The topic of motors and motor control techniques is wide and varied. Part one of this two-part article will cover the principles of alternating-current (AC) motor operation and control. Part two will cover design considerations for AC motors with variable speed drives and electronically communicated variable-speed direct-current (DC) motors (ECMs), followed by a comparison table of suggested advantages and disadvantages and design considerations for each type, and concluded with a case study comparing the use of AC motors with variable speed drives (VSDs) with ECMs.
The most common type of motor used in HVAC fan and pump applications is the three-phase AC induction motor (IM). DC motors also have applications in the building and HVAC systems — in particular, many elevator designs employ DC motors. Compared to DC motors, AC motors are simpler in design, are more rugged, and have a longer life under normal operating conditions.
The Lorentz Force Law relates the direction and speed of the rotor windings as they pass through the stator magnetic field to the force applied at the circumference of the rotor and to the resulting torque.
When a charged particle, q, is placed in a magnetic or an electric field, it is acted upon by a force. This is analogous to a force acting on a mass placed in a gravitational field(1).
The Lorentz Force Law describes the combination of forces on a charged particle when placed in an area that contains both an electric field and a magnetic field.
Lorentz Force Equation: F ⃗=qE ⃗+qv ⃗ X B ⃗ [Eq 1]
Two forces are present in this equation:
The force due to the electric field, F ⃗=qE ⃗ [Eq 1a]
The force due to the magnetic field, F ⃗=qv ⃗ X B ⃗ [Eq 1b]
As indicated in equation 1, the variables are vector quantities, while is a scalar quantity. The values are defined as follows:
B ⃗ , Magnetic field strength T, Wb/m2
E ⃗ , Electric field strength V/m, N/C
F ⃗, Force (N)
q, charged particle (C)
v ⃗ , velocity of charged particle (m/s)
For this discussion we will put aside the force contribution from an electric field and concentrate on the force resulting from a magnetic field. One difference between the effect of an electric field and the effect of a magnetic field on a charged particle is that while an electric field acts on a charged particle at rest, a magnetic field acts on a charged particle in motion and motion of a charged particle in a magnetic field is the nucleus of motor rotation. Referring to equation 1b we note that the magnetic force, F, is the result of the cross-product between a charged particle in motion, qv, and the magnetic field strength, B, where F, v, and B are all vectors. A magnetic field can be visualized by lines of magnetic force originating on the north pole of the field and terminating on a south pole where the line density per area represents the strength of the field. A charged particle moving through this field will move in one of three ways relative to the field lines: parallel, perpendicular, or skew. The magnitude of the force acting on the moving charged particle is summarized below:
The above bullet points can be described mathematically by the fact that the cross-product of two vectors results in a third vector that is perpendicular to the first two vectors and whose magnitude equals the product of the first two vector magnitudes multiplied by the sine of the angle between the first two vectors. In our case, the magnitude of F ⃗ is
F =q|v ⃗ ||B ⃗ | sin(θ), , where θ equals the angle between v ⃗ and B ⃗..
In the principles of induction motor operation, the charged particle, q, in motion, v, represents current flow in the rotor windings and the magnetic field, B ⃗., represents the magnetic field setup by the magnetized poles of the stator.
Another form of the equation is:
F=BIL sin(θ), [Eq 2]
In this form of the equation, B, again, represents the strength of the stator magnetic field, I represents the current flowing in the rotor windings, L represents the length of the windings, and represents the continuously, varying angle formed between the direction of the stator magnetic field and the direction of the alternating current flowing in the rotor as it moves through the stator magnetic field. The resulting force acts in a direction perpendicular to both the stator magnetic field and the direction of current flow in the stator windings. This force, acting at a fixed distance from the rotor center-line, produces a torque about the rotor, causing it to rotate.
When a force is applied about an axis (referred to as the axis of rotation) at a fixed distance, such as tightening a nut with a wrench, this force is referred to as torque.
Torque equation: τ=rFsin(θ),, where θ represent the angle formed between the direction of the force and the line falling along the radius. In the wrench example above, the radius represents the length from the center of the nut to the end of the wrench where the force, F, is applied.
In the operation of a motor, the applied force, F, is determined from equation 2 above, (reprinted here for reference) and the radius corresponds approximately to the radius of the rotor.
