Brushed DC motors

Fig. 12.10. Brushed DC motors of similar ratings using ferrite (a) or NdFeB (b) permanent magnets are compared. The disk geometry is illustrated in (c) [43].

Classic brushed DC motors present limitations due to the need for brushes to press against the collector, with the consequence of friction resistance. At higher... 

In separately excited DC machines (Fig. 5.3d) the excitation is obtained by means of field windings which have a supply separated from the rotor windings, and this gives a separated control of speed and torque, according to what is required in an electric vehicle. Separately excited brushed DC motors were widely... 

Fig. 5.3 Different methods of supplying current to the excitation windings for brushed DC motor, a parallel DC machines, b series DC machines, c compound DC machines, d separately excited DC machines...

DC-Machines Brushed DC-Motors Separately, Parallel, Series, and Compound Excited DC-Motors... 

The brushless DC motor (BLDC motor) is really an AC motor The current through it alternates, as we shall see. It is called a brushless DC motor because the alternating current must be of variable frequency and so derived from a DC supply and because its speed/torque characteristics are very similar to the ordinary with brushes DC motor. As a result of the brushless DC not being an entirely satisfactory name, it is also, very confusingly, given different names by different manufacturers and users. The most common of these is self-synchronous AC motor , but others include variable frequency synchronous motor , permanent magnet synchronous motor , and electronically commutated motor (ECM). 

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. 

Service life is another important criterion when configuring a drive system. In commutator equipped machines (DC motors), the brushes are subject to abrasion. Service time here is on the order of 2,000 to 8,000 hours, with correct current load, correct contact pressure, and purity of the cooling air influence service time considerably. The commutator is also subject to wear and should be checked regularly and, when required, overhauled. 

The working principle of a DC motor is relatively simple, but its mechanical design is quite complicated. The rotor s conductors are permeated by the stator s magnetic field. Commutation (guidance of current density in the rotor) occurs mechanically. To this end, switches (brush collectors) are affixed to the axle that are activated by the rotation of the... 

MoS2 with Mo and Ta Surveyor DC motor brushes on moon landing vehicles... 

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. 

A simple dc motor consists of a field magnet and an armature. The armature is placed between the poles of the magnet. The armature is made up of a loop of wire and a split ring known as a commutator. The loop is comiected to the commutator. Current is supplied to the motor through carbon blocks called brushes. 

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. 

Older motor drive systems generally consist of a DC brush motor, a power conversion unit (PCU), and operator controls. A frequent problem with the motor itself Is worn brushes these should be replaced at regular Intervals as recommended by the manufacturer. In troubleshooting an extruder drive, one should follow the procedure recommended by the manufacturer of the drive. A typical troubleshooting guide for a DC motor is shown in Table 11.2. 

Torque may also be maintained in a controlled strain rheometer such as shown in Figure 8.2.2 by using feedback from the torque sensor to adjust the motor velocity or position. However, control is typically difficult because of the sample response. Ideally one should include the viscosity of the sample in the control algorithm. Performance can be improved significantly by avoiding the sample and closing the feedback loop around a torque sensor on the motor, such as motor current to a dc motor (Michel, 1988). However, brush fiiction in the motor limits the lowest torque levels to 10 to 10- N-m. 

Motor Two brushed DC servo motors Drive Differential drive Maximum Speed 1.7m/s Computing System... 

Then we will use a 12v DC motor. After a quick search of supplier catalogs, we could find a 2500rpm and 15.68N.mm motor, which with two four time reductions can supply the requirements and some extra power needed to move the brushes. This t q)e of motor can be bought for US 3 to US 5, if purchased in large quantities. 

The synchronous rotor carries the rotating independently excited field windings of the machine. For good control response, these must be fed with DC current via slip rings and these must be correctly sized for the low speeds and stall duties required by a hoist. The synchronous motor rotor will comprise of wound poles, pole bolting system, damper bars, slip rings/brush-less excitation and interconnections. 

Brushless DC drives have an advantage in that the motor does not contain brushes. As a result, the drive is less maintenance intensive. The motor contains permanent magnets the size of the magnets determines the horsepower capability of the motor. The maximum power available today is around 600 hp. A schematic of the brushless DC drive is shown in Fig. 3.6. 

Universal Motors. Universal motors are essentially series motors that operate with nearly equivalent performance on direct current or alternating current up to 60 Hz. They differ from dc series motors in that they have different winding ratios and thinner iron laminations. A dc series motor runs on alternating current, but inefficiently. A universal motor, however, runs on direct current with essentially equivalent ac performance, but with poorer commutation and brush life than an equivalent dc series motor. 

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