FREQUENCY INVERTERS AND EVERYTHING ABOUT THEM

A frequency inverter is a device for regulating the speed of electric motors. Changes in speed are made by a simultaneous change of frequency and voltage, or, after reaching nominal voltage values, only by changing the frequency.

Use

Inverter control is used wherever different permanent speeds of electric motor need to be achieved, or where it is necessary to make fluid or step changes in the speed of rotation or to control the output torque directly. Inverters are also often used for controlled start-up and stopping without current or mechanical shocks in applications with high inertia (for soft start-ups, cheaper soft starter components can be used). When the drive is started up with an inverter, there is a significant reduction in current and torque shocks.

Not least, inverter deployment facilitates considerable energy savings, especially in applications where speed control replaces the need for a throttle valve or overflow (fans, pumps). 

By controlling the speed of your technology, you can also optimise productivity and/or improve production quality. You could see the cost of acquiring the inverter returned to you several times over in a relatively short amount of time.

Supply voltage

Frequency inverters are designed to control three-phase electric motors. On input, the inverter is powered by alternating voltage (single-phase or three-phase), the voltage in the internal circuits is regulated, and on output it is converted by a power inverter to three-phase alternating voltage at the required frequency. Depending on the type of input voltage, inverters can be classified as follows:

• inverters with single-phase power input
  • in our network, usually 1AC230 V and three-phase output for motors, which can be powered by 3AC230 V with a delta connection
    
    • we recommend checking this option on the motor label or in its technical documentation
  • a drawback may be relatively high current consumption from a single-phase inverter supply at higher outputs (from approximately 1.5 kW)
  • an advantage is the substantially lower cost of the inverter
    
• inverters with three-phase power input
  
  • in our network, usually 3AC400 V, in some industrial networks 3AC500 V, and exceptionally also 3AC230 V, and three-phase output for motors at the corresponding voltage
  • the only downside in this case is the higher purchase price of the inverters

Load types

• constant load
  • applications where the load does not change according to the speed (e.g. conveyors, machine tools, mills and crushers, etc.)
  • often referred to as HO – High Overload
• quadratic load
  • applications where there is a quadratic increase in load when the speed increases (typically fans and centrifugal pumps)
  • often referred to as LO – Low Overload

Control methods

• linear characteristics
  
  • the simplest method of control, suitable for dynamically undemanding applications with a constant load
  • also used in the parallel control of multiple motors via a single inverter
  • the change of voltage and frequency in this case is linear
  • available with all the inverter series we supply

• quadratic characteristics
  
  • designed exclusively for quadratic load drives
  • this method can also be used in the parallel control of multiple motors with a quadratic load
  • available with all the inverter series we supply

• vector control without speed feedback
  
  • for dynamically more demanding applications, without speed feedback from the rotary sensor on the electric motor
  • the advantages are the high torque from very low speeds, speed adjustment if there is a load change, and better dynamics
  • available with Micromaster 440 and Sinamics G120

• vector control with speed feedback
  
  • for dynamically more demanding applications, with speed feedback from the rotary sensor on the electric motor
  • the inverter must be fitted with an encoder
  • the advantages are nominal torque even at zero revolutions, the fact that the speed is independent of the load, and better dynamics
  • this is the optimal method for controlling an induction motor
  • available with Micromaster 440 and Sinamics G120 with encoder

• FCC – control with active current limiting control
  
  • used for small motor powers characterised by the large resistance of the stator winding (typically up to 4 kW)
  • limits the peak current so as to avoid drive outages due to current overload
  • available with the entire Micromaster 440 series and Sinamics G120

• multi-point characteristics
  
  • used in special cases where the linear characteristics can be defined by multiple independent points
  • available for all of the inverter series referred to above

• direct torque control without feedback
  
  • designed for applications where a target torque value is set
  • the drive speed is automatically adjusted so that the torque corresponds to the required value (i.e. it is not possible to independently control the speed and the torque at the same time)
  • available with Micromaster 440 and Sinamics G120

• direct torque control with feedback
  
  • designed for applications where a target torque value is set
  • the drive speed is automatically adjusted so that the torque corresponds to the required value (i.e. it is not possible to independently control the speed and the torque at the same time)
  • torque is achieved even at zero revolutions
  • a rotation sensor must be installed on the motor and the inverter must be fitted with the encoder module
  • available with Micromaster 440 and Sinamics G120 with encoder

• other special methods
  
  • e.g. ECO mode, special pump control methods for the Micromaster 430, special mode for textile machines with the Micromaster 440, etc.

