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How Does a DC Motor Work? Understanding DC Electric Motors

March 23, 2021

DC electric motors offer a combination of unique performance capabilities and a simplicity of control that makes them hard to beat for variable-speed applications. While AC motors and controls have developed recently to the point of competing with many of its strengths, the DC electric motor remains prevalent in many industries, especially when a compact, efficient, and highly responsive motor is required.

This article is your guide to the fundamentals of DC electric motors, giving you the information you need to decide for yourself if they're the right fit for your application.

DC Motor Operating Principles

Electric motors operate on the basic principle that a wire carrying an electrical current will experience a physical force when it passes through a magnetic field. This magnetic field is typically created in the stationary part of the motor (the stator). It exerts a force on the wire windings attached to the rotary part of the motor (the armature), which is attached to the output shaft.

By switching the direction of the magnetic force based on the rotor position, the rotor armature is alternately pulled toward one direction and then pushed in the opposite direction, creating continuous rotary motion. This process is called commutation.

The critical difference between DC and AC motors is that a DC motor requires a particular commutator device to function. In contrast, an AC motor uses the alternating current of the input power to drive the switching effect.

In a brushed DC motor, the commutator has a conducting brush that presses against the rotor and contacts different parts of the armature depending on the rotor position, effectively using the rotor as a switch for changing the direction of the electromagnetic field. A brushless DC motor uses an electronic sensor instead, usually a Hall effect sensor, to identify the rotor position and manage commutation, removing any need for electrical contact with the rotor.

Types of DC Motors

While there are many DC motor variations, there are two key categories, each with several distinct motor types.

Permanent Magnet DC Motors

A permanent magnet DC motor is one that uses magnets in either the armature or the stator. These motors can be brushless or brushed type.

Brushless DC Motors

If the permanent magnets are on the rotor, there's no need to pass electricity via brushes to create the magnetic field, and this configuration is known as a brushless motor. Because of the lack of brush friction and wear and tear, brushless DC motors will give you exceptional service life, performance, quiet operation, and efficiency (around 90%), making them ideal for noise- and energy-sensitive applications such as HVAC, robotics, and battery-operated devices. A particular type of brushless DC motor known as a stepper motor can rotate in steps and apply holding torque, making it useful for robotics and servo-type control.

The use of permanent magnets limits brushless DC motors to relatively small applications. Because they require an electronic sensor for commutation, they are significantly more costly than comparable size brushed motors, but the power savings can quickly pay back that difference.

Brushed Permanent-Magnet DC Motors

When the stator uses permanent magnets, electricity must reach the rotor windings to energize it, and this is what brushes are for. Because the brush contact points on the rotor are built so that they alternately switch the direction of the rotor's magnetic field, there is no need for electronic commutation. This lowers the cost of brushed DC motors, but brushes reduce efficiency to around 75%. Wear and tear on the brushes also creates a need for periodic replacement and makes the motor less reliable.

You will find permanent magnet brushed DC motors in small, cost-sensitive applications such as consumer electronics and household appliances.

Winding-Field DC Motors

Winding-field DC motors use electromagnets on both the rotor armature and the stator field windings, without any permanent magnets, making them cost-effective for larger industrial applications. These motors use commutation brushes that require periodic replacement. There are three main types of winding-field motors.

Series DC Motors

Series-wound motors have the armature and stator windings connected in series, meaning that the full current passes to the armature. This enables much greater speed and torque but makes it challenging to achieve speed control as the speed changes proportionally to the load. Series-wound motors are typically used in applications requiring high starting torque and power without the requirement for precise speed control, such as starter motors and power tools.

Shunt DC Motors

Shunt DC motors have the armature and field windings in parallel, enabling them to be powered independently. This makes speed control relatively easy and gives them the unique ability to maintain a constant speed, mostly irrespective of changes in the load. The trade-off is that the current is split between the armature and field windings, resulting in less torque than series-wound DC motors. Shunt DC motors are typically used in applications requiring constant or controllable speed, such as mixers, pumps, conveyors, and hoists.

Compound DC Motors

Compound motors compromise between series and shunt DC motors, featuring both winding types in one package. This makes them the best of both worlds if you require both high torque and reasonable speed control.

DC Motor Controls

For most DC motors, speed control is relatively easy compared to AC motors. Because the motor's speed is proportional to the voltage in the motor windings, a simple PWM voltage adjusting circuit is technically all you need.

While this circuit may be enough for some applications, it cannot adjust for changes in the speed brought about by variations in the load. A suitable speed controller will track the motor's actual speed, adjusting the voltage up or down to maintain the desired speed.

It's also easy to reverse the direction of a DC motor by reversing the voltage's polarity at the motor terminals. A simple H-bridge circuit design is commonly used to achieve this without the use of mechanical switches.

DC speed controls do not require any tuning, and their simplicity and low cost make DC motors an excellent choice for many types of variable-speed applications.

Selecting a DC Motor

The process of sizing a DC motor is similar to any other electric motor. While there may be many different variables you need to consider, start by determining if there are any physical size limitations where the motor will be placed, especially in portable devices and household appliances. Next, determine the voltage you have available and the speed and torque your application requires. Compare this information to the options you have available.

For smaller applications, brushless motors give you outstanding efficiency in a compact and robust package, while brushed permanent-magnet motors are more cost-effective. Recently, the increased focus on energy efficiency has driven down the cost of brushless motors, and they make an excellent choice in almost any small-scale setting.

Larger industrial motors will likely be the winding-field type, as these do not require the use of expensive permanent magnets. If you're prioritizing high starting torque and do not need to manage the motor speed closely, a series DC motor is ideal. If you require the speed to be consistent and controllable, shunt DC motors are a better choice.

Summary

Now that you understand the fundamentals of DC motors, you'll be in a great position to determine which type best meets your requirements. We offer a comprehensive selection of DC motors and controls to choose from, so you can build the electric motor drive that delivers the performance you require.

Have questions? Get in touch with our experts.

Connect with our expert team via email or phone.

1-800-890-7593
customerservice@emotorsdirect.ca

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