Engineering Tech

A DC MACHINE DC [ Direct current ] Motor is an electric motor that supplies power to machines as it uses electrical energy to perform its main function.

The introduction of this motor in the 1870s paved the way for the second Industrial Revolution.

The primary principle that makes a DC Machine motor work is based on how Magnets react to each other: like magnetic poles repel and unlike magnetic poles attract. [ When you look at a DC motor, you will see a coil of wire (armature) and a horseshoe magnet (stator) as part of its basic components. Every time an electric current run through the coil, an electromagnetic field is generated and aligned to the center of the coil. As you switch the current on or off, the magnetic field is also turned on or off. ]

DC Motor Classification Chart

Types of DC machine

The types of DC motor include:

• Permanent Magnet DC Motor (PMDC Motor)
• Separately Excited DC Motor.
• Self Excited DC Motor.
• Shunt Wound DC Motor.
• Series Wound DC Motor.
• Compound Wound DC Motor.
• Short shunt DC Motor.
• Long shunt DC Motor.
• Differential Compound DC Motor

PMDC Motor [ Permanent Magnet DC Motor ]

The ability to produce motion from electricity is possible thanks to the electric motor. They are a diverse class of machines that provide power for a staggering amount of applications, and currently rule automation, manufacturing, commercial products, and more. The versatility of these motors comes from the many types of electric motors available, and this article will explore a promising design, the Permanent Magnet Motors While initially developed early on, this motor is quickly becoming a capable alternative to industry standards thanks to the advancements of the 21st century.

This motor, its working principles, and its applications will be investigated in this discussion and will show why this motor has gained so much attention in recent years.

What are Permanent Magnet Motors?

Permanent magnet motors are an advanced motor similar to both Induction Motor and Servo motor in design (more information on these two designs can be found in our articles All about induction motor and servo motors).

They are composed of a stator – the outside housing – and a rotor – the moving component connected to the output shaft of the motor. Much like other AC motor, the permanent magnet motor harnesses the physics of electromagnetism to generate torque, and they do this by using Permanent Magnet Motors (usually rare earth magnets) embedded in their rotor. This design deviates from most other electric motors, where the rotor either generates its own magnetic field via induction or a through the use of DC power source or is simply composed of a ferromagnetic metal.

The magnets in a permanent magnet motor, when properly arranged in relation to the stator, can provide speeds equal to the excitation current frequency, and so are considered a Synchronous motor (see our related article all about Synchronous motor to learn more). These motors must be paired with an electronic component that smooths out the torque of this motor, and is why these machines have only recently hit their stride as a viable design.

How do Permanent Magnet Motors Work?

The fundamental operation of a permanent magnet motor is like most electric motors; the outer stator holds windings of coils fed by a power source, and the rotor freely rotates based on the forces imparted by the stator coils.

Many of the same basic principles for induction motors hold true for permanent magnet motors, and more information can be found in our article all about induction Motor. This is not to say that they are purely AC machines; in fact, for most of their life, they have been implemented as permanent magnet DC motors (PMDCM) for small applications. However, The power of PMDCMs are quite weak, and this article will mainly focus on permanent magnet AC motors (PMACM), as they come in larger sizes, offering greater horsepower, and can meet induction motors eye-to-eye in terms of strength, efficiency, and amount of uses.

The defining feature of PMACMs – the permanent magnets within their rotor – are acted upon by the rotating magnetic field (RMF) of the stator windings, and are repelled into rotational motion. This is a deviation from other rotors, where the magnetic force must be induced or generated in the rotor housing, requiring more current. This means that PMACMs are generally more efficient than induction motors, as the rotor’s magnetic field is permanent and does not need a source of power to be used for its generation.

This also means that they require a ( Variable Frequency drive ) VFD, or PM drive) to operate, which is a control system that smooths out the torque produced by these motors. By switching the current on and off to the stator windings at certain stages of rotor rotation, the PM drive simultaneously controls torque and current and uses this data to calculate rotor position, and therefore the speed of the shaft output.

They are synchronous machines, as their rotational speed matches the speed of the RMF. These machines are relatively new and are still being optimized, so the specific operation of any one PMACM is, for now, essentially unique to each design.

