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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Fine copper wire mesh or gauze provided better surface contact with less segment wear, but gauze brushes were more expensive than strip or wire copper brushes. Then, is a shunt motor usable with AC? The higher resistance of carbon also results in a greater voltage drop of 0. At near running speed, a centrifugal switch shorts out all commutator segments, giving the effect of a squirrel cage rotor.

Structure And Characteristics Of AC Commutator Motors | Nidec Corporation

On the other hand, AC commutator motors have the following disadvantages: There is less sparking with carbon as compared to copper, and as the carbon wears away, the higher resistance of carbon results in fewer problems from the dust collecting on the commutator segments. However, these hard metal brushes tended to scratch and groove the smooth commutator segments, eventually requiring resurfacing of the commutator.

Replacing mktors copper and mica segments is commonly referred to as “refilling”.

Friction between the segments and the brushes eventually causes wear to both surfaces. Because of these characteristics, they are used for home appliances such as electric vacuum cleaners and tools such as electric drills Fig. Each conducting segment of the commutator is insulated from adjacent segments. Also, the high resistance brush was not constructed like a brush but in the form of a carbon block with a curved face to match the shape of the commutator.

As the shaft rotates, the commutator reverses the flow of current in a winding. When the motor fails it is discarded and replaced.

Two or more fixed brushes connect to the external circuit, either a source of current for a motor or a load for a generator. A sensor keeps track of the rotor position and semiconductor switches such as transistors reverse the current. For mathematical concept, see Commutator. On large industrial machines say, from several kilowatts to thousands of kilowatts in rating it is economical to replace individual damaged segments, and so the end-wedge can be unscrewed and individual segments removed and replaced.


On large industrial equipment, the commutator may be re-surfaced with abrasives, or the rotor may be removed from the frame, mounted in a large metal latheand the commutator resurfaced by cutting it down to a smaller diameter. To conduct sufficient current to or from the commutator, the brush contact area is not a thin line but instead a rectangular patch across the segments.

This motor has superior commutation below synchronous speed, inferior commutation above synchronous speed. The situation was so bad that motors could not be designed ahead of the actual construction. The contact point where a brush touches the commutator is referred to as the commutating plane. Views Read Edit View history. Thus, the magnetic flux density becomes low when the current is conducted to the armature, and when it becomes high, the current to the armature is stopped.

This is known as a universal motor.

AC Commutator Motors

Mica was used on early machines and is still used on large machines. Universal motor It is possible to design small under watts universal motors which run from either DC or AC.

Figure below A smaller motor air gap and fewer field turns reduces lagging reactance in series with the armature improving the power factor. During operation, it was necessary to adjust the position of the brush ring a.c.commutatir adjust the commutation to minimise the sparking at the brushes.

AC commutator motors may be either single-phase or poly-phase. If AC is applied to this a.x.commutator coil, its phase of current is lagged behind that of the armature. The single-phase AC version suffers a double line frequency torque pulsation, not present in poly-phase motor. Disposable, molded type commutators commonly found in smaller DC motors are becoming increasingly more common in larger electric motors.

In a generator, the mechanical torque applied to the shaft maintains the motion of the armature winding through the stationary magnetic field, inducing a current in the winding.

The commutator on small motors say, less than a kilowatt rating is not designed to be repaired through the life of the device. In other projects Wikimedia Commons. The addition of a compensating winding yields the compensated series motorovercoming a.c.cimmutator commutator sparking.


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. AC asynchronous induction IM.

In Chapter 1, you have read about electromagnet-based DC motors. So even for a rotor which is at rest and initially requires no compensation for spinning field distortions, the brushes should still be advanced beyond the perfect degree angle as taught a.c.ckmmutator so many beginners textbooks, to compensate for self-induction.

Self-induction — The magnetic fields in a.c.commmutator coil of wire join and compound together to create a magnetic field that resists changes in the current, which can be likened to the current having inertia.

If the polarity of the electromagnet remains the same as shown in Fig.

The effect can be considered to be analogous to timing advance a.c.commuttor an internal combustion engine. This is less of a problem as speed increases, a.c.cpmmutator shares the arcing a.c.commuutator sparking between commutator segments The lagging reactance and arcing brushes are only tolerable in very small uncompensated series AC motors operated at high speed.

Improving Gain Sensitivity and Bandwidth Having introduced the general negative feedback structure, we will now demonstrate that negative feedback has a beneficial effect on two Instead, the rotation of the rotor induces field effects which drag and distort the magnetic lines of the outer motos stator. The ratio of copper to carbon can be changed for a particular purpose. With the wide availability of alternating current, DC motors have been replaced by more efficient AC synchronous or induction motors.

Large motors may have hundreds of segments. The segments are held onto the shaft using a dovetail shape on the edges or underside of each segment. The field induces a current flow into the shorted armature whose magnetic field opposes that of the field coils. This process was known as ‘rocking the brushes’.