Sensorless Control Without Loss Of Performance

To significantly reduce energy costs and greenhouse gas emissions, massive changes in drive technology are necessary. As a consequence of the introduction of mandatory efficiency standards from the Ecodesign guideline, many users are inefficiently converting asynchronous machines (ASM) to grid-fed synchronous machines (SM). In order to be able to control these motors efficiently with a converter, however, it is necessary to know the rotor position of the synchronous machine exactly at all times. Traditionally, an encoder is used that recognizes the rotor position of the motor and passes this information on to the converter.

Sensors cost money

However, this approach has some disadvantages for the drive system. Not only that the donor incurs procurement costs. In addition, it must be installed; In addition, plugs and housing cabling are necessary. The converter itself requires evaluation electronics. These direct component costs naturally increase the system price.

At the same time, sensors are very sensitive components. The sensor must be mounted on the motor shaft. Systems with encoders have a reduced robustness, especially in motor-mounted applications with higher loads such as vibration and dust. Failure of the sensor can paralyze the entire drive. These are all factors that increase costs and limit the areas of application. For this reason, it would be ideal to be able to operate the drive system without an encoder.

With encoderless systems on the market, however, compromises in the performance of the control had to be accepted. Typical requirements for the performance of such motor-mounted applications are

• a high starting torque
• fast accelerations
• Delays
• Use of new motor topologies to meet future IE5 requirements at low cost.

That is why there was regularly a dilemma between a system with encoders and good performance, but higher costs and less robustness, or a system without encoders, but with significant losses in performance and efficiency. A Gordian knot that has never been solved.

This is how the pulse injection procedure works

The new pulse injection process in the Inveor frequency inverters from Kostal eliminates this dilemma. The Inveor is able to operate any type of synchronous machine without a process-related load limit in the entire speed range without an encoder. Since the method determines the rotor position regardless of speed and load, it achieves a performance that is comparable to that of systems with encoders such as resolvers or encoders.

The good properties of the pulse injection process are particularly useful when the rotor is at a standstill or at low speeds, when no induced counter-voltage can be measured. By evaluating high-frequency signals, the anisotropy of the machine can be determined, which enables conclusions to be drawn about the rotor position.

Simply put, the anisotropy describes the design-related asymmetry of the rotor. While the determination of the rotor position in the de-energized state is comparatively simple, it has so far been increasingly difficult or even impossible to determine the rotor position exactly with encoderless methods with increasing, torque-generating current.

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What the pulse injection method can do better than other methods

For other sensorless anisotropy methods available on the market, there is a principle-related load limit from a certain saturation point. This means that magnetic saturation effects occur under heavy load, which make a reliable determination of the rotor position impossible. Depending on the type of machine, this effect can occur well before the nominal current is reached, which means that regular operation of the motor is no longer possible.

This limit does not exist for the pulse injection method from Kostal, since a new approach takes the disturbing influence of this saturation into account in the algorithm. This means that the hardware solution with encoder 1: 1 could be replaced by a software solution. As a result, the control procedure itself no longer acts as a limiting factor in relation to the overload capacity. Instead, the limit to the overload is determined only by the maximum permissible currents from the motor and converter.

There is no comparable control method on the market that delivers a control performance, especially in the low speed range, without a sensor, which is comparable to encoder-based systems and can also be used on almost any motor.

Supplementary to the topic The advantages of the pulse injection method over others

Many encoderless systems often operate controlled and not regulated for low speeds. So there is no precise knowledge of the current rotor position and load situation. The result: loss of performance. In addition, applications that require this information do not work - such as heavy load start-up. Other injection-based methods on the market show restrictions with regard to the variety of usable motors and the overload behavior. These methods are often only adapted to an engine from this supplier. The Kostal process can be used for all engines.

Easy commissioning of the frequency inverters

The new pulse injection process is used as standard in all Kostal frequency inverters of the new performance class Inveor MP and Inveor MP Modular. All synchronous machines can be operated with maximum energy efficiency. With their robust design, these inverters are ideal for motor-mounted applications. Thanks to the universal motor adaptation concept, the inverters can be mechanically adapted to almost any industrial motor.

Commissioning the system is also extremely easy. The Inveor relies on field-oriented control, whereby only the input of the motor nameplate data by the user and the approx. 10-second automatic motor identification (Selfcom) is required in order to be able to operate the motor reliably and energy-efficiently at all operating points. For more complex applications and application-specific adaptation of the converter, a motor of the desired series can be measured precisely on the test bench and other parameters can be determined automatically in order to ensure maximum-efficient operation.

All power ranges from 0.55 kW to 5.5 kW are currently covered. The power range up to 30 kW will be available by the middle of the year. The areas of application in which the process is used are broad and range from pump / fan applications to logistics applications such as conveyor belts and simple servo applications. Thanks to the sensorless pulse injection process in the Inveor frequency inverters, the motors become more economical, more robust and, on top of that, gain in efficiency. (ud)

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