Permanent Magnet DC Motor (PMDC)
The Permanent Magnet DC motor or shortly known as PMDC motor is another type of brushed DC motor. It has the usual armature like the rest of brushed DC electric motors explained above. However, there is no stator or field winding, the magnetic field is generated using a permanent magnet placed in the stator.
When the armature windings carrying the input current is placed inside the N & S poles of the magnet. The magnetic field interacts with it & the armature experience rotational force. The permanent magnet generates a fixed magnetic field which is designed at the time of the construction & it can’t be changed after that. However, the strength of the magnet reduces with time. There is an extra excitation field found in some designs that helps increase its magnetic strength when it reduces. PMDC does not need field excitation to generate the field flux as it is produced by the permanent magnet. This increases its efficiency as no extra power is consumed for excitation. The absence of the field windings greatly reduces the size of overall motor. Therefore PMDC motors have compact designs. They are also very cheaper & best for low power applications.
Brushless DC Motor
As the name suggests, Brushless or BLDC Motor is an another main type of DC motor that does not have any carbon brushes & commutators assembly. It means the input power is not supplied to the rotating part of the motor but to the stator of the motor which in this case is made of multiple windings & the rotor is made of a permanent magnet.
It has multiple stator windings each situated at a different angle to generate flux in different directions. The input is switched between the stator’s windings to generate a magnetic field that pushes & pull on the rotor’s magnetic field causing it to rotate in its direction. A Hall Effect sensor is used to detect the position of the rotor & switch the input to the correct stators winding respectively.
Since the DC input to the stator needs to be switched, such electric motors used electronic commutations instead of mechanical commutation using switching devices such as thyristors. These switches are controlled using microcontroller to precisely switch the input between the stators windings. It essentially switches the DC input into a 3-phase supply which generates a smooth rotating magnetic field. The brushless motor speed depends on the frequency of the AC power supplied by the controller. Which is why it is also called synchronous motor The controller used for brushless motor is more sophisticated & very expensive. It does no operate without its controller which also offers the precise speed control & positioning of the rotor. But the cost of the controller is far greater than the motor itself. Since there are no brushes, there are no electrical or electromagnetic noises & the sparks generated in mechanical commutation. It helps in increasing the life span of the motor as well as the efficiency of the motor. The energy dissipated in brushes is converted into mechanical output. And they are also maintenance free.
Coreless or Ironless DC Motors
As the name suggests, such DC motors have no laminated iron core. The rotor winding is wound in skewed or honeycomb shape to form a self-supporting hollow cage often made with using epoxy. The rotor made of permanent magnet sets in the hollow rotor. The coreless design eliminates the issues & losses associated with the iron cores of the traditional motors. For example, such electric motors have no iron losses which increases the efficiency of the motor up to 90%. The design also reduces the winding inductance which reduces the sparks generated between the brushes & commutator thus increasing the lifetime of the motor. It also reduces the mass & inertia of the rotor which also increases the acceleration & deceleration rate of the motor.
Special Motors
There are several types of special electric motors that are the modified versions of other motor designed for special purposes. Some of these electric motors are given below.
Servo Motors
Servo motor is a special type of motor used for pushing/Pulling or lifting or rotating an object at some specific angle. Servo motor can be designed to run on AC as well as DC power supply. Servo motor that runs on DC supply is called DC servo motor While those that runs on AC is called AC servo motor. It is a simple motor with a controller & multiple gears to increase its torque.
These motors are rated in kg/cm (kilogram per centimeter). It specifies much weight the servo can lift at a specific distance. E.g. a servo rated 3kg/cm can lift a load of 3kg that is at 1cm away from its shaft. The weightlifting capacity decreases with an increase in the distance. The servo motor has a gear assembly, controller, a sensor and a feedback system. The gear assembly is used to decrease the speed & increase its torque significantly. The controller is used for comparing the input signal (desired position) & signal from the sensor (actual position of the servo) which is acquired through the feedback system. The controller compares these two signals & eliminated the error between them by rotating the motor shaft. Servo motors have three wires. Two of them are used for providing the power supply while the third one is used for controlling the servo’s position. It is controlled by providing a pulsating signal through a microcontroller using PWM (pulse width modulation). The servo can rotate 90° in either direction making it a total of 180° rotation. Neutrally it is in the middle position at 90°. It can rotate by varying the pulse width between 1ms & 2ms where 1ms corresponds to 0°, 1.5ms corresponds to 90° while 2ms corresponds to 180° angle of the shaft.
Direct Drive
Direct drive motor or also known as torque motor is another type of motor that produces high torque at low speed even when it is stalling. The payload is directly connected to the rotor thus eliminating the use of gearbox, belts, speed reducers etc. It is a brushless permanent magnet synchronous motor with no commutators & brushes. Since there is no mechanical wear & tear, it reliable & has a long lifetime. The fact that it has less mechanical parts means it require less maintenance and low cost.
Linear Motors
The linear motor has an unrolled stator & rotor that offers a linear force instead of rotational force. If you slice any motor & lay it at on a surface, you will get a linear motor. The armature windings are designed in a linear fashion which carries 3 phase current to generate a magnetic field. the magnetic field does not rotate instead moves in a straight line. The magnetic field interacts with the magnetic field generated by the at permanent magnet lying below it. The interaction between them generates a linear force upon each other thus the armature moves forward or backward. It is an AC powered motor with a controller such as in servo motor. The power is supplied to the primary part of the motor that contains windings. It generates its own magnetic field whose polarity depends on the phase of the AC supply. The secondary part of the motor is permanent magnet whose magnetic field interacts with the magnetic field of the primary part & as a result attracts & repels it by generating a linear force. The amount of current determines the force while the rate of change of current determines the speed of the primary part. Linear motors are used in robotics, medical device & factory automation etc.
Stepper Motor
A stepper motor or a stepping motor is a brushless DC motor whose full rotation is divided into a number of equal steps. Such motor rotates in steps (fixed degrees) instead of rotating continuously. Such stepping movement offers great precision which is utilized is robotics.
The stepper motor operates on pulses. Each pulse moves the motor by one step. The precision of the motor depends on the number of steps per revolution. The steps size is determined during its design. However, the speed of the motor can be controlled by applying the pulse train of variable frequency. The controller inside the servo motor moves forward or backward the rotor by one step upon each pulse. It is used for its accurate & precise positioning. It offers full torque at standstill. It has less maintenance requirement due to brushless design. Thus they are very reliable & has long lifetime. Stepper motor due to its precise positioning is used in industrial machines used for automatic manufacturing of products, CNC based machines. It also found applications in medical instruments & machinery as wells as in security cameras. Stepper is widely used in electronic gadgets & other smart electronic systems.
Universal Motor
The Universal motor is a special type of motor that can run on AC as well DC power supply. it is a brushed series wound motor where the field windings is connected in series with the armature windings. They offer maximum starting torque with a high operating speed. Since the windings are connected in series, the direction of the current through both windings remains the same even if the current direction reverses multiple times in a second. Although, the motor may run slower on AC due to the reactance of the windings.
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