Permanent Magnet Motors Specifications

PMACMs are rated similarly to induction motors, and a refresher on the basic specifications common to these motors can be read about in our articles on Induction Motors. Below are some important specifications specific to PMACMs, which can help designers choose the right motor for their job.

Phase type

PMACMs are, most of the time, powered by a three-phase AC input meant to produce a rapid RMF, making them type of Three phase motor. It is important to understand the phase of the motor at hand, as single-phase motors are inherently not self-starting, and three-phase motors usually come in higher-rated voltages/torques. More information can be found in our articles on single phase motor.

Poles & motor cogging

The poles of a motor are simply the north-south magnetic points on the stator and rotor. In PMACMs, these poles are permanent in the rotor and are switched in the stator to produce rotation. A phenomenon known as motor cogging can occur, where the constant overcoming of attraction and repulsion of the permanent magnets causes unwanted jerking during rotor spinning. 

Cogging usually happens upon startup of the motor and can cause vibrations, noise, and uneven rotation. Increasing the number of poles in a PMACM helps reduce this issue as well as the torque-ripple effect (more information on torque ripple can be found in our article on reluctance motors). PMACMs therefore typically have more poles than induction motors, suggesting that they need a higher input frequency to achieve similar rotation speeds.

Saliency & closed-loop feedback

It is necessary for these motors to come with specialized control system equipment that allows them to function most effectively. In PMACMs, saliency is the difference in inductance at the motor terminals as the rotor is rotated. This difference can misalign the rotor and stator, which can cause unwanted cogging/failure. Closed-loop feedback is used to address this issue by tracking the exact rotor position via sensors and then changing the input current and speed to ensure that the motor rotation remains continuous.

Applications and Selection Criteria

Since these motors are still being designed, it is difficult to provide a fool-proof method for selection. It is more useful to highlight the benefits of these motors over current designs, and also their pitfalls which may be cause to choose another, more conventional motor.

The most enticing advantage of PMACMs is that they sport higher efficiency, thanks to their simplified rotor. This efficiency is exceptional with small torque loads and can save many kWh of energy in these arrangements. These savings also increase with motor size, allowing PMACMs to compete with conventional induction motors for high speed, high torque applications. The higher power density of PMACMs combined with their high-speed capabilities and efficiency can give induction motors such as the classic Squirrel cage and wound motors a run for their money.

They also tend to have a smaller footprint and are great for retrofitting older systems with newer, smaller, and more powerful PMACMs. While more expensive than induction motors in their initial product cost, PMACMs and their energy savings can realize a full return on investment in a little over a year. They are also synchronous, which allows them to work in applications where induction motors cannot. PMACMs also run cooler than induction motors, which increases their reliability and lifespan.

Separately Excited DC Motor.

In this section will discuss about separately excited DC motor. Direct current (DC) motors converts electrical energy into mechanical energy and they works on dc supply.

In this section we will discuss about the separately excited dc motor. Like other DC motors, these motors also have both stator and rotor. Stator refers to the static part of motor, which consists of the field windings. And the rotor is the moving armature which contains armature windings or coils. Separately excited dc motor has field coils similar to that of shunt wound dc motor. The name suggests the construction of this type of motor. Usually, in other DC motors, the field coil and the armature coil both are energized from a single source. The field of them does not need any separate excitation. But, in separately excited DC motor, separate supply Provided for excitation of both field coil and armature coil. Figure below shows the separately excited dc motor.

Here, the field coil is energized from a separate DC voltage source and the armature coil is also energized from another source.

Armature voltage source may be variable but, independent constant DC voltage is used for energizing the field coil. So, those coils are electrically isolated from each other, and this connection is the specialty of this type of DC motor.

Equations Of Voltage, current and power for DC motors

In a separately excited motor, armature and field windings are excited form two different dc supply voltages. In this motor,

• Armature current Ia = Line current = IL = I
• Back emf developed , Eb = V – I Ra
where V is the supply voltage and Ra is the armature resistance.
• Power drawn from main supply , P = VI
• Mechanical power developed ,

Pm = Power input to armature – power loss in armature

Operating characteristics of Separately excited dc motor

Both in shunt wound dc motor and separately excited dc motor field is supplied from constant voltage so that the field current is constant. Therefore these two motors have similar speed -armature current and torque – armature current characteristics. In this type of motor flux is assumed to be constant.

