A.c. motors

These are series motors and are relatively compact and lightweight compared to an a.c. motor. The use of such motors is therefore common for hand tools and home appliances and also for such applications that require a high speed (above 3200 r.p.m.) which is not possible in an a.c. machine. Likely applications are polishers, grinders and mixers. This motor runs equally well on both a.c. and d.c. sources of supply. 

Since the motor s fixed parameters can now be varied to suit a particular load requirement, there is no need to pre-match a motor with the load. Now any motor can be set to achieve the required characteristics to match with the load and its process needs. Full-rated torque (TJ at zero speed (during start) should be able to pick up most of the loads smoothly and softly. Where, however, a higher 7 s, than is necessary, a voltage boost can also be provided during a start to meet this requirement. (See also Section 6.16.1 on soft starting.) The application of phasor (vector) control in the speed control of an a.c. motor is shown in a block diagram in Figure 6.12. 

This field is very large and a detailed study of the subject is beyond the scope of this handbook. We will limit our discussions to the area of this subject that relates to the control of a.c. motors and attempt to identify the different solid-state devices that have been developed and their application in the control of a.c. motors. ()nly the more common circuits and configurations are discussed. The brief discussion of the subject provided here, however, should help the reader to understand this subject in general terms and to use this knowledge in the field of a.c. motor controls to achieve from a soft start to a very precise speed control and, more importantly, to conserve the energy of the machine which would be wasted otherwise. For more details of. statie controllers see the Further reading (Sr. nos. 2, 4., 5, 8 and 12) at the end of the chapter. To... 

Power MOSFF.Ts and IGBTs can handle much higher switching frequencies, compared o a thyristor. In an a.c. motor control, fast switching is mandatory and therefore transistors are preferred. 

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... 

A power diode reetifier unit feeding a fixed d.e. power to an inverter unit to control an a.c. motor, or a thyristor rectifier unit, directly controlling a d.e. motor, both contain... 

This part deals w ith three- and single-phase a.c. machines, and their protective switchgears. However, reference is inade to and comparisons drawn of a d.c, motor with an a.c. motor, to assist a user to inake a proper choice of machine. 

Oscarson, G. L., Application of A-C Motors and Controls to Centrifugal Compressors, Bui. 200-TEC-1120, Electric Machinery Co., div. Dresser-Rand (1955). 

Elevators of this type are of a configuration similar to the geared drive systems mentioned above. The difference lies in the increase in the physical size of the drive as the required operating torque of a gearless d.c. motor is produced at a fraction of the rpm of a geared a.c. motor. 

When the power supply voltage is higher than the nominal one, the motor results in being oversized even if it is not. A voltage increase of 5-10 per cent causes increases in the maximum torque by 10-20 per cent in a.c. motors. 

A cage rotor is used on single-phase a.c. motors, the turning force being produced in the way described previously for three-phase induction motors and shown in Fig. 2.49. Because both windings carry currents which are out of phase with each other, the motor is known as a split-phase motor. The phase... 

A single-phase a.c. supply produces a pulsating magnetic field, not the rotating magnetic field produced by a three-phase supply. All a.c. motors... 

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