Guide to Industrial Automation -- Components and Hardware: AC and DC Motors

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.An electric machine is a link between an electrical system and a mechanical system. The process of converting energy from one of these forms to the other is electromechanical energy conversion. In these machines, the process is reversible. If the conversion is from mechanical to electrical, the machine is acting as a generator, and if the conversion is from electrical to mechanical, the machine is acting as a motor.

Three types of electrical machines are used extensively for electromechanical energy conversion: DC, induction, and synchronous motors. Other types of motors are permanent magnet (PM), hysteresis, and stepper motors. Conversion from electrical to mechanical energy is based on two electromagnetic principles: when a conductor moves within a magnetic field, voltage is induced in the conductor; simultaneously, when a current-carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. In a motor, an electrical system makes current flow through conductors placed in the magnetic field and a force is exerted on each conductor.

If the conductors are placed on a structure that is free to rotate, an electromagnetic torque is produced, making the structure rotate. This rotating structure is called a rotor. The part of the machine that does not move and provides the magnetic force is called the stator. Usually this is the outer frame of the machine or motor with the exception of special cases such as powered rollers.

Both stator and rotor are made of ferromagnetic (iron-rich) materials. The iron core is used to maximize the coupling between the coils of wire, increasing the magnetic flux density in the motor and therefore allowing its size to be reduced. In most motors, slots are cut on the inner periphery of the stator and outer periphery of the rotor and conductors are placed in the slots. If a time-varying electrical signal is placed on the stator or rotor (or both), it will cause a mechanical torque to be exerted by the rotor. The conductors placed in the slots are interconnected to form windings; the winding through which the current is passed to create the major source of magnetic flux is called the field winding, although in some motors the main source of magnetic flux is a PM.

Electric motors are used in many different applications of automated systems, from blowers, pumps, and fans to conveyors, robotics, and actuators. They may be powered by AC supplied from a power grid within the plant or a motor drive, or DC from batteries or a converter. Motors may be classified by their construction method, their source of power, or their application and the type of motion they provide. In the industrial field they are generally standardized as to size and horsepower or wattage range.

6.1 AC Motors

A typical AC motor consists of two parts: a stator having coils supplied with AC current to produce a rotating magnetic field and an inside rotor attached to an output shaft. The rotor is provided a torque by the rotating field that is generated by the alternating current.

AC motors often include designations relating to their physical construction such as TE (totally enclosed), FC (fan cooled), and PM. Other information, such as frame size, also describes motors physically, including mounting options, sealing methods, and shaft sizes. A good motor catalog will describe these options well.

Synchronous Motors

A synchronous motor is an AC machine with a rotor that rotates at the same speed as the alternating current that is applied. This is accomplished by exciting the rotor's field winding with a direct current. When the rotor rotates, voltage is induced in the armature winding of the stator; this produces a revolving magnetic field whose speed is the same as the speed of the rotor. Unlike an induction motor, a synchronous motor has zero "slip" while operating at speed.

Slip rings and brushes are used to conduct current to the rotor.

The rotor poles connect to each other and move at the same speed; hence, the name synchronous motor. Synchronous motors are used mainly in applications where a constant speed is desired and are not as common in industrial applications as induction motors.

One problem with synchronous motors is that they are not self starting. If an AC voltage is applied to the stator terminals and the rotor is excited with a field current, the motor will simply vibrate.

This is because as the AC voltage is applied it is immediately rotating the stator field at 60 Hz, which is too fast for the rotor poles to catch up to. For this reason synchronous motors have to be started by either using a variable frequency supply (such as a drive) or starting the machine as an inductive motor. If a drive is not used, an extra winding can be used called a "damper" winding. In this instance, the field winding is not excited by DC but is shunted by a resistance. Current is induced in the damper winding, producing a torque; as the motor approaches synchronous speed, the DC voltage is applied to the rotor and the motor will lock onto the stator field.