Torque, as applied to induction motors, can be defined as a force acting at a distance r (rotor radius) where the axis of rotation corresponds to the rotor axis. Torque is measured in Newton-meters (Nm).
The Torque output of an induction motor is related to and dependent upon the value of slip as shown in the following equation:
(ksE_2^2 R_2)/(R_2^2+ (sX_2 )^2 )
Where:
E2 = induced rotor voltage
R2 = resistance of rotor windings
X2 = reactance of rotor magnetic field
S = slip where s=(%s)/(100%) ,(0≤s ≤1)
k= 3/(2πN_s^' ) and N_s^'= N_s/60
From the above torque equation, it can be shown that:
Recall the following from physics:
The rotational speed of the stator magnetic field is determined by two components.
As described above, the AC current flowing in the windings of the motor’s stator generates a rotating magnetic field. The stator field rotates with a speed that is directly proportional to and is in time-phase with the electrical frequency of the AC line current and is, therefore, referred to as synchronous speed. The synchronous speed is determined by the frequency of the electrical line current flowing in the stator windings and the quantity of poles in the stator.
Synchronous speed, n_s= 120f/p
or, less commonly n_s= 60f/pp
Where:
f = the frequency of the electrical line current in Hertz (Hz) or cycles per second
p = number of poles. (Always occurs in multiples of 2). In a multi-phase motor, p refers to the number of poles per phase
pp = number of pole pairs.
Synchronous speed is measured in rotations per minute (RPM)
The equation shows that synchronous speed is proportional to the electrical frequency, f, and is inversely proportional to the number of poles, p.
Examples:
A 2-pole, 60Hz motor rotates at
120 * 60 / 2 = 3600 RPM
A 4-pole, 60Hz motor rotates at 1
20 * 60 / 4 = 1800 RPM
An AC induction motor is comprised of an outer, fixed structure called a stator. The interior of the stator is comprised of metallic poles, and an axial motor shaft that supports and is connected to a solid metal cylinder called a rotor. The end of the motor shaft is connected to the load, such as a fan. AC motors are designed with an even number of poles per electrical supply phase (2-pole, 4-pole, 6-pole, etc.). They are manufactured to run on either a three-phase power supply or a single-phase power supply. A 2-pole, single-phase motor will have only two poles in the stator – a north pole and a south pole. A 2-pole, three-phase motor will have six poles in the stator – a north pole/south pole pair for each of the three electrical supply phases.
In image 1, conductive wiring is wrapped around one of the slots a fixed number of times (e.g. clockwise) and then continues on to wrap around the opposing slot in the opposite direction (e.g. counterclockwise). Instead of wire, conducting metal bars may be used. The wiring or rod laid in the slots is known as a winding. The windings are connected in a manner that allows a pair (2) of opposite polarity magnetic poles to be established. One end of the winding is connected to an external AC circuit (phase conductor) and the other end of the winding is connected to the neutral point in a wye-connected 3-phase motor or to an opposite-polarity pole of an adjacent phase. When AC current flows in the windings of the poles a magnetic field is established between the north and south poles of each pole pair. The magnitude (strength) of the magnetic field is directly proportional to the value of current flowing in the windings — when the current in phase A reaches a maximum, the associated magnetic field reaches a maximum.
Each of the three AC phases (A, B, and C) is 120 electrical degrees out of phase (time-shift) with the other two phases. This spacing of 120 degrees allows for only one phase at a given instant to be at its maximum positive value (north pole). The operation of the AC induction motor is based upon principles of electromagnetic laws, including Faraday’s Law of Induction, Ampere’s Law, and the Lorentz Force Law. In an AC motor, two rotating electromagnetic fields are formed that result in the rotation of the rotor. AC line current flowing in the windings of the IM’s stator establishes a rotating, time-varying, magnetic field within the interior of the IM. The time-varying nature of the stator magnetic field (specifically, the portion of the field located in the air-gap between the face of the stator poles and the face rotor) interacts with the windings of the rotor to induce a magnetic field within the moveable rotor. Rotor motion occurs when the magnetic field around each rotor winding (initially stationary) acts to align its magnetic north pole with the nearest magnetic south pole of the stator’s rotating field. The same is true for south poles in the rotor field act to align themselves with the north poles of the stator field. The effects of electromagnetic interaction cause the rotor field to rotate at a slower speed than the stator field. The difference in speed between the two fields is referred to as the slip speed (or slip). The slip determines the magnitude of the rotational force acting on the axial line of the rotor. This rotational force is known as torque. If the slip (speed) falls to zero, there is no torque acting on the rotor and the rotor will slow down eventually coming to rest.
