AC MOTOR AND DC MOTOR


 

AC MOTOR  and DC MOTOR

 

While both A.C. and D.C. motors serve the same function of converting electrical energy into mechanical energy, they are powered, constructed and controlled differently. 1 The most basic difference is the power source. A.C. motors are powered from alternating current (A.C.) while D.C. motors are powered from direct current (D.C.), such as batteries, D.C. power supplies or an AC-to-DC power converter. D.C wound field motors are constructed with brushes and a commutator, which add to the maintenance, limit the speed and usually reduce the life expectancy of brushed D.C. motors. A.C. induction motors do not use brushes; they are very rugged and have long life expectancies. The final basic difference is speed control. The speed of a D.C. motor is controlled by varying the armature winding’s current while the speed of an A.C. motor is controlled by varying the frequency, which is commonly done with an adjustable frequency drive control.

 

 

 

AC Motors

AC motors are powered with alternating current and convert electrical energy into mechanical energy.  There are three types of alternating current motors with three-phases. AC induction motors the most commonly used, for AC voltage, the voltage on which they run, is readily available at any outlet.  All AC motors, no matter their type, are comprised of a stator, which produces the magnetic field, and a rotor, which is made to rotate by the magnetic field that is induced from the current generated by the stator. 

When choosing the right AC motor for your application there are two key components to keep in mind; the running speed, or how fast the motor will turn as measured in RPMS, and the starting torque, or how much force is needed (if any) to start the motor.  By providing these specifications to an experienced engineer, you can be sure you will receive the most effective and cost efficient AC motor for your application.

AC vs. DC

The main difference between AC motors and  DC motors can be found in the name itself for,  DC motors run on direct current as opposed to alternating current on which AC motors run.

With direct current, energy is obtained from batteries or cells and generates a constant voltage.  This constant voltage then stimulates the steady flow of electrons in one direction.  It is this constant and precise flow of energy that makes direct current most suitable for applications that require precise and stable operation, such as speed and torque control applications.

With alternating current, energy is obtained through both AC generators and mains to generate a varying voltage.  It is because of this varying voltage that the electrons of an alternating current, unlike those of a direct current, continuously change direction, moving forward and backward.  Also in contrast to direct current, which is unable to produce voltage capable of traveling over long distances, the voltage of alternating current is safe to transfer over long distance and provide greater power, making alternating current ideal for both home and industrial applications.

 

 MOTOR WITH BRUSH OR WITHOUT BRUSH?

 

 

To Brush                               When it comes to a loosely defined range of basic applications, one could use either a brush or brushless motor. And like any comparable and competing technologies, brush and brushless motors have their pros and cons.

 

On the pro side, brush motors are generally inexpensive and reliable. They also offer simple two-wire control and require fairly simple control or no control at all in fixed-speed designs. If the brushes are replaceable, these motors also boast a somewhat extended operational life. And because they need few external components or no external components at all, brush motors tend to handle rough environments reliably.

 

For the downside, brush motors require periodic maintenance as brushes must be cleaned and replaced for continued operation, ruling them out for critical medical designs. Also, if high torque is required, brush motors fall a bit flat. As speed increases, brush friction increases and viable torque decreases.

 

However, torque may not be an issue in some applications and could actually be desirable. For example, electric toothbrushes require higher speeds with decreasing torque, which is good for the brush and your teeth and gums.

 

Other disadvantages of brush dc motors include inadequate heat dissipation caused by the rotor limitations, high rotor inertia, low speed range due to limitations imposed by the brushes, and electromagnetic interference (EMI) generated by brush arcing.

 

Or Not To Brush                               BLDC motors have a number of advantages over their brush brothers. For one, they’re more accurate in positioning apps, relying on Hall effect position sensors for commutation. They also require less and sometimes no maintenance due to the lack of brushes.

 

They beat brush motors in the speed/torque tradeoff with their ability to maintain or increase torque at various speeds. Importantly, there’s no power loss across brushes, making the components significantly more efficient. Other BLDC pros include high output power, small size, better heat dissipation, higher speed ranges, and low-noise (mechanical and electrical) operation.

 

Nothing is perfect, though. BLDC motors have a higher cost of construction. They also require control strategies that can be both complex and expensive. And, they require a controller that can cost almost as much as if not more than the BLDC motor it governs.

