Grinder: Excessive Sparks From Motor

The armature is the power-producing part of the motor, located inside the field. Defects in the windings may show up as sparking at the brushes. It also may be apparent by one, or more, burned places on the commutator. Any other faults within the armature may also show up as sparking.

To examine the armature, first remove the wheel cover and handle from the grinder. Remove the screws securing the gear box assembly to the grinder and then separate the gear box assembly from the grinder. Remove the brush holders by taking out the lower cover, releasing the tension on the springs and then removing the brushes. Some models have caps on the side for easy remove of the brushes. Take off the gear case cover and pull the armature out of the field, inside the grinder.

Clean the armature with electrical contact cleaner. Now, inspect for any burn marks, indicating a short. If you notice any burn marks, replace the armature. Check the commutator for missing bars and replace if you notice any are. Clean the commutator with 600-grit aluminum oxide sandpaper by rotating the armature in the travel of direction, only to provide a smooth, even surface. Use a plastic scribe to clean in between the bars on the commutator.

To check for a bad armature, you can perform the 180-degree test. With the armature removed from the unit, stand it up on a bench. With an ohmmeter, attach the two ends of it to both sides of the commutator to measure the resistance of the windings. As you move it around the commutator, the reading number is not important. You are looking for a consistence in reading, indicating all the windings are good. If it varies radically, reading a zero or open circuit, then that indicates a bad armature. The last test is a bar-to-ground test, which is done by placing one lead of the armature to the tip of the shaft of the armature and the other end of the ohmmeter to the commutator. If the armature is bad, install a new one.

Remove the nut securing the armature gear and then remove the armature gear itself. Take off the gear box cover and bearing by placing the cover on top of the bench vice, with the bench vice open enough for the cover to sit on. Tap the top of the armature shaft with a rubber mallet to remove the bearing and cover. Install the cover and bearing on the new armature and use a socket big enough that it only makes contact with the bearing race, to avoid damage. Now, tap it in place with a hammer. Slide the armature in the field, inside the grinder. Reinstall the gear box assembly and then the brushes. Reapply the tension springs and reinstall the lower cover back on the grinder. Reinstall the grinder wheel over and the handle.

The brushes ride on the commutator, supplying the electrical current to the armature. If the brushes become worn, wear unevenly, suffer damage or have a heavy build up of dirt and debris, they can cause excessive sparking. To check the brushes to see if they require replacement, remove the screws securing carbon brush cover on the back of the angle grinder and take off the cover.

Locate the brushes and pull back the tension spring. Remove the brushes and inspect the carbon brushes for any signs of damage, such as chips, cracks or anything else out of the ordinary. Clean them with electrical contact cleaner, along with the brush holder. Using fine sandpaper, you can gently sand the face of the brush down to smooth it out. If the carbon brushes are getting close to a 1/8 of an inch, or less, replace them.

Install the carbon brushes back in the holders and reapply the tension springs. Install the cover. Some models have brush caps installed on the side of the grinder for easy access. Use a screwdriver to remove these and inspect the brushes.

Some models will have a tension spring that holds down the brush mounted on a post. To replace this type of spring, remove the rear cover to access the spring and remove the paddle and lock button. Remove the second half of the rear cover and then disconnect the electrical wire at the brush. Pull the spring out of the way and pull the brush arm back. Remove the brush spring and install the new brush spring by using needle-nose pliers. Insert the pliers into the inside tab of the spring and turn 180-degrees counter clockwise and slide it back onto the post. Pull the spring back, out of the way.

Reinstall the brush back into the brush arm and fold the brush arm back down. Connect the electrical connector for the brush and then pull the spring down onto the arm. Reinstall the second half of the rear housing, making sure all wires are tucked out of the way. Reinstall the lock button, ensuring the tab on the button faces the armature. Reinstall the paddle and finally, install the top half of the rear cover.

Some models have a different set up. Remove the rear cover to access the brush holders. Release the tension on the brush by moving the tension spring to the side and then remove the brush. With the brush removed, move the tension spring back. Take out the electrical connections at the brush holder and then remove the screws securing the brush holder. Inspect the brush holders for distortion or anything else out of the ordinary. Install the new brush and secure it with the screws. Reattach the electrical connections. Move the tension spring out of the way and install the brush, then apply the tension spring on top of the brush. Finally, reinstall the cover.

Other models have brush caps located on the side of the grinder for easy access. Remove them with a screwdriver, pull the brush and spring out, replace the spring (if necessary) and reinsert it back in the cap, with the brush and reinstall into the grinder.

The brush holder supports the tension spring and houses the brush on the commutator. Any physical damage to the holder, or heavy build up of dirt in and around the brush holder, can impact its ability to support the brushes. Remove the lower cover to gain access to the brush holders. You may have to remove a paddle and lock button on some models. Release the tension on the brush by moving the tension spring to the side and then remove the brush. With the brush removed, move the tension spring back. Remove the electrical connections at the brush holder and then take out the screw securing the brush holder. Inspect the brush holders for distortion, or anything else out of the ordinary. Install the new brush holder and secure it with the screws. Reattach the electrical connections. Move the tension spring out of the way and install the brush and then apply the tension spring on top of the brush. Reinstall the lower cover and, if required, the paddle and lock button.