Three-Phase AC Synchronous Motors

The stator of a three-phase synchronous motor has a distributed winding called the armature winding. It is connected to the AC supply and is designed for high voltage and current. DC is then applied to the rotor coils of the motor through slip rings and brushes from a separate source. This creates a continuous field, and the rotor will then rotate synchronously with the alternating current applied to the stator.

Synchronous motors can be further divided by two different construction types: high-speed motors with cylindrical rotors and low-speed motors with salient pole rotors. The nonsalient pole or cylindrical motor has one distributed winding and a uniform air gap between the rotor and stator. The rotor is generally long and has a small diameter. These motors are often used in generators.

Salient pole motors have concentrated windings on the motor poles and a nonuniform air gap. The rotors are shorter and have a greater diameter than cylindrical rotor synchronous motors. Salient pole motors are often used to drive pumps or mixers.

One use for a synchronous motor is its use in a power factor correction scheme; these are referred to as synchronous condensers.

This method uses a feature of the motor where it consumes power at a leading power factor when its rotor is overexcited. It appears to the supply to be a capacitor, and can then be used to correct the lagging power factor that is usually presented to the electric supply by inductive loads. Since factories are charged extra for their electricity consumption if the power factor is too low, this can help correct a plant's power profile. The excitation is adjusted until a near unity power factor is obtained (often automatically). Motors used for this purpose are easily identified as they have no shaft extensions.

Single-Phase AC Synchronous Motors

Small single-phase AC motors can also be designed with PM rotors.

Since the rotors in these motors do not require any induced current, they do not slip backward against the stator frequency; instead, they rotate synchronously. Because they are very accurately synchronized with the applied frequency, which is carefully regulated at the power plant, these motors are often used to power mechanical clocks, chart recorders, or anything else that requires a precise speed.

Hysteresis synchronous motors use the hysteresis property of magnetic materials to produce torque. The rotor is a smooth cylinder of a magnetic alloy that stays magnetized but can be demagnetized fairly easily as well as remagnetized with poles in a new location. The stator windings are distributed to produce a sinusoidal magnetic flux.

Because of the hysteresis of the magnetized rotor, it tends to lag behind the rotating field. This creates a constant torque up to the synchronous speed, a useful feature for some applications. A hysteresis motor is quiet and smooth running; however, it is more expensive than a reluctance motor of the same rating.

A reluctance motor has a single-phase distributed stator winding and a cage-type rotor, often called a "squirrel cage." This is a cylindrical-shaped rotor with bars spaced around the periphery. In a reluctance motor, some of these teeth are removed. The stator of a single-phase reluctance motor has a main winding and an auxiliary starting winding. When the stator is connected to a single-phase supply, the motor starts as an induction motor. A centrifugal switch is then used to disconnect the auxiliary winding at about 75 percent of the synchronous speed. The motor continues to gain speed until it is synchronized with the rotating field. Reluctance motors are generally several times larger than an equivalent horsepower motor with DC excitation; however, because it has no slip rings, brushes, or field winding, it is low cost and fairly maintenance-free. A single-phase squirrel cage motor is shown in FIG. 43.


Centrifugal Switch Rotor Cooling Fan Bearing Power Stator

FIG. 43 Single-phase "squirrel cage" motor.


Asynchronous Motors

Induction motors are the most rugged and widely used motor for industrial applications. An induction motor has a stator and rotor with a uniform air gap between their windings. The rotor is mounted on bearings and is made of laminated sheets of ferromagnetic metal with slots cut on the outer surface. The rotor winding may be of the squirrel cage type or the wound rotor type. The stator is also made of laminations of high-grade sheet steel with distributed windings. In induction motors, alternating current is applied to both the stator and rotor windings.

Three-Phase AC Induction Motors

Windings of both the stator and the rotor of a three-phase motor are distributed over several slots in the laminated sheets. Terminals of the rotor windings are connected to three slip rings; using stationary brushes, the rotor can then be connected to an external circuit. Power applied to the three-phase windings of the stator and rotor produce rotating fields 120° apart electrically, as shown in the waveform for three-phase power in Section 2. A cutaway diagram of a three-phase induction motor is shown in FIG. 44.

Current is induced in the rotor by the rotating fields of the stator.

