AC Motor Drives (part 1)

Introduction

The term AC drives covers a wide range of drive types. When talking mostly industrial terms, an AC drive could also be considered a variable frequency drive (VFD), adjustable-speed drive (ASD), variable-speed drive (VSD), and "inverter." If a technician was discussing ASDs and the factory contained mostly DC equipment, then ASD or VSD would refer to a DC drive.

The term inverter is actually the final or power section of the drive-and is considered an acceptable term for the entire unit.

There are actually many different types of AC drives, but all of them have one concept in common-they convert fixed voltage and frequency into a variable voltage and frequency output.

Though they don’t meet the strict definition of an AC drive, reduced volt age starters (e.g., soft starts) and wound rotor slip recovery units fall into the variable-speed category. Soft starts immediately deliver line frequency to the motor but at a reduced voltage value for a specified period of time.

The result is reduced motor torque, as the motor is accelerating.

Load commutated inverters (LCIs) and cycloconverters are also part of the variable-frequency drive category. Cycloconverters actually use SCRs in very large horsepower amounts that require regeneration. Though the output of a cycloconverter may not be considered variable, the unit does alter the frequency output, thereby reducing the speed. This unit would actually step down the frequency to 1/2 or 1/3 of the line frequency. This frequency would then be applied to motors of 30- or 20-Hz design. The cycloconverter got its start in the 1930s but is not much in demand today because of its complexity and cost of circuitry.

The primary focus of this section will be on three common types of VFDs: the variable voltage input or inverter (VVI, sometimes referred to as the six step drive), the current source inverter (CSI), and the pulse width modulated (PWM). The VVI and PWM would be considered a voltage source inverter, while the CSI would be considered a current source inverter.

Several other types of drives fall under the category of voltage source inverters. Vector (or flux vector) drives and sensorless vector drives will be considered later in this section.

If not already done, it would be helpful to review the section on AC induction motors in Section 3. That section will provide a foundation upon which to build the basic concepts of variable-frequency drives.

--- Simple VFD/fan application: M, Replaced with a Drive; Air In, Air, Outlet, Damper

---

--- Motor speed formula (including slip) Motor Slip:

Shaft Speed = 120 x F Slip for NEMA B Motor = 3 to 5 % of Base Speed which is 1800 RPM at Full Load F = Frequency applied to the motor; P = Number of motor poles P

Example:

Shaft Speed = 120 x 60 Hz 4 - 50 = 1750 RPM

---

Basic Theory of Major Drive Types

The easiest way to understand drives is to take a brief look at what a drive application looks like. A simple application with a fixed speed fan using a motor starter. The three-phase motor starter can be replaced by a VFD, allowing the fan to be operated at variable speed.

If the fan can operate at virtually any speed required (below maximum motor speed), the air outlet damper can be "fixed" open. Using the fixed speed motor starter method, the only means of varying the air out was by adjusting the air outlet damper.

Now the question is: How does this induction motor work with a drive? As mentioned earlier, a standard three-phase induction motor can be con trolled by a VFD. There are two main elements that are controlled by the drive, speed, and torque. To understand how a drive controls these two elements, we will take a short review of AC induction motor characteristics.

--- the formula for determining the shaft speed of an induction motor.

As notice, this formula includes a characteristic called slip. The slip speed in a motor is actually termed base speed. As indicated earlier, a VFD controls two main elements of a motor-speed and torque.

The speed of a motor is conveniently adjusted by changing the frequency applied to the motor. The VFD adjusts the output frequency, thereby adjusting the speed of the motor. The torque of a motor is controlled by a basic characteristic of every motor-the volts per hertz ratio (V/Hz). A review of this ratio is shown.

--- AC motor linear volts per hertz ratio

If an induction motor is connected to a 460-V power source at 60 Hz, the ratio is 7.67 V/Hz. As long as this ratio is kept in proportion, the motor will develop rated torque.

--- a typical speed/torque curve for a motor. This curve represents a motor operated at a fixed voltage and frequency source.

A VFD provides many different frequency outputs, as shown. At any given frequency output of the drive, another torque curve is established. A typical operating point is where the curve intersects the 100% level, indicating rated torque.

