The term “high power” can often be misconstrued when discussing motors and actuators, so let’s clarify its meaning at the beginning of this article. Designers are typically looking for a motor with high torque, when asking for “high power”. This is explained when we consider that power (P) is the result of motion (υ) and force (F), or when a rotating machine is involved, angular velocity (ω) and torque (T). Due to this, a small torque/force will create quite of bit of power when the velocity is high, and similarly a very high torque/force will create no power when there is no motion. Hope that clears it up!

What is a high power density motor?

High power density motors (HPDM) are motors that have a high ratio of “power” to the volume of the motor. This means that even the smallest of motors can have a high power output, HPDM can also be referred to as low speed high torque motors. They are typically lightweight, small, with high amounts of torque and lower cogging torque.

High power density motors are a common component in the engineering field, and they are required for a vast amount of applications. It is normal for a customer or professional to provide a manufacturer with a set of drawings that contain all the necessary details for the project, minus the motor or actuator. They will also outline the specifications that they need from the motor, which are always impossible for the motor to meet, all for an extremely low price! Nevertheless, there are some options when it comes to high power density motors, and an acceptable solution can always be found. Let’s explore some of the options that are available in today’s world.

What type of HPDM is right for me?

As with any component, there are many factors to consider when choosing a motor to suit the application. First let’s start with the different types of motor technology that can be used. brushed Direct Current (DC) motors, stepper motors and a lot of Alternating Current (AC) induction motors can be quite cost-effective, but they need to be bigger in size to generate the same amount of power and torque than higher performance brushless permanent magnet (PM) motors, with the brushless PM motors also being significantly more efficient. Thus, they produce less heat and last longer, the biggest drawback with these motors is their cost.

Brushless motors are used in drills for their cool operating temperatures and high performance.  Source: FDominec/CC BY-SA 3.0Brushless motors are used in drills for their cool operating temperatures and high performance. Source: FDominec/CC BY-SA 3.0

The brushed DC and AC motors are the cost-effective choices, and the high performance brushless PM that has neodymium magnets will cost a lot more. This cost is reduced slightly by the reduced heat output from the PM motors and therefore there is no cooling apparatus needed, but the bottom line is your pocket will be significantly lighter if you choose to go with this option. One word of warning when going with brushless PM motors is that they are very sensitive to temperature. The neodymium magnets can easily get damaged at higher temperatures, and this typically happens when cheaper magnets have been used to make the motor.

As a compromise between performance and cost, reluctance motors can be used, which generally offer better efficiency in a smaller package than brushed and AC motors. They may, however, have lower efficiency and a larger overall size, but this would be at a greatly reduced price, particularly for high volume original equipment manufacturer applications. Reluctance motors operate without magnets and can operate at higher internal temperatures than the high performance brushless motors. These temperatures can be regulated with safety requirements in some situations, which limits the allowed operating temperatures. There are exceptions to this, such as when you have cooling provided by the load medium, like water in an automotive water pump. This may allow the motor to operate at a higher internal temperature while maintaining safe operating conditions.

Electric motors can change their operating efficiency with speed, but they can also change it with torque load. A motor that is lightly loaded is typically less efficient than a motor that is at rated load and power. This is especially true for shunt DC motors, reluctance motors and AC induction motors. As loads are increased, these motors will keep the torque constant, but the efficiency will drop.

What motors should be used in various applications?

I’m so glad you asked! The heat that is generated from the motors in the last paragraph increases disproportionately with load. This can sometimes be used to its advantage depending on the application. If the motor is needed to produce short periods of high power/torque and has sufficient time periods for cooling in between, this is a situation where the power/torque performance of the motor is being prioritized more than the efficiency. In these cases, smaller, lower-rated and cheaper motors can be used to satisfy the high load requirement for short periods of time.

The ratio of rated torque to peak torque must also be taken into account as it can vary greatly within motor technology. Brushed DC motors can have very little margin, while some brushless motors can have up to five times or more peak versus rated torque. Reluctance motors once again come somewhere in the middle, with about three times or more peak torque versus rated torque.

If cooling is an issue, and the motor must run at a cooler internal temperature or operate at a certain efficiency, it is a good idea to look at motors that have a larger physical size than those who are more compact. The larger motors dissipate heat much better and efficiency is function of the internal heat in the motor’s windings and magnets, so larger motors generally have better efficiency as well.

There are many different factors to consider when choosing a high power density motor for a particular application. The decision can be complex and believe it or not a lot of engineers that specify certain motors are not qualified to do so! In general, suppliers can be helpful when choosing, and beware that manufacturers will always try to steer you towards one of their products, no matter if it is the best option or not. There is very rarely a simple or straightforward answer when it comes to high power density motors, but by assessing the conditions and all of the technology available, a solution can absolutely be obtained.

Do you have any experience with high power density motors? I would love to hear your thoughts and experiences in the comments below!

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