As the rotor rotates, the relative speed of the rotor and fields decreases as the motor speeds up. If the rotor speed were to reach the rotating field speed, the rotor would provide no torque. The difference between the rotor speed and the synchronous speed is called slip.

When loaded, standard motors have between 2 and 3 percent slip; a three-phase 60-Hz motor typically runs at 1725 to 1750 rpm as opposed to a calculated speed of 1800 rpm.

Induction motors are the most commonly used AC motors in industrial automation and are produced in standard frame sizes up to about 500 kW or 670 horsepower. This makes them easily interchangeable, though European and North American standards are different.


FIG. 44 Three-phase AC induction motor. Rotor, Terminals, Motor Shaft, Stator, Mounting Base, Cooling Fins


Single-Phase AC Induction Motors

Most single-phase induction motors have squirrel cage rotors and a single-phase distributed stator winding. Some single-phase induction motors use a wound rotor, but these are far less common. The squirrel cage motor takes its name from its shape-a ring at either end of the rotor connected by bars running along its length, forming a cage shape.

Single-phase induction motors are classified by the methods used to start them. Some common types are resistance-start or split-phase, capacitor start, capacitor run, and shaded pole.

The split-phase induction motor has a main winding and an auxiliary winding on the stator. The auxiliary winding is used for starting as described in the reluctance synchronous motor. The two windings are placed 90 electrical degrees apart and the currents of the two windings are therefore phase shifted from each other. This produces a starting torque; the auxiliary winding can then be removed from the circuit using a centrifugal switch as described previously.

If a capacitor is placed in series with the auxiliary winding, a greater phase angle is created, creating a higher starting torque. This method of starting is known as a capacitor start motor. The cost of this motor is slightly higher than that of the split-phase type, though because the circuit is only used for starting an inexpensive AC electrolytic capacitor can be used.

In a capacitor run motor, the starting capacitor and auxiliary winding are not removed from the circuit while at full speed. This requires a different kind of capacitor, usually an AC paper-oil type.

Although the capacitor is more expensive than the electrolytic type, the centrifugal switch is removed, reducing the cost. Starting torque is not as high as that of the capacitor start type; however, the motor is quieter running.

If both optimum starting and optimum running torque are desired, a combination starting method called capacitor-start capacitor run can be used. This places an electrolytic capacitor in series with the auxiliary winding and a smaller value paper-oil-type capacitor in series with the main winding. This is a more expensive motor than the others; however, it provides the best performance.

Shaded pole motors use the salient pole construction method described previously in the synchronous motor. The main winding is wound on the salient poles, but a short-circuited copper turn is placed between the main coil and the rotor, "shading" the magnetic flux as it rotates. This creates a small starting torque. This method is used in low torque applications, such as fans or small devices.

A resistance start motor is a split-phase induction motor with a resistance inserted in series with the start-up winding, creating a starting torque. The resistance provides assistance in the starting and initial direction of rotation without producing excess current. Starting torque in a resistance start motor is higher than that of a shaded pole or capacitor run motor, but not as high as a capacitor start.

6.2 DC Motors

A DC motor places the armature winding on the rotor and the field windings on the stator, which is the opposite of the AC motors described previously. It is designed to run on DC power, though it alternates the direction of current flow in the windings through commutation. The stator has salient or projecting poles excited by one or more field windings; these produce a magnetic field that is symmetrical around the pole axis, also called the field or direct axis.

The voltage induced in the armature winding alternates by using a commutator-brush combination as a mechanical rectifier. Alternatively, a brushless DC motor uses an external electronic switch synchronized to the position of the rotor.

The field and armature windings can be connected in a variety of ways to provide different performance characteristics. The field windings can be connected in series, in shunt (parallel with the armature), or as a combination of both, called a compound motor. DC motors can also have a PM.

FIG. 45 Brushed DC motor. Magnet, Ball Bearings, Spring-Loaded Brush, Electrical Terminals, Commutator, Stator Windings

Brushed DC Motors

The field winding is placed on the stator to excite the field poles and the armature winding is placed on the rotor. The commutator consists of a split ring connected to each end of the rotor windings. DC voltage is then applied to the brushes; as the rotor turns, the brushes alternately contact the different halves of the ring, changing the direction of the current flow and thereby creating an alternating field.

