Current sources, motor control

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 buyers of motor vehicles have been substantially positive concerning the need to have cleaner running vehicles. Although the required emission control devices and other mandated safety equipment have increased the cost of new motor vehicles, sales have not been significantly effected. The current environmental awareness and concern are evidence of the general population s new found knowledge and acceptance of both mobile and stationary source emission controls. 

In instances where a capacitor is connected directly across the terminal of the motor, the capacitor can act as a source of excitation current after the control device is opened. In order to prevent this the capacitor rating should not exceed 90 per cent of the motor no-load magnetizing current. 

Solid-State Overload Relays. These overload relays are of two types, self-powered and externally powered.The self-powered relay (Fig. 5.137) uses currents induced by the current flowing to the motor for power, while the externally powered units contain an electronic power supply and require a separate source of control power to operate. The advantage of the setf-powered units is that they are sinyler, require no extra wiring, and are self-protected from short circuits. 

For example, if a worker turns on a stirrer in a reactor, he or she may use a local or control room indicator to verify that current is flowing to the agitator motor. Errors may arise at several points in the input process. At the sensory stage, there may be so many sources of information that the worker may be unable to scan em all in the time available. This can be a particular problem when a large number of alarms occur following a major process disturbance. 

Direct Current (DC). This current is transmitted for industrial uses only in exceptional situations. The most common sources of direct current are storage batteries and industrial devices called rectifiers, in which alternating current is changed (rectified) to direct current, as is used in electrolytic cells for the manufacture of chlorine gas, magnesium, aluminum, and a few other chemicals. The direct current is flowing from the source through the user application and back to the source, in one direction. The motor is primarily used for speed control of selected equipment. 

A variable field d-c motor generator was used by the submitters. In lieu of such equipment, the checkers used a 120-volt d-c source with a heavy-duty resistor in the circuit. Occasionally, a film of polymeric material may form on the anode and reduce the current flow. This condition may be corrected by reversing the current flow for a period of 5-10 seconds. A suitable control circuit is diagrammed in Fig. 1. 

Formerly Director of the U.S. Environmental Agency s Office of Mobile Source Air Pollution Control, currently an environmental consultant in the motor vehicle pollution field.Address 2800 N Dinwiddie Street, Arlington, Virginia 22207, U.S. A. 

There are a number of types of m-g sets, each having its own characteristics, advantages, and disadvantages. A simplified schematic diagram of an m-g is shown in Fig. 10.124. The type of motor that drives the set is an important design element Direct-current motor drives can be controlled in speed independently of the frequency of the AC power source from which the DC is derived. Use of a DC motor, thereby, gives the m-g set the capability to produce power at the desired output frequency, regardless of variations in input frequency. The requirement for rectifier conversion hardware, control equipment, and commutator maintenance are drawbacks to this approach that must be considered. 

The transfer of motor loads may require special consideration, depending on the size and type of motors used at a plant. If the residual voltage of the motor is out of phase with the power source to which the motor is being transferred, serious damage may result to the motor. Excessive current draw also may trip overcurrent protective devices. Motors above 50 hp with relatively high load inertia in relation to torque requirements, such as flywheels and fans, may require special controls. Restart time delays are a common solution. 

---