 

 

INDUCTION MOTOR & SYNCHRONOUS MOTOR

All induction motors are asynchronous motors. The asynchronous nature of induction-motor operation comes from the slip between the rotational speed of the stator field and somewhat slower speed of the rotor. A more-specific explanation of how this slip arises gets into details of the motor internals.

 

Most induction motors today contain a rotational element (the rotor) dubbed a squirrel cage. The cylindrical squirrel cage consists of heavy copper, aluminum, or brass bars set into grooves and connected at both ends by conductive rings that electrically short the bars together. The solid core of the rotor is built with stacks of electrical steel laminations. The rotor contains fewer slots than the stator. The number of rotor slots must also be a nonintegral multiple of stator slots so as to prevent magnetic interlocking of rotor and stator teeth when the motor starts.

It is also possible to find induction motors containing rotors made up of windings rather than a squirrel cage. The point of this wound-rotor configuration is to provide a means of reducing the rotor current as the motor first begins to spin. This is generally accomplished by connecting each rotor winding to a resistor in series. The windings receive current through some kind of slip-ring arrangement. Once the rotor reaches final speed, the rotor poles get switched to a short circuit, thus becoming electrically the same as a squirrel cage rotor.

The stationary part of the motor windings is called the armature or the stator. The stator windings connect to the ac supply. Applying a voltage to the stator causes a current to flow in the stator windings. The current flow induces a magnetic field which affects the rotor, setting up voltage and current flow in the rotor elements.

A north pole in the stator induces a south pole in rotor. But the stator pole rotates as the ac voltage varies in amplitude and polarity. The induced pole attempts to follow the rotating stator pole. However, Faraday’s law says that an electromotive force is generated when a loop of wire moves from a region of low magnetic-field strength to one of high magnetic-field strength, and vice versa. If the rotor exactly followed the moving stator pole, there would be no change in magnetic-field strength. Thus, the rotor always lags behind the stator field rotation. The rotor field always lags behind the stator field by some amount so it rotates at a speed that is somewhat slower than that of the stator. The difference between the two is called the slip.

 

The amount of slip can vary. It depends principally on the load the motor drives, but also is affected by the resistance of the rotor circuit and the strength of the field that the stator flux induces.

 

A few simple equations make the basic relationships clear.

 

When ac is initially applied to the stator, the rotor is stationary. The voltage induced in the rotor has the same frequency as that of the stator. As the rotor starts spinning, the frequency of the voltage induced in it, fr, drops. If f is the stator voltage frequency, then slip, s, relates the two via fr = sf. Here s is expressed as a decimal.

 

When the rotor is standing still, the rotor and stator effectively form a transformer. So the voltage E induced in the rotor is given by the transformer equation

 

E = 4.44 f N фm

 

where N = the number of conductors under one stator pole (typically small for a squirrel-cage motor) and фm = maximum magnetic flux, Webers. Thus, the voltage Er induced while the rotor spins depends on the slip:

 

Er = 4.44 s f N фm = s E

 

Explanation of synchronous motors

 

A synchronous motor has a special rotor construction that lets it rotate at the same speed — that is, in synchronization — with the stator field. One example of a synchronous motor is the stepping motor, widely used in applications that involve position control. However, recent advances in power-control circuitry have given rise to synchronous-motor designs optimized for use in such higher power situations as fans, blowers, and to drive axles in off-road vehicles.

 

There are basically two types of synchronous motors:

 

• Self-excited — Using principles similar to those of induction motors, and

 

• Directly excited — usually with permanent magnets, but not always

 

The self-excited synchronous motor, also called a switched-reluctance motor, contains a rotor cast of steel that includes notches or teeth, dubbed salient poles. It is the notches that let the rotor lock in and run at the same speed as the rotating magnetic field.

 

To move the rotor from one position to the next, circuitry must sequentially switch power to consecutive stator windings/phases in a manner analogous to that of a stepping motor. The directly excited synchronous motor may be called by various names. Usual monikers include ECPM (electronically commutated permanent magnet), BLDC (brushless dc), or just a brushless permanent-magnet motor. This design uses a rotor that contains permanent magnets. The magnets may mount on the rotor surface or be inserted within the rotor assembly (in which case the motor is called an interior permanent-magnet motor).