This field never fully aligns with the salient poles of the stator, which keeps the rotor moving.

More than one set of rings and poles can be and often are used in larger DC motors. The distance between the centers of adjacent poles is known as pole pitch, while the difference between the two sides of the coil is called coil pitch. If the coil pitch and pole pitch are equal, it is called a full-pitch coil. A coil pitch that is less than a pole pitch is known as a short pitch or fractional pitch coil. AC motors often have short pitch coils, while DC motors have full-pitch coils. FIG. 45 illustrates the construction of a brushed DC motor.

Disadvantages of Brushes

Because brushes constantly wear as they press against the commutator rings, they eventually have to be replaced. Brushes also create sparks as they cross the insulating gaps in the commutator. At high speeds the brushes have a harder time maintaining contact with the commutator; this also creates sparking. Sparking can pit the commutator surface, creating irregularities and making the contacts of the brushes bounce, which causes even more sparking. This can overheat and eventually destroy the commutator and brushes. Brushed DC motors also create quite a bit of electrical noise because of this sparking, and maximum speed is limited.

Many of the problems created by the brushes are eliminated in brushless motors, which last longer and are more efficient in their use of energy.

Some of the problems of the brushed DC motor are eliminated in the brushless design. In this motor, the mechanical "rotating switch" or commutator brush gear assembly is replaced by an external electronic switch synchronized to the rotor's position. Brushless motors are typically 85 to 90 percent efficient or more (higher efficiency for a brushless electric motor of up to 96.5 percent was reported by researchers at the Tokai University in Japan in 2009), whereas DC motors with brushes are typically 75 to 80 percent efficient.

Brushless DC Motors

The brushless DC motor replaces the brushes and commutator with an electronically alternating pulse that is synchronized to the position of the rotor. Hall effect sensors are used to sense the position of PMs on the rotor and the driving coils are activated sequentially. Coils are usually arranged in groups of three, acting very similarly to a three phase synchronous motor.

Another method of sensing rotor position is by detecting the back-EMF in the inactivated driving coils. This allows the drive electronics to sense both speed and position of the motor. These motors are often used in applications where very accurate speed control is required.

Brushless DC motors last much longer than those with brushes and run cooler than AC motors. They are very quiet from an electrical noise standpoint as well as audibly. Since they do not create sparks like motors with brushes, they are better suited to chemical or explosive environments.

Coreless or Ironless DC Motors

A motor capable of very rapid acceleration is the coreless or ironless motor. This motor makes use of a very lightweight rotor by making it almost entirely of the windings themselves with no steel or ferromagnetic material in the rotor. This method of construction can be used for brush and commutator or brushless motors. The rotor can either be placed inside the stator magnets or form a cylindrical basket shape outside the stator. Windings for these rotors are often encapsulated in epoxy for physical stability. These types of motors

are also typically rather small. They also tend to generate quite a bit of heat since there is no metal to act as a heat sink; this often necessitates an additional cooling method, such as forcing air over the rotor windings.

Universal Motors and Series Wound DC Motors

DC motors with the field and armature windings placed in series allow the motor to run on either AC or DC power. These motors are called universal or series wound motors. Though very flexible as far as power usage, they have several disadvantages when comparing them with standard AC or DC varieties.

As a universal motor increases its speed, its torque output decreases, making it impractical for high-speed high torque applications. Without a load attached, these motors also tend to "run away," potentially damaging the motor. A permanent load such as a cooling fan is often attached to the shaft to limit this problem. The high starting torque can be useful in some starting applications.

Universal motors operate better using DC than AC and are best for intermittent use. Accurate speed control can also be problematic.

6.3 Linear Motors

Linear motors operate in a similar manner to standard electric motors except that the rotor and stator are placed next to each other in a linear fashion, or "unrolled." Generally linear motors are classified as either low or high acceleration. AC linear induction motors (LIMs) are used for high acceleration applications. Typically they use a powered stator winding with a conducting plate as the rotor carrying the load.