--- Motor speed/torque curve: Operating Point Synchronous Base Speed

--- Drive frequency output vs. motor torque: Operating Point

The VFD provides the frequency and voltage output necessary to change the speed of a motor. A block diagram of a basic PWM drive (pulse-width modulated) is shown. All PWM drives contain these main parts, with subtle differences in hardware and software components. A PWM drive will be used as an example here, then the characteristics of a VVI and CSI drive will be compared with the PWM.

Although some drives accept single-phase input power, a three-phase unit will be considered for illustration purposes. (To simplify illustrations, the waveforms in the following drive figures only show one phase of input and output.)

--- PWM drive (VFD) block diagram -- Input Converter ( Diode Bridge) DC Bus (Filter) Output Inverter (IGBT's)

Three-phase power is applied to the input section of the drive, called the converter. This section contains six diodes, arranged in an electrical bridge.

These diodes convert AC power to DC power. Because diodes are used in the converting process, the next section sees a fixed DC voltage.

The DC bus section accepts the now converted, AC-to-DC voltage. The role of this section is to filter and smooth out the waveform. The "L" and "C" indicate inductors and capacitors. (Note: Many drive manufacturers install only one DC bus inductor, along with the filter capacitor(s). Some manufacturers install line reactors, "L" ahead of the drive converter. Later in this section, innovations and technology improvements will be reviewed in detail.) As shown, the diodes actually reconstruct the negative halves of the wave form onto the positive half. An average DC voltage of ~650-680 V is seen, if the drive is a 460-VAC unit. (Line voltage × 1.414 = DC bus voltage.) The inductor and the capacitor(s) work together to filter out any AC component on the DC waveform. The smoother the DC waveform, the cleaner the output waveform from the drive.

Once filtered, the DC bus voltage is delivered to the final section of the drive, called the inverter section. As the name implies, this section actually inverts the DC voltage back to AC-but in a variable voltage and frequency output. Devices called insulated gate bipolar transistors (IGBTs) act as power switches to turn on and off the DC bus voltage at specific intervals.

In doing so, the inverter actually creates a variable AC voltage and frequency output. Control circuits, called gate drivers, cause the control part of the IGBT (gate) to turn "on" and "off" as needed.

As seen, the output of the drive does not provide an exact replica of the AC input sine waveform. It actually provides voltage pulses that are at a constant magnitude in height. The IGBTs switch the DC bus voltage on and off at designated intervals.

--- PWM output waveform (voltage and current) Line to Neutral Voltage Line to Neutral Current

--- Frequency and voltage creation from PWM Output Frequency (Hz) ON =

Voltage Output OFF, Off Time = Frequency Output Switch Frequency (Carrier) (Hz)

The control board in the drive signals the gate driver circuits to turn on the waveform positive half or negative half IGBT. This alternating of positive and negative switches recreates the three-phase output.

The longer the IGBT remains on, the higher the output voltage; the less time the IGBT is on, the lower the output voltage. Conversely, the longer the IGBT is off, the lower the output frequency.

The speed at which IGBTs are switched on and off is called the carrier frequency or switch frequency. The higher the switch frequency, the more resolution each PWM pulse contains. Typical switch frequencies are 3000 × 4000 times per second (3-4 kHz), but some manufacturers use a carrier as high as 16 Hz.

As you can imagine, the higher the switch frequency, the smoother the output waveform (the higher the resolution). However, there is a disadvantage. Higher switch frequencies cause decreased efficiency of the drive.

The faster the switching rate, the faster the IGBTs turn on and off. This causes increased heat in the IGBTs.

Now that the standard PWM VFD has been reviewed, the next step will be to review the characteristics of VVI and CSI VFDs. This will be followed by an analysis of how PWM compares with VVI and CSI drives.

VVI (Variable Voltage Inverter)-Input

This design takes the supply voltage (e.g., 230 or 460 V), rectifies it, and sends the variable voltage to the DC bus and then to the inverter section.

The inverter section then inverts (changes DC to AC) the variable voltage DC to a variable-voltage and variable-frequency AC. The inverter section contains power semiconductors such as transistors or thyristors (SCRs).

