Electric motors are crucial components in everyday pumping applications, powering pumps that move fluids across a wide variety of industries. Whether for a water supply, wastewater or a cooling system in an industrial setting, electric motors are the source of power behind the mechanical pump moving the fluids. The basics of the electric motor are important to understand when selecting the right motor for an application. This article will cover the types of motors used in pumping, the different phase types and the critical components.
Types of Electric Motors Used in Fluid Pumping
The type of motor needed for a pumping system depends on the application and the requirements needed. When selecting the motor type, one should determine what power, torque, efficiency, speed and control is required for the pump application. The most common motor types for fluid applications are as follows:
1. Induction motors (asynchronous motors): Induction motors are commonly found in pumping applications. These types of motors are simple to set up, are cost-effective and have been proven to be long-lasting. Induction motors create power by inducing an electromagnet current from the stator to the rotor.
2. Synchronous motors: Synchronous motors work at a constant speed, which is synchronized with the frequency of the power supply. This is a critical feature that makes them suitable for applications where consistent speed is required. Synchronous motors typically have a better efficiency offering than induction motors. These higher efficiency motors are more complex than the asynchronous motors, typically, and as a result may cost more.
3. Permanent magnet (PM) motors: PM motors are designed with embedded magnets in the rotors. This creates a more efficient magnetic field, improving energy loss compared to other motor types. PM motors are becoming more popular in variable speed applications due to their smaller sizes, higher torque capabilities and improved efficiency.
4. Direct current (DC) motors: DC motors are much less common in pumping applications than alternating current (AC) motors. DC motors are typically used in specific applications that are portable, or very specialized, in pumping applications. Another use for DC motors in pumping applications is when controlled precision speed is needed. Brushless DC motors are becoming more popular in small, variable speed pumping applications due to the maintenance requirements being lower than alternative motors.
Electrical Phases: Single-Phase vs. Three-Phase Power
Electric motors typically work in single-phase or three-phase electrical systems. The phase requirement is driven by the power supply from the utility company. Depending on the supply, it can impact the performance of the motor, efficiency of the motor and cost of operating the motor.
1. Single-phase power: Typically found in residential settings, single-phase motors operate at 120 volts (V) or 240V. Single-phase motors are usually used in small pumping systems in homes, small businesses, farms and rural areas. These motors can be less efficient and lack the starting torque of a three-phase motor.
- Single-phase motors commonly need more components than a three-phase motor to start properly. A common component found is a start capacitor or centrifugal switch, which allows rotation to begin.
2. Three-phase power: Three-phase motors are different than single-phase, as there are three different alternating currents supplying the motor. Viewing the currents, each phase is 120 degrees, adding up to a total of 360 degrees. The configurations allow for a continuous power supply, which in turn allows for simpler power operation, better efficiency and higher torque than single-phase motors. Most of the time, three-phase electrical supply is used in commercial and industrial settings.
- Three-phase motors are available in various power ratings. They are frequently used in high-load applications such as water/wastewater treatment, pumping of industrial chemicals and irrigation.
Major Components of an Electric Motor
The internal components of an electric motor are important factors in understanding how the motor works. With this knowledge, one could diagnose an issue, perform maintenance and develop motor specifications for a pump. Some of the key components to an electric motor include:
1. Stator: The stator is the part of the motor that is stationary, assembled with laminated steel cores and copper or aluminum windings. When an electric current is supplied directly to the windings, it creates a magnetic field that rotates, inducing current to the rotor.
2. Rotor: The rotor is the moving part of the motor, placed inside the stator. In an induction motor, the rotor is generally in a squirrel-cage design made of conductive bars. In a PM motor or synchronous motor, the rotor may have magnets that have been embedded, or wound windings for
better efficiency.
3. Bearings: Bearings play a role as a support item by holding the rotor in place, along with allowing the rotor to rotate with minimal friction. The bearings have a critical role for the lifespan and efficiency of the motor. In pumping applications, sealed bearings or water-resistant bearings are used to prevent contaminants from entering the bearings, which allows for longer bearing life.
4. Shaft: The motor shaft transfers the rotational power from the rotor to the pump end. Usually, the shaft is directly connected to the impeller, but in some cases, it is connected with a coupling or belt.
5. Enclosure: The enclosure is the shell or protection surrounding the internal components of a motor (stator/rotor). It is there to protect from outside hazards such as debris or water. Three common types of enclosures are used on motors for pumping systems:
- Open drip-proof (ODP): For indoor environments with few contaminants present
- Totally enclosed fan-cooled (TEFC): For environments where dust and moisture are present
- Explosion-proof enclosures: For hazardous locations with flammable vapors or gases
6. Cooling system: An electric motor will generate heat while being operated. To help with cooling, a fan may be placed on the shaft of the motor, which creates an air stream over the internals to cool the motor. Other times, a water jacket is present to run a coolant over the internals. These cooling methods prevent damage to the motor due to overheating, creating longevity.
Motor Selection Criteria for Pumping Applications
Selecting the right motor for a pump application is critical. Several different issues should be considered.
Type of load: For example, centrifugal pumps require motors that can handle variable torque loads, while positive displacement pumps require motors with high starting torque.
Speed control: Inverter-duty motors compatible with variable frequency drives (VFDs) are ideal for applications requiring variable speeds, such as constant pressure applications.
Efficiency: Super high-efficiency motors can reduce energy costs, especially in systems with continuous operation.
Environmental conditions: Wet, dusty or corrosive environments require motors with appropriate enclosures and materials.
Voltage and phase: The electrical infrastructure available for an application may dictate whether a single-phase or three-phase motor is appropriate.
Electric motors are critical in pumping systems. They allow for reliable power and efficient use of the power supplied. From simple single-phase induction motors found in household goods to complex pumping systems requiring highly efficient synchronous or permanent magnet motors, choosing the right motor for the pumping application is crucial.
Effective pump system design and operation is possible with a solid understanding of motor types, electrical phases and internal components.
A follow-up to this article next month will cover the benefits of variable speed motors over the conventional induction motor.
