Motors alternating current motor, constant speed

Synchronous Alternating-Current Motors These motors run in exact clock synchronism with the power system. For most modern power systems, these are truly constant-speed motors. 

I have made the statement earlier in this chapter that alternating current motors operate at a constant speed. I wrote this because I ve read it in a book. Yet, I know from personal experience that this is wrong. I know that my wood-turning lathe will slow down if I press too hard on the spinning wood with my cutting chisel. The lathe is run by a 1750 rpm, 60 Hz, 120 V, 3/4 hp AC motor. But I also know that if I run the motor at a slower speed for a minute or two, the motor will get hot and trip off on its internal thermal trip switch. I will then have to wait 10 minutes for the motor to cool off before I hear the click of the trip resetting. 

Alternating-Current Squirrel-Cage Induction Motors These motors are by far the most common constant-speed drives. They are relatively simple in design and therefore both low in cost and highly reliable. Representative prices are shown in Fig. 29-1 for various speeds and horsepowers. 

This is a very good motor for direct connection to certain loads, particularly where constant speed is required. NEMA defines it as a synchronous machine which transforms electrical power from an alternating-current system into mechanical power. It usually has direct-current field excitation by a separately driven direct-current generator or one directly connected to the motor. This motor remains synchronous with the supply frequency and is not affected by the load. Proper application requires consideration of the following ... 

Use of wound-rotor induction motors has been largely in continuous-duty constant-speed supplications where particularly high starting torques and low starting currents are required simultaneously, such as in reciprocating pumps and compressors. These motors are also used where only alternating current is available to drive machines that require speed adjustment, such as types of fans and conveyors. 

Most of the pumps we see, both at home and at work, are centrifugal pumps. And most of these pumps are driven by constant-speed, alternating-current motors. 

The three basic types of drives are alternating current (ac), direct current (dc), and hydraulic. While a number of drives have been used in extruders, the most common are dc silicon control rectified (SCR) and ac adjustable frequency drives. A dc SCR drive is a sohd-state dc rectifier connected to a dc motor. The base speed is about 1 percent, but reduces to 0.1 percent when a tachometer is added to the drive. These drives are very reliable, can handle high starting torques, can maintain a constant torque through a speed range of 20 1, and are relatively easy to maintain (that is, replace brushes). However, since the drives have brushes, they are limited to noncorrosive polymers. 

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