To deliver variable voltage to the inverter, the input rectifier section or front-end consists of a controllable rectifier-SCRs. The control logic fires the SCRs at the appropriate time during the sine wave, thereby providing the variable voltage to the DC bus. --- shows a block diagram of a VVI drive.

--- VVI drive block diagram: C, + L M Variable DC Bus AC Input Controllable Rectifier Inverter

Some of the advantages of a VVI drive include good speed range, ability to connect multiple motors to the drive (within drive current limitations), and a fairly simple control regulator. However, there are some limitations.

One of the major disadvantages, in terms of AC drives, is the input power factor. It decreases as the speed of the drive/motor decreases. This is due to the "controllable" rectifier front end being constructed of SCRs. This issue is identical to that of DC drives.

Another disadvantage is the inability of the drive to "ride through" a low input voltage situation. The term power loss ride-through is defined as the ability of a drive to ride through a low- or zero-voltage input and still remain in operation. This may be 2 to 3 cycles of the AC Sine wave, or more. (Note: Each AC cycle lasts about 16 ms). At low operating speeds (low hertz output), the SCR rectifier is not constantly keeping the DC bus charged at full potential. This means that the motor has voltage to draw from, in the event of a low line input.

Other disadvantages include the requirement of an input isolation trans former or line reactor. This is needed because of the SCR control technology (line spike generation). In addition, the generation of additional output harmonics is a possible result of the technology being applied to the AC motor.

The VVI drive has one additional disadvantage-the characteristic of low speed motor cogging (shaft pulsations/jerky motion). Though not an issue at mid to high speeds, cogging at low speeds can cause equipment problems. This limitation is best illustrated.

--- VVI voltage and current waveforms Voltage Line to Neutral Current (Line) VVI Voltage and Current Waveforms

As seen, the voltage waveform approximates a series of steps. Because of this characteristic, this drive is sometimes called a six-step drive. During low-speed operation (>15-20 Hz), the rotor actually searches for the next available magnetic field in the stator. The result is a jerky rotation of the motor shaft. Because of this, gears or gear reducers connected to the motor shaft will suffer additional friction and wear. At high speeds, the inertia of the motor will provide continuous movement of the motor shaft. Therefore, cogging is not a problem.

As shown in the current waveform, there are several spikes that occur at regular intervals. These spikes, or transients, are caused by the SCRs gating on, or triggering. The DC bus filter circuit (shown by an L and C) does reduce the effects of these spikes, but they are not eliminated. These spikes translate into additional motor heating and inefficiency.

The VVI drive was one of the first AC drives to gain acceptance into the industrial drives market. It may be considered one of the most economical drives in the 25- to 150-HP range if a 6:1 speed range is acceptable (operation from 10-60 Hz). This type of drive is also widely used in high-speed drive applications-400 to 3000 Hz.

CSI (Current Source Inverter)

This type of AC drive (sometimes referred to as current source input) has basically the same components as a VVI drive. The major difference is that it’s more of a current-sensitive drive as opposed to a VVI, which is more of a voltage-sensitive drive.

This design also takes the supply voltage (e.g., 230 or 460 V), rectifies it, and sends the variable voltage to the DC bus and then to the inverter section. As with the VVI, the CSI drive inverter section inverts (changes DC to AC) the variable-voltage DC to a variable-voltage and variable-frequency AC. The inverter section is made up of power semiconductors such as transistors or thyristors (SCRs).

To deliver variable voltage to the inverter, the input-rectifier section also consists of a controllable rectifier-SCRs. The control logic fires the SCRs at the appropriate time during the sine wave, thereby providing the variable voltage to the DC bus. -- a block diagram of a CSI drive.

--- CSI block diagram: L, M; Variable DC Bus AC Input Controllable Rectifier Inverter

Some of the advantages of a CSI drive include high efficiency, inherent short-circuit protection (due to the current regulator within the drive), inherent regenerative capability back to the AC line during overhauling load situations, and the capability of synchronous transfer (bringing other motors on-line during full-voltage output).

