How do you specify the best motor? It depends

Selecting the most appropriate motor depends on many things for an engineer

By Jeff Thomas January 24, 2024
Figure 1: This is one of six 2,500-horsepower motors driving three-stage vertical turbine pumps providing 65 million gallons per day of raw water from Lake Huron to water treatment plants in the cities of Midland, Saginaw and Bay City, Michigan. It also serves five other very small communities. Courtesy: Lockwood Andrews & Newnam Inc.


Learning Objectives

  • Learn about the typical types of motors: induction, synchronous, stepping, specialty.
  • Know the NEMA motor types and their main differences.
  • Consider the client’s requirements and general trade-offs when selecting a motor.

Motor insights

  • Motor selection is a complex, detailed process that requires the engineer to understand the application.
  • There are many factors to consider before specifying a motor, such as guidelines, regulations and energy efficiency requirements.

This article has been peer-reviewed.Navigating the complexities of motor selection is about more than matching horsepower. It’s about deciding what makes the most strategic sense for the entire electrical distribution system. Choosing the right motor hinges on its intended application, load requirements, available voltage and the surrounding environment. Whether it’s a corrosive setting, a high-temperature zone or an air conditioner in an office building, the motor needs to be up to the task.

Motors come in many different sizes and configurations, from tiny, precision control motors to immense propulsion motors. No matter the size, they all share common characteristics and operating parameters.

The history of motors is closely linked to the evolution of electricity. In 1821, Michael Faraday made a pivotal discovery when he found that a current-carrying conductor could rotate around a magnet, essentially creating the first motor. Commercialization started in the late 19th century when Nikola Tesla pioneered the development of the alternating current (AC) motor while Thomas Edison aided in the invention and commercialization of the direct current (DC) motor.

DC motors

DC motors are easier and cheaper to control than AC motors. Controlling the speed and direction of a DC motor is simpler and more cost-effective than AC motors, especially in scenarios where precise motion control is required.

Figure 1: This is one of six 2,500-horsepower motors driving three-stage vertical turbine pumps providing 65 million gallons per day of raw water from Lake Huron to water treatment plants in the cities of Midland, Saginaw and Bay City, Michigan. It also serves five other very small communities. Courtesy: Lockwood Andrews & Newnam Inc.

Figure 1: This is one of six 2,500-horsepower motors driving three-stage vertical turbine pumps providing 65 million gallons per day of raw water from Lake Huron to water treatment plants in the cities of Midland, Saginaw and Bay City, Michigan. It also serves five other very small communities. Courtesy: Lockwood Andrews & Newnam Inc.

DC motor functions include:

Higher starting torque: Generally speaking, DC motors provide a greater torque at startup than comparable AC motors. In a DC series motor, the field is connected in series with armature. This connection allows the torque to be to be proportional to the square (hyperbolic) of the current, making them suitable for applications that require a strong initial force.

Ability to start and stop quickly: Being able to control the speed of a DC motor is advantageous in applications that demand precise motion control, such as in robotics or conveyor systems.

Good at reversing: The direction of rotation in a DC motor can be easily reversed by simply changing the polarity of the power supply. This is useful for bidirectional operations such as elevators, baggage claim belts and other similar applications

Variable speeds with voltage input: The speed of a DC motor can be varied smoothly from zero to its maximum, providing dynamic control options. By adjusting the voltage input, you can directly control the motor’s speed, making it versatile for various applications.

Entrenchment in the control space: DC motors are foundational in control systems because of their ease of control and reliability. Many legacy systems, particularly in industries like manufacturing, still rely heavily on DC motor setups.

Use in emerging markets: There’s surging interest in DC motors in the fields of automotive drives, particularly in electric direct compatibility of DC motors with battery-driven systems and photovoltaic sources. Without the need for inversion makes them ideal candidates for these developing areas.

More maintenance.

A commutator and brushes to achieve an alternating current internally to the motor: These brushes are sacrificial. The carbon they are typically made from creates a small arc every time the commutator goes by. This makes them a bad choice for explosive environments and each arch removes a small amount of the brush. They are usually spring loaded to maintain contact with the commutator as they age. Research varies here, but between 1,000 and 3,000 hours on a set of brushes is about normal. One alternative is brushless DC motors. These use a different architecture which requires the alternating magnetic fields to be created externally to the motor. In other words, a controller is required to alternate the DC voltages supplied to the field magnets of the motor. This adds complexity, but also control. Hard disk drives are a common user of brushless direct current motors.

