Electric Motors

Electric motors are the most common driver for all types and sizes of pumps. They come in an array of styles with specially designed features. There are two types of motors used in the water and wastewater industry namely induction and synchronous motors. The great majority of motors with sizes from

fractional to 3,000 hp are induction motors. In larger sizes, (1,000 and larger horsepower) synchronous motors are also used. These motors have a power factor of approximately unity, and are therefore able to help the power factor of the pumping station. They typically cost of a synchronous motor is more than an induction motor and requires special starters. A life cycle analysis is required to justify the use of synchronous motor over induction motors unless otherwise preferred by the Owner. If variable speed

pumps are required, induction motors are more compatible with VFDs while it is more complicated to match the VFD with synchronous motors. Motors are available in vertical or horizontal position.

There is confusion regarding how prime movers such as engines, turbines and especially electric motors are rated. All prime movers are rated based on their output shaft horsepower at a given speed, also called as brake horsepower (bhp). The rating for prime movers is NOT based on the electrical power input. Pumps are rated based on the required bhp measured at the input shaft of the pump. If the pump is directly driven by the motor, the nominal standard bhp rating of the motor shall be equal to or greater than the required non-overloading bhp of the pump without encroaching into the service factor of the motor.

The pump bhp is determined using the water or hydraulic horsepower divided by the pump efficiency.

The nameplate rating of motor indicates maximum output power, while the input power accounts for motor losses and motor efficiencies. Motor efficiencies, are generally in the 95 to 96 percent efficiency range.

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The speed of the pump and motor directly affect the hydraulics of the pump. The design engineer should be familiar with the motor speed terminology, standard synchronous speed, and the effects of electrical frequency (50 Hz versus 60 Hz) on the overall speed.

The motor speed is sometimes described in terms of poles. Based on the number of poles and power supply frequency, the design engineer can derive the nominal speed of the motor. The following equation can be used to estimate the motor speed based on the number of poles and frequency of the power supply. The number of poles is only available in even increments.

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Based on the above equation, the frequency of the power supply also affects the speed of the motor.

This relationship is used when the design engineer is designing a station to be installed in country with 50 Hz power supply. If the manufacturer only supplies pump curves at 60 Hz speed, the design engineer must evaluate the pump with a 50 Hz power supply. Below is a table derived from the motor speed equation. The speeds identified are nominal speeds; motor slip is not shown for clarity.

Table 4-1: Standard Synchronous Speed at 60 Hz and 50 Hz

# of Poles 60 Hz 50 Hz

Furthermore, when using the motor speed table, maximum running speed of an induction motors are typically 1 to 2 percent below the listed speed due to slippage. For example, the speed of a 4-pole induction motor is approximately 1780 RPM. Synchronous motors, however, their maximum speeds are equal to its synchronous speed because the slip is zero. The speeds are designed to meet the speeds indicated on the above table.

Design Considerations

For some installations, electric motors can be supplied with a number of various monitoring sensors.

Unless otherwise preferred by the Owner, typically, MWH designs the monitoring system based on the horsepower required by the pump. The following is a list of monitoring criteria guidelines relative to the HP of the electric motor:

Less than 50 HP

 No temperature monitoring

 No vibration monitoring 50 to 90 HP

 Provide temperature switches for windings and bearings.

 No vibration monitoring unless required by Client.

100 to 350 HP

 Provide local resistance temperature detectors (RTDs) on windings and bearings.

 No vibration switches or monitoring unless requested by the Client, or other special conditions (vertical motors on wells often get these).

400 to 900 HP

 Provide local resistance temperature detectors (RTDs) on windings and bearings.

 Pre-alarm switch set at vibration amplitude of 150% of the vibration amplitude at normal operating range.

 Shut down switch set at vibration amplitude of 200% of the vibration amplitude at normal operating range.

 Vibration switch shall be provided with a time delay to ignore the transient vibration during startup.

The timer shall be built into the switch or be part of the PLC.

1000 HP and greater

 Provide local and remote temperature detectors (RTDs) on windings and bearings.

 Pre-alarm switch set at vibration amplitude of 150% of the vibration amplitude at normal operating range.