However, there are also some limitations. As with a VVI drive, the input power factor decreases as the speed of the drive/motor decreases. Also, this drive has a limited speed range due to low-speed motor cogging (shaft pulsations/jerky motion). This drive is also unable to "ride through" a low input voltage situation. This drive also has the requirement of an input isolation transformer, due to the SCR control technology (line spike generation).

Unlike the VVI drive, the CSI drive cannot operate more than one motor at a time. The motor is an integral part of the drive system and its characteristics must be matched to the drive. (Usually the motor and drive are sold as a complete package.) Multiple motors would cause malfunctions in the drive-control system. In addition, the motor normally requires a feed back device (e.g., tachometer) to provide information to the drive current regulator.

Also related to motors is the requirement for the motor to always be connected to the drive. This means that the drive cannot be tested without the motor connected. In some cases, one additional disadvantage is the drive size. Typically, it’s physically larger than other drive types because of internal power components.

As mentioned earlier, the VVI and CSI drives produce low-speed cogging.

--- CSI voltage and current waveforms Voltage Line to Neutral Current (Line)

As seen, line notching or spikes developed from the gating of SCRs in the drive front end. Compared with a VVI drive, the voltage waveform is somewhat closer to the sine wave voltage required by the motor. The current waveform appears to simulate a trapezoid. In addition, there are times when no current flows. These gaps in current cause the rotor to search for the next available magnetic field in the stator. This characteristic, like that of the VVI, results in jerky rotation of the motor shaft at low speeds (<15-20 Hz).

As with the VVI drive, the DC bus filter circuit (shown by an L) does reduce the effects of these spikes, but they are not eliminated. Here again, these spikes translate into additional motor heating and inefficiency.

CSI drives are the latest addition to the line-up of AC variable-frequency drives. They are usually used in applications requiring 50 HP or larger.

These VFDs are well suited for powering pumps and fans because of the inherent synchronous transfer capability. The cost of a CSI drive may be less than either a VVI or PWM in powering pumps, fans, or similar applications. However, the efficiency of the CSI drive matches that of the DC drive and may not provide a total energy-saving package compared with the PWM drive.

PWM (Pulse-Width Modulated)

As seen earlier, the power conversion principle of this drive is different from that of VVI and CSI. One of the major differences is that of a fixed diode front end, not a controllable SCR front end. This fixed diode bridge provides a constant DC bus voltage. The DC bus voltage is then filtered and sent to the inverter section. Another difference between PWM and the other types is the operation of the inverter section. The inverter in the PWM drive has a dual purpose-it changes fixed-voltage DC to variable voltage AC and changes fixed frequency to variable frequency. In the other types, the inverter's primary purpose is to change the fixed-frequency to a variable-frequency output.

PWM drives use several types of power transistors; IGBTs, and GTOs (gate turn-off-SCRs) are examples. These semiconductors offer the advantages of PWM technology without the expense of commutation circuits. (Com mutation circuits are required to turn off the SCRs once they start con ducting. They are found in early VVI or CSI units.) Another major difference is the actual voltage output of the inverter itself.

The DC bus voltage is fixed and approximately equal to the RMS value of the drive input voltage (e.g., 460 V × 1.414 = 650 V). By chopping or modulating the DC bus voltage, the average voltage (output voltage) is increased or decreased. The output voltage value is controlled by the length of time the power semiconductors actually conduct. As seen earlier, the longer the on time for the semiconductors, the higher the output volt age. The longer the off times occur in the process, the lower the frequency output. Thus the inverter accomplishes both variable voltage and frequency. ---34 shows a block diagram of a PWM drive.

--- PWM block diagram: C, + L M Fixed DC Bus AC Input Diode Rectifier Inverter

Some of the advantages of a PWM drive include high efficiency, the capability of optional common bus regeneration (operating several inverter sections off of one DC bus), and a wide controllable speed range (in some cases up to 200:1, with no low speed cogging under 20 Hz operation).

The PWM drive offers other advantages, such as power loss ride-through capability, open circuit protection, and constant input power factor. This is due to the fixed diode front end and DC bus inductor. Constant power factor is not seen by CSI, VVI, or DC drives.

