Grinder: Trips the Power Source Circuit Breaker

The armature – part of the motor that is located inside the field – is the only moving part of the assembly. An armature that is shorting out can cause the circuit breaker to trip. The armature will have to be removed, examined and tested to determine if it is bad. The following procedure will vary on all makes and models. Remove the gear box assembly from the grinder. Remove the brush holders by taking off the lower cover, and then releasing the tension on the springs.

Now, take out the brushes. For some models, they will have caps on the side for easy removal of the brushes. Remove the armature from the field, inside the grinder. Clean the armature with electrical contact cleaner. Once cleaned off, inspect the armature for burn marks which indicates a short. If any burn marks are present, replace the armature.

Check the commutator for any missing bars. If there are, you must replace the commutator. Clean the commutator with 600-grit aluminum oxide sandpaper by rotating the armature only in the travel of direction 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 then 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 consistent reading, indicating all the windings are good. If it varies radically, reading zero or open circuit, then the armature is not good. Another test is reading. If the ohmmeter reads 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 ohmmeter to the tip of the shaft of the armature.

Remove the but securing the armature gear, followed by the armature gear. Remove the gear box cover and bearing by placing the cover on top of a 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 onto the new armature and use a socket that only makes contact with the inner bearing race, to avoid damage. Tap it in place with a hammer.

Reinstall the armature gear and nut. Some armatures are sold with all the parts intact, already as one unit. Slide the armature into 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 cover and the handle.

Worn out carbon brushes can cause the circuit breaker to trip. To inspect the wear on the carbon brushes, remove the lower cover. Locate the brush holders and move the tension spring to the side in order to remove the brush. Brushes 1/8 of an inch, or less, are worn and will need to be replaced. Install the new brushes into the holders and reapply the springs. Reinstall the lower cover.

For some models, remove the side cover and then the paddle and lock button. Remove the second side cover. Now, locate the brush holders and pull the tension spring to the side and take out the electrical connector and then the brush. Replace brushes 1/8 of an inch, or less, as they are worn out. Install into the brush holder and reposition the spring on top of the brush. Reconnect the electrical connection and then reattach the side cover. Reinstall the lock button and paddle and close with the other side cover.

There are also models that have brush caps located on the side of the grinder, for easy access. Remove these with a screwdriver and pull out the brush. Brushes 1/8 of an inch, or less, need to be replaced. Insert the new brush into the cap and reinstall the cap.

The field is an electromagnet on the outside of the motor, that surrounds the armature. The field generates the magnetic field. The field can short-out, causing the circuit to trip. To inspect the field for burn marks, melting or any other damage that could lead to the short/tripping the power, begin by removing the gear box assembly from the grinder. Remove the wheel cover, handle, lower cover and then the tension spring from the brushes. Pull the brushes out of the brush holders.

On some models, there will be two lower covers, a paddle and a lock button to remove. Other models, you will need to remove the brush caps located on the side of the grinder in order to remove the brushes. Pull the armature out of the field and remove the baffle plate. Remove the power switch by holding it in the OFF position, pressing the switch lever forward. When you hear the click, you can remove the power button. Disconnect the wire connections and remember where they attach to for reassembly. Remove the field from the grinder housing. You may have to hang the housing on the table to free the field as it may be tightly secured. Clean the field with electrical contact cleaner and then inspect the field for burn marks, indicating it is shorting out. If you notice burn marks on the field, it probably needs to be replaced.

You can perform one simple test on the field, to check for continuity with a multimeter. Set the multimeter to the ohmmeter and place a lead on each end of the field. If the reading is infinity, the field is bad and requires replacement. Install the field into the body of the grinder, in the correct position for the wires to reattach to their proper connections. You may need a long screwdriver to position the wires, with the openings, during the installation of the field. Reconnect all of the wires back to the switch and brush holders. Push the switch lever forward and reinstall the power switch, making sure the lever on the switch catches the lever. Reinstall the baffle plate and then slide the armature back into the field. Reinstall the gear box assembly back on the grinder and then the brushes back into the holders. Reapply the tension springs, or the brush caps, and install the lower cover. If applicable, reinstall the two lower covers along with the paddle and lock button.

The power cord supplies power to the grinder. Examine the cord for any cracks, splits, cuts, burn marks or anything else out of the ordinary that will cause a short. If you notice any of these, then replace the cord. A cord that shorts out can cause the circuit breaker to trip. If the power cord looks good, it may have come loose inside the grinder, causing it to ground out. To replace a power cord, remove the lower cover, or side cover, on the grinder – depending on the model. Remove the cable clamp. First, check that the connections where the power cord is connected on the switch are attached and tight. If one has come loose, repair the wire end and reconnect it. If the cord requires replacement, disconnect it. Now, strip approximately ½ of an inch of insulation from the new cord (if it has not come that way), twist the wire, crimp the new connectors on the wire ends and reattach to the switch. Place the switch in the housing, secure the cable with the clamp. Reinstall the lower cover or side cover.

The switch controls the power to the motor. A shorted-out switch can trip the circuit breaker. To check and replace a switch, remove the lower cover. On some models, you will have to remove the side cover, lock button and paddle. Remove the power cord clamp and locate the switch. Pull the switch lever away from the switch and remove the switch from the grinder. Remove the wires, making note of where they go. Inspect the switch for any burn marks, or melting, indicating a short. Attach an ohmmeter to the switch wires to check for continuity. Press the switch in – the ohmmeter should give you a reading. Now, release the switch and it should say infinity or O.L. If you are not getting these readings, the switch is inoperable and requires replacement. Attach the wires to the new switch according to the appropriate terminals. Install the switch back into the housing and make sure to reposition the switch lever to the button on the switch. Tuck away all wires and then reinstall the cord clamp. Reinstall the lower cover or, on some models, install the lock button, paddle and the side cover.

An extension cord that is too long and/or not the proper gauge rating for your particular grinder can cause the circuit breaker to trip. This is due to high resistance in the extension cord, causing excessive heat. The wrong extension cord also puts stress on the power tool, which shortens its life. Make sure to use a heavier-gauge cord for any power tool you use and try to keep them as short as possible in length.