The difference between the mechanical speed of the rotor and the synchronous speed of the stator magnetic field is referred to as slip and is typically indicated as a percentage of the synchronous speed.
N_s=((N_s-N))/N_s x 100%
There are four primary types of variable-speed motor control methods used to control induction motors operated by a VFD. The first two methods are classified as a scalar control method, and the last two are classified as field-oriented control also known as vector control.
The main difference between these control methods is the algorithm used to calculate the voltage required by the motor at any given moment.
The control methods listed above utilize either an ‘open-loop’ control system or ‘closed-loop’ control system. In an open-loop system, a parameter is set, and the fan will operate at that speed regardless of changes in conditions such as static pressure. In a closed-loop (‘feedback’) system, a sensor monitors output conditions and feeds a signal back to the controller indicating adjustments required to meet the current load conditions. For instance, if the static pressure in a duct changes, a feedback signal will tell the controller the adjustments necessary to meet the new static pressure.
The most common of the above control methods is the V/f method. The V/f method is also the only control method that allows for multiple motor control by a single VSD. Multiple motor control means all motors connected to the same VFD will be controlled in unison. They will start and stop at the same time and all will follow the same speed signal.
V/F represents a ratio fixed by a motor’s rated voltage and rated frequency. For a motor rated at 460 volts, 60 Hertz, the ratio is 460V/60Hz ≈ 7.67 V/Hz.
V/F control is an induction motor control method that maintains a constant V/f ratio. V/F control uses a voltage-controlled oscillator to adjust the VFD output voltage and frequency signal to adjust motor speed (rotor RPM) while maintaining the motor rated V/f ratio. Maintaining this ratio as the voltage varies allows for a constant motor flux to prevent a weakening or a saturation of the magnetic field. If, for instance, the voltage remains fixed and only the frequency is reduced, then saturation of the magnetic field occurs, resulting in potential motor damage.
A VSD using the V/f method is an open-loop system. A VSD using the V/f method with an encoder is a closed-loop system.
The process of vector control, also referred to as field-oriented control, (FOC) allows an AC motor to be controlled like a DC motor. This control method applies one or both transforms — the Clarke Transform(3) and the Park Transform(4) — to transform the stator 3-phase current in a stationary refence frame to a 2-phase current-in a rotating reference frame. The 2-phase current is resolved into the two components comprising stator current:
The Clark Transform is used for induction motor control while both the Clark and Park Transforms are used successively for control of synchronous AC motors. Synchronous AC motors are not discussed in this article
Vector control allows for independent control of both components of the stator current (the magnetizing current and the torque-producing current). A VSD using vector control can control torque as well as speed.
Vector control is used to achieve independent control of motor speed and motor torque, similar to how a DC motor is controlled.
With knowledge of the principles of AC motor operation and control in hand, stay tuned for part two of this article. Part two will cover design considerations for AC motors with VSDs and electronically communicated variable-speed DC motors, followed by a comparison table of suggested advantages and disadvantages and design considerations for each type, and concluded with a case study comparing the use of AC motors with VSDs, with ECMs.
Footnotes
(1) Equivalence of force equations for a gravitational field and an electric field:
Gravitational field:
Electric field:
(2) A magnetic pole cannot exist independently and must always be paired with another magnetic pole of opposite polarity. [Gauss’ Law for Magnetism]. A magnetic field always flows from the ‘north’ pole to the complimentary ‘south’ pole. The number of poles is always a multiple of 2.
(3) Clarke Transform - converts the time domain components of a three-phase system (in abc frame) to two components in an orthogonal stationary frame
(4) Park Transform - converts the two components in the frame to an orthogonal rotating reference frame (dq0)
(5) Speed regulation:
(6) IGBT. Insulated Gate Bipolar Transistor. A solid-state switch combing the high-speed switching ability of Field-effect transistor and the high-power rating of a bipolar transistor.
What Are Motor Drives?
What Are the Types of Motor Controllers and Drives?