Like the VVI drive, the PWM also allows multi-motor operation (within the current capability of the drive). However, there are a few limitations.

Extra hardware is required for line regenerative capability (discussed later in this section). Also, the regulator is more complex than a VVI. However, microprocessor control has nearly eliminated significant economic differences between the two drives.

As mentioned, low-speed cogging is not an issue with PWM drives.

--- PWM voltage and current waveforms Line to Neutral Voltage Line to Neutral Current

--- has been seen before. Of particular interest is the fact that there is no line notching or spikes developed, thanks to the diode front end. The voltage waveform, which could be superimposed on the modulations, very closely approximates the sine wave voltage required by the motor. If the carrier frequency is high (8-16 kHz), the quality of low-speed operation is improved. The higher the carrier frequency, the smoother the motor operation. (Remember-carrier frequency is the speed at which the power semiconductors are switched on and off.) Another benefit of high carrier frequencies is that of reduced audible noise. The higher the frequency, the less motor noise is generated. Audible motor noise can be an issue with low switching rates (e.g., 1-3 kHz). The current waveform, though it contains some ripple, is the smoothest of the three types of drives. It closely approximates the AC sine wave. The efficiency is therefore very high with little motor heating.

Continued improvements in drive technology have enabled PWM drives to deliver a response almost equal to that of DC servos. High response applications such as machine tools and robots require very precise control of motor speed and torque. PWM flux vector drives provide this type of capability and are covered later in this section.

AC Drives-Braking Methods

Braking methods of DC motors has already been reviewed earlier in this section. In this section, attention will be given to AC drive braking methods, which, for the most part, are similar to DC drive braking methods, with a few exceptions. ---36 is a review of the stopping methods of an AC motor, with a minor variation.

--- Braking methods for AC drives 100% 50% 25% Time (Seconds) Dynamic Braking Ramp to Stop DC Injection Regeneration Coast to Stop

As indicated, the easiest way of bringing an AC motor to a stop is the simple method of coast-to-stop. This is followed by the next fastest means, called ramp-to-stop.

During this method, the drive actually forces the motor down to a stop by systematically reducing the frequency and voltage. This is done in a deceleration ramp format, which is set through a parameter in the drive. It should be noted that the motor will contain energy or inertia that must be dissipated-in this case voltage. The DC bus circuit will have to absorb the back fed voltage. When this happens, the DC bus voltage rises-possibly to a point of a voltage trip (called over-voltage fault or DC bus fault). A typical drive will automatically protect itself by shutting down at ~135% of nominal DC bus value. ( For example, a 460-VAC drive will carry ~650 VDC on the bus. The trip point would be ~878 VDC.) The DC bus of a typical AC drive will take on as much voltage as possible without tripping. If an over-voltage trip occurs, the operator has three choices-increase the deceleration time, add DC injection braking, or add an external dynamic braking package. If the deceleration time is extended, the DC bus has more time to dissipate the energy and stay below the trip point. This may be a trial-and-error approach (keep setting the deceleration time until the drive does not trip). A few of the recent drives offered on the market automatically extend the deceleration time, without an operator having to do so. If 30 s is a deceleration time and the drive stops the motor in 45 s, the motor cannot be stopped in 30 s without DC injection braking or external hardware (e.g., dynamic braking). If a 30-second stop is required by the application, DC injection braking is a possibility.

DC Injection Braking

As the name implies, during this braking process, DC voltage is "injected" into the stator windings for a preset period of time. In doing so, a definite north and south pole is set up in the stator, causing the same type of magnetic field in the rotor. Braking torque (counter torque) is the action that results, bringing the motor to a quicker stop, compared with ramp. The rotor and stator dissipate the energy within itself through heat. This method of braking is usually used in lightly loaded applications, where braking is not often used. Repetitive operation of injection braking can cause excessive heat buildup, especially in high-inertia applications, such as flywheels or centrifuges. Excessive heat can cause permanent damage to the stator windings and rotor core.

Dynamic Braking

If DC injection braking cannot bring the motor to a stop in the required time, then dynamic braking will need to be added. A typical dynamic braking system for an AC drive.

