When a motor is down, it’s vital to replace it quickly to maintain your production levels supported by the motor or replenish your backup motor inventory. Choosing the right motor is just as important. Motor selection often begins with an understanding of the application requirements. While there are numerous technical considerations, the following key factors will provide a foundation for selecting the right electric motor.
The type of input power available at the motor’s operating location will influence the type of motor needed. AC power is readily available from wall sockets and lines connected to the grid, while DC is commonly used when sourcing power from a battery bank. While AC power can be converted to DC and vice versa, extra equipment is required to facilitate the conversion, which comes with an additional cost. This cost must be weighed against the fundamentally different strengths and weaknesses of AC and DC motors arising from differences in their design, which make them suitable for specific tasks. These differences will be explored throughout the rest of this article.
AC power is generally supplied in either single-phase or three-phase, and while three-phase motors would generally be the first choice to use, three-phase is not always available in smaller industrial settings or farms. Single-phase motors can be a great alternative but do have some downsides. They are typically only available in smaller sizes, create more harmonics which lead to noise/vibration, and are less efficient. If only single-phase power is available, most VFDs (Variable Frequency Drives) can convert a single-phase input to a three-phase output, making it possible to operate a three-phase motor on single-phase power.
The primary performance consideration for an electric motor is usually whether the application requires control of the speed and torque. AC motors are more difficult to control, with low starting torque and a non-linear torque-speed curve, requiring complex controls to manage motor performance, especially at low speeds. Because of this, they are widely used in applications where precise speed and torque control is not required, such as appliances, HVAC systems, pumps, and compressors.
Conversely, DC motors offer great starting torque and a more linear torque-speed curve, making it relatively easy to manage the motor performance across the entire speed range. This speed can be easily controlled by varying the input voltage, which requires a simple circuit, making DC motor speed controls relatively simple and inexpensive. This ability to easily and precisely manage the motor performance means that DC motors are widely used in applications such as manufacturing, robotics, and actuation. Their high starting torque, especially for series-wound DC motors, makes them ideal as starter motors for high inertia loads.
For constant-speed applications, speed control is often not needed, as the base speed of the motor can be modified by trading it for torque using a gearbox at an appropriate ratio. AC induction motors are typically used in these types of applications, using a soft starter to ramp up the speed smoothly and prevent damaging current surges when the motor is switched on.
For applications such as power tools which require high speed and portability, universal motors are a special type of motor that can operate on both AC and DC power. They have a high base speed (typically approaching 20,000 RPM) which gives them a high-power density (ratio of power to volume) but prohibits their use with certain types of gearboxes. Additionally, universal motors
In terms of power efficiency, DC motors generally have the advantage, as permanent magnets are more efficient than electromagnets. Brushless DC motors are even more efficient, since they have no friction from brushes or slip rings pressing against the rotor, or power losses from brush arcing.
By comparison, AC induction motors have significantly lower efficiency, with power losses mainly occurring in the form of heat produced in the windings during induction. The difference can be as much as 30%, making DC motors far more suited for applications where power must be strictly managed.
The allowable size of an electric motor may be limited by the available mounting space, especially when the motor, gearbox and load are fitted inside a small package. Generally speaking, DC brushed and brushless motors offer a higher power density compared to AC motors, especially for sub-horsepower applications. Universal motors also offer a very high power density for high-speed applications such as power tools and home appliances.
The maintenance required for a motor, and the time and effort required to carry it out, have a great impact on the total cost of ownership of the motor. For DC motors, the primary maintenance requirement is for the changing of brushes, which typically wear out over several thousand hours or less, depending on how the motor is operated. While this is not a task that occurs frequently, removing and disassembling the motor can be difficult and costly, resulting in a significant amount of lost productivity in applications that are always ‘on’.
AC motors are generally more reliable with low maintenance requirements, and their service lifetime is often limited only by the bearing life. Because they have no need for commutators, brushes, or slip rings, they don’t have parts that wear out on a regular basis. This makes them ideal for applications where the motor must be continuously available to operate, is not easily accessible, or will operate without supervision for long periods of time.
DC motors are generally more expensive than AC motors of the same power output due to their greater complexity. The use of commutators and permanent magnets increases their manufacturing cost, especially in larger sizes. This means they are more commonly found in smaller applications, whereas AC motors are more cost-effective for medium to large applications.
When planning the purchase of an electric motor, you may find it useful to compare the total cost of the entire package, including any motor controls required. Because AC motor speed control is more complex, requiring the use of a VFD, the addition of speed controls can make the cost of an AC motor solution the equivalent or more of its DC counterpart, especially in smaller sizes where the difference in the cost of the motor alone is relatively small.
If cost is a large factor for your project, leverage the eMotors Direct cross-reference tool. If an alternative motor exists for the product you have selected, the tool will suggest alternative brands with the same specifications. This will allow you to price shop for the most economical motor. The cross-reference tool can be found on each product page.
By focusing on these key criteria, you will be able to quickly narrow down your search and speed up the process of selecting an electric motor for your application. Even with this knowledge, it can be complex selecting the right combination of products for your application. Consult with a motor or electrical expert if you are unsure what products you require.
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