Pulse width modulation

The most popiilar form of motor speed control for adjustable-speed pumping is the voltage-controlled pulse-width-modulated (PWM) frequency synthesizer and AC squirrel-cage induction motor combination. The flexibility of apphcation of the PWM motor drive and its 90 percent- - electric efficiency along with the proven ruggedness of the traditional AC induction motor makes this combination popular. 

Figure 6.7(a) Approximate characteristics of vital parameters after pulse width modulation... 

All these drives are based on pulse width modulation (PWM) and hence would produce overvoltages at the inverter output and require overvoltage protection for cable lengths of 100 m (typical) and above, depending upon the steepness of the wave (Section 6.14.1). 

Both V and/can be varied with the help of pulse width modulation (PWM) in the inverter circuit. The converter unit normally is an uncontrolled pow-cr diode rectifier. 

This is Ihe most commonly used inverter for Ihe control of a.c. motors and is shown in Figure 6.28(a). The fixed d.c. voltage from the uncontrolled rectifier converter acts as a voltage source to the inverter. The voltage in Ihe inverter unit is varied to Ihe required level by using a pulse width modulation, as noted earlier. Through Ihe switching circuit of Ihe inverter Ihe frequency of the... 

The inverters are either voltage source or current source (see Figure 7-7a and b). There are other variations, but they apply to drivers smaller than the ones used with compressors. However, pulse-width-modulated (PWM) (see Figure 7-7c), transistorized units are less complicated and are relatively maintenance-free with reliable units available to at least 500 hp. For all but the smaller compressors, the current source inverter is the one typically used. With a six-step voltage source, a rule of thumb has been to size the motor at two-thirds of its rating so as not to exceed the insulation temperature rise. For current source motors, the output torque is not constant with decreased speed, which fortunately is compatible with most compressors, as torque tends to follow speed. For current source drives, one needs to upsize the motor captive transformer by approximately 15% to account for harmonic heating effects. 

Figure 7-7. Output from three inverters (A.) Voltage source, (B.) Current source, (C.) Pulse width-modulated source.

Figure 7-10. Schematic of the rectifier and inverter circuit for a pulse width-modulated inverter.

Pulsation control, 84 Pulsation snubbers, 85 Pulse-width-modulated unit, 278... 

Pulse width modulation Each half-cycle of output is made up of pulses of varying duration but equal magnitude. 

This square wave ultimately controls the speed of the drive for the carriage via pulse width modulation. The gain, Kcar was selected to be. 1. 

For all these reasons, commercial instruments, based on the classical P + PD + PI control (see for example ref. [8]), are of little utility except for r>lK. Their heater control methods sometimes employ pulse width modulation techniques of a square wave. Thus, they introduce vast amounts of noise to sensitive regions of the experiment. 

The SG1524A advanced regulating pulse width modulator can be configured to create a voltage mode controlled buck regulator. This type... 

FIGURE 9.7 Pulse-width-modulated waveform from an adjustable speed drive output. 

Flow control via pump speed adjustment is less common than the use of throttling with valves, because most AC electric motors are constant-speed devices. If a turbine drive is used, speed control is even more convenient. However the advent of the pulse-width modulated (PWM) adjustable speed drive with sensorless flux-vector control has brought adjustable speed (AS) pumping into the mainstream of everyday applications. 

The transistors of the output-side inverter are pulse-width modulated in such a way that in every phase, a near-sinusoidal current is created after smoothing by motor inductivity. The frequency of this current can be varied to adjust the speed of the motor. The switching frequency of small to medium frequency converters is on the order of 2 to 8 kHz and adjustable. In frequency converter operation, this frequency is quite audible. The larger the motor, the smaller the switching frequency selected. In large drives, switching frequency drops to 400 Hz. 

Sensor measurements are recorded in situ by a battery-powered, eight-channel, miniature digital tape recorder (MDTR) (Oxford Medical Systems Ltd.) with range, 0-1.023 V resolution, 0.1% scan period, 15 s (alternatively 5 s) duration (C120 cassette), 25 h (alternatively 8 h) and data capacity, 6000 scans. Direct (bandwidth 100 Hz) or pulse-width modulated analog signals (range, 120 mV bandwidth, 0-8 Hz and resolution, 1-2 %) can be recorded on the additional three tracks of the same cassette. 

AC electric drives require more sophisticated converters when they are supplied with DC sources, because electric machines requires periodic voltage and current waves with a variable frequency depending on the load requirements. In Fig. 5.8, the scheme of an example of three-phase induction motor driven by a pulse-width-modulated inverter is reported. In this scheme a three-phase bridge connection with six power modules is shown to form the so-called inverter. Each power module can be composed by a number of power switches connected in parallel to carry higher currents. Across each power switch (IGBT) a parallel diode is connected to provide a return path for the phase current when the power module is switched off. 

The voltage waveform which feeds the motor is controlled by pulse-width modulation (PWM) of each of the six power modules, by means of control signals optimizing the motor phase currents. The PWM is controlled evaluating the motor... 

The microcontroller reads and interprets the signals from the sensor element and reports to the climate control system using a PWM pulse-width modulation signal. This signal communicates the level of pollution to the climate control, which in response initiates the actuation of the fresh-air inlet. The use of a microcontroller enables us to enhance the performance of the climate control system by programming it to behave in accordance with the wishes of the customer. The software determines, for example, the total fresh air inlet closed versus open time, the structure of the PWM signal output, sensor behavior at start up, and many more variables. 

DAS. The heater control system is based on PWM (pulse width modulation) running at 0.5 Hz, which represents a compromise between the desire for a high frequency to provide better control and a low frequency to allow the computer to complete overhead tasks without disrupting control (reeNote 5). 

The control signal calculated by the PID algorithm (see Note 3) is translated into a heating profile by way of pulse width modulation. The heater receives a pulse train (frequency of 0.5 Hz) with variable on time. During the on segment of each 0.5 s pulse, the heater is on at lull power. The integrated power over the lull pulse is controlled to achieve the desired temperature. The thermal capacitance of the capillaries and the heater itself is sufficient to damp out any temperature fluctuations such that the temperature measured inside the capillaries is steady (does not show any 0.5 Hz ripple). 

Forced Pulse-Width Modulation Mode (FPWM)... 

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