--- AC drive dynamic braking: C + L M Fixed DC Bus AC Input Diode Rectifier Inverter Chopper Module DB Resistor

This form of stopping uses a fixed, high-wattage resistor (or bank of resistors) to transform the rotating energy into heat. When the motor is going faster than commanded speed, the energy is fed back to the DC bus. Once the bus level increases to a predetermined point, the chopper module activates and the excess voltage is transferred to the DB resistor. The chopper is basically a sensor and is constructed of a transistor or IGBT switch device. The DB resistor is not mounted within the drive box or inside a drive cabinet. It’s always mounted in an area where the heat developed cannot interfere with the heat dissipation requirements of the drive.

As previous indicated, the main stopping power of a DB system occurs when the resistor is cold, during the first few seconds of the process. Once the resistor heats up, the amount of braking torque diminishes. The number of times per minute DB is engaged will also determine the effectiveness of braking torque. Duty cycle, as it’s called, is the number of times per minute the DB resistor is used. Many DB circuits consider a maximum of 10% duty cycle (6 s on, 54 s off-time to cool).

--- Regenerative AC drive (two IGBT bridges)

Regenerative Braking (Four Quadrant)

The process of regenerative braking has already been discussed. However, this type of braking uses different components in the AC drive, compared with DC. The end result is still the same-generation of voltage back to the AC line in synchronization with utility power.

To accomplish this, a second set of reverse-connected power semiconductors is required. Some AC drives use two sets of fully controlled SCRs in the input converter section. The latest AC regenerative drives use two sets of IGBTs in the converter section (some manufacturers term this an active front end).

The reverse set of power components allows the drive to conduct current in the opposite direction (taking the motor's energy, and generating it back to the line). A block diagram of a regenerative braking (four quadrant) system.

As expected with a four-quadrant system, this unit allows driving the motor in the forward and reverse directions, as well as regeneration in both the forward and reverse directions. The control board contains the microprocessor that controls the status of the forward and reverse IGBT bridges. When the speed of the motor is faster than commanded, the motor's energy is fed back into the DC bus. The regeneration circuit senses the increase in reverse voltage and turns on the reverse IGBT bridge circuit.

In this method, the reverse IGBTs need to be able to conduct in the reverse direction. Therefore, if power is removed from the drive, the microprocessor and the reverse IGBTs would not operate. Therefore, this method would not be used for emergency stop situations. However, one method of working around this issue is to include brake resistor and chopper across the DC bus. This provides the best of both worlds, regeneration and e-stop capability.

--- Closed loop flux vector AC drive (block diagram) Speed Control Torque Control Modulator V F Motor T

Drives (AC)-Torque Control

Up until now, standard PWM voltage-controlled drives have been discussed. In this type of drive, the voltage and frequency applied are the controlling variable, when talking about motor torque produced. Torque produced is actually a product of the amount of slip in the motor. The motor has to have a certain amount of slip present to produce torque. As the motor load increases, slip increases and so does torque. This type of AC drive is termed a volts per hertz drive, primarily because of the two control ling elements-volts and hertz. It’s also given the label of a scalar drive.

The drive technology of today has moved beyond a "motor turner" philosophy. Drive systems of today need to accurately control the torque of an AC induction motor. Controlled torque is required by automation systems such as wind-unwind stands, process-control equipment, coating lines, printing, packaging lines, hoists and elevators, extruders, and any place where standard motor slip cannot be tolerated (typically 3-5%). Enter the realm of controlled-slip drives-called flux vector or simply, vector drives.

Flux Vector Drives

One of the basic principles of a flux vector drive is to simulate the torque produced by a DC motor. As indicated in the DC drive section, one of the major advantages of DC Drives, and now Flux Vector Drives - is full torque at zero speed. Up until the advent of flux vector drives, slip had to occur for motor torque to be developed. Depending on motor design, 30-50 rpm of slip might be needed for torque to be developed. An output frequency of 3-7 Hz may be needed from the drive before the motor actually starts turning. With flux vector control, the drive forces the motor to generate torque at zero speed.

