or remote switching or control.
Individual units of switchgear do not in general fulfill all the requirements of the three basic functions (protection, control, and isolation). Therefore, it generally consists of a combination assembly of an overload/short-circuit protection device to fulfill the first requirement joined to a disconnecting means for the second, and combined to a system to provide local or remote switching or control for the third requirement.
Electrical protection of low-voltage systems is normally incorporated using devices known as circuit breakers or fuses. Circuit breakers are available in the form of thermal-magnetic devices, instantaneous trip devices, solid state (electronic current sensing) devices, or residual-current-operated tripping devices.
When circuit breakers are used, the second requirement, disconnecting means, is generally incorporated into this device. Fuses are available in single element, dual element, high or low peak, current limiting, one-time, and time-delay styles. In addition, fuses are available in plug or screw-in styles for low voltage and amperage (to 30 amp) or cartridge fuses for larger amperages and higher voltages.
Unlike circuit breakers, fuses must incorporate a separate means of disconnection from the line power. The disconnecting means for fuses is placed before the fuses to allow a safe removal and replacement process for two reasons:
- Overvoltage protection
- Undervoltage protection, which is provided by specific devices (lightning and various other types of voltagesurge arrester, relays associated with contactors, remotely controlled circuit breakers, and with combined circuit breaker/isolators).
Motor Starting Methods: Full Voltage Starting
With an electrical service, if a motor is planned to start using a full voltage or across-the-line motor starter (Figure 1), the required motor inrush burden and system capacity are typically known and provided.
When a generator is used, though, motor starting, particularly full voltage or across-the-line starting, is probably the most complex interaction that occurs between a generator set and its loads because the results are a function of alternator capability (including both the stator and exciter), voltage regulator capability, engine capability, and governing functions as well as the motor itself and the diverse characteristics of the loads actually driven by the motor.
Thus, much of the following discussion will be oriented towards the use of generators for motor starting. Starting a motor, especially at full voltage, demands varying levels of both kW and kVAR as a motor is starting and accelerating its load to its rated speed. Consequently, a critical evaluation of
generator set ability when starting one or more motor loads demands an evaluation of the ability of the entire system to serve all these time and magnitude-varying needs.
While most motor loads can be considered easy to start, it’s risky to simply assume they won’t push a generator set or electrical service to or beyond its limits, potentially leaving a part of the other loads effectively unserved or causing an overload or shutdown of the genset or service. Complicating the
problem is the fact there is no single standard that can be used to provide a basis for all the necessary validation and design work.
The industry currently depends on a critical evaluation of hardware primarily based on the requirements in NEMA MG1-Part 32 and NFPA 110 for fire protection systems. All electric motors are designed for a Locked Rotor Code (LRC) or Locked Rotor Amps (LRA) condition, often referred to
simply as the Code or code letter.
This is the amount of kVA or amperes at a specific voltage the motor requires to overcome the starting drag and inertial forces needed to accelerate the motor and its load. When starting motors, large voltage and frequency dips may occur if the generator set isn’t sized properly.
Other loads, particularly electronic loads such as uninterruptible power supplies (UPS) and computers connected to the generator output, may be more sensitive to voltage and frequency dips than the motor or motor starter and cause problems. For example, a rate of change greater than 1 Hz/sec in generator frequency may cause some static UPS units to malfunction. If the load on the generator set is a single large motor, particularly one requiring a high degree of starting torque, a vast number of problems can occur.
These include sustained low-voltage operation that can cause overheating, extended load acceleration times, opening of circuit breakers or motor protection devices, activation of engine-generator protection devices, unit shutdowns, and more.
Motor loads cause difficulty because a motor draws high current when started at full voltage. As shown in Figure 1, starting current is typically six times (600%) the motor’s rated full load current and this inrush current stays high until the motor reaches approximately 75% of its rated speed, when it begins to fall rapidly.
When a motor is started on normal and properly designed utility power, the high inrush current will cause only a small and momentary voltage dip because the utility power source has a higher available primary voltage level. However, when a motor is started on genset power, the high inrush current can result in a large voltage dip that can inhibit the motor from reaching its operating speed.
Motors have typical starting characteristics with a high inrush current that causes a momentary dip in the generator voltage. The motor must develop greater torque than required by the load. The torque demand for most submersible and deep well vertical turbine pumps lies below the available torque as also shown on the curve.
The difference between the torque developed by the motor and the torque required by the load determines the rate of acceleration. Since torque is proportional to voltage, any reduction in voltage means a proportional reduction in torque.
The information shown earlier in Table 2 can be used to estimate the number of amps per HP to start a motor per assigned code letter under full voltage (across-the-line) starting conditions.
A properly sized generator set or electrical service will support the high starting kVA requirements of the motor while maintaining adequate output voltage for the motor. Thus, it can develop adequate torque to accelerate the load to rated speed.
All standby gensets use synchronous generators with exciters and many are available with permanent magnet generator (PMG) excitation systems. The PMG provides excitation power independent of the generator terminal voltage. As such, it can maintain full excitation even during transient loading such as motor starting.
Full excitation power results in a less extensive voltage dip and improved recovery times. When a motor is run intermittently, the transient response time must be considered. When an intermittent motor is started, the voltage dip that occurs must still be in an acceptable range that does not adversely affect other loads which are already running.
All loads following the initial intermittent load (like a pump) must include the intermittent load as part of its total as well. By considering this, more kVA may be needed and thus a larger genset may be required.
This concludes this edition of The Water Works. We will continue this topic discussion in July with an overview of reduced voltage motor-starting methods.
Until then, keep them pumping!
Ed Butts, PE, CPI, 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 firstname.lastname@example.org.