Archive for the ‘DC Drive’ Category

Sensor-less Vector Control

April 22, 2009
This sensor-less vector control performs the vector control of the induction motor without use of the speed sensor. Conventionally, there has been the V/f control without the speed sensor. However, this sensor-less vector control provides the simple control feature of the V/f control and the high performance of the vector control. The following describes the features of the sensor-less vector control.

(1) Sensor installation and wiring construction are not required.
(2) This control is applicable to motors, in which the sensor cannot be installed, such as two-axis motors or super high-speed motors, and other motors, which require special sensors, such as explosion-proof motors.
(3) This vector control technology is used for parallel drive of multiple motors, which is difficult to control by the conventional vector control.
(4) This sensor-less vector control provides excellent stability and large start-up torque when compared to the V/f control.
(5) The torque can be limited, ensuring stable rapid acceleration and deceleration.

Normal and Reverse Winding Operations

April 22, 2009

Torque Control

April 22, 2009
In winding machines, the winding materials are controlled at a specified tension. Therefore, the host PLC calculates the torque (reference) to be output from the motor. Additionally, the drive unit controls to output a torque corresponding to this torque reference. Furthermore, operation is made with speed control when the winding is completed or winding of next materials is started.

On the other hand, if operation based on the torque reference sent from the host PLC continues in case of a fault, such as material breakage, overspeed may result. In such case, the control is automatically changed to the speed control. (Torque control with speed limit function) The following describes how to use the torque control for operation with normal rotation and positive torque.

Auto Field Weakening Control

April 22, 2009
Operation shown in Fig. (a) to make the magnetic flux constant is used for general operation method of the induction motor. Operation is performed with the magnetic flux and ID_REF made constant. At this time, the induced voltage is calculated by multiplying the speed by the magnetic flux. The voltage is then increased in proportion to the speed.

In the auto field weakening control, when operating at a higher speed, the induced voltage is controlled at a constant level based on the magnetic flux reference in inverse proportion to the speed feed back after the voltage has reached the rated voltage.
If the speed exceeds the start speed of the field weakening control, the induced voltage becomes constant and the motor output shows the constant output characteristics. (Fig. (b)).

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Braking a drive system (2) – Finish

February 11, 2009
Braking methods

Brake motors
Motors fitted with mechanical brakes stop the load quickly and efficiently and provide holding torque at standstill. Their disadvantage is that the linings wear and require replacement from time to time. Brake lining wear can be reduced by combining mechanical braking with any of the electrical braking methods listed below.

Countercurrent braking
This involves switching a motor to the opposite rotational direction. After eceleration to standstill, the motor starts in the opposite direction unless the current is disconnected at the right moment. This method creates a very high braking torque, resulting in a large amount of heat being developed in the motor. Temperature monitors should always be used to protect the windings.

DC injection braking
DC braking can be performed with or without a frequency converter. With a frequency converter, a stop command makes the frequency converter switch to supplying the motor with direct current, developing a braking torque. The same effect can also be achieved using suitable DC excitation equipment. This method gives a considerably longer braking time than countercurrent braking, but its heat losses are much lower, so more frequent braking is possible. Still, its use is confined to applications in which braking accounts for a relatively small proportion of the running time.

Flux braking
Flux braking is a method based on increasing motor losses in a controlled way. This method is available in frequency converters based on DTC (Direct Torque Control). When braking is needed, the flux in the motor is increased, which in turn increases the motor’s capability to brake. When braking is not needed, DTC brings the motor flux down to its nominal value. Unlike DC braking, the motor speed remains controlled during braking.

Brake chopper and braking resistor
The braking chopper is an electrical switch that connects the DC bus voltage to a resistor, where the braking energy is converted to heat. During deceleration, the motor changes to generator operation and supplies energy back through the inverter. As brake energy cannot be fed back to the supply via the normal diode bridge, the brake chopper will turn on at a certain level and feed energy out via the brake resistor. Here, the energy is converted to heat and wasted, unless a separate heat recovery system is installed; additional ventilation for the room may be required. The chopper works even during loss of AC supply. This method is used when the braking cycle is needed occasionally, when the amount of braking energy with respect to motoring energy is small, or when braking operation is needed during mains power loss. Other solutions may be considered when the braking is continuous or regularly repeated.