Linear synchronous motors (LSMs) are used for larger motors requiring high speed or high torque. They also use a powered stator winding but use an array of alternating pole magnets mounted to the load-bearing frame as a rotor. These motors have a lower acceleration than the LIM type.

6.4 Servomotors and Stepper Motors

Servomotors are specially designed and built for use in feedback control systems. This requires a high speed of response, which servomotors achieve by having a low rotor inertia. Servomotors are therefore smaller in diameter and longer than typical AC and DC motor form factors. They must often operate at low or zero speed, which makes them typically larger than conventional motors with a similar power rating. Peak torque values are often 3x continuous torque ratings, but may be as high as 10x.

Servo power ratings can range from a fraction of a watt to several hundred watts. Within a specific power range, different inertias may also be specified by some motor manufacturers. They are used in a wide variety of industrial applications, such as robots, machine tools, positioning systems, and process control. Both AC and DC servomotors are used in industry.

Brushless servomotors often use sinusoidal commutation to produce smooth motion at lower speeds. If the more traditional trapezoidal or "six-step" DC commutation method is used, motors tend to "cog" or produce a jerky motion at low speed, partially because of the low inertia of servomotors. Motors rotate because of the torque produced by the interacting magnetic fields of the rotor and stator. The torque is proportional to the magnitudes of the fields multiplied by the sine of the angle between them. Maximum torque is produced when the rotor and stator angles are at 90°. Torque can then be controlled by varying the angle between the two waveforms.

To detect the relative positions of the rotor and stator, a commutation encoder can be used to find the phase angles relative to each other.

These are incremental encoders with additional tracks for regulating motor commutation.

Servomotors are driven by servo drives that provide precise velocity, torque, and position control by using encoder, resolver, and/ or current signals that comprise the feedback components of a servomechanism. Additional components of a servomechanism actuator are a home switch to establish a reference position and overtravel switches to prevent actuator or tooling damage.

DC Servos

DC servomotors may be separately excited or PM DC motors. The principle of operation is the same as described in the DC motor section 3.6.2 previously. They are normally controlled by varying the armature voltage, which has a large resistance, ensuring that the torque-speed ratio is linear. The torque response is very fast in these motors, making them ideal for quick changes in position or speed.

AC Servos

AC servos are robust in construction and have a lower inertia than DC servomotors; however, they are nonlinear in their torque-speed response. They also have lower torque capability than DC servos of a similar size.

Most AC servos are two-phase squirrel cage-type motors. The stator has two distributed windings displaced 90° electrically. One winding, the reference or fixed phase winding, is connected to a constant voltage source. The other winding is called the control phase and is supplied with a variable voltage at the same frequency as the reference phase. For industrial applications, the frequency is usually 60 Hz. The control phase voltage is supplied from a servo amplifier, which controls rotation direction by shifting the phase plus or minus 90° from the reference voltage.

FIG. 46 shows a typical AC servomotor with its electrical cable connections. A gearbox is often bolted to the motor flange.

The squirrel cage rotor has a high resistance like the DC rotor windings; varying this resistance provides different torque-speed characteristics. Lowering the resistance decreases the torque at low speed and increases it at higher speeds, making the curve very nonlinear. This is not desirable in control systems.

Two-phase AC servomotors are built as high-speed low torque actuators and are usually geared down severely to achieve the desired result. Typical speeds of these motors are 3000 to 5000 rpm.

FIG. 46 AC servomotor. Brake, Feedback, Power

Stepper Motors

A stepper motor is a DC motor that rotates a specific number of degrees based on its construction, that is, number of poles. It converts digital pulse inputs to shaft rotation; a train of pulses is made to turn the motor shaft by steps. This allows the position to be controlled precisely without a feedback mechanism. Typical resolutions of commercially available stepper motors range from a few steps per revolution to as many as 400. They can follow signals of up to 1200 pulses per second and may be rated up to several horsepower.