DC Drive
Types of Motors That Rely on DC Drives and Their Applications
AC Drive
Types of Motors That Rely on AC Drives and Their Applications
Servo Motor Drives and Controllers
Servo Motor Applications
Stepper
Stepper Applications
Need Your Motor Drives Repaired
Motor drives come in a wide variety of forms. These electronic or electrical devices are used to power a range of machines, robots, equipment and other applications. If you’re looking to purchase new motor drives or repair old ones, you’ll need to know what motor drives are, along with the different types of drives and how they are commonly used.
Motor drives are electronic devices that control torque, position outputs and motor speed. As power goes into the motor, the drive modifies it so that your motor has the needed output. The terms “motor controllers” and “motor drives” are often used interchangeably since controller circuits are usually combined with drive circuits to produce a single unit.
There are a few different types of motors and motor drives that are common today. The four basic available motor drive types are stepper, AC, DC and servo. These motor drives each have input power types that are tailored to the output functions of their applications. Learn more about what the drives in these motors do and how they are commonly used below:
In its basic function, a DC drive converts alternating current (AC) into direct current (DC) to power a DC motor. DC drives are power modules that serve as the interface between a DC motor and a controller. Often, the motor comes with the controller integrated alongside the drive circuit, helping to power the control signals that communicate with the drive.
There a few different types of DC motor drives, with the most common type of drive coming with two SCRs (Silicon Controlled Rectifiers) that use a single-phase AC input to generate a half-cycle DC output. This type of power generation is referred to as a half-bridge method. More complex and powerful DC drives incorporate six SCRS to utilize a full-bridge method. When relying on a full-bridge method, the six SCRs will use a three-phase AC input to generate the DC output.
Occassionally, DC drives are referred to as variable speed drives. They have this name due to the way most types of DC drives adjust shaft speed. A DC Drive is typically characterized by its dependable speed regulation, broad speed range and how the drive transfers voltage to the motor.
There are a variety of DC motors that rely on DC drives. All of these motors use the same operation to power them, with the motor rotation coming from the application of power through current-carrying conductors that are installed inside a magnetic field. The differences between DC motors arise when looking at how and where the electromagnetic fields are created. To give you an idea of where you might expect to use a DC motor, take a look at some of the main types below:
Linear motors, like their name suggests, generate forces in only one direction. They create mechanical force though permanent rare-earth magnets that generate magnetic flux, which then interacts with the current in the conductors. Linear motors can accelerate quickly, position themselves with great precision and operate at fast speeds.
You can find linear motors in a variety of equipment types. Their ability to help with speed control makes them especially useful for applications where precise speeds are a priority. They are used in looms, sliding doors and baggage handling machines. Additionally, they are often installed in roller coasters to help with acceleration and speed control.
A brushed motor is a type of DC drive that uses physical touch to commutate and produce mechanical energy. The brushes in the motor are made with carbon or other types of material to be used as electrical contacts. While the shaft rotates, the spring-loaded brushes make contact with the commutator. In essence, the motor’s brushes help the DC power supply connect to the rotor assembly that contains the output shaft, commutator rings and armature.
These brushed motors have been around for more than a century, with companies and individuals still trusting them due to their significant ratio of torque to inertia. They are known for their reliability and affordability. You can often find brushed motors used in vehicles to control the power windows, windshield wipers and seat positioners. They also appear in battery-powered pumps, X-rays and welding equipment. In a more industrial setting, brushed motors are used in industrial equipment that requires quick bursts of power.
Unlike brushed motors, brushless motors don’t use physical touch to cause commutation to occur. As you might expect, they don’t use brushes to generate motion. Instead, they use magnets placed around the rotor that are then attracted to the stator’s powered coil windings. As a result of this design, the motor creates its torque with electromagnetism. The motor’s rotation speed can be adjusted by simply changing the direction and size of current contained in the stator coils.
Due to their efficiency and durability, brushless DC motors are best used in applications where a motor is going to need to be running for an extended period. Some examples of applications include hard drive disks, washing machines, computer fans and air conditioners.
An AC drive converts AC inputs to DC, much like the DC drive. However, after this first conversion is completed, the DC is converted back into AC that fuels the motor. Essentially, AC drives are frequency inverters or amplifiers that serve as an interface between an AC motor and controller. The drives make the voltage compatible with the motor by converting the controller’s step and direction inputs into the appropriate voltage.
Sometimes, AC drives are referred to as variable frequency drives, since the majority of AC motor drives adjust input frequency. Whatever name you refer to them as, the AC drive’s function stays the same, with the AC drive regulating a motor’s torque and output speed.
