Željko.V. Despotović1, Vladimir.M.Šinik2 , Aleksandar.I. Ribić1

1Mihajlo Pupin Institute-University of Belgrade, Serbia, zeljko.despotovic@pupin.rs 2Technical Faculty “Mihajlo Pupin”-Zrenjanin, Univesity of N.Sad, Serbia

Vibratory resonance conveyors (VRC) are widely used devices in various technological processes for transporting and dosing particulate materials

The conveying process is based on a sequential throw movement of particles

VRCs having electromagnetic vibratory actuator (EVA) are very popular because of their high efficiency and easy maintenance

In the processing industry often are in use mechanisms based on the vibratory conveyors having electromagnetic drives

Vibrations of a “load-carrying element” (LCE), in which the material is placed, induces the movement of material particles, so that they resemble a highly viscous liquid and the material becomes easier for conveying Application of an electromagnetic vibratory drive in

combination with a power converter provides flexibility during work.

It is possible to provide operation of the vibratory drive in the region of mechanical resonance.

Resonance is highly efficient, because large output displacement is provided by small input power.

The SCR converters are used for the EVA standard power output stage.

Their usage implies a phase angle control

SCR converters have a very negative impact on the power supply network

A varying firing angle provides a controlled AC or DC injection current of EVA or controls its mechanical force.

Today, a great deal of work is done on implementing switch mode power converters (SMPC)

Application of SMPC enables accomplishing the amplitude and/or frequency control of VRC.

The use of SMPC makes the excitation of a resonant VRC independent of the supply network frequency.

In this paper the considerations will be focused on the electromagnetic drives having asymmetrical switching topology

This topology has many advantages, such as reliability, easy control, and minimal losses

For generation of voltage in the intermediate DC circuit the use is made of the input diode rectifier (diodes D3-D6), bank of electrolytic capacitors of equivalent capacitance , and input inductor which is set on the network side.

In addition to the output current of asymmetrical SMPC, DC link circuit essentially determines the input current waveform of the entire regulated vibratory conveying drive.

The influence of the sine and triangular waveforms of the output current which is fed, through an asymmetric half-bridge, to EVA coil has been analyzed.

Simulation circuit of switching converter for driving of EVA with single phase full-wave diode rectifiier (DC voltage source Vs).

Typical waveforms of the characteristic voltages and currents in a DC link circuit in relation to EVA current, in the cases of the sine and triangle half-wave output current.

Characteristic waveforms of voltages and currents in DC link for hard and soft switching operations; (a)-sinusoidal half-wave of EVA current , (b)-triangular half-wave of EVA current

5swf kHz

The simulated waveforms of a converter having sinusoidal output current half-wave of variable driving frequency of EVA coil: (a) 40Hz, (b) 60Hz, (c)50Hz

The simulated waveforms of a converter having triangular output current half-wave of variable driving frequency of EVA coil: (a) 40Hz, (b) 60Hz, (c) 50Hz

The experimental results presented in this section, obtained with a real case, demonstrate the influence of a regulated VRC on the supply network.

The EVA was driven by an asymmetric half-bridge IGBT converter.

Since the VRC can operate with both sinusoidal and triangular signals driving EVA, the experimental results for both cases are presented.

The characteristic waveforms for the case of sinusoidal half-wave current drive of EVA

driving frequency of EVA coil 30Hz:

(a) CH1: supply network voltage and CH2: input current,

(b) CH1: EVA current, CH2: input current and CH3: DC- link voltage.

The characteristic waveforms for the case of sinusoidal half-wave current drive of EVA

driving frequency of EVA coil 70Hz:

(a) CH1: supply network voltage and CH2: input current,

(b) CH1: EVA current, CH2: DC- link voltage and CH3: input current

The characteristic waveforms for the case of sinusoidal half-wave current drive of EVA;

driving frequency of EVA coil 50Hz:

(a) CH1: supply network voltage and CH2: input current,

(b) CH1: input current and CH2: DC- link voltageFrom these results it can clearly be concluded that the input current is a string of unipolar current pulses. It should be mentioned that this is the worst case in practice.

This shape of input current is a consequence of synchronization of the output current with the network voltage.

The characteristic waveforms for the case of triangular half-wave current drive of EVA;

driving frequency of EVA coil 55Hz (SUPER-RESONANT OPERATING):

(a) CH1: input current and CH2: DC-link voltage,

(b) CH1: input current, CH2: supply network voltage, CH3: DC-link voltage and CH4: EVA current

FFT

The characteristic waveforms for the case of triangular half-wave current drive of EVA;

driving frequency of EVA coil 45Hz (SUBRESONANT OPERATING)

(a) CH1: input current and CH2: DC-link voltage,

(b) CH1: input current, CH2: DC-link voltage, CH3: EVA current and CH4: supply network voltage.

FFT

The characteristic waveforms for the case of triangular half-wave current drive of EVA;

driving frequency of EVA coil 50Hz (RESONANT OPERATING):

(a) CH1: input current and CH2: DC-link voltage,

(b) CH1: input current, CH2: DC-link voltage, CH3: EVA current, and CH4: supply network voltage.

FFT

From the obtained simulation and experimental results for the described switching converter controlling a VRC system, one can conclude that for both cases of the driving current half-wave (sinusoidal or triangular) the input current, taken by the drive from the supply network, is far from being sinusoidal.

Moreover, this is a pulse current having a very unsuitable harmonic content. The worst case is when the output frequency of the converter is 50Hz, i.e. when

the driving current pulses are in synchronism with the network voltage frequency.

Under these conditions it may occur that the input current is represented by a string of very sharp unipolar pulses of 50Hz frequency.

In a power network, regulated VRCs generate mainly bipolar current pulses, but under certain conditions they can also generate DC pulsating current pulses.

Owing to this, they represent significant sources of higher harmonics which adversely affect the operation of other consumers.

The impact on a power network is more drastic if a number of regulated VRCs is used.

In fact, this is quite common in practice, where, for technological reasons, a processing line is feed from a number of conveying devices in order to form a final mixture of a required homogeneity.

The authors gratefully acknowledge the constructive comments and suggestions of anonymous reviewers.

This investigation has been carried out with the financial support of the Serbian Ministry of Science- project No: TR33022.