Controlled mains bridge – anti-parallel thyristor solution
The diode rectifier bridges can be replaced by two thyristor controlled rectifiers, allowing the power flow to be reversed, effectively feeding mechanical energy back to the supply network, saving energy. However, the DC bus voltage is always lower than the AC supply voltage in order to maintain a commutation margin, which may cause a drop in torque. Additionally, the cos phi varies with loading, total harmonic distortion is higher than in IGBT regenerative units (see below), and the braking capability is not available during mains power loss.

Controlled mains bridge – IGBT solution
The IGBT, or insulated gate bipolar transistor, is a type of semiconductor power switch that can replace the anti-parallel thyristor. It has a low amount of supply current harmonics in both motoring and regeneration, as well as high dynamics during fast power flow changes on the load side. It also offers the possibility to boost the DC voltage higher than the respective incoming AC supply. This can be used to compensate for a weak network or increase the motor’s maximum torque capacity in the field weakening area. The IGBT solution is useful when the braking is continuous or repeating regularly, when the braking power is very high, when space savings can be achieved compared to the braking resistor solution, when network harmonics limits are critical, or when energy savings are targeted.
Common DC bus
When a process consists of several drives where one motor may need braking capability while others are operating in motoring mode, the common DC bus solution is a very effective way to reuse the mechanical energy. A common DC bus solution uses the DC bus as the channel to move braking energy from one motor to benefit the other motors.

Braking a drive system (1)

February 11, 2009
When is braking needed?

In many applications, being able to stop safely and precisely is as important as being able to start and accelerate quickly. Obvious examples include cranes, elevators and ski lifts, but quick and precise stopping is also necessary in machine tools, feed equipment and many other processes.

Design of the braking system

Evaluating the braking requirement goes back to the mechanical fundamentals of the process. Typically, there will be a requirement to brake the mechanical system within a specific time. There may also be sub-cycles in the process where a moving load forces the motor to operate as a generator. Any devices used for braking must be dimensioned for the braking power required. This depends on braking torque and speed; the higher the speed, the higher the power. In electrical braking systems, this power is transferred as a certain voltage and current; the higher
the voltage, the lower the current needed for the same power.

How DTC Works (2) – Finish

December 19, 2008
Speed Control

Step 5 Torque Reference Controller

Within the Torque Reference Controller, the speed control output is limited by the torque limits and DC bus voltage. It also includes speed control for cases when an external torque signal is used. The internal torque reference from this block is fed to the Torque Comparator.

Step 6 Speed Controller

The Speed Controller block consists both of a PID controller and an acceleration compensator. The external speed reference signal is compared to the actual speed produced in the Motor Model. The error signal is then fed to both the PID controller and the acceleration compensator. The output is the sum of outputs from both of them.

Step 7 Flux Reference Controller

An absolute value of stator flux can be given from the Flux Reference Controller to the Flux Comparator block. The ability to control and modify this absolute value provides an easy way to realise many inverter functions such as Flux Optimisation and Flux Braking.


How DTC Works (1)

December 19, 2008

Figure 1, below, shows the complete block diagram for Direct Torque Control (DTC).

Walk around the block

Figure 1: DTC comprises two key blocks: Speed Control and Torque Control

The block diagram shows that DTC has two fundamental sections: the Torque Control Loop and the Speed Control Loop. Now we will walk around the blocks exploring each stage and showing how they integrate together.
Let’s start with DTC’s Torque Control Loop.

Torque Control Loop

Step 1 Voltage and current measurements
In normal operation, two motor phase currents and the DC bus voltage are simply measured, together with the inverter’s switch positions.

Step 2 Adaptive Motor Model
The measured information from the motor is fed to the Adaptive Motor Model. The sophistication of this Motor Model allows precise data about the motor to be calculated. Before operating the DTC drive, the Motor Model is fed information about the motor, which is collected during a motor identification run. This is called auto-tuning and data such as stator resistance, mutual inductance and saturation coefficients are determined along with the motor’s inertia. The identification of motor model parameters can be done without rotating the motor shaft. This makes it easy to apply DTC technology also in the retrofits. The extremely fine tuning of motor model is achieved when the identification run also includes running the motor shaft for some seconds.
There is no need to feed back any shaft speed or position with tachometers or encoders if the static speed accuracy requirement is over 0.5%, as it is for most industrial applications. This is a significant advance over all other AC drive technology. The Motor Model is, in fact, key to DTC’s unrivalled low speed performance.
The Motor Model outputs control signals which directly represent actual motor torque and actual stator flux. Also shaft speed is calculated within the Motor Model.