A flux vector drive features field-oriented control-similar to that of a DC drive where the shunt field windings continuously have flux, even at zero speed. The motor's electrical characteristics are simulated in the drive controller circuitry called a motor model. The motor model takes a mental impression of the motor's flux, voltage, and current requirements for every degree of shaft rotation. Due to the way the drive gathers information for the motor model, it would be termed a closed loop drive. Torque is indirectly controlled by the creation of frequency and voltage on the basis of values determined by a feedback device. A block diagram of a closed loop, flux vector-controlled AC drive.

To emulate the magnetic operating conditions of a DC motor, that is, to perform the field orientation process, the flux vector drive needs to know the spatial angular position of the rotor flux inside the AC induction motor. With flux vector PWM drives, field orientation is achieved by electronic means rather than the mechanical commutator and brush assembly of the DC motor.

During field orientation, information about the rotor status is obtained by feeding back rotor speed and angular position. This feedback is relative to the stator field and accomplished by means of a pulse encoder. A drive that uses speed encoders is referred to as a closed-loop drive. In addition, the motor's electrical characteristics are mathematically modeled with micro processors, processing the data. The electronic controller of a flux vector drive creates electrical quantities such as voltage, current, and frequency.

These quantities are the controlling variables, which are fed through the modulator and then to the AC induction motor. Torque, therefore, is controlled indirectly.

The advantages of this type of drive include good torque response (<10 ms). Some manufacturers consider this response as the limiting response of standard AC induction motors because of the inherent inertia of the machine. Other advantages include full torque at zero speed (at ~0.5 Hz output).

Note: Special caution must be taken when an "off-the-shelf" motor is used to pro vide full torque at zero speed. A specialized cooling system may be needed, in addition to the internally mounted fan. This is due to drastically reduced airflow. The motor is developing full torque and increased heat buildup.

Accurate speed control is possible because of the pulse tachometer feed back. This speed control approaches the performance of a DC drive. Accurate speed control would be stated as ±5% of rated torque.

Depending on point of view, there may be several drawbacks to this type of control. They may be considered drawbacks when compared with the next version of vector control discussed-sensorless flux vector control).

--- Sensorless flux vector control block diagram ( ABB Inc.) Speed Control Torque Control Motor Speed Reference

Sensorless Vector Drives

To achieve a high level of torque response and speed accuracy, a feedback device is normally required. This can be costly and adds complexity to the traditionally simple AC induction motor. Also, a modulator is used, which is a device that simulates the AC sine wave for output to the motor. A modulator, slows down communication between the incoming voltage and frequency signals and the ability of the drive to quickly respond to signal changes _ Although the motor is mechanically simple, the drive is electrically complex. A simple sensorless flux vector control scheme, which is achieving increased recognition in recent years.

Sensorless flux vector control is similar to a DC drive's EMF control. In a DC drive, the armature voltage is sensed, and the field voltage is kept at constant strength. With sensorless flux vector control, a modulator is used to vary the strength of the field, which is in reality, the stator.

The role of sensorless flux vector fits generally in between the standard PWM open loop control method and a full flux vector, closed loop control method. As previously indicated, the role of sensorless flux vector control is to achieve "DC-like" performance, without the use of a shaft position feedback device. This method provides higher starting and running torque, as well as smoother shaft rotation at low speed, compared with standard V/Hz PWM drives. Additional DC-like performance comes from benefits such as a wide operating speed range and better motor-speed control during load variations.

As indicated, there are many advantages of sensorless flux vector over standard PWM control, a main advantage being higher starting torque on demand. However, standard sensorless flux vector drives may not accomplish torque regulation or full continuous torque at zero speed, without more complex circuitry. Several drive manufacturers use a software design that estimates rotor and stator flux. The result of the flux calculations (estimations) is current that produces a type of regulated motor torque.

If more accurate torque control is required by the application, even more sophisticated control technology is needed. Though more complex in design, high-speed digital signal processors and advanced micro circuits make the electronics design easier to manage. These newer designs also don’t require a feedback device and provide the smooth control of torque, as well as full torque at zero speed.

