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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All but very small AC commutator motors employ compensating windings. Namely, this motor has a feature of being capable of continuing the rotation with AC power, too.

The contact point where motlrs brush touches the commutator is referred to as the commutating plane. Negative Feedback, Part 2: While starting, armature windings connected to commutator segments shorted by the a.c.commhtator look like shorted transformer turns to the field.

Series AC motors smaller than hand drills and kitchen mixers may be ac.commutator. Very small universal motors may be uncompensated. Carbon brushes tend to wear more evenly than copper brushes, and the soft carbon causes far less damage to the commutator segments. Low starting current produces high starting torque. If AC is applied to this field coil, its phase of current is lagged behind that of the armature.

The ratio of copper to carbon can be changed for a particular purpose. Among these motors, what do you think will happen if AC power is applied to a series motor? 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. The effect can be considered to be analogous to timing advance in an internal combustion engine.

One variety, notable for having an adjustable speed, runs continuously with brushes in contact, while another uses repulsion only for high starting torque and in some cases lifts the brushes once the motor is running fast enough.


A sensor keeps track of the rotor position and semiconductor switches such as transistors reverse the current. Figure below A smaller motor air gap and fewer field turns reduces lagging reactance in series with the armature improving the power factor.

Speed can be controlled by rotating the brushes with respect to the field axis. While such a motor will rotate, operation is marginal.

What Are Motors?

Motors and generators suffer from a phenomenon known a.c.commmutator ‘armature reaction’, one of the effects of which is to change the position at which the current reversal through the windings should ideally take place as the loading varies. Replacing the copper and mica segments is commonly referred to as “refilling”.

This article is about the electrical component. Hysteresis is a lagging behind of the magnetic field strength as compared to the magnetizing force.

On the left is an exaggerated example of how the field is distorted by the rotor. Thus, although the current passing through the brush was more or less constant, the instantaneous current passing to the two commutator segments was proportional to the relative area in contact with the brush. Many other insulating materials are used to insulate smaller machines; plastics allow quick manufacture of an insulator, for example. Brushes and the Brush Gear, p.

The output was taken from a pair of curved copper wires which were moved to dip into one or other pair of mercury wells.

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.

Large motors may have hundreds of segments. The different brush types make contact with the commutator in different ways. These effects can be mitigated by a Compensation winding in the face of the field pole that carries armature current.

This results in considerable arcing and sparking at the brushes as the armature begins to turn. In a motor the commutator applies electric current to the windings.


This means that two adjacent segments are electrically connected by the brush when it contacts both. Thus, the offsetting effects result in a nearly constant speed from DC to 60 Hz. This opposing field helps to reverse the lagging self-inducting current in the stator. Motors as large as those employed in a kitchen mixer, or larger, use compensated stator windings.

Operating life of these machines is much longer, limited mainly by bearing wear.

Alternating-current commutator motor | electrical engineering |

At high speed this motor provides more power than a same-size induction motor, a.c.ccommutator is not maintenance free. For a single armature winding, when the shaft has made one-half complete turn, the winding is now connected so ac.commutator current flows through it in the opposite of the initial direction.

Since a commutator motor can operate at much higher speed than an induction motor, it can output more power than a similar size induction motor.

Because copper brushes have the same hardness as the commutator segments, the rotor cannot be spun backwards against the ends of copper brushes without the copper digging into the segments and causing severe damage.

The worn brush is pulled out and a new brush inserted. It is possible to design small under watts universal motors which run from either DC or AC. At near running speed, a centrifugal a.c.com,utator shorts out all commutator segments, giving the effect of a squirrel cage rotor.

The faster the rotor spins, the further this degree of field distortion. If AC power is applied when cast metal or soft steel is used for the stator core, current is generated due to electromagnetic induction this current is called eddy currentgenerating a large amount of heat.

Published under the terms and conditions of the Design Science License.