Step 3 Torque Comparator and Flux Comparator
The information to control power switches is produced in the Torque and Flux Comparator. Both actual torque and actual flux are fed to the comparators where they are compared, every 25 microseconds, to a torque and flux reference value. Torque and flux status signals are calculated using a two level hysteresis control method. These signals are then fed to the Optimum Pulse Selector.

Step 4 Optimum Pulse Selector

Within the Optimum Pulse Selector is the latest 40MHz digital signal processor (DSP) together with ASIC hardware to determine the switching logic of the inverter. Furthermore, all control signals are transmitted via optical links for high speed data transmission.
This configuration brings immense processing speed such that every 25 microseconds the inverter’s semiconductor switching devices are supplied with an optimum pulse for reaching, or maintaining, an accurate motor torque.
The correct switch combination is determined every control cycle. There is no predetermined switching pattern. DTC has been referred to as “just-in-time” switching, because, unlike traditional PWM drives where up to 30% of all switch changes are unnecessary, with DTC each and every switching is needed and used.
This high speed of switching is fundamental to the success of DTC. The main motor control parameters are updated 40,000 times a second. This allows extremely rapid response on the shaft and is necessary so that the Motor Model (see Step 2) can update this information.
It is this processing speed that brings the high performance figures including a static speed control accuracy, without encoder, of ±0.5% and the torque response of less than 2ms.

DTC Questions & Answers (2)

December 18, 2008

What is the difference between DTC and traditional PWM methods?
• Frequency Control PWM and Flux Vector PWM
Traditional PWM drives use output voltage and output frequency as the primary control variables but these need to be pulse width modulated before being applied to the motor.
This modulator stage adds to the signal processing time and therefore limits the level of torque and speed response possible from the PWM drive. Typically, a PWM modulator takes 10 times longer than DTC to respond to actual change.
• DTC control
DTC allows the motor’s torque and stator flux to be used as primary control variables, both of which are obtained directly from the motor itself. Therefore, with DTC, there is no need for a separate voltage and frequency controlled PWM modulator. Another big advantage of a DTC drive is that no feedback device is needed for 95% of all drive applications.

Why does DTC not need a tachometer or position encoder to tell it precisely where the motor shaft is at all times?
There are four main reasons for this:
• The accuracy of the Motor Model.
• Controlling variables are taken directly from the motor.
• The fast processing speeds of the DSP and Optimum Pulse Selector hardware.
• No modulator is needed.
When combined to form a DTC drive, the above features produce a drive capable of calculating the ideal switching voltages 40,000 times every second. It is fast enough to control individual switching pulses. Quite simply, it is the fastest ever achieved. Once every 25 microseconds, the inverter’s semiconductors are supplied with an optimum switching pattern to produce the required torque. This update rate is substantially less than any time constants in the motor. Thus, the motor is now the limiting component, not the inverter.
What is the difference between DTC and other sensorless drives on the market?
There are vast differences between DTC and many of the sensorless drives. But the main difference is that DTC provides accurate control even at low speeds and down to zero speed without encoder feedback. At low frequencies the nominal torque step can be increased in less than 1ms. This is the best available.
How does a DTC drive achieve the performance of a servo drive?
Quite simply because the motor is now the limit of performance and not the drive itself. A typical dynamic speed accuracy for a servo drive is 0.1%s. A DTC drive can reach this dynamic accuracy with the optional speed feedback from a tachometer
How does DTC achieve these major improvements over traditional technology?
The most striking difference is the sheer speed by which DTC operates. As mentioned above, the torque response is the quickest available. To achieve a fast torque loop, ABB has utilised the latest high speed signal processing technology and spent 100 man years developing the highly advanced Motor Model which precisely simulates the actual motor parameters within the
Does a DTC drive use fuzzy logic within its control loop?
No. Fuzzy logic is used in some drives to maintain the acceleration current within current limits and therefore prevent the drive from tripping unnecessarily. As DTC is controlling the torque directly, current can be kept within these limits in all operating conditions.
A drive using DTC technology is said to be tripless. How has this been achieved?
Many manufacturers have spent years trying to avoid trips during acceleration and deceleration and have found it extraordinarily difficult. DTC achieves tripless operation by controlling the actual motor torque.
The speed and accuracy of a drive which relies on computed rather than measured control parameters can never be realistic. Unless you are looking at the shaft, you are not getting the full picture. Is this true with DTC?
DTC knows the full picture. As explained above, thanks to the sophistication of the Motor Model and the ability to carry out 40,000 calculations every second, a DTC drive knows precisely what the motor shaft is doing. There is never any doubt as to the motor’s state. This is reflected in the
exceptionally high torque response and speed accuracy. Unlike traditional AC drives, where up to 30% of all switchings are wasted, a drive using DTC technology knows precisely where the shaft is and so does not waste any of its switchings. DTC can cover 95% of all industrial applications. The exceptions, mainly applications where extremely precise speed control is needed, will be catered for by adding a feedback device to provide closed loop control. This device, however, can be simpler than the sensors needed for conventional closed loop drives.
Even with the fastest semiconductors some dead time is introduced. Therefore, how accurate is the auto-tuning of a DTC drive?
Auto-tuning is used in the initial identification run of a DTC drive. The dead time is measured and is taken into account by the Motor Model when calculating the actual flux. If we compare to a PWM drive, the problem with PWM is in the range 20-30Hz which causes torque ripple.

