Can any electric motor be variable speed?

Electric motors play a vital role in numerous industries and applications, powering everything from industrial machinery to household appliances. One common question that often arises is whether all electric motors can operate at variable speeds. In this blog post, we'll delve into the concept of variable speed in electric motors and explore the factors that determine their ability to operate at different speeds.

Honestly, this is one of my favorite questions. We are really going to focus in on industrial electric motors. These are motors that move equipment such as fans, pumps, conveyors and extruders. There are other electric motors such as ones for automobiles and kids’ toys. Those are outside of the scope of this blog.

To comprehend the concept of variable speed motors, it's crucial to grasp the basic principle of electric motors. Electric motors convert electrical energy into mechanical energy through the interaction of magnetic fields. The speed at which an electric motor operates is determined by the frequency of the alternating current (AC) supplied to it. Traditionally, electric motors have operated at a fixed speed determined by the power supply frequency. AC industrial electric motors can be single speed or two speed. Single speed motors are more common, so we will focus on those.

There are a few basic components that make up an industrial electric motor. The first is the frame, which is the body or housing of the motor. The frame contains the stator. The stator is made up of electrical steel and magnetizing material (typically copper wire) that, when electricity is applied, creates a magnetic field, or flux. This field is cycled through the motor through phases, creating a rotating magnetic field. Motors can have a single phase or three -phases. We will focus on the three phases because single-phase motors initiate the rotation using a capacitor.

The magnetic field rotating around the stator is only doing part of the work. The next major component to discuss is the rotor. This rotating component is made up of thin electrical steel sheets, called laminations, that are stacked together and occasionally skewed in a pattern to allow for the magnetic field to push and pull and force rotation of the rotor. The rotor is balanced on a shaft supported by two bearings. There is a slight air gap between the surfaces of the stator and the rotor to allow for minimal drag, which is only present due to the bearings rotating.

These main components: stator (stationary component creating magnetic field) and rotor (rotation component held by the shaft and bearings) are contained in the frame of the motor. The frame is the enclosure to protect the raw components from the environment.

While some electric motors are designed to operate at a fixed speed, advancements in technology have led to the development of variable speed motors. Variable speed motors, as the name suggests, allow users to adjust the speed of the motor according to their specific needs. These motors provide greater control and flexibility in various applications, enabling energy savings and improved performance.

So, how do you adjust the motor speed? And why would you want to adjust the motor speed? Let’s begin with the “why.” Varying the motor speed is very much dependent on the application. Some applications, such as centrifugal pumps, fans or compressors - commonly known as variable torque applications - should be run at a reduced speed to optimize the overall performance of the driven equipment. Running at partial speed can result in significant energy consumption reduction, which allows the end customer to reduce their overall electricity bill.

On the other hand, constant torque applications, like mixers and conveyors, require torque to move loads at nearly any speed. Reducing the speed of the motor for these applications would really be important at start up and shutdown. Reducing the shocking loading of the system by having controlled starts and stops is important to protect the equipment. Commonly downtime is expensive for these applications, so operators really want to protect equipment.

Now, we can talk about how to reduce motor rotational speed. Variable speed drives (also known as variable frequency drives, VSDs, VFDs, inverters or simply drives) can control the frequency and voltage being fed to the motor. By controlling these two parameters, an operator can precisely control the speed of the motor.

A drive can’t be added to any motor, and not all electric motors can be easily converted to variable speed operation. Motors designed to be operated with a drive are built with an insulation system adequate to withstand the voltage spikes that occur coming out of the drive. Also important is that the motor has the ability to cool itself adequately to prevent damage. Permanent magnet synchronous motors and brushless DC motors inherently possess the ability to operate at variable speeds due to their construction, and all ABB Baldor-Reliance three-phase motors (ID motors) are capable of running with a drive. 

Not all electric motors can operate at variable speeds by default. While certain motors are designed specifically for variable speed applications, others may require additional components and control systems to enable speed adjustments. It's essential to consider factors such as motor design, type, control systems and load characteristics to determine if an electric motor can be converted to variable speed operation. If the motor isn’t designed and manufactured appropriately, operating it with a drive could lead to premature failure.

Advancements in technology have made variable speed motors increasingly accessible and beneficial in various industries. Their ability to optimize energy consumption, improve process control, and enhance overall performance makes them an attractive option for many applications. When considering the use of variable speed motors, it is recommended to consult with experts or motor manufacturers to ensure compatibility and achieve the desired functionality.

Remember, the ability of an electric motor to operate at variable speeds depends on its design, construction, and the presence of appropriate control systems. With the right components and considerations, the advantages of variable speed motors can be harnessed to optimize efficiency and productivity in numerous applications.

To learn more about ABB’s Baldor-Reliance ID motors, please view our webinar series and videos and download our white papers.