Mar 062014
 

Induction three-phase motors are widely used in industry because of their simple structure, extremely high reliability and low cost. The speed of the induction three-phase motor is determined by two factors: the frequency of the current and the number of poles. With conventional electric drives the frequency is constant – about 50 Hz, and once selected, the construction of the electric motor is unaltered, which leads to invariant speed of the motor. There are also motors with two sets of windings, where the velocities may be four, but they remain unaltered and do not allow smooth adjustment. In industry, however, we often need to smoothly adjust the speed of motors. Areas, where this is required, are extremely large: in metal-cutting and metal-processing machines, hoisting and hauling devices, compressors, pumps, fans, conveyor belts, etc. To implement smooth speed control of induction motors a frequency inverter is required.

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This is a device that converts the constant applied voltage and frequency of the mains supply into varying voltage and frequency. It is mounted between the power supply and the motor and allows a simple standard motor to become a flexible, mouldable system with speed-variable drive. There are also additional benefits from the usage of a frequency inverter in the system, which are also important. They are many and diverse – starting from the fact that the motor can be run smoothly, without increasing the inrush current; implementation of highly-efficient dynamic braking; raising the cosφ, without using capacitors; essentially reducing energy costs for motors with variable loading mode (for example, in pumps and fans, the mode of controlling the fluid which they propel, is through various types of valves and shutters, where the power and respectively the energy consumption of the motor stays unaltered, whereas much of the efficient power is lost to overcome the barrier in the path of the fluid, however if using a frequency inverter to control the speed of the electric motor, the power consumption is reduced in proportion to the pressure of the propeller, resulting in significant energy savings); we have full control over the operation of the motor – voltage, current, shaft speed, torque, acceleration time, deceleration time, etc.; thanks to the smooth operation mode, the service life of each component of the electrical equipment increases and finally we come to the possibility of overload protection.
A frequency inverter consists of three basic parts:

  1. Rectifier – converts the feed AC voltage into pulsating DC voltage through diode modules.
  2. DC filters – consist of filter inductors, capacitors and other filters.
  3. Transistor Inverter – converts the DC voltage into PWM (pulse-width-modulated) AC voltage.
    Besides the above-listed power components, the frequency inverter also has a control system implemented by a programmable microprocessor controller that also provides the sinusoidal waveform.

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Frequency inverters are divided into scalar and vector, depending on how the electric motor is controlled. The method of calculating the value of the voltage determines the inverter type. In scalar frequency inverters, the voltage is a function of frequency, which is calculated as a linear interpolation on several pillars. The user of the inverter is able to determine the value of those points. The scalar is a quantity whose value can be expressed with a single number. Since we change a single value, we call these inverters scalar.

Управляващ панел на честотен инвертор
In vector frequency inverters, the user must preset the electrical parameters of the controlled motor.

The value of the voltage is determined by modeling the transients in the induction motor. The vector inverter demands that the user has in-depth knowledge of the theory of electric drives. In some modern inverters some of the setup parameters (macros) are preset, which facilitates the setting of the inverter. The scalar inverter also has a good quality of controlling the motor speed, even when you just use the default settings.
Frequency Inverters can be used for both three-phase and single-phase induction motors.
Along with the undoubted benefits from the collaboration between frequency inverters and induction motors, there are some problems that need to be considered when using them.

presence of high harmonics at the output of the frequency inverter. Harmonic distortionincreasesthe lossesin the steeland the electricallossesin the motor windings. The availableradio-frequencyinterferencecan spreadboth over cablesandover the air. Due to the fact that thecablesbetweenthe frequency inverter and the motorare an extremelypowerfulsource of potentialinterference, powerandsignal cablescan act astransmission antennas, which is unacceptable according to theEMCdirectivesadopted by the EuropeanUnion.StandardIEC 61800-3:2004definesthe requirements that drives mustmeet in terms ofharmonic distortion in order tobe certifiedin the EuropeanUnion.

To avoid these problems the following measures can be taken: install the equipment in a closed and properly earthed metal control panel; place a filter for suppressing radio frequency interference of the power supply; use shielded cables, possibly with metal braid of each wire; cables prone to interference and supply cables must be installed away from each other.

voltage rush on the terminals of the motor. At the output of the inverters with width- pulse modulation, voltage with a high carrier frequency (up to 20kHz) is produced. When the distance between the frequency inverter and the motor is longer than 20 meters, a wave with a steep front runs in the cable, and its pulse form can create amplitude of the first harmonic, significantly exceeding the rated voltage for which the motor terminals are designed. This rush causes damages to the cable insulation, leading to premature aging of the terminals, increases the vibrations of the motor shaft, increases the heating of the coils and reduces the engine torque up to 5%. Laboratory studies have shown that the first few turns of the coil take voltage of up to 1.4kV, such as the gradient of the increase du / dt is so large that it exceeds the dielectric strength of the insulation and there is a risk of breakthrough. To avoid this problem we should provide a relatively small distance between the frequency inverter and the IM controlled by it.

overheating of the induction motor at low speeds of rotation. As we know the frequency inverter allows us to rotate the induction motor at any speed. The standard induction motor is designed in such a way that the rotation of the rotor rotates the cooling blades, located axially on it. However, when by using a frequency inverter we reduce the motor speed below 50% for an extended period of time, we must consider that this leads to poor cooling and respectively to overheating of the IM.

 

Bearing electroerosion. Insulated gate bi-polar transistor IGBT leads to a very high rate of increase of voltage pulses, on the order of du / dt> 10kV / s, which leads to highly asymmetric magnetic system, leading to the emergence of voltage of the shaft of the induction motor, consequently leading to the flow of high pulsating currents through the bearings of the motor which close the circuit through the grounded housing, thereby deteriorating the quality of the oil, cause arcing in the bearings, which in turn forms caverns and leads to an increased vibration level. This problem is most pronounced when through the frequency inverter frequencies exceeding the synchronous one are reached. If, however, this operation mode is necessary, you must provide wear-resistant and high-speed bearings, which is good to be electrically isolated from the IM and have independent cooling.
vibration of the motor. The high-frequency harmonics create magnetostriction in the magnetic core, cause induction of higher harmonics of the electromotive force in the rotor winding, which interact with the main magnetic flux and create additional parasitic mechanical shaft torque. This creates increased noise and vibrations of the rotor. In order to avoid this problem special filtering devices are placed, which we will further discuss.
Additional protective equipment to overcome the problems of adjustable AC drives using a frequency inverter.

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Installation of external filters, throttles, limiting the high-frequency components to standard values, which increases the life of the motor.
Using an induction motor with high-quality insulation.
Using the bearings with dielectric coating and insulated outer and inner ring as well as ceramic rolling parts.
Using independent cooling with drives with low speed rotation of the shaft.
Placing thermistor protection against overheating of the stator winding.
Using a shielded cable with a minimum length. This avoids the effect of wave voltage of the terminals of the motor. Most preferably, the inverter and the motor have common housing.
Mounting feedback encoders on the shaft, which carry information about the phase angle of the rotor and the rotation speed.

From all that we’ve discussed so far, it has become clear how useful frequency inverters can be when they are used competently by professionals.

 

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