DTC Questions & Answers (2)

Operation

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
controller.

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.

Source: www.abb.fi