There are a few different AC motors that rely on AC drives. Some of the main types of AC drives and the motors they power can be found below:
If you want a reliable motor that keeps its speed consistent, synchronous motors are ideal. These motors and their AC drives can maintain a precise speed even with a full load. Since the stator’s rotating magnetic field maintains a speed that’s equal to the speed of the rotor, a synchronous motor doesn’t have any slip.
In practice, synchronous motors are used in machines that require exceptional accuracy. Some high-precision drill machines use them to ensure their drilling is as precise as possible. Other examples of machines that use synchronous motors include metering pumps, timers, clocks, speed controllers and electromechanical robots.
Induction motors don’t use any kind of physical connection from stator windings to induce the current into the rotor windings. They are well known for their ability to generate a significant amount of power, their variable speed control capabilities and their adaptability to a broad variety of environments. Induction motors usually have some slip to them, where they lose their exact speed tracking.
Induction motors are the most used AC motors and drives in day-to-day processes. You’ll find induction motors in kitchen appliances, air conditioners, vehicles, industrial machines and water pumps. They are used in so many household devices due to their flexibility in meeting varying load demands from many electrical applications.
For greater speed control and torque output that’s low-speed, a sensorless vector drive takes sole control over the frequency and voltage that’s fed into the motor. This type of drive is the closest to DC motors in their effect. As their name suggests, they do not use any feedback sensors, such as resolvers or encoders.
Sensorless vector drives are used in several types of industrial applications. They’re often used in applications that have extremely high inertia or when an application needs a high level of steady-state speed accuracy.
A servo motor is one of the most efficient motors that users can control easily to get the correct output. This type of motor relies on a servo drive that helps to generate precise movements to rotate or push the parts of your machine. A servo drive modifies the input power by taking the alternating or constant current source and turning it into a pulsed current output that ranges in pulse frequency and duration. These drives help to control positions, torques and motor speeds.
There are two primary sizes of servo motors — small and standard. As you might expect, the standard-sized motor offers greater speed and power than the motor system. Small servo motors are typically used by companies that have limited space and don’t need to have a huge amount of power. Large servos are constructed with metal parts to handle heavier work, while small servo motors are built with plastic parts.
Typically, Typically, servo motors and drives are used for motion control applications in the construction and manufacturing industries. Their main applications involve assisting a machine as it conducts a task that’s meant to be done frequently and in a particular way. In practice, they often help power spindle drive systems, conveyors, machine tools and robotics. To gain a broader understanding of servo motors, consider some of their applications below:
A stepper motor drive and controller modify alternating or constant current sources to a stepped current output that then adjusts the stepper motor’s input power. Due to their design, they also are called step amplifiers and pulse drives. Stepper motor drives monitor and adjust input power through permanent magnets. They also use carefully placed poles in both the stator and rotor that employ DC currents to help create stepwise rotation.
Since a stepper’s rotational output is not continuous, the input power has to be controlled in a way that helps the groups of stator poles either energize or de-energize. The stepper’s drives and controllers are crucial for the needed control that helps the stepper function properly. More specifically, the controller integrated with a stepper drive circuit sends the relevant control signals to the motor drive.
This type of motor is primarily used in industries like construction and manufacturing. They control torques, position and motor speeds of various types of machinery. They’re quite popular due to their open feedback design and precision. While they aren’t suited for high-speed applications, their precise, repeatable movements still work at a wide range of speeds. Stepper motors are known for how reliable, easy to use and reversible they are.
Steppers have several applications where they are commonly used. Companies regularly employ small-wattage steppers for micro-positioning, robots, machine tools, electric watch drives, medical equipment, printers, computer control systems and hard drive disks. Large-wattage steppers can be found in military equipment, conveyors, scientific exploration devices and machine tools.
With all of the information above, you should now have a basic understanding of the types of motor drives available to you, giving you a better idea of which motors drives are best for your needs. Whether you’re using DC motors, AC motors, servo motors or stepper motors, you’ll eventually have to perform some type of maintenance on them.
When you need motor drives repaired, you’ll want to reach out to an expert in the industry. Global Electronic Services offers comprehensive repair services and solutions that help your business continue to operate smoothly. Contact Global Electronic Services today with any questions you might have about motor drives or with inquiries about service.