Motors voltage sources

Starter type Motor voltage Motor current Line or source current Motor torque Source voltage dip... 

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 inverter may be a current source inverter, rather than a voltage source inverter (.Section 6.9.4) since it will be the rotor current that is required to be vtiried (equation (1.7)) to control the speed of a wound rotor motor, and this can be independently varied through the control of the rotor current. The speed and torque of the motor can be smoothly and steplessly controlled by this method, without any power loss. Figures 6.47 and 6.48 illustrate a typical slip recovery system and its control scheme, respectively. 

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. 

Induction Motor Fed from a Voltage Source Inverter... 

A 250 kW three-phase, fonr-pole, 50 Hz, 415 V, indnction motor is fed from a voltage source inverter. The motor has the following parameters for its star-wonnd windings. Find the cnrrents and air-gap voltage in the circnit. 

Figure 15.12 Instantaneous voltages and currents in an induction motor supplied from a voltage source inverter.

Induction motor fed from a voltage source inverter Eiltering of Power Line Harmonics Protection, Alarms and Indication References... 

Fung, D.G. and Berg, H.G. (1995). Powering the flagellar motor of Escherichia coli with an external voltage source. Nature 375, 809-812. 

Pulse width modulation at a fixed frequency has no minimum on-time restriction. Therefore, current peaking and torque ripple can be minimized. For analysis, a d.c. motor can be modeled as a RL circuit, resistor and inductor, or in series with a voltage source. If the motor current is assumed continuous, then the minimum and maximum motor current can be represented by the following equation ... 

Section 8.2.3 also considers the effect of an abrupt parametric fault in one phase of the inverter s load. In this case study, the load of the three-phase inverter is an RL-network in delta configuration often used in studies of three-phase PWM voltage source inverters. The study may be extended by replacing the RL-network by a sophisticated BG-model of an induction motor (see for instance [31, Chap. 8]). Studies of the three-phase diode bridge rectifier typically assume a resistive load in parallel to a filter capacitor. 

The harmonic losses developed in the motor by the drive are very small in a typical mine hoist low speed synchronous motor powered from a Variable Speed Drive. In this type of configuration the Voltage Source Inverter (VSI) and Active Rectifier Unit (ARU) typically increase the motor losses by the order of 1-1.5% of the total motor losses. The low harmonic losses in this case is due to the motor s high reactance and low base frequency, the inverter technology and the switching frequency. 

For the induction motor in the application in Table 1, the motor specified has a 9% margin (i.e., is 91% loaded on RMS). A similar 9% margin applied to the drive transformer for the AC drive application would require a drive transformer rated 2,675 kVA for the 3,000 kW induction motor application. This result has sometimes lead to questions by the client who is concerned with a drive supply transformer rated at a lower power than the hoist motor it is supplying. The results also demonstrate the benefits of Voltage source IGBT drives over DC drives in reducing the kVA demand on mine hoist applications. 

The application of PWM Voltage source inverter drives to induction motors for mine hoisting applications is now a common solution for mine hoist drives. The drives have the benefit of having unity power factor, and can be designed to be compliant with the IEEE 519 guidelines for harmonic content. 

FIG. E-10 Motor current of induction motor started at full voltage. (Source Reliance Electric.)... 

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