 

The permanent magnets are the salient poles of this design and prevent slip. A microprocessor controls sequential switching of power on the stator windings at the proper time using solid-state switches, minimizing torque ripple. The principle of operation of all these synchronous-motor types is basically the same. Power is applied to coils wound on stator teeth that cause a substantial amount of magnetic flux to cross the air gap between the rotor and stator. The flux flows perpendicular to the air gap. If a salient pole of the rotor is aligned perfectly with the stator tooth, there is no torque produced. If the rotor tooth is at some angle to the stator tooth, at least some of the flux crosses the gap at an angle that is not perpendicular to the tooth surfaces. The result is a torque on the rotor.Thus, switching power to stator windings at the right time causes a flux pattern that results in either clockwise or counterclockwise motion.

 

 

One other type of synchronous motor is called a switched reluctance (SR) motor.

 

 

 

Its rotor consists of stacked steel laminations with a series of teeth. The teeth are magnetically permeable, and the areas surrounding them are weakly permeable by virtue of slots cut into them. Thus the rotor needs no windings, rare-earth materials, or magnets.

 

Unlike induction motors, there are no rotor bars and consequently no torque-producing current flow in the rotor. The absence of any form of conductor on the SR rotor means that overall rotor losses are considerably lower than in other motors incorporating rotors carrying conductors. Torque produced by the SR motor is controlled by adjusting the magnitude of current in the stator electromagnets. Speed is then controlled by modulating the torque (via winding current). The technique is analogous to the same way speed is controlled via armature current in a traditional brush-dc motor.

 

An SR motor produces torque proportional to the amount of current put into its windings. Torque production is unaffected by motor speed. This is unlike ac-induction motors where, at high rotational speeds in the field-weakening region, rotor current increasingly lags behind the rotating field as motor rpm rises.

 

 

 BRUSH & BRUSHLESS MOTOR

 

 

Brushless DC Motor

 


 

 

Brushless DC motors were developed from conventional brushed DC motors with the availability of solid state power semiconductors. So, why do we discuss brushless DC motors in a chapter on AC motors? Brushless DC motors are similar to AC synchronous motors. The major difference is that synchronous motors develop a sinusoidal back EMF, as compared to a rectangular, or trapezoidal, back EMF for brushless DC motors. Both have stator created rotating magnetic fields producing torque in a magnetic rotor.

 

Synchronous motors are usually large multi-kilowatt size, often with electromagnet rotors. True synchronous motors are considered to be single speed, a submultiple of the powerline frequency. Brushless DC motors tend to be small– a few watts to tens of watts, with permanent magnet rotors. The speed of a brushless DC motor is not fixed unless driven by a phased locked loop slaved to a reference frequency. The style of construction is either cylindrical or pancake. (Figures
and below)

 

Cylindrical construction: (a) outside rotor, (b) inside rotor.

 

The most usual construction, cylindrical, can take on two forms (Figure above). The most common cylindrical style is with the rotor on the inside, above right. This style motor is used in hard disk drives. It is also possible to put the rotor on the outside surrounding the stator. Such is the case with brushless DC fan motors, sans the shaft. This style of construction may be short and fat. However, the direction of the magnetic flux is radial with respect to the rotational axis.

 

Pancake motor construction: (a) single stator, (b) double stator.

 

High torque pancake motors may have stator coils on both sides of the rotor (Figure above-b).

 

Lower torque applications like floppy disk drive motors suffice with a stator coil on one side of the rotor, (Figure above-a). The direction of the magnetic flux is axial, that is, parallel to the axis of rotation.

 

The commutation function may be performed by various shaft position sensors: optical encoder, magnetic encoder (resolver, synchro, etc), or Hall effect magnetic sensors. Small inexpensive motors use Hall effect sensors. (Figure below) A Hall effect sensor is a semiconductor device where the electron flow is affected by a magnetic field perpendicular to the direction of current flow.. It looks like a four terminal variable resistor network. The voltages at the two outputs are complementary. Application of a magnetic field to the sensor causes a small voltage change at the output. The Hall output may drive a comparator to provide for more stable drive to the power device. Or, it may drive a compound transistor stage if properly biased. More modern Hall effect sensors may contain an integrated amplifier, and digital circuitry. This 3-lead device may directly drive the power transistor feeding a phase winding. The sensor must be mounted close to the permanent magnet rotor to sense its position.