There are several different types of stepper motors, including single and multiple stack variable reluctance motors and PM types.

Variable reluctance motors operate by exciting the poles of the stator, causing the rotor to align itself with the magnetic field. The poles may be energized in combinations, allowing the rotor to line up between stator poles as well as directly with them. Multiple stack versions arrange the poles in several levels or "stacks," allowing finer resolution positioning by phasing from stack to stack.

PM steppers use magnets for the rotor poles. They have a higher inertia than variable reluctance motors and therefore cannot accelerate as fast; however, they produce more torque per ampere of stator current.

FIG. 47 shows a four-pole stepper arrangement with PMs; the A, B, C, and D poles are energized in sequence in one polarity after which the polarities are reversed to achieve eight positions per revolution.

Hybrid stepper motors use a combination of variable reluctance and PM motor techniques. This provides maximum power in a small package size. Hybrid stepper motors are probably the most commonly used type of stepper in industrial automation.

Though steppers can be a lower-cost alternative to servos for positioning applications since feedback is not required, stepper motors do not provide nearly as much torque as servomotors, especially at higher speeds.

Command signals for stepper motors are usually low power logic circuits using TTL or CMOS transistors, power amplification stages are placed between the pulse train generators and the motors.

FIG. 47 Stepper motor diagram.

6.5 Variable Frequency Drives

Variable frequency drives (VFDs) are solid-state power converters.

They first convert an incoming AC voltage into DC, then reconstruct an AC waveform by switching the DC power rapidly at the desired frequency and voltage to approximate a sinusoidal signal. The rectifier that converts the incoming voltage to DC is usually a three phase full wave bridge; single-phase power may also be used for smaller VFDs. FIG. 48 is a diagram of this system.

In order to deliver a consistent torque value while varying speed, the applied voltage must be adjusted proportionally with the frequency. If a motor is rated for 480VAC at 60 Hz, the voltage must be reduced to 240VAC for 30 Hz, 120VAC for 15 Hz, and so on. This is sometimes called volts per hertz control. Additional methods such as vector control and direct torque control allow the magnetic flux and mechanical torque of the motor to be controlled more precisely.

The stage that converts the DC back into a sinusoidal form is known as an inverter circuit. This circuit usually uses pulse-width modulation (PWM) to adjust both output voltage frequency and voltage as required. This is illustrated in FIG. 49.

Newer drives often use special transistors called IGBTs, or Insulated Gate Bipolar Transistors. These are electronic switches that operate over a wide current range, have high efficiency and fast switching, making them ideal for PWM.

A microprocessor is used to control the operation of the VFD. Typically there are a range of parameters that can be set to control the operation of the drive: acceleration and deceleration, maximum speed and velocity set points, and peak current are some of the more common values. Digital I/O connections for start/stop, alarms, and speed preset selection are also common. These may be hardwired or communications based. Analog values may also be interfaced with the drive physically as in a 0 to 10 V or 4 to 20 mA signal or via mapping communication values from a controller.

FIG. 48 VFD system. Input AC (Sinusoidal) Power Variable Frequency Power Mechanical Power Operator Interface

VFD Motor

An OIT may also be mounted on the front of the drive for setting parameters and viewing operational data, such as current or speed.

These may be built into the drive or removable so that it can be shared between VFDs. Like servo systems, VFDs can also be used with feedback devices such as encoders and resolvers to improve control; however, typically a controller such as a PLC or DCS is used as an intermediary between the device and the drive.

VFDs can be operated at speeds above the speed listed on the nameplate of the motor, depending on the application. At ranges above 150 percent, it is usually recommended that a gearbox be used.

Another consideration when planning a system using a VFD is the distance of the motor from the drive. At distances over 150 ft or so, a phenomenon called reflected wave can occur because of the rapid switching of the transistors. This can cause high voltages to be present in the cabling and motor. There are a number of ways to mitigate this, including filters and using inverter duty motors, but ideally the drive should be located relatively close to the motor.

FIG. 49 Pulse width modulation. Low Frequency and Voltage High Frequency and Voltage

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