Control options, motor behaviour

All of our inverters offer a change of frequency from 0 to 650 Hz. This is considerably more than the permitted speed range for induction electric motors. Consequently, it is possible to regulate the speed continuously in both directions of rotation and at the full speed permitted by the specific motor characteristics.

When dimensioning the drive, it is necessary to take into account the fact that a motor combined with an inverter does not reach the full nominal power values, but is typically 10-15% below that level. In addition, when the motor decelerates, the power is reduced; when it accelerates, the torque drops.

The inverters allow you to regulate the motor as follows:

1. continuous control

2. control at set frequencies (for simple inverters up to three fixed frequencies, for higher series six fixed frequencies or more)

Method of operation

Inverters can be controlled in several different ways:

• manual control using a control panel
  
  • it is necessary to purchase an add-on control panel for the front of the inverter
  • the control panel is equipped with a monochrome LCD display and buttons for start, stop, an increase or decrease in value, reversing, stepping and programming the inverter

• manual or automatic control via external controls with output on the inverter’s controlling screw terminal
  
  • the inverter has its own source of DC24V control voltage for digital inputs, so there is nothing easier than connecting the switches
  • digital inputs (0/1) can be set to call up the function you require when connected (e.g. start/stop in any direction, reversing, fast stop activation, preset fixed frequency in any direction, malfunction reset, increase or reduction of frequency, step activation, etc.)
  • with analogue input, speed can then be controlled by means of a potentiometer or other analogue source (voltage 0-10 V, for higher inverter series also with a current of 0-20 mA or 4-20 mA)
    
    • analogue input can be calibrated to the required range
    • for analogue input, a DC10 V source is available for inverters
    • all these inputs can also be used to connect automatic switches (end and position sensors, etc., for switching digital inputs or various measuring components with analogue output for speed control via analogue inputs)

• remote control from a PLC or other control system
  
  • inverters can be controlled via various control protocols over standard buses (Profibus, Profinet, serial interface, etc.)
  • communication options over standardised buses vary between the series; some can be integrated directly in the inverter, others requite the purchase of communication interfaces

• automatic control via feedback
  
  • another very common control method is speed control via feedback in order to achieve and maintain the required target value
    
    • a built-in PID controller is used for this purpose
  • example in practice:
    
    • imagine a water supply system in which you want to maintain a constant pressure (the required quantity)
    • because there is different water offtake from the system at different times, you need the pump drive to respond flexibly to changes in system pressure (the actual quantity)
    • therefore, you integrate a pressure sensor with output of 0-10 V (or 0-20 mA) into the system
    • you direct this signal to the inverter and activate the control via feedback (the minimum difference between the required and actual quantity)
    • you fine-tune the required values at the inverter input, thus automatically adjusting the drive to the immediate conditions
      
      • this method is, among other things, a very effective way of saving electricity costs and we recommend that it be deployed instead of throttle valves and similar
      • feedback control is not limited to controlling the pressure, but essentially also continuously controls any target value (pressure, temperature, flow, weight, speed, torque, etc.) that can be affected by motor speed in the application and measured in real time (with output from measurements in the form of a voltage or current signal)

Inverter programming 

Inverters are programmable, meaning that customers can adapt the drive’s features to their needs. Most users are sure to want to adjust, for example, the range of minimum and maximum speeds, adjust the acceleration and deceleration rate, set the control method, set the functions of the control inputs and outputs, etc. All of these and many other parameters are customisable to your requirements.

To programme the drive, use either an add-on control panel or an add-on communication interface for PC communication with Siemens Starter or Siemens Drive Monitor software.

In principle, basic configuration of the inverter is mandatory (to put it into operation quickly). Here, the motor’s nameplate values are transferred to the inverter memory and some basic drive parameters (minimum and maximum frequency, control mode, etc.) are selected. This step is necessary for the proper operation of the drive and the protection functions built into the inverter.

