How to Test an Armature with a Multimeter

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Are you pursuing a career in electrical engineering? Then one of the things you need to understand is the armature. Did you know that rotary motions are produced by forces acting at a distance from the rotation’s axis?

An armature features a winding around itself that conducts current within the magnetic field produced by the permanent magnet or electromagnet. In real life, the number of windings around the armature is way more than one. That’s because they are wound severally to raise the force because of the magnetic field on the armature itself.

Further, slip rings are utilized to change the course of current after a half rotation to keep it rotating in the same course. 

Learning More About Armature

Keep in mind that an armature is the part of an electric generator and motor that involves the principal current-carrying windings. In tiny and small motors, the armature is normally composed of a number of coils of wire wound on a soft iron core and installed on a drive shaft. 

Meanwhile, in bigger AC motors, the armature is often the stationary part. When a current flow in the armature winding is a motor, it engages with the magnetic field created by the field windings offering an increase to a torque between the rotor and stator. The armature in the generator is rotated within a magnetic field, offering lift to an electromotive force in the windings. 

One of the things you need to keep in mind about this component is that it’s arranged to cut the lines of a magnetic field at proper angles. Both the armature and the field might be on either the rotor or stator. A voltage is induced in a conductor by a basic principle of electromagnetism, which moves in the magnetic field. A conductor in a magnetic field encounters a force and is more likely to move whenever a current flows through it.

So, how can you make the best use of that effect? The electrical and magnetic elements within electrical machines should interact in the most efficient way possible. What’s more, the armature is normally wound on an iron core that focuses the maximum amount of flux lines.

The field coils are wound as well on iron cores to create the maximum flux for a specific current. Note that the iron for both the armature and the field is often laminated and composed of slices. That stops the currents from eddying and circulating in the iron itself and producing wasted heat because of changing magnetic flux within the machine. 

You should supply the armature with a current, especially if it’s the rotor in a motor, not to mention there should be a way of taking current from it if it’s a generator. The same thing applies to the field, particularly if it’s electrically fueled.

The arrangement of the rotating contact could be either a set of commutators or slip rings. Such rotate under fixed contacts. Such rotations under fixed contacts, and we referred to them as brushes, composed of carbon and held in place by springs. 

Every now and then, the brushes must be changed whenever the carbon wears away. 

DC Motor and the Armature Reaction 

Before we dive in, let’s briefly discuss what’s armature reaction. In case you didn’t know, an armature reaction indicates the impact of armature magnetomotive power on the fundamental wave of the main pole magnetic field under a symmetrical load.

It’s believed that the only magnetomotive force functioning on the DC generator is produced by the stator magnetic field. Nonetheless, the magnetic field produced by the current in the armature winding is referred to as the armature magnetic field. Further, the armature magnetic field’s axis intersects the axis of the main magnetic field vertically. 

Whether it’s in the generator or motor, the weakening and distortion of the magnetic field happen. The reaction caused by the magnetomotive armature force is what we referred to as armature reaction. 

Now going back to the DC motor, you will find two sources of magnetic fluxes existing: main field flux and armature flux. The impact of armature flux on the key field is the armature reaction. That reaction adjusts the distribution of the magnetic field, impacting the machine’s overall operation.

Take note that the impacts of the armature flux could be offset by including a compensating winding to the main poles or in other motors, including intermediate magnetic poles linked in the armature circuit. 

When it comes to winding circuits, particularly in lap winding, you will find as many current paths as possible between the brush connections because poles are present in the field winding. You will only find two paths in a wave winding, and you will find as many coils in series as half the number of the poles.

Hence, a wave finding is more preferable for low voltages and big currents for a given rating of the machine. 

Testing an armature

Using a digital multimeter, you can take a look at the resistance value of the series windings connected between the two commutator bars of each coil. 

Change the multimeter to ohms setting and take the necessary measurements of the resistance from the commutator bars, particularly 180 degrees away from each other. Make sure you also rotate the armature and remove the resistance value between each set of two bars on the commutator.

Remember that it’s not possible to identify the exact resistance value of the armature. However, all measurements must amount to the same number. Do you see the resistance amount vary substantially? Then, there may be a problem with the windings themselves. 

To be exact, a lower resistance value could indicate there might be a shorted wire inside the coil. A sudden surge in resistance value might suggest the wire is burned or broken that can cause huge interruption to the circuit.

Final thoughts

Are you ready to test your armature? We hope we have provided you with the information you need about armature and its working principle. Feel free to share your thoughts to us by leaving your comments below.

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