Current, electric motors requirements

Most electronic equipment shares the television set s need for a number of differing voltages for the operation of individual components. This alone may be sufficient justification for the inclusion of a direct current to alternating current converter in fuel cell power systems. In addition, alternating current electric motors are more suitable in most applications. They tend to operate at a rotational speed controlled by the frequency of the current. If completely unloaded they speed up to this fixed velocity and accelerate no further. Many types of direct current motors, if operated unloaded, will continue to accelerate until they fail. A belt driven fan operated by an alternating current motor is undamaged by the failure of the belt. A direct current motor will require a special safety circuit to shut it down in case of belt failure. If the belt and the safety circuit both fail, the motor will speed up until it destroys itself. 

The centrifugal pump directly driven by a variable-speed electric motor is the most commonly used hardware comoination for adjustable speed pumping. The motor is operated by an electronic-motor speed controller whose function is to generate the voltage or current waveform required by the motor to make the speed of the motor track the input command input signal from the process controller. 

In addition to electrical braking, a mechanical brake, as discussed in Section 6.20.1(A) may also be essential if the motor is required to be stopped completely because, at any value of excitation current, the motor will never reach a standstill condition. The heat of braking up to the standstill condition (N = 0) is roughly equal to one start and is expressed by equation (6.9). 

The following table lists some common laboratory devices along with the current and power requirements for the operation of the device.1 This information is important to consider when instrumentation is being installed, relocated, or used on the same circuit. Common 120-V circuits in laboratories are typically rated at 10 or 15 A. Note that the current draw often spikes to a high level in first few microseconds after a device is energized. This is especially true for devices that have electric motors. 

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. 

With the addition of the flow fields and current collectors, the PEM fuel cell is now complete. Only a load-containing external circuit, such as an electric motor, is required for the electric current to flow, the power having been generated by passing hydrogen and air on either side of what looks like a piece of food wrap that is painted black. 

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