Factory default values in the inverter are consistent with a 4-pole induction motor with the same power as that defined for the inverter. The control of start/stop, reversing and malfunction reset is preset for digital inputs; continuous speed control is set for analogue input. The linear characteristics are also preset.

After basic set-up, numerous programmable parameters can be adapted to the required drive properties.

Drive braking methods

Inverters are programmable, meaning that customers can adapt the drive’s features to their needs. Most users are sure to want to adjust, for example, the range of minimum and maximum speeds, adjust the acceleration and deceleration rate, set the control method, set the functions of the control inputs and outputs, etc. All of these and many other parameters are customisable to your requirements.

To programme the drive, use either an add-on control panel or an add-on communication interface for PC communication with Siemens Starter or Siemens Drive Monitor software.

In principle, basic configuration of the inverter is mandatory (to put it into operation quickly). Here, the motor’s nameplate values are transferred to the inverter memory and some basic drive parameters (minimum and maximum frequency, control mode, etc.) are selected. This step is necessary for the proper operation of the drive and the protection functions built into the inverter.

Factory default values in the inverter are consistent with a 4-pole induction motor with the same power as that defined for the inverter. The control of start/stop, reversing and malfunction reset is preset for digital inputs; continuous speed control is set for analogue input. The linear characteristics are also preset.

After basic set-up, numerous programmable parameters can be adapted to the required drive properties.

Use in automated systems

Protective features

Modern inverters are equipped with a range of protective features not only for their own protection, but also for motor protection. The most common include:

• protection against short circuiting (ground and interphase)
• overload protection for the motor and inverter, and more
• motor winding temperature control via feedback from temperature sensors in the winding
  
  • in the latest series, there is also an optional “safety integrated” feature, considerably simplifying the design and production of drive systems by integrating safe torque limiting functions, safe speed limitation, safe braking, or safe brake control

Inverter features – EMC (electromagnetic compatibility)

Inverter features – communication

Add-on inverter features

• input network (commutation) chokes
  • used to reduce the feedback to the network by means of greater harmonic distortion reduction and to prolong the life of the inverter capacitors
• output motor chokes
  
  • reduce the stress load on the motor windings
  • at the same time, they reduce the capacitive currents, which additionally stress the power parts of the inverter using long cables
  • their use requires suitable inverter configuration, specifically the switching frequency
  • the output voltage is rectangular
• braking resistors
  
  • used to quickly brake drives with greater inertia
  • the energy returning during rapid motor braking is radiated by the resistor in the form of waste heat
    
    • available only for Micromaster 440 and Sinamics G120 with PM240
• add-on network filters
  
  • used to achieve a higher class of interference suppression
• sinus filters
  
  • restrict the steepness of the voltage and capacitive currents
  • replace the output (motor) chokes
  • use is similar to motor chokes, but with higher efficiency
  • the output voltage is sinusoidal
  • the disadvantage is the higher acquisition costs

Summary

When you are choosing an inverter, we will of course assist you if necessary, but it is still essential to clarify some basic information before ordering:

1. power supply – consider whether you are choosing an inverter with single-phase or three-phase input power supply (only up to 3 kW; over 3 kW the inverters are always powered in three phases)
2. power – essential basic information on the drive power is needed to select the right inverter
3. application type – essential for selecting a suitable inverter series; for simple, dynamically undemanding applications, one of the basic inverter series will suffice, for more demanding applications, it is necessary to select Micromaster 440 or Sinamics G120 series inverters
4. control method – for simple applications, a basic series of inverters is sufficient; for applications requiring vector control or some other more sophisticated method of control, the higher Micromaster 440 or Sinamics G120 series is required
5. method of operation – clarify how you want to control the inverter (manually, via a control panel or via a communication interface)
6. energy recovery requirement – if you would like to save on the running costs of your equipment and this is equipment where greater inertia is braked fairly frequently, choose an inverter offering energy recovery (Sinamics G120 with a PM250 or PM260 power unit)
7. inverter features – verify your requirements as to the number of digital and analogue inputs, the communication interface, the possibility of connecting braking resistors, and other optional equipment