AC motors

AC motors are robust, well entrenched and versatile machines with a track record in many industrial applications. They are commonly used in air handlers, pumps, blowers and vacuum systems. Basically, anywhere there is need for continuous rotation to produce work of some kind. Their inherent reliability and compatibility with existing systems make them a “go-to” choice. Strengths include:

They are very reliable and known for their long-lasting durability: Their simpler construction, with fewer moving parts, often means a longer life span with fewer breakdowns. For example, while AC and DC motors typically have a rotor, a stator, bearings and a housing, the DC motor adds a commutator and brushes. The commutator is essentially a series of contacts spaced around the shaft that the brushes make contact (commute) with as they pass. This action effectively chops the DC current, reverses it and sends alternating pulses to the rotor. This induces a changing magnetic field that interacts with the stator magnets, causing rotation. This is exactly how an AC motor works, we just leave all the chopping and reversing out by providing an alternating current source.

Up to a certain size, they’re disposable: Like many commodity appliances, it’s cheaper to replace smaller motors than to have them rewound or repaired. For example, how often do you repair the motor in a tabletop fan? Even the blower in the air handler of many small to medium units is simply replaced rather than repaired. And let’s be realistic —most equipment motors run to fail with very little to no maintenance. Bearings are sealed, totally enclosed fan-cooled (TEFC) motors have no accessible parts and they seem to run forever. Until they stop.

They have a large installed base: AC motors have dominated many industries for decades and are used in conveyance, air-handlers and water pumps, for example. This widespread use means lots of knowledge, spare parts and expertise are available.

Able to match existing infrastructure: In many facilities, the electrical infrastructure is set up to support AC power. This makes integrating AC motors straightforward without needing converters.

They have variable speeds with frequency input: An AC motor’s speed can be controlled by adjusting the frequency of the input power. Variable frequency drives (VFDs) are commonly seen coupled with motors in a variety of applications to provide precise control over motor speed and torque. VFDs also provide significant energy savings by allowing the motor to run where it needs to rather than at full speed. According to some studies, electric motors are responsible for about 45% of the total consumption of electric energy. If we focus the analysis on the most energy-consuming sector, the industrial one, the percentage attributable to motors rises to around 67%. It becomes easy to see why energy conservation through motors is important.

Figure 2: The vast range of motor sizes is shown. Courtesy: Lockwood Andrews & Newnam Inc.

Figure 2: The vast range of motor sizes is shown. Courtesy: Lockwood Andrews & Newnam Inc.

AC induction versus AC synchronous motors

With designs tailored for specific tasks, motors are essential components in various applications. The choice is usually between induction or synchronous motors. The debate over induction versus synchronous motors arises from their distinct operational characteristics, with operators weighing factors such as efficiency, cost, maintenance and application-specific performance to determine the most suitable motor for a given task.

So, which to choose? Each has its own uses and advantages.

Induction motors are asynchronous, which means the rotor (the spinning part inside the motor) doesn’t spin at the same speed as the magnetic field created by the stator (the stationary part outside the rotor). It is always a bit behind.

The rotor current is induced by the stator current. The rotor doesn’t get its electric current directly. Instead, the current in the stator produces a changing magnetic field, which then creates (or “induces“) a current in the rotor, making it spin.

Induction motors, therefore “slip.” Because the rotor is always trying to catch up to the stator’s magnetic field but never quite does, there’s a small difference in their speeds. This difference is called “slip.”

Induction motors operate at a lagging power factor. Simply, the power used by the motor doesn’t perfectly match the power coming in from the electrical supply. This mismatch is referred to as a “lagging” power factor.

Induction motors are less expensive to manufacture and maintain. Because they don’t have certain parts that wear out quickly, like brushes or commutators that makes them simpler, more durable and cheaper to produce and to keep running.

Most motors in commercial applications are induction motors. Because of their cost-effectiveness, reliability and simplicity, induction motors are a prime choice for many commercial tools and machines.

Synchronous motors, in contrast, maintain a steady speed regardless of load, running in synchronization with the power source. They are designed for precision, efficiency and improved power management, but they often come at a higher initial cost.

