What is the difference between an ac motor and a dc motor?
There is a wide variety of electric motors, suited for many industrial applications and equipment. Engineers and designers have a lot of choice when selecting a motor for their application. The table below shows an overview of the most common electric motor types. The global classification of electric motors is divided in mainly two different branches: AC motor and DC motors. This article will focus on four different sub groups:
- AC Induction motors
- Synchronous brushed DC motors (with commutator)
- Brushless DC motors
- Permanent Magnet Synchronous sinewave AC motors (PMSM)
1. AC Induction Motor
Induction motors are widely used as industrial drives. The stator of the induction motor consist of wound poles which can be connected directly to the grid or via a VFD. The AC current through these poles will induce a magnetic field in the airgap and the rotor. As AC current is used for this type of motor, often when people refer to the term “AC motor” they actually mean an induction motor. The magnetic field will induce eddy currents in the rotor, more or less in the same way as a transformer does. These induced currents in the rotor will create a magnetic field which generates the torque.
The majority of the induction motors have electrical shorted rotors, called squirrel cages. This is the reason for the other nick name of the induction motor: ‘squirrel cage motor’. Induction motors utilize NO permanent magnets, only copper wire and laminated steel. The rotor cage mostly consists of aluminium bars, but in the past 10 years also copper bars are used in order to boost efficiency.
Single and three phase models
They are available in single and three phase models. The three phase motors are mainly for larger loads, while smaller loads are often driven by single phase induction motors. When connected directly to the grid the motor will speed up to a certain fixed speed, which is a bit slower than the rotating frequency of the applied electric field (this phenomena is called slip). This property is the reason why inductions motor are also sometimes called asynchronous motors. The achieved speed depends on the amount of pole-pairs and the applied electric frequency. The load on the shaft will only modestly lower the speed of the motor (in the order of a few percent).
For a lot of applications this constant speed is suitable. Because of modern electric drive technology it is now also possible to achieve a variable speed by using a variable-frequency drive (VFD). Since varying the electric frequency (as mentioned before) the revolutions per minute of the shaft will also vary proportional. The introduction of the VFD has improved the functionality and the energy-efficiency of the induction motor.
AC Induction Motor advantages and disadvantages
Induction motors have a lower overall efficiency and lower torque density when compared to permanent magnet motors. Due to the lack/absence of permanent magnets it is easier and more straight forward to apply field weakening, which enables a large RPM range for these motors. The induction motors are wide-spread in many industrial and household applications where torque density and controllability are of less importance. They are self-starting, reliable and economical.
2. Synchronous brushed DC motors (with commutator)
As the name suggest, brushed DC electric motors are powered with a direct current (DC). The motors are mechanically commutated. Brushed DC motors utilize a permanent magnet or wound stator poles. The permanent magnet or wound stator poles generate the main stationary magnetic field of the motor.
Rotor Brushed DC motors
Brushed DC motor rotors are constructed with wound poles which are connected via a commutator with brushes to a voltage or current supply. When the rotor poles are carrying current, a magnetic field is generated. This rotor field will try to align with the stator field and therefore generate torque. The pattern of the current in the individual coils of the rotor will be dictated by the commutator and the brushes (image).
Stator Brushed DC motors
The Stators of Brushed DC motors are constructed with either wound or permanent-magnet poles. The wound poles of the stator have to be supplied with a specific voltage/current. This creates an additional control feature and provides a motor with specific torque/speed characteristics. Typical configurations are series, shunt and compound.
Brushed DC motor. Image credit: ZGC Motor
Commutator Brushed DC motors
The commutator acts like a mechanical switch as it will cause current to flow through alternating rotor poles as function of the rotor position. This results in continuous torque per revolution. The brushes continuously slide over the commutator and this causes them to wear. This negatively affects the lifetime and maintenance sequence of the brushed DC motor and provides a major drawback for this type of motor. Due to the introduction of more sophisticated electronics (like VFD’s) the mechanically commutated rotor field with the brushes has been transformed to an electronic commutated principle (i.e. Brushless DC).
Controllability of Brushed DC motors
In the early days, when electronics were not commonly used, these types of motors where frequently used for different applications. A simple controllable DC voltage supply could drive the motor. Later options like, current control, feedback systems and PWM drives facilitated even higher controllability of brushed DC motors. The use of permanent magnets enables a relative high torque density and good efficiency for these type of motors.
Advantages and Disadvantages Brushed DC motors
The performance of the brushed DC motors when it comes to starting and regulating speed is good. It has a relative high torque density. The range of speed regulation is wide and it runs smoothly. The electromagnetic interference is small and the overload capability is strong. But the structure of the DC brushed motors are a disadvantage. The sliding contact between the commutator and the brush causes sparks and mechanical wear. Because of this DC brushed motor have relatively short life expectancy, reliability concerns and high maintenance cost.
