Voltage waveforms

As standard practice, all motors are designed for a balanced and virtually sinusoidal supply system, but it may not be feasible to obtain the designed supply conditions in practice. Hence, a motor is designed with a certain in-built capacity to sustain small amounts of voltage unbalances and some degree of harmonic quantities, such that the voltage waveform may still be regarded as sinusoidal. 

A supply system would normally contain certain harmonic quantities, as di.scussed in Section 23.5.2. The influence of such quantities on an induction motor is also discussed in Chapter 23. To maintain a near-sinusoidal voltage waveform, it is essential that the harmonic voltage factor (HVF) of the supply voltage be contained within 0.02 for all 1-0 and 3-0 motors, other than design /V motors and within 0.03 for design N motors, where... 

The CDF can be controlled by controlling the period of conduction, in other words, the pulse widths (periodic time period, T remaining the same). Thus the a.c. output voltage in an IGBT inverter can be controlled with the help of modulation. The modulation in the inverter circuit is acliieved by superposing a cairier voltage waveform... 

Inverter natural voltage waveform before modulation, improved to a near sinusoidal waveform, with the use ol L and C. 

Triangular voltage waveform of fixed amplitude Variable frequency and modulated voltage output (V/f) as desired. 

By the immediate first current zero it is assumed that the contacts have travelled sufficiently apart to achieve the required deionization and have built up adequate dielectric strength to withstand at least 0.95 V, . If the circuit does not interrupt at the immediate current zero at a which is so near to the point of chopping Vt , the interruption will take place only by the next current zero at point h and result in another 260 strikes by then. To study more accurate behaviour of an intenupter, with the number of restrikes and the formation of the actual transient voltage waveforms on current chopping, oscillograms similar to those during a short-circuit test may be obtained (Section 14.3.6). 

Its operation ean be seen as analogous to a meehanieal flywheel and a one-piston engine. The L-C Alter, like the flywheel, stores energy between the power pulses of the driver. The input to the L-C Alter choke input filter) is the ehopped input voltage. The L-C Alter volt-time averages this duty-cyele modulated input voltage waveform. The L-C Altering funetion ean be approximated by... 

When the eore s flux is eompletely emptied prior to the next cyele, it is referred to as the discontinuous-mode of operation. This is seen in the induetor eurrent and voltage waveforms in Figure 3-4. When the eore does not eompletely empty itself, a residual amount of energy remains in the eore. This is ealled the continuous mode of operation and ean be seen in Figure 3-5. The majority of boost-mode eonverters operate in the diseontinuous mode sinee there are some intrinsie instability problems when operating in the eontinuous mode. 

Snubbers are passive networks that delay the risetime of the voltage waveform. Historieally, snubbers have been used to keep power deviees within their forward- and reverse-biased safe operating areas (FBSOA and RBSOA) or to eontrol RF emissions from the power supply. They are essentially lossy tank eireuits (L-C eireuits with R). Using them offered more of an advantage than the loss ineurred. Semieonduetor eomponents are more rugged today and the traditional need for the RFC snubber for proteetion has lessened, but oeea-sionally a snubber is still needed. 

Power factor correction circuits are intended to increase the conduction angle of the rectifiers and to make the ac input current waveform sinusoidal and in phase with the voltage waveform. The input waveforms can be seen in Figure C-2. This means that all the power drawn from the power line is real power and not reactive. The net result is that the peak and RMS current drawn from the line is much lower than that drawn by the capacitive input Alter circuit traditionally used. 

In terms of strietly passive reaetive loads, the power faetor is the resulting phase between the voltage and the eurrent waveforms. In power supplies though, it is the distortion to the voltage waveform resulting from the time whieh input reetifiers eonduet. Power faetor is measured from 0 to I where I is where all the power is used by the load (purely resistive). The typieal eapaeitive input filter found in power supplies has an average power faetor of 0.5 to 0.7. 

I will be using a seeond order, eommon-mode filter. The diffieulty in eonsider-ing an input eondueted EMI for this power faetor eorreetion eireuit is its variable frequency of operation. The lowest instantaneous frequency of operation occurs at the crests of the sinusoid voltage waveform. This is where the core requires the longest time to completely discharge the core. The estimated frequency of operation has been 50 kHz, so I will use this as an assumed minimum frequency. 

The power convertor must provide the AC motor with low-harmonic voltage waveform and simultaneously allow the amplitude to be adjusted. This avoids magnetic saturation of the motor as the frequency is adjusted. For constant torque, from maximum speed to base speed, the voltage is adjusted proportional to frequency. Above base speed, the motor is usually operated at constant horsepower. In this region the voltage is held constant and the flux density declines. Also, the convei tor must limit the starting current, ensure operation at favorable slip, and provide a path fitr reverse power flow during motor slowdown. 

FIG. 30 Top Square voltage waveform applied to the tip. Bottom. Corresponding changes in tip deflection (converted to force after multiplying by the lever spring constant). There is a net attractive force for both the positive and the negative cycles, but it takes time to reach the final force value. Note that the square-wave voltage is not symmetrical around zero. An offset is applied to compensate for the contact potential difference between the tip and the surface. This offset is dependent on humidity and is equal to the potential difference between the tip and the sample. (From Ref. 78.)... 

The use of electrically-gated solute injection into the electrophoresis system simplifies the chip design as electrical connections are easy to implement as compared to the microfluidics part of the chip. Voltage waveform manipulation via hardware and software are relatively easy to control and implement. 

Note that ceramic capacitors have such low ESRs that it is not very practical to talk about the current passing through them. So manufacturers typically provide a derating based on the changing voltage waveform across them. Typically, if the rated DC voltage is VR, then the peak-to-peak voltage of the waveform is not allowed to exceed 80% of VV In addition, its RMS value is not allowed to exceed 28% of VR. 

Figure 3.5 Rectifier voltage waveforms of a 2 kW silicon full-bridge converter (a) without and (b) with active snubber circuitry. (From [6], 1991 IEEE. Reprinted with permission.)...

VSIN, ISIN - Sinusoidal voltage or current source. Typical voltage waveform v(t) = 5 sin(2000t + 30°)... 

The Transient Analysis uses the sinusoidal voltage waveform when it performs the simulation. This circuit would not work if an AC Sweep were used. To set up a Transient Analysis, select PSpice and then New Simulation Profile from the Capture menus ... 

In this section we will demonstrate the use of a Zener diode, as well as the piece-wise linear (PWL) waveform voltage source. The PWL source can be used to create an arbitrary voltage waveform that is connected by straight lines between voltage points. Wire the circuit shown ... 

EXERCISE B-7 Find the output voltage waveform and the transfer curve for the circuit below. Let the input be a 15 volt triangle wave. Use the source Vtri to create a 1 Hz triangle wave. 

The input and output waveforms are shown on the following left screen capture. Twenty cycles were simulated, but only one is shown in the following left screen capture to make the distortion easily seen. The right screen capture shows the Fourier components of the output voltage waveform. 

In this section we will demonstrate the use of an ideal operational amplifier and the pulsed voltage waveform. Wire the circuit shown below. 

Click the OK button to select all of the runs. First we will plot the input voltage waveform. Add the trace V(VIN). Zoom in on the traces to see the ripple in more detail ... 

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