What kind of stability will a DTC drive have at light loads and low speeds?
The stability down to zero speed is good and both torque and speed accuracy can be maintained at very low speeds and light loads.

We have defined the accuracies as follows:
Torque accuracy: Within a speed range of 2-100% and a load range of 10-100%, the torque accuracy is 2%.
Speed accuracy: Within a speed range of 2-100% and a load range of 10-100%, the speed accuracy is 10% of the motor slip. Motor slip of a 37kW motor is about 2% which means a speed accuracy of 0.2%.
What are the limitations of DTC?
If several motors are connected in parallel in a DTC-controlled inverter, the arrangement operates as one large motor. It has no information about the status of any single motor. If the number of motors varies or the motor power remains below 1/8 of the rated power, it would be best to select the scalar control macro.
Can DTC work with any type of induction motor?
Yes, any type of asynchronous, squirrel cage motor.

DTC Questions & Answers (1)

December 18, 2008

What is Direct Torque Control?
Direct Torque Control – or DTC as it is called – is set to replace traditional PWM drives of the open- and closed-loop type in the near future.

Why is it called Direct Torque Control?
Direct Torque Control describes the way in which the control of torque and speed are directly based on the electromagnetic state of the motor, similar to a DC motor, but contrary to the way in which traditional PWM drives use input frequency and voltage. DTC is the first technology to control the “real” motor control variables of torque and flux.
What is the advantage of this?
Because torque and flux are motor parameters that are being directly controlled, there is no need for a modulator, as used in PWM drives, to control the frequency and voltage. This, in effect, cuts out the middle man and dramatically speeds up the response of the drive to changes in required torque. DTC also provides precise torque control without the need for a feedback device.
Why is there a need for another AC drive technology?
DTC is not just another AC drive technology. Industry is demanding more and existing drive technology cannot meet these demands.
For example, industry wants:
• Better product quality which can be partly achieved with improved speed accuracy and faster torque control.
• Less down time which means a drive that will not trip unnecessarily; a drive that is not complicated by expensive feedback devices; and a drive which is not greatly affected by interferences like harmonics and RFI.
• Fewer products. One drive capable of meeting all application needs whether AC, DC or servo. That is a truly “universal” drive.
• A comfortable working environment with a drive that produces much lower audible noise.
Who invented DTC?
ABB has been carrying out research into DTC since 1988 foll owing the publication of the theory in 1971 and 1985 by German doctor Blaschke and his colleague Depenbrock. DTC leans on the theory of field oriented control of induction machines and the theory of direct self control. ABB has spent over 100 man years developing the technology.