Unlike induction motors, synchronous motors often get their rotor current from external sources or magnets, rather than relying on induction from the stator. Synchronous rotors turn in sync with the stator frequency. The rotor’s speed matches exactly with the speed of the stator’s magnetic field, which means there’s no difference or “slip” in their speeds.

Small horsepower synchronous motors are used for precision control (timing). When precise movement or timing is essential, like in clocks or some manufacturing processes, these motors are ideal because they move in sync with the electrical supply’s frequency.

Synchronous motors are more energy efficient because they often convert more of the electrical power into useful mechanical power, wasting less energy. They operate between a unity (1.0) power factor and 0.8 leading power factor. This means they can better match the power they use with the power they receive or even give some power back to the electrical grid, helping to balance electrical systems and potentially reduce costs. While they offer benefits like precision and efficiency, they usually cost more initially than induction motors.

Motors have standards and guidelines

NFPA 70: National Electrical Code (NEC) provides the standards for determining the electrical load for a specific installation and the appropriate sizing and safety considerations. The NEC guidelines also detail the proper sizing, protection, control and installation of motors and their associated circuits.

The National Electrical Manufacturers Association (NEMA) also designates motor types based on their torque-speed characteristics:

  • Type A: General-purpose motors, with medium starting torque and normal starting current. Suitable for most applications.

  • Type B: Standard design with high slip, low starting torque and low starting current. Common in applications like fans and pumps.

  • Type C: High starting torque with low starting current. Often used with hard-to-start load applications such as compressors.

  • Type D: High slip motors with high starting torque. Commonly used in applications like hoists, oil well pumps and punching presses.

  • Type E: High-efficiency version of Type B motors.

NEMA guidance

Motor enclosure types: Each motor is also housed in different protective enclosures fitted to the specific operational environments. These different types of enclosure ensure motor safety, longevity and optimal performance. They are:

Open drip-proof: Allows for ventilation through the motor but is designed to prevent liquids and solids (like water drops) from entering from above or the sides.

TEFC: Completely sealed against external contaminants. It uses an external fan to cool the motor, which blows outside the motor casing without interacting with internal components.

Totally enclosed nonvented: Like TEFC, but doesn’t have the external cooling fan, relying on the motor’s housing to dissipate heat. It’s suitable for applications where minimal contamination is possible.

Totally enclosed air over: Designed to be cooled by external airflow, often from a system in which the motor is installed, like a fan blade that blows air over the motor.

Totally enclosed wash down: Suitable for environments requiring regular cleaning or washing, preventing water and contaminants from entering the motor during washdown processes.

Explosion-proof: Built to withstand and contain internal explosions without causing external hazards, commonly used in environments with flammable gases or dust.

Hazardous location: Like explosion-proof casings, these enclosures are designed for locations with specific hazardous conditions and have certain design attributes unique to the particular hazard they’re addressing. There are also several classifications of hazardous locations motors. Class I deals with gases and vapors, Class II with dust and Class III with fibers and “flyings” (particles, debris, dust).

Motor design letter classification: NEMA uses a motor design letter classification system to categorize the torque and currents for motors. These letters (design A, B, C, etc.) give insight into a motor’s performance, particularly its starting torque, starting current and slip. NEMA tables detail how these ratings relate to energy and horsepower.

Figure 3: Located in a commercial mechanical room, 15 horsepower motors are used for chilled water pumps. Courtesy: Lockwood Andrews & Newnam Inc.

Figure 3: Located in a commercial mechanical room, 15 horsepower motors are used for chilled water pumps. Courtesy: Lockwood Andrews & Newnam Inc.

Other factors to consider

The duty cycle: The duty cycle describes the operation time and rest period of a machine or system. For continuous duty, the motor runs continuously without stopping. Periodic duty consists of identical run and rest cycles with constant load. The motor starts, runs for a specific period, stops, rests and then the cycle repeats. Short-time duty means the motor runs continuously with a constant load for a short time and doesn’t repeat once the operation is completed. Most commercial applications, fans, pumps, etc. are continuous duty motors.

Service factor and insulation class: The service factor is a motor’s cushion for temporary overload; for instance, a service factor of 1.2 means the motor can handle 20% more power than its rated capacity without incurring damage. Insulation class tells us how much heat the motor’s internal wiring insulation can withstand before deteriorating, with each class (like A, B, F, H) indicating a specific temperature limit.