3. Brushless DC motors (BLDC)
As the name implies, brushless DC motors do not use brushes. In comparison to brushed DC motors, brushless DC motors use a very different method of commutation. Instead of brushes, commutation is done electronically. The construction of most brushless DC motors is based on a stator with wound poles and a multipole permanent magnet rotor. The multipole permanent magnet rotor runs past a Stator with wound poles. The permanent magnets of the rotor react to the changing magnetic field of the stator. The stator is fixed, there are no moving wires. Therefore there is no need for sliprings, commutators or brushes with the brushless DC motor.
Stator poles –Three phase system
Most of the time the stator poles are wound in a three phase configuration. The three phase system of the stator windings is connected to a circuit of electronic switches (transistor, FET’s, etc). The switching circuit is supplied with a DC voltage or by a rectified AC voltage source. The simple switching of the phases causes square wave/trapezium formed currents, therefore the back EMF of the brushless DC motor is also more or less square wave/trapezium formed. This causes an increase of torque ripple when driving these motors. More speed can be achieved by phase advancing. Since the use of VFD technology with the brushless motor, it became possible to generate a sinusoidally formed current.
To be able to turn-on the correct phase at the right moment a feedback system is needed. Hall sensors, can be positioned to detect the position of the permanent magnet rotor with respect to the stator. Although hall sensors are most commonly used, there are also more accurate position sensor systems available. With the above system the rotation of the motor can be kept synchronous with the applied electric frequency.
Advantages and Disadvantages Brushless DC motors
Brushless DC motors have many advantages as compared to the brushed DC motors, like low maintenance cost, less operating noise, better performance and higher efficiency, lower EMC. They are available in compacter sizes and provide high torque to weight ratio. But compared to other motors, brushless DC motors also have few shortcomings. The cost of the motor and electronic controller is comparatively higher. Torque ripple as mentioned before. Next to that, they are a bit more complicated.
Image credit: Microchip Technology Inc.
4. Permanent Synchronous Sinewave (PMSM) AC motors
Terms like PMSM AC motor or brushless AC motor or EC motor (Electric Commutation) are also frequently used for this type of motor. The working principle of this motor is a stator part, which holds a three phase wound coil system and a rotor construction with a multipole permanent magnet array. These high power density motors are very efficient, meeting IE4 and even IE5 energy efficiency classes. There are various constructions of permanent magnet synchronous sinewave AC motor, for example inner runners, outer runners, axial flux motor, transversal motor, etc.
Because of the magnetic design the Back EMF is sinusoidal, in combination with a sinusoidal current, a smooth torque can be generated. A relative sophisticated drive like a power amplifier or a VFD is needed to produce the sinusoidal current. These types of drives are becoming more and more standard and more straight forward to use. A PMSM AC motor also runs synchronous with the electric applied frequency (divided by the amount of pole-pairs).
The use of strong permanent magnets causes constant iron losses in the laminations stack during rotation. These iron losses limit the maximum velocity of the motor as does the higher voltage needed for higher velocity (mostly limited by the applied voltage 400Vac_rms). These parameters have to be kept in mind. For all the above mentioned motor types the applied current introduces dissipation losses in the coils. These losses have to be thermally managed in the structure. This also requires a good thermal design, because the better the thermal design the higher the torque density. As more power can be applied and dissipated without the motor overheating.
High Torque Density
Combine high strength permanent magnets with an excellent magnetic design and you can achieve a very high torque density PMSM motor. For example a 20 kg (44 lb) PMSM AC motor including housing and bearing can deliver comparable power/torque output when compared to a 55 kg (121 lb) AC induction motor. Based on standard/passive cooling methods for both. PMSM AC motors do require continuous position feedback, either by use of sensors or today even sensorless control is possible. To increase the velocity capabilities, some PMSM AC motor designs have the possibility of field weakening.
Usage growth PMSM AC motor
The technology of PMSM AC motors in combination with motor drives that enable electronic commutation have been available for a while. But as, motor drives supporting PMSM motors are becoming increasingly better available, more companies are starting to integrate permanent magnet AC motors into their applications. The usage is increasing signifyingly. This is because of the compact design capability, high torque density, high efficiency and long lifetime expectancy. The PMSM AC motor gives companies the possibility to build applications that are smaller in size, operate more efficiently, more controllability, with increased reliability and as such run at lower (operating) costs.
Advantages and Disadvantages of PMSM AC motors
While PMSM AC motors will always need a drive and tend to be slightly more expensive compared to an AC induction motor, they often are a better alternative. They have higher torque density, are much more efficient are significantly smaller and as lighter than comparable induction motors.