DC motors

Motor Driver Small DC-Motor Motor Driver fj Tsmall DC-Motor... 

To measure friction and shear response, one has to laterally drive one surface and simultaneously measure the response of a lateral spring mount. A variety of versions have been devised. Lateral drives are often based on piezoelectric or bimorph deflection [13, 71] or DC motor drives, whereas the response can be measured via strain gauges, bimorphs, capacitive or optical detection. 

The silicon-controlled rectifier with a dc motor has become predominant in adjustable-speed drives for almost all commonly used conveyors when speed adjustment to process conditions is necessary. The low cost of this control device has influenced its use when speed synchronization among conveyors is required. This can also be done, of course, by changing sheave or sprocket ratios. 

Compound-wound dc motors have both series and shunt fields. The addition of a small series field helps provide the proper amount of no-load to fuU-load speed regulation or droop. Shunt or compound-wound motors are apphed widely to many adjustable-speed drives. They are important for drives requiring accurate speed regulation and adjustment. 

A dc motors inherent speed-torque curve can be varied widely by adjusting the relative amounts of shunt and series fields. The series field may also be connected to aid or buck the shunt field. The usual practice is to connect the series field so that it adds to the shunt field (cumulative compound), which gives a stable, drooping speed with increasing load. 

The great flexibility of dc motors both through inherent design charac teristics and through the way in which they are operated makes them ideally suited to adjustable-speed drives, particularly regulated drive systems. 

One of the oldest adjustable-speed drives is the Ward-Leonard system. This consists of an ac to dc motor-generator set and a shunt or compound-wound dc motor. Speed is adjusted by changing the generator voltage. A functional equivalent of this drive uses an adjustable-voltage rectifier feeding a dc motor. This system has only one rotating machine in contrast to the three of a conventional Ward-Leonard system. 

Direct-Current Motor Control Control for dc motors runs the gamut from simple manual line starters to elaborate regulating systems. Only the starting problems are considered here since variable-speed drives and regulating systems are discussed elsewhere. 

The major differences between ac and dc starters are necessitated by the commutation limitation of dc motors, which is the ability of the individual commutator segments to interrupt their share of armature current as each segment moves away from the brushes. Normally 250 to 275 percent of rated current can be commutated safely. Since motor-starting current is limited only by armature resistance, line starting can be used only for veiy small [approximately 1492-W (2-hp)] dc motors. Otheiwise, the commutator would flash over and destroy the motor. External resistance to limit the current must be used in starting to prevent this. 

The electrical power supplied to electrical motors can be from a direct-current (dc) source or an alternating-current (ac) source. Because dc motors are more... 

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. 

Solid-State DC Drives. The controlled-thyristor rectifier and separate-field DC motor is the solid-state motor drive in greatest use. The combination provides control over at least a 10 1 speed range, plus an additional two to three times by field weakening. Depending upon the power level, the rectifier is operated directly from the AC supply lines, or via a transformer. Typical speed regulation of 2% can be accomplished with a single control system. The horsepower and speed limitations are set by the DC motor, not by the semiconductor rectifiers. The DC motor and rectifier can be combined to any required power level. 

Commercial solid-state DC motor drives fall into three general groups. Drives operating from single-phase lines are available in fractional horsepower sizes up to about 3 hp. Three-phase drives are available in horsepower sizes from 5 to 500 hp. Drivers above 500 hp are generally classed as special. 

Figure 3-17. Rectifier-DC motor driver (a) unidirectional or field-reversing (b) bidirectional operation with armature voltage reversing [10].

In contrast, the latest DC motors are very powerful and with integral feedback they also exhibit close speed control to 2 rpm within range 50-700 simple indications of speed and motor load are also possible. The trials of two such home-made units around the laboratories have met with enthusiastic and universal welcome, a clear sign of incipient demand for their commercialisation. 

The electric connection to the electrode is made by connecting a copper wire from the brass holder to the threaded portion of the hemisphere electrode. The brass holder is machined to fit snuggly into the steel shaft of a rotator. The rotation of the electrode is provided by a timing pulley connected to a variable speed DC motor. A graphite slipring contact located on the top of the shaft is used to provided electric contact to the RHSE during the experiments. 

Noise emission control is another area where the application of electronics goes hand in hand with environment protection. Special electronic circuits on DC motors ensure that the noise level can be reduced at the source rather than through costly insulation. 

Ford s Think NEV came in two and four passenger models with a range of 30 miles. It had a 72 volt battery with a 5,000 watt DC motor. The charging time was 4 to 8 hours and it had regenerative braking with hydraulic drum brakes. The price range was 8,000 to 10,000. 

Nevco s Gizmo has a top speed of 40 mph with a motor 12 peak HP series DC motor and deepcycle lead add batteries for a 48V electrical system. A fiberglass body is used with hydraulic disc brakes. The range is 45 miles per charge. The base configuration with the 45 mile battery pack is 8,650. 

Figure 10.12 Pow/er and torque as a function of motor speed for a 100 kW DC motor w/ith a maximum speed of 1750 rpm w/ith a full voltage on the field and at the maximum motor current. The motor can be operated at screw speeds up to 2100 rpm by weakening the field voltage...

Most extruders manufactured today are built using AC vector motor systems instead of the older DC motor systems. AC motors operate at about 5% higher efficiencies, require less maintenance, and are lower in cost than a DC motor. Typical efficiencies for AC and DC motors rated at 83 kW are provided in Table 10.3. It is... 

Table 10.3 AC and DC Motor Efficiencies for Motors Rated at 83 kW at Fuii Speed (Nominai 1750 rpm) and as a Function of Load [20]...

AC vector motors typically operate with a power factor In the range of 0.90 to 0.98 [20]. The power factor for a DC drive system can be considerably lower and as low as 0.5. Replacing an older DC motor with an AC vector motor can be economically attractive due to an Increase In both the efficiency and the power factor. 

Smaller rotary positive displacement compressors are controlled with external bypass. Such equipment usually has a built-in relief valve that opens at a pressure short of damaging the equipment, but the external bypass still is necessary for smooth control. Large units may be equipped with turbine or gas engine drives which are speed adjustable. Variable speed gear boxes or belt drives are not satisfactory. Variable speed dc motors also are not useful as compressor drives. Magnetic clutches and hydraulic couplings are used. 

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