For motors, understanding the parameters of efficiency, slip and the power factors is vital for ensuring efficient motor operation, optimizing performance and managing operational costs.

Efficiency: Efficiency is the ratio of the useful power output (mechanical power) to the power input (electrical power) of the motor. A motor with higher efficiency converts a larger portion of the electrical energy it receives into mechanical energy, wasting less as heat. It’s desirable to use high-efficiency motors, especially in continuous operations, as they reduce energy costs in the long run.

Slip: In induction motors, there’s a phenomenon where the rotor’s frequency lags behind the stator frequency (F). This delay, which is linked to the physical attributes of the motor’s components, is termed “slip.” Slip is crucial for torque production in an induction motor. A motor with zero slip (i.e., rotor speed equals the magnetic field speed) would produce no torque. In typical scenarios, the slip can range anywhere from 5% down to a mere ½%, shedding light on the intricate balance of power and performance in motor design.

Power factor: For motors, maintaining a high-power factor ensures optimal efficiency and reduced electrical losses.

The power factor is the cosine of the phase angle between the current and voltage in an AC circuit. It measures how effectively electrical power is converted into useful work. It ranges between 0 and 1.

A power factor of 1 means all the electrical power is effectively converted into work, while a lower power factor indicates inefficiencies and results in a higher current draw for a given power output.

It’s important to note that a low power factor can lead to increased power loss in the distribution system and can incur additional utility costs. Correcting the power factor (using capacitors, for instance) can lead to more efficient power use and reduced electricity costs.

Supplementary components: Other things to consider include mounting methods, lifting rings for larger motors, sensors for monitoring things remotely, availability of service and spare parts, familiarity with the maintenance staff, standardization of units across a large enterprise and other logistical concerns.

Motor starters and controls

The systems and devices employed to initiate, control and protect electric motors are varied. Picking the right one depends on the purpose.

Motor starters are used to safely start and stop a motor by providing the necessary initial current and then cutting off or limiting it as needed. These are the different types:

Direct online starters are suitable for small motors. The motor is directly connected to the power supply. In smaller motors, this is often a simple capacitor to provide that extra boost to get things running. Many an air conditioner has failed in the hottest part of the summer due to a bad capacitor. Fortunately, they’re relatively inexpensive and easy to replace.

Figure 4: Large medium-voltage variable frequency drive. Courtesy: Lockwood Andrews & Newnam Inc.

Figure 4: Large medium-voltage variable frequency drive. Courtesy: Lockwood Andrews & Newnam Inc.

Star-delta starters turn the motor on in a “star” configuration for reduced voltage and current, then switch to “delta” for normal operation. Used for larger motors to reduce starting current.

Soft starters gradually increase the voltage to the motor, ensuring a smooth start and reduced initial current. VFDs fall into this category.

Variable frequency drives

While traditional motor starters turn a motor on or off, a VFD controls the speed of the motor by varying the frequency of the supplied voltage. This reduces the mechanical and electrical stresses on a motor during startup, leading to extended equipment life and reduced maintenance costs. This functionality means that a VFD not only starts and stops a motor, but dynamically adjusts speed and torque during operation. VFDs can also be integrated so they can be controlled and monitored remotely, allowing operators to adjust motor operations and receive performance data or alerts from a distance. In many applications, VFDs can replace or work with traditional motor starters to provide more precise and energy-efficient control.

VFDs are most used in pumping systems; heating, ventilation and air conditioning fan and blower systems; conveyor belts; and other machinery where variable speed and torque are beneficial. They can improve the power factor by ensuring that the motor only draws the current it needs, leading to more efficient power usage and potential cost savings on electricity bills.

Overall, selecting the appropriate motor for industrial applications is extremely important. The right motor choice guarantees optimal efficiency, leads to energy savings and reduces operational costs. The right motor ensures the reliability of processes, reducing unforeseen downtimes that can disrupt production schedules. By minimizing wear and tear through the proper motor selection, maintenance intervals can be extended, leading to further cost savings.

Ultimately, the right motor can extend the equipment’s overall life span, ensuring a better return on investment. In the end, the correct motor selection directly impacts productivity, operational expenses and the financial health of an industrial operation.

Author Bio: Jeff Thomas, PE, CEM, CEA, CHC, is VP and Business Group Director at Lockwood Andrews & Newnam Inc.