Testing an Industrial AC and DC Variable Speed Drives

[NiCeMaN]

Hi all,
i did this test in the lab. can anyone help me with some doubts that i have?

below is how the system was setup.
Power circuit of lab drive setup

The shafts of motors M1 and M2 are coupled directly. The AC VSD (A1) is a two-quadrant drive, and hence cannot provide braking. Motor M1, therefore, operates always in the motoring mode. The DC VSD of motor M2 can provide operation in four quadrants. It is set for torque control (Fig. 2). If the set torque of the DC drive is of the opposite direction to the direction of rotation selected on the AC drive, the DC
drive will operate in the braking mode. Consequently, M1 operates as a motor, and M2 – as a generator.

fig2
Torque/Speed Characteristics of AC Drive
1. On the control panel of AC drive, select a frequency in the range of 15 Hz – 20 Hz. This is the parameter of the first characteristic to be taken. The selected direction of rotation is clockwise (positive).
2. Start the AC motor by depressing the START pushbutton.
3. On the control panel of DC drive, switch to local control (LOC) by depressing the LOC/REM pushbutton.
4. Start the DC drive by depressing the START p/b.
5. On the DC drive control panel, set the reference torque at a negative value corresponding to the braking of the AC motor. The torque is displayed as a percentage of the motor nominal value, and can be varied in the range of 0 to (-80%).
6. In the table below, record the displayed values of frequency f (AC drive control panel), torque T and speed n (DC drive control panel).
7. Record the drive speed for at least four other frequency values, selected to cover evenly the remaining range up to 60 Hz. The torque is maintained constant by the torque controller of the DC drive.
8. Repeat steps 5, 6 and 7 for at least four other torque values, selected to obtain measurement points spread evenly in the range of 0 to (- 80 %). The set of frequency values at which the speed is measured remains the same for each torque value.

tab1
Torque/Speed Characteristics of a DC Generator
1. The procedure is similar to that described in the previous section, except that the parameter is the dc machine terminal voltage Vt, displayed on the control panel of the DC drive. The frequency is adjusted to obtain the required Vt at a given value of T.
2. Select at least four values of Vt spread evenly over the range of 0 – 380 V. These are the parameter values for which a family of characteristics will be taken.
3. With the AC drive on, set the torque at a value within the available range of 0 to (-80%). Adjust the AC drive frequency until the selected value of Vt is obtained. For each value of Vt, record the displayed speed n.
4. Repeat step 3 for at least three other values of T, making sure that all torque values are spread evenly over the full range of 0 to (- 80 %).

tab2
Output Voltage and Current Waveforms of a PWM VSD.
1. On the oscilloscope, monitor the phase current and the phase voltage of the induction motor. Keep the motor loaded with a torque of at least (-50%) (as displayed on the DC drive).
2. Sketch the waveforms for three widely spaced frequencies in the range of 25 – 60 Hz. Note the differences between the voltage waveforms. Note that, in your report, you are required to include the scale of the presented graphs.
3. Measure the amplitude of pulses in the voltage waveforms.
4. Note the response of the phase current to the voltage pulses.

30hz

40hz

50Hz

Anyone can tell my why the current waveforms are of the shape shown above and why the amplitude of the pulses in the output voltage is constant and how the output voltage magnitude is varied?

Thanks

 

[FvM]

The VSD (or VFD) is performing PWM control of the motor voltage. The pulse magnitude is defined by the DC bus voltage, which is effectively constant. The average motor voltage and subsequently the motor current is set by the variable pulse width, which can't be well recognized in your oscilloscope display.

Ideally, the motor current waveform would be sine-shaped, achieving maximum motor efficiency.

 

[NiCeMaN]

so is there a way i can change the magnitude of the output voltage?

 

[FvM]

If you mean the magnitude of the averaged motor voltage, ideally a sine waveform, yes it can be varied up to a maximum according to the bus voltage. A VSD will change the magnitude automatically with the output frequency. A linear V~f characteristic is the standard, but you can also select other characteristics like V~f² ("fan" characteristic). In addition, the nominal output voltage can be adapted to the motor.

 

[NiCeMaN]

How about the current waveform(blue).
Any Idea why the shaped is as recorded?

Thanks

 

[FvM]

I can't see the waveform, because it's hidden in part. As said, it should be sinoidal, ideally. The spikes are just measurement artefacts, most likely from the SCR DC motor controller. If the waveform has more a triangular than a sine shape, as it seems to me, it may be either due to the motor characteristics or a simplified VSD design. Setting the motor voltage higher than nominal can e.g. cause distorted current waveforms due to partial saturation.

 

[NiCeMaN]

On the waveforms, do you think the Air-Gap is kept constant?

Usually if the airgap is constant if each flux line crosses the air gap twice, the mmf drop across the air gap must be equal to half of the total or Ni/2.

You got any idea on it?
thanks

 

[FvM]

I would like to see the waveform before jumpinmg into conclusions. Of course the specific design of the motor's magnetic path may be a reason for creating harmonic currents and deviations from sine current waveform. But without knowing the driving voltage waveform, it's just a speculation.