• Speed – armature current (N – Ia) characteristics: 

We know that speed of dc motor is proportional to back emf / flux i.e Eb / φ . When load is increased back emf Eb and φ flux decrease due to armature resistance drop and armature reaction respectively .However back emf decreases more than φ so that the speed of the motor slightly decreases with load.

• Torque – armature current ( τ – Ia) characteristics :

Here torque is proportional to the flux and armature current . Neglecting armature reaction, flux φ is constant and torque is proportional to the armature current Ia . τ – Ia characteristics is a straight lien passing through the origin. From the curve we can see that huge current is needed to start heavy loads. So this type of motor do not starts on heavy loads.

Speed control of separately excited DC Machine motor

Speed of this type of dc shunt motor is controlled by the following methods:
I. Field control methods: Weakening of field causes increase in speed of the motor while strengthening the field causes decreases the speed. Speed adjustment of this type of motor is achieved from the following methods:
II. Field rheostat control: – Here a variable resistance is connected in series with the field coil. Thus the speed is controlled by means of flux variation.
Reluctance control involving variation of reluctance of magnetic circuit of motor.
Field voltage control by varying the voltage at field circuit while keeping armature terminal voltage constant.
III. Armature control methods: Speed adjustment of separately excited DC motor by armature control may be obtained by any one of the following methods :
i. Armature resistance control: – Here, the speed is controlled by varying the source voltage to armature. Generally, a variable resistance is provided with the armature to vary the armature resistance.
ii. Armature terminal voltage control involving variation of variation of voltage in armature circuit.

Self Excited DC Motor

Energy can neither be created nor destroyed, but can be transformed from one form to the other” is the fundamental law of universe. A DC machine transforms electrical energy to mechanical (motor) or vice-versa. The working principle in both the cases remains the same. The DC motor finds its applications in fields of engineering and technology ranging from an electric shaver to parts of automobiles, in all small or medium sized motoring.

[A]. SHUNT WOUND SELF EXCITED DC Machine MOTOR

SHUNT WOUND TYPE

In this case , the field winding are exposed to the entire terminal voltage as they are connected in parallel to the armature winding as shown in the figure. The shunt wound dc motor is a constant speed motor, as the speed does not vary here with the variation of mechanical load on the output. It comes under the category of Self excited DC Machine Motor.

[B]. SERIES WOUND SELF EXCITED DC MOTOR

SERIES WOUND TYPE

In  this case, the entire armature current flows through the field winding as its connected in series to the armature winding. The series wound self excited dc motor is diagrammatically represented for better understanding. In a series wound dc motor, the speed varies with load. And operation wise this is its main difference from a shunt wound dc machine motor.

[C]. COMPOUND WOUND SELF EXCITED DC MOTOR

COMPOUND WOUND SELF EXCITED TYPE

The compound excitation characteristic in a dc motor is obtained by combining the operational characteristic of both the shunt and series excited dc motor. The compound wound self excited dc motor or simply compound wound dc motor essentially contains the field winding connected both in series and in parallel to the armature winding. The excitation of compound wound dc motor can be of two types depending on the nature of compounding.

Cumulative Compound DC machine Motor, in which the shunt field flux assists the main field flux, produced by the main field connected in series to the armature winding. φtotal = φseries + φshunt.

Differential compound dc motor, in which the arrangement of shunt and series winding is such that the field flux produced by the shunt field winding diminishes the effect of flux by the main series field winding. The net flux produced in this case is lesser than the original flux and hence does not find much of a practical application. φtotal = φseries – φshunt

Both the cumulative compound and differential compound dc motor can either be of short shunt or long shunt type depending on the nature of arrangement.

Short Shunt DC Motor

If the shunt field winding is only parallel to the armature winding and not the series field winding then its known as short shunt dc motor or  specifically short shunt type compound wound dc motor. Shunt is basically an arm that is connected in parallel, so it is c a Short Shunt DC Motor. DC Shunt motor is used in devices where Speed control is essential. Its major application is in centrifugal pumps as they produces constant flux and

Long Shunt DC Motor

If the shunt field winding is parallel to both the armature winding and the series field winding then it’s known as long shunt type compounded wound dc motor or simply long shunt dc motor machine.