Fixed speed AC motors

The popularity of this drive was, and still is to a large extent, due to the simplicity of the drive. In simple terms, the eddy-current drive consists of a fixed speed AC motor driving a steel drum see Fig. 3.1. 

Some early DC extruder drives used fixed-speed AC motors to drive DC generators that produced the variable voltage for the DC motor. Nowadays, the DC motor drives usually operate from a solid-state power supply, since this power supply is generally more cost-effective than the motor generator set. The DC motor drive can be simpler and cheaper than the variable frequency drive, even when the higher cost of the DC motor is included. The smaller number of solid-state devices tends to give the DC drive a better reliability than the variable frequency drive. Brushes and commutator maintenance is the principal drawback to the use of DC motors. If the drive has to be expiosion-proof, the additional expense associated with this option may be quite iarge for a DC drive, more so than with a variable frequency AC drive or a hydraulic drive. A schematic of the DC drive is shown in Fig. 3.4. 

Installation only of a larger control valve, or larger valve trim, will save zero energy regardless of the result of the above calculation. In addition to modification of the control valve, the upstream centrifugal pump must also be modified. Assuming that the pump is driven by a fixed-speed AC motor, the size of the pump s impeller has to be reduced. The smaller impeller plus the larger control valve will result in ... 

The function of an inverter is to convert a DC input voltage to an AC output voltage. The output voltage and frequency may be fixed or variable depending on the application. Inverters find several applications, some of them are in variable speed AC motor drives, uninterruptible power supplies (UPS), mobile AC power supplies (operating on a battery), induction heating, etc. They are also used as the front stage in DC-to-DC converters. 

Pumps are driven by either fixed-speed or variable-speed motors. Variable-speed motors are becoming increasingly common. Flow can be controlled by varying the pump speed with the motor and thus eliminating parasitic energy losses across a flow control valve. However, because of the extra electronic components needed for a variable-speed AC motor, 95 percent of the motors we work with are fixed-speed alternating-current motors. I will only discuss fixed-speed, alternating-current (AC) motors in this chapter. 

A motor designed for operation on a high-performance, variable-frequency drive must have considerable flexibility inherent in its construction to accomplish the variety of tasks it will be called upon to perform. A comparison of the standardized NEMA enclosures for fixed-frequency AC motors to the wide variety of DC motor constructions available demonstrates the difference in the fundamental design approach. Since hig -performance, variable-frequency drives will typically be used in DC-like applications as opposed to converting fixed-frequency AC (pumps and fans, etc.) to variable speed, it can be assumed that more DC-like construction will be required in definite-purpose AC motors. 

Most induction ac motors are fixed-speed. However, a large number of motor applications would benefit if the motor speed could be adjusted to match process requirements. Motor speed controls are the devices which, when properly applied, can tap most of the potential energy savings in motor systems. Motor speed controls are particularly attractive in applications where there is variable fluid flow. In many centrifugal pump, fan, and compressor applications mechanical power grows roughly with the cube of the fluid flow. To move 80 percent of the nominal flow only half of the power is required. Centrifugal loads are therefore excellent candidates for motor speed control. Other loads that may benefit from the use of motor speed controls include conveyers, traction drives, winders, machine tools and robotics. 

Fixed speed. Advise synchronous speed desired (i.e., 3,600, 1,800, etc.) for AC motors. For DC motors, advise desired basic speed at full, load, and maximum speed by field control. 

PrImary-Voltage-Control-AC Motor Driver. Induction motor torque at any slip s is proportional to primary V. Rotor-power dissipation is equal to s times the air-gap power. These two relationships define the boundary of operation of an induction motor with primary voltage control of speed. As the speed is reduced (s increased) at constant torque, the air-gap power remains fixed, but the power divides between rotor circuit dissipation and mechanical shaft power. 

A steam turbine is a machine with an ancient genealogy. It is a direct descendant of the overshot water wheel, used to kick off the industrial revolution in England, and the windmill still used in Portugal. Turbines are widely used in process plants to drive everything from 2-hp pumps to 20,000-hp centrifugal compressors. They are versatile machines, in that they are intrinsically variable-speed devices. Electric motors are intrinsically fixed-speed machines. It is true that there are a variety of ways to convert AC (alternating-current) motors to variable speed, but they are all expensive and complex. 

But, we are working with an ordinary AC (alternating-current) motor—which is a fixed-speed device. There are then three methods available to control the temperature in the evaporator ... 

Answer Nothing. An AC motor is a fixed-speed machine. It will continue to spin at 3600 rpm. 

DC motor drives have always offered high torque at all speeds and exact control of motion speed. AC induction motors have reliably converted electricity into rotary power for many years, and recently adjustable-frequency controls add variable-speed capability. While AC motors were originally relegated to relatively simple tasks, such as varying the flow rates of fans or pumps, advances in both motor and control technologies have allowed their use in higher performance operations. They are reliable sources of fixed-speed and variable-speed rotating power. Electric drives with appropriate closed-loop control operate only when required. However, to avoid unsuccessful apphca-tions, it is important to properly match the load, motor, and controller. 

Until recently, most unwinders were supplied with an AC motor, direct coupled, or with an air clutch, running at a fixed payoff speed, or possibly with a mechanical variable-speed mechanism that was manually adjusted. Loop control consisted of a switching device (either mechanical or photoelectric) to turn the... 

The use of an adjustable-frequency power supply (ac inverters) in conjunction with the induction motor results in an adjustable-speed drive system.The speed of the motor is adjusted by controlling the output frequency of the ac inverter.Thus the induction motor can be used on many adjustable-speed apphcations. The speed-torque motor performance at different input motor frequencies is Ulustrated by Fig. 5.127a. This permits the apphcation of induction motor adjustable-speed systems to loads such as fans and pumps for flow control with considerable power savings over fixed-speed systems with dampers or valves for flow control. Figure 5.127Z) shows the performance of a 10-hp motor adjustable-frequency system driving a fan load. 

AC motor Most of our pumps are driven by alternating current (AC), three-phase motors. Such motors that drive pumps are usually fixed-speed drivers. Direct current (DC) motors are rarely used in process plants. 

---