The Direct Torque Control Method

The idea of vector control without feedback (i.e., open loop control) has been researched for many years. A German doctor, Blaschke, and his colleague Depenbrock published documents in 1971 and 1985 on the theory of field-oriented control in induction machines. The publications also dealt with the theory of direct self control. One manufacturer in particular, ABB Inc., has taken the theory and converted it into a refined hardware and software platform for drive control. The result is similar to an AC sensor less vector drive, which uses a direct torque control scheme. The theory was documented and tested in lab experiments for more than 30 years. However, a practical drive ready for manufacture was not possible until the development of application-specific circuitry.

Specific circuits such as the DSP (digital signal processor) and ASIC (application specific integrated circuit) are imbedded in IC (integrated circuit)

chips. These chips perform a certain function in the overall production of direct torque control. The controlling variables are motor magnetizing flux and motor torque.

With this type of technology, field orientation is achieved without feed back using advanced motor theory to calculate the motor torque directly and without using modulation.

There is no modulator used in direct torque control and no need for a tachometer or position encoder for speed or position feedback of the motor shaft. Direct torque control uses the fastest digital signal processing hardware available and a more advanced mathematical understanding of how a motor works.

The result is a drive with a torque response that is as much as 10 times faster than any AC or DC drive. The dynamic speed accuracy of these drives are many times better than any open-loop AC drive. It’s also com parable with a DC drive that uses feedback. One drives manufacturer indicates this drive is the first universal drive with the capability of performance like either an AC or DC drive. It’s basically the first technology to control the induction motor variables of torque and flux.

A block diagram of direct torque control. It includes the basic building blocks upon which the drive does its calculations, based on a motor model.

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--- The direct torque control (DTC) method (ABB Inc.) Speed Reference Torque Reference Torque Reference Controller Speed Controller DT C Core Calculated Speed Ac Motor Mains Torque and Flux Hysteresis Control Motor Model Torque and Flux actual values Optimal switching logic Switch Positions Voltage Current PID Inverter

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The two fundamental sections of direct torque control are the torque control loop and the speed control loop. During drive operation, two output-phase current values and the DC bus voltage value are monitored, along with the IGBT switch positions. This information is fed to the adaptive motor model.

The motor model calculates the motor data on the basis of information it receives during a self-tuning process (motor identification).

During this automatic tuning process, the drive's motor model gathers information such as stator resistance, mutual inductance, and saturation coefficients, as well as the motor inertia. In many cases, the motor is operated by the drive automatically for a short period of time, to gather the information required.

The output of this motor model is the representation of actual motor torque and stator flux for every calculation of shaft speed. The values of actual torque and actual flux are fed to their respective comparators, where comparisons are performed every 25 µs.

The optimum pulse selector is a fast digital signal processor (DSP) that operates at a 40-MHz speed. Every 25 µs, the inverter IGBTs are sent information for an optimum pulse for obtaining accurate motor torque. The correct IGBT switch combination is determined during every control cycle.

Unlike standard PWM control, in this control scheme there is no "predetermined" IGBT switching pattern. The main motor control parameters are updated as much as 40,000 times per second. This high-speed processing brings with it static speed control accuracy of ±0.5% without an encoder.

It also means that the drive will respond to changes in motor torque requirements every 2 ms.

The speed controller block consists of a PID controller and a circuit that deals with dynamics of acceleration. The external speed reference signal is compared with the actual speed signal given by the motor model. The resulting error signal is fed to the PID section of the speed controller. The flux reference controller contains circuitry that allows the drive to produce several dynamic motor features. Flux optimization is performing just-in-time IGBT switching. This IGBT switching method is controlled by a hysteresis block, which controls the switching action-when to switch, for how long to switch, and which IGBT switches are to be used. This reduces the resulting audible noise emitted from the motor and reduces energy consumption. In addition, flux braking is also possible, which is a more efficient form of injection braking.

Field-oriented control is a term commonly used by one manufacturer when describing continuous torque control. Similar to the direct torque control method, an advanced motor reference model acquires motor parameters during actual operation. An auto-tuning procedure determines the motor values to be used in the motor reference model. These voltage values are fed back to an adaptive software control block, which controls output cur rent, thereby controlling torque. The proper amount of slip is provided, thereby maintaining field orientation (precise stator flux control).

[cont to part 2 >>]