 Shut down switch set at vibration amplitude of 200% of the vibration amplitude at normal operating range.

 Vibration switch shall be provided with a time delay to ignore the transient vibration during start up.

The timer shall be either built into the switch or be part of the PLC.

 If required by the Client, vibration monitoring equipment should be included in the design instead of vibration switches to have the ability provide alarms and to trend data over time.

4.1.1.2. Engines

For some installations, electrical power may not be available or the design criteria may require a backup power in the event of a power failure event. In these situations, diesel engine drivers are a viable option as the prime mover for the pump or to power an onsite generator. As a prime mover, the engine can be coupled to the pump using a gear box, or in the case of vertical turbines, a right angle gear drive. In very few instances, a combination motor and engine gear drive with clutch can be used so that one pump can be driven by either the engine or electric motor. The engine-generator sets may be used to provide

backup electrical power to motor drivers during power outages. Engines can be powered by natural gas, digester gas, LPG, diesel fuel or gasoline. There are a large number of manufacturers of all types and sizes of engines for almost any pumping requirement.

Engine drivers are more costly than electric motors for the following reasons:

 Initial capital cost

 Space requirements

 Weight/vibration attenuation require massive foundations

 Generally must be located in a secure building

 Noise – particularly external exhaust noise

 Lubrication requirements

 Starting requirements

 Engine cooling/building cooling requirements

 Increased maintenance costs

 Drive lines and gearing required between engine and pump

 Fuel storage requirements

Recommended continuous duty shaft speeds for engines, 100-400 HP are usually in the range of 600-1200 RPM, a geared increaser may be required to meet pump operating speeds. For small engines, particularly gasoline engines, direct drive applications are common. In such situations, for horizontal pump installations, a simple clutch may be provided. Generally, either parallel or right-angle gear drives are required to match pump speeds. In a vertical pump configuration where an engine is to serve as standby to a motor drive, a combination right-angle drive is installed between the discharge head and the motor.

Critical frequency vibrations occur when the speeds of two closely connected rotating units are identical.

It is good practice to select units with speed no closer than 25%; i.e. gear ratios 4:3 or 3:4.

Senior mechanical engineering staff should be consulted during design of an engine-driven pumping station and should be responsible for detailed design of the engine, pump and appurtenances.

Drive Lines and Gears

Whether used as a prime mover (directly driving the pump) or as a standby in combination with an electric motor drive, the drive line and gearing between the engine and pump must be carefully designed. The engine must be mounted securely on a concrete pad of a mass of about 4 times the engine’s weight, and then it is normally isolated from the concrete by means of heavy-duty isolators to minimize transfer of vibrations to the concrete base. This allows some small movement of the output shaft which is to be connected to a pump or gear box shaft. It is necessary to provide a flexible coupling or couplings. To insure proper lubrication of the coupling(s), the connecting shaft (drive line) is generally installed at about a 2 to 3 degree offset from exact alignment. A common method for providing this eccentricity is to the use of two U joint-type couplings, such as the Eaton-spicer. On larger units, gear couplings, such as Koppers Fast, have been used with success.

Figure 4-1: Diesel Engine and Enclosure 4.1.2. Variable Frequency Drives (VFD)

Where pump stations are to operate against an infinitely varying flow and head, VFDs are used. By modulating the frequency of the electrical power to the electric motor, the speed of the unit can be adjusted. Variable speed motors should be considered in the following situations:

 If pumped-flow must match influent flow from either a limited capacity sump or a sewer collection system, water treatment plant or a water conveyance system.

 If the pumped-flow must match varying water demands, such as in a closed distribution system or water/waste water effluent

 If the pump station is required to meet a broad operating flow or head range

In some instances such as for the water conveyance pumping system where flow varies with demand but with fore bays and terminal reservoirs, a combination of constant speed pumps two VFDs to trim flows can be used. The only drawback is that the control system would be more complicated as compared to using all variable speed drives. In which case, the capacities of variable speed pumps are normally equal to or 150% greater than the constant speed pumps depending on how steep the system head curve.