In Chapter 1, you have read about electromagnet-based DC motors. You have seen various connection methods there such as shunt, series, and separate field. Sep 22, AC commutator motors, like comparable DC motors, have higher starting torque and higher speed than AC induction motors. The series motor operates well. If a d.c. armature is placed in a rotating magnetic field, the emf induced in the individual armature conductors is proportional to the relative speed of field and.
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Mica was used on early machines and is still used on large machines. Two or more fixed brushes connect to the external circuit, either a source of current for a motor or a load for a generator. Views Read Edit View history. High power, high current commutated equipment is now uncommon, a.c.commutqtor to the less complex design of alternating current generators that permits a low current, high voltage spinning field coil to energize high current fixed-position stator coils.
The result of this a.c.co,mutator that as the commutator segment passes from under the brush, the current passing to it ramps down more smoothly than had been the case with pure copper brushes where the contact broke suddenly. In a real motor or generator, the field around the rotor is never perfectly uniform. A commutator consists of a set of contact bars fixed to the rotating shaft of a machine, and connected to the armature windings. An Introduction to Negative Impedance Converters An introduction to the properties and purposes of a negative resistance device.
Figure below Uncompensated series AC motor. This article is about the electrical component. Such requirements are common with traction, military, aerospace, nuclear, mining, and high speed applications where premature failure can lead to serious negative consequences.
Each conducting segment of the commutator is insulated from adjacent segments. The addition of a compensating winding yields the compensated series motorovercoming excessive commutator sparking.
Brushes and the Brush Gear, p. Retrieved from ” https: Published under the terms and conditions of the A.c.comjutator Science License. These effects can be mitigated by a Compensation winding in the face of the field pole that carries armature current. When the motor fails it is discarded and replaced.
In the coils of the rotor, even after the brush has been reached, currents tend to continue to flow for a brief moment, resulting in a wasted energy as heat due to the brush spanning across several commutator segments and the current short-circuiting across the segments.
As the a.c.commmutator and commutator wear down, the spring steadily pushes the brush downwards towards the commutator.
Friction between the segments and the brushes eventually causes wear to both surfaces. Spurious a.ccommutator is an apparent increase in the resistance in the armature winding, which is proportional to the speed of the armature, and is due to the lagging of the current.
When a commutated motor or generator uses more a.c.commutatro than a single brush is capable of conducting, an assembly of several brush holders is mounted in parallel across the surface of the very large commutator. Instead, the rotation of the rotor induces field effects which drag and distort the magnetic lines of the outer non-rotating stator. If a DC series motor equipped with a laminated field is connected to AC, the lagging reactance of the field coil will considerably reduce the field current.
Disposable, molded type commutators commonly found in smaller DC motors are becoming increasingly more common in larger electric motors. However, if you apply AC power to a series motor designed to be used with DC power, there occurs a problem.
AC commutator a.c.commutafor, like comparable DC motors, have higher starting torque and higher speed than AC induction motors. The single-phase AC version suffers a double line frequency torque pulsation, not present in poly-phase motor.
If the polarity of the electromagnet remains the same as shown in Fig. However, with a series motor, the polarity of the electromagnet is also reversed as shown in Fig.
AC commutator motors may be either single-phase or poly-phase. The arcing and sparking is mitigated by placing a compensating winding the stator in series with the armature positioned so that its magnetomotive force mmf cancels out the armature AC mmf. A sensor keeps track of the rotor position and semiconductor switches such as transistors reverse the current. However commutator motors are not as maintenance free as induction motors, due to brush and commutator wear.
The contact point where a brush touches the commutator is referred to as the commutating plane. Hysteresis is a lagging behind of the magnetic field strength as compared to the magnetizing force.
The success or failure of a motor design was not known until after it was actually built at great expense and tested. One of these was the development of ‘high resistance brushes’, or motrs made from a mixture of copper powder and carbon. Speed can be controlled ,otors rotating the brushes with respect to the field axis. Then, is a shunt motor usable with AC?
When an induction motor drives a hard starting load like a compressor, the high starting torque of the repulsion motor may be put to use.
A repulsion motor Figure below consists of a field directly connected to the AC line voltage and a pair of shorted brushes offset by 15 o to 25 o from the field axis. For mathematical concept, see Commutator.
The situation was so bad that motors could not be designed ahead of the actual construction. Carbon brushes, being made of a softer material, wear faster and may be designed to be replaced easily without dismantling the machine. Small line operated appliances, such as drills, vacuum cleaners, and mixers, requiring to 10, rpm use universal motors. Metallic copper brushes can be found in toy or very small motors, such as the one illustrated above, and some motors which only operate very intermittently, such as automotive starter motors.
In other projects Wikimedia Commons. This parallel holder distributes current evenly across all the brushes, and permits a careful operator to remove a bad brush and replace it with a new one, even as the machine continues to spin fully powered and under load. This process was known as ‘rocking the brushes’. It is possible to design small under watts universal motors which run from either DC or AC.
Similarly the segment coming into contact with the brush has a similar ramping up of the current.