AC Motors vs. DC Motors - What's the Difference?
There are several key differences between AC motors and DC motors, besides the obvious one that relates to how each of these components is powered. Below is a brief presentation of what each of these types of motors is, followed by a summary of the differences between them.
To learn more about the different types of motors, consult our buying guide to motors.
What are AC Motors?
AC Motors are electromechanical devices that convert electrical power in the form of alternating voltage and current into mechanical energy. AC Motors come in different varieties which can be characterized as being either Induction Motors (which are asynchronous) or Synchronous Motors, and which contain a stator and rotor. Induction motors can be either Single Phase or Polyphase, while synchronous motors include Reluctance Motors and Hysteresis Motors. See our related guide, Types of AC Motors, to learn more about each of these.
What are DC Motors?
DC Motors can convert the electrical energy that is supplied to it in the form of direct current into mechanical rotational energy. The same device can be used in reverse to produce DC electrical power from the rotation of the motor shaft. When used in that manner, the device is functioning as a generator. There are several key types of DC motors available. These include Permanent magnet DC Motors, Series Wound DC Motors, Shunt DC Motors, Compound DC Motors, and Brushless DC Motors. Our related guide, Types of DC Motors, contains more information about each of these types.
How Do AC and DC Motors Differ From Each Other?:
While AC and DC motors both create mechanical energy in the form of a rotating motor shaft, there are some key differences:
Input Power
AC motors operate from an input electrical signal that is an alternating current and voltage which changes in amplitude and direction as the input AC waveform completes a cycle. AC Motors can be operated either from a single-phase power source, of a polyphase source featuring multiple voltage inputs that operate at a phase angle difference from each other (commonly 120o or 2π/3 radians in the case of three-phase power). DC motors are powered from a unidirectional current (one that does not change direction with time) supplied from a DC power source. The general prominence of AC power means there may be a need for conversion to DC power when using a DC motor, such as using an AC-DC converter or DC power supply.
Magnetic Field
In polyphase AC motors, as the stator coils are supplied with an alternating current, a rotating magnetic field, or RMF, is produced which, through Faraday’s law of induction, generates an EMF in the rotor coils. That EMF results in a current in the rotor and a net torque to be applied, causing it to rotate, and which also generates a rotating magnetic field. Induction motors exhibit a phenomenon known as slip, wherein the speed of the rotor (Nr) is less than synchronous speed of the rotating field of the stator (Ns). The Slip is expressed mathematically as:
In a DC motor, a permanent magnet or a set of field coils produce a magnetic field that does not rotate. Current is supplied to the coils of the armature, which results in the armature’s rotation.
Direct vs. Indirect Connection Design
With an AC motor, energizing the stator coils through a direct connection to a polyphase AC power source is all that is needed to produce rotation of the rotor. The principle of electromagnetic induction generates the current in the rotor without the need for a direct electrical connection.
With a DC motor, current needs to be supplied to both the stationary field coils (unless a permanent magnet is used) as well as to the armature. To accomplish this, brush-type DC motors make use of a set of spring-loaded carbon brushes which press against a commutator ring that carries the current to the armature coils and to the field coils as the armature rotates. Depending on whether the field coil connection is done in parallel with the armature coil (shunt motor) or in series with the armature coil (series wound motor), the resulting DC motor configuration will exhibit different performance characteristics.
The use of brushes and a commutator has several impacts to the operation of DC motors:
Brushes are subject to wear from mechanical friction, meaning that repair and brush replacement is inevitable, which impacts motor placement due to the requirement for accessibility.
Brush contact with the commutator can result in sparks and arcing which can cause pitting and damage to the commutator and can also be an ignition source – a concern in some environments where there is a risk of exposure to flammable vapors or gases.
Brush friction is a cause of a reduced efficiency for DC motors that use them, as some of the input energy is consumed in friction and not used to generate motion.
Brushed DC motors create more noise and generate dust from the wearing of the brush, which is typically a carbon or graphite material.
Speed Control
In an AC motor, the speed of the motor is controlled by the input frequency of the alternating current supplied to the stator coils and is directly proportional. As the frequency increases, the speed of the motor increases. Variable frequency drive controllers are used to adjust the input frequency as desired to produce the desired motor rpm.
For DC motors, the speed of the device is controlled by varying the voltage and current that is applied to the armature coils or windings, or by adjusting the current that flows to the field coils (hence impacting the strength of the magnetic field for the field coil). The speed-current relationship is again a proportional one.
Startup Mechanism
Polyphase AC motors are designated as self-starting, requiring no additional electronics beyond the variable frequency control for speed. Single-phase AC motors, as well as DC motors, both require a start-up mechanism for controlling start-up conditions. As an example, in large DC motors, the back EMF generated in the armature is proportional to the speed of the armature and is therefore small at start-up. This condition can cause a large current flow to the armature, potentially causing burnout. Thus, controlling the input voltage ramp-up at start-up is needed for these motors.
Performance
AC motors are often used for their high-speed and variable torque, but typically torque will exhibit a drop as the motor speed increases. DC motors can produce high torque and are valuable where speed control is needed. DC motors can provide a more constant torque over the speed range, and generally provide faster response to load changes that AC motors. Depending on the configuration of the coil connection (series versus parallel), different performance across load value for DC motors can be obtained. Series motors exhibit higher starting torque but have a steeper drop-off in speed as the load increases. Parallel or shunt DC motors provide lower starting torque but have a flatter speed vs. load relationship and can, therefore, provide a constant speed almost independent of the load applied.
AC motors suffer from efficiency issues because of the induction current loss and the slip mentioned earlier. DC motors that use permanent magnets can be some 30% more efficient as they do not have to consume power to create an electromagnet, but there is some loss in efficiency due to the energy loss from the friction of brushes. Brushless DC motors are more efficient than those with brushes, but the efficiency gains are primarily at the low-load or no-load areas of the motor performance curve.
Other Considerations
For a given amount of mechanical work output, AC motors are usually larger than DC motors, with brushless DC designs being the smallest. AC motors have a long service life while DC motors require more maintenance for those designs that use brushes and commutators which feature mechanical wear. Electronically Commutated Motors (ECMs) are a form of brushless DC motor that eliminates mechanical commutation and brushes in favor of electronic commutation and control, therefore improving useful life, reducing power consumption, running cooler, and providing better performance.
Слова:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|