 

Hall effect sensors commutate 3-φ brushless DC motor.

 

The simple cylindrical 3-φ motor Figure above is commutated by a Hall effect device for each of the three stator phases. The changing position of the permanent magnet rotor is sensed by the Hall device as the polarity of the passing rotor pole changes. This Hall signal is amplified so that the stator coils are driven with the proper current. Not shown here, the Hall signals may be processed by combinatorial logic for more efficient drive waveforms.

 

The above cylindrical motor could drive a harddrive if it were equipped with a phased locked loop (PLL) to maintain constant speed. Similar circuitry could drive the pancake floppy disk drive motor (Figure below). Again, it would need a PLL to maintain constant speed.

 

Brushless pancake motor

 

The 3-φ pancake motor (Figure above) has 6-stator poles and 8-rotor poles. The rotor is a flat ferrite ring magnetized with eight axially magnetized alternating poles. We do not show that the rotor is capped by a mild steel plate for mounting to the bearing in the middle of the stator. The steel plate also helps complete the magnetic circuit. The stator poles are also mounted atop a steel plate, helping to close the magnetic circuit. The flat stator coils are trapezoidal to more closely fit the coils, and approximate the rotor poles. The 6-stator coils comprise three winding phases.

 

If the three stator phases were successively energized, a rotating magnetic field would be generated. The permanent magnet rotor would follow as in the case of a synchronous motor. A two pole rotor would follow this field at the same rotation rate as the rotating field. However, our 8-pole rotor will rotate at a submultiple of this rate due the the extra poles in the rotor.

 

The brushless DC fan motor (Figure below) has these feature:

 

Brushless fan motor, 2-φ.

 

  • The stator has 2-phases distributed between 4-poles
  • There are 4-salient poles with no windings to eliminate zero torque points.
  • The rotor has four main drive poles.
  • The rotor has 8-poles superimposed to help eliminate zero torque points.
  • The Hall effect sensors are spaced at 45o physical.
  • The fan housing is placed atop the rotor, which is placed over the stator.

 

The goal of a brushless fan motor is to minimize the cost of manufacture. This is an incentive to move lower performance products from a 3-φ to a 2-φ configuration. Depending on how it is driven, it may be called a 4-φ motor.

 

You may recall that conventional DC motors cannot have an even number of armature poles (2,4, etc) if they are to be self-starting, 3,5,7 being common. Thus, it is possible for a hypothetical 4-pole motor to come to rest at a torque minima, where it cannot be started from rest. The addition of the four small salient poles with no windings superimposes a ripple torque upon the torque vs position curve. When this ripple torque is added to normal energized-torque curve, the result is that torque minima are partially removed. This makes it possible to start the motor for all possible stopping positions. The addition of eight permanant magnet poles to the normal 4-pole permanent magnet rotor superimposes a small second harmonic ripple torque upon the normal 4-pole ripple torque. This further removes the torque minima. As long as the torque minima does not drop to zero, we should be able to start the motor. The more successful we are in removing the torque minima, the easier the motor starting.

 

The 2-φ stator requires that the Hall sensors be spaced apart by 90o electrical. If the rotor was a 2-pole rotor, the Hall sensors would be placed 90o physical. Since we have a 4-pole permanent magnet rotor, the sensors must be placed 45o physical to achieve the 90o electrical spacing. Note Hall spacing above. The majority of the torque is due to the interaction of the inside stator 2-φ coils with the 4-pole section of the rotor. Moreover, the 4-pole section of the rotor must be on the bottom so that the Hall sensors will sense the proper commutation signals. The 8-poles rotor section is only for improving motor starting.

 

Brushless DC motor 2-φ push-pull drive.

 

In Figure above, the 2-φ push-pull drive (also known as 4-φ drive) uses two Hall effect sensors to drive four windings. The sensors are spaced 90o electrical apart, which is 90o physical for a single pole rotor. Since the Hall sensor has two complementary outputs, one sensor provides commutation for two opposing windings.