DC Motor Power Density Basics

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This article is part of the Power Management Series: Diving into Power Density

Members can download this article in PDF format.

What you will learn:

  • Specificities of high power density motors (HPDM).
  • Applications are best served with HPDMs.
  • Why BLDCs are perhaps the most preferred among HPDM architectures.

High power density is an important feature in most motors, even in a tiny motor with high power output capability. A high power density motor (HPDM) is a motor that has a high power to volume ratio.

The smallest motors can even have high power output. HPDMs are low speed, high torque motors. These motors are generally light, small, and have large amounts of torque and lower cogging torque. Cogging torque is defined as the torque required to overcome an opposing torque created by the magnetic attraction force between the rotor magnets and the iron teeth of the stator.

In HPDMs, many motor manufacturers will alloy cobalt with iron (CoFe); however, it is a high priced alloy. The mineral cobalt is used because it can achieve the highest maximum saturation magnetization compared to all other materials. These motor designs will achieve high maximum flux density, which can significantly reduce the size and weight of electric motor cores.

The permanent magnet direct current motor (PMDC), an HPDM, has poles made of permanent magnets. These magnets are radially magnetized and mounted on the inner area of ​​the cylindrical steel stator. The motor stator acts as a return path for the magnetic flux. The rotor is constructed with a DC armature plus brushes and commutator segments (see picture).

Applications for high power density motors

Drone

A main application for an HPDM is a drone. Brushless direct current (BLDC) permanent magnet motors, for example, can provide high power density for drone designs. These types of motors, with a high power-to-mass-to-volume ratio, can easily power drone propellers/rotors.

There are even DC industrial drone motors for heavy loads with a thrust capacity of 50 kg or more. These motors typically use lightweight carbon fiber propellers.

Automotive

Another important type of lightweight battery-powered application is a high power density automotive system. Such designs are capable of reduce the size, weight and component count of the automotive fuel system. Designers can create power modules capable of driving HPDMs, enabling lightweight and efficient power supply designs for all types of electric vehicles (EVs).

Medical

HPDMs in the medical field enable smaller medical devices and applications. These types of motors work in important applications for portable or wearable devices, such as miniature pumps and medical implants. Here are some examples of DC motors requiring high power density:

  • Surgical hand tools, which require small, high performance motors that operate with less vibration and noise due to reduced EMI/RFI.
  • Hospital infusion pumps, which require high efficiency, high torque density motor solutions. These will reduce the size and increase the life of the backup battery, especially for mobile clinical pumps. These motors will reduce the size of the pump as the HDPMs have been reduced to single digit millimeter diameters.
  • Aerospace applications, which use small, lightweight, yet powerful motors that can also be used in applications requiring precision and the ability to withstand harsh environments.

Industrial

A major application area is in industrial power tools designed with BLDC motors. High power density BLDC motors are used for positioning, servoing, actuation and variable speed. In such applications, motor running stability and precision motion control will be the primary goals for industrial and manufacturing sectors.

Electric screwdrivers, hand drills, nail guns and pruners are just some of the specific applications. HPDMs in these types of devices will also reduce packaging size and weight in the overall design.

Summary

In many applications, space and weight must be kept to a minimum. Power density is a critical factor in meeting these design constraints.

If designers are looking for the best power density in their designs, with one of the key components being a motor, the best choice would most likely be a brushless DC motor architecture. Standard BLDC motors have a decisive advantage over a brushed DC or AC motor due to their excellent power density.

Some BLDC motor manufacturers have incorporated new BLDC motor drive technologies that can increase power density. Savings in size, weight, power and cost are achievable by combining the driver and power amplifier sections of the motor controller into a single solution.

Read more articles in the Power Management Series: Diving into Power Density

Reference

1. “Permanent Magnet DC Motor (PMDC) – Construction, Operation and Speed ​​Control”, tutorialspoint.com.

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