In the market of speed control units for fan drives, it is self-evident that they must work efficiently and without noise emissions. To fulfil these requirements, and to be able to offer an ideal system consisting of motors and frequency inverters, Ziehl-Abegg AG in Kuenzelsau, Germany, has developed the range of Fcontrol frequency inverters. These frequency inverters with an integrated all-pole effective sinusoidal filter, enable the motor to run efficiently and free of electro-magnetic noise.
In particular, with Fcontrol, parallel operation of multiple motors with one frequency inverter is possible without problems, screened motor leads are not necessary, and cable length is not limited. Due to the sinusoidal output voltage and output current, the Fcontrol frequency inverter can also be described as an “electronic transformer”.
Recognising hazards when operating with frequency inverters
Due to the continuous increase in the efficiency of electronic components, the range of powerful frequency inverters offered on the market has significantly increased. Frequency inverters have become less expensive and are enjoying increased popularity.
It is important to know that standard frequency inverters without all-pole effective sinusoidal filters have block shaped (square wave) or pulse width modulated (PWM) output voltages. As outlined in the European standard for rotating electrical machines, DIN IEC/TS 60034-17, this type of power supply waveform presents potential hazards for motor bearings and windings. In particular, small motors, like those integrated into fans, are at danger. The standard describes the increased stress on the motor windings through voltage peaks, and the hazard of bearing currents through capacitive coupled voltage, which can provoke the premature failure of the motor bearings.
In the classic combination of a standard frequency inverter without a sine filter, and one motor and with short, shielded motor-leads, operation is generally unproblematic. In this situation, the motor is usually designed specifically for inverter speed control, and includes special measures, such as improved motor winding insulation and ceramic bearings.
If motors (small fan motors) are operated using these frequency inverters and have not been adapted for frequency inverter operation, or if groups of motors are operated in parallel using frequency inverters without sine filters, motor winding failures and problems in the motor bearings can result.
External sine filters for retrofitting –
Integrated sine filter in Ziehl-Abegg Fcontrol
Most manufacturers of standard frequency inverters do not provide sine filters in their devices. The frequency inverters are generally intended for switch cabinet installation and the manufacturers assume that the user will install an external sine filter in the switch cabinet if necessary.
An external sine filter is not all-pole and merely filters the voltages between phases. The all-pole effective sinusoidal filters as used in the Ziehl-Abegg Fcontrol frequency inverters filter the voltage not only between phases but also between each phase and earth.
The integrated sine filter ensures the sinusoidal shape of the output voltage and the output current. It’s even possible to refer to the Fcontrol frequency inverter as an "electronic transformer".
Why frequency inverters without sine filters endanger motors
As mentioned earlier, the efficiency of electronic components has sharply increased. This especially applies to the IGBT modules (Insulated Gate Bipolar Transistor) used in frequency inverters. These are used as electronic switches to generate the PWM voltage from the frequency inverter’s dc link.
In frequency inverters without sine filtration, this unfiltered PWM voltage is applied directly to the motor's terminals. The PWM voltages rise steeply, so that square-wave voltage impulses from the PWM voltage expose the motor to clock-frequencies of 2...16kHz as below:
1.) Bearing currents
Due to the high slope rate in the PWM common-mode voltage and the high clock frequencies, there is a danger that arcing could occur from the parasitic motor capacitances (rotors – stators). The result of such arcing is the ejection of metallic particles from the ball bearings into the lubricant. Bearing currents cause so-called ‘spark erosion’, which results in damage to the motor bearing ball races. The consequence is premature bearing wear.
2.) Voltage reflections
The high-frequency voltage impulses in the PWM voltage output spread through the motor lead in a wave-like manner. Voltage impulses are reflected at the motor terminals like waves striking a solid surface. Over-voltages occur due to the addition of reflected voltage peaks with outgoing voltage peaks. Measurements demonstrate that such reflections mainly occur in long motor leads. If several motors are operated by a frequency inverter in parallel, multiple reflections can arise. Multiple voltage peak superimpositions are the result, giving rise to even greater voltage peaks. These voltage peaks or ‘spikes’ severely stress the motor winding.
3.) Motor temperature rise
Harmonic waves from the PWM signal lead to losses in the motor’s laminated core. Depending on the motor’s structure/cooling, these losses can lead to the motor becoming overheated.
4.) Motor noise
Through the higher clock frequencies used in the IGBT modules, high-frequency noises often arise that are considered quite irritating.
The benefits of using the Fcontrol frequency inverter with integrated, all-pole effective sine filter
• The sinusoidal shape of the frequency inverter’s output voltage provides a restriction to the voltage rise rate. Transient waves are prevented, there is no limitation to the motor cable length. The stress on the motor insulation is no greater than that when the motor is operating with a direct mains connection.
• Radiated electro-magnetic noise from motor cables is reduced, such that shielded motor cables are not necessary.
• Bearing currents do not occur in the motors. No capacitive compensating current in motors. No danger to the motor bearings.
• Elimination of disturbing harmonic oscillations that arise due to the frequency inverter’s switching principle. This also prevents additional temperature rises in the motor, losses in the motor are reduced, resulting in energy savings.
• No electromagnetic excitation in the connected motors, this means silent motor operation is possible
• Unlimited parallel operation of multiple motors is feasible without any problem
Conclusion
Many benefits result from using the all-pole effective sine filter equipped Fcontrol frequency inverter in place of standard inverters. The most important of these is the elimination of the potential motor hazards associated with the use of standard frequency inverters, and the resultant increased reliability of the industrial plant so equipped.
