Cordless Drill: Motor Gets Hot

The armature is the power-producing unit of the motor. Causes of the armature getting too hot are heavy dirt build up inside, which can block the cooling ports and not provide enough air to keep cool, excessive sparks from the brushes, which can cause the commutator to overheat and transfer the heat to the armature or working in hot environments, overloading the motor.
When the armature becomes too hot, the insulation on the windings can break down from the excessive heat and cause the armature to ground. Grounding is a common fault, which occurs when part of the winding becomes connected to the metal core of the armature.
To check and repair this, remove all the screws on the cover to the case and remove the cover. Then, remove the two transmission screws on the side of the drill that the cover is attached too. Remove the hold-down on the motor. Lift the motor and transmission assembly out and pull the armature and field away from the transmission. Now, pull the armature out of the field. If you notice there is a heavy build up of dirt, you can clean it with thinner and then dry thoroughly. Test it or reassembly it back in the field if you are confident it just needed a cleaning.
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 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 of 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 one by placing one lead of the ohmmeter to the tip of the shaft of the armature and the other to the commutator.

If the armature is bad, install a new one. Install the armature into the field. The field will have a notch on it, indicating the front, which faces the transmission. Reattach it to the transmission. Install the one brush in the housing and then reinstall the motor/transmission assembly back in the housing. Install the other brush in the cover and then put the cover back on the housing. Reinstall the two transmission screws and the end cap.

If a bearing becomes seized, it can cause the motor to spark excessively and generate a lot of heat. A seized bearing can also spin inside the housing, burning up the plastic housing of the drill. To replace a bearing, remove the end cap cover on the housing and the two transmission screws, securing the transmission to the housing on the side of the drill where the cover is located on the housing. Remove the cover. Pull the motor and transmission out of the housing and separate. Remove the armature from the field. Remove the bearing with a bearing puller and install a new one. Reinsert the armature back into the field then reattach to the transmission and then reinstall the transmission/motor assembly back into the housing. Ensure all brush holders and brushes are back in place. Put the cover and the transmission screws securing the transmission to the housing. Reinstall the end cap.

If the bearings become worn it can make a squealing sound or even a grinding noise. Sometimes a seized bearing can make this noise as it will spin inside the housing when seized causing the squealing. To replace a bearing remove the end cap, cover on the housing and the 2 transmission screws securing the transmission to the housing on the side of the drill where the cover is located. Remove the screws securing the cover and pull the cover off. Pull the motor and transmission out of the housing and separate the 2. Remove the armature from the field. Remove the bearing from the armature with a bearing puller. Install the new bearing on to the armature shaft. Reinsert the armature back into the field then reattach it to the transmission and reinstall the transmission/motor assembly back into the housing. Make sure all brush holders and brushes are back in place. Reinstall the cover and the transmission screws securing the transmission to the housing. Reinstall the end cap.

The holders hold the brush in place on the commutator. Any physical damage to the holder, or a heavy build up of dirt on its inside, may interfere with its ability to hold equal pressure on the commutator and cause more sparks, resulting in an overheating motor. Since it is very rare that the commutator is perfectly round in electric motors, the brushes must move in and out of their holders in order to maintain proper and effective contact. A visual examination and testing of the free action of the brush with the fingers are usually sufficient to reveal this condition.
Usually a thorough cleaning or a complete replacement will prevent excessive sparking at the brushes causing the motor to run cooler. Remove the end cap on the drill. Next, remove the two screws holding the transmission to the motor on the side of the transmission where all of the housing screws are facing upwards, on the case, securing the case to the housing. Take out the screws securing the cover on the case and remove the cover. One of the brushes is located in the cover. Remove the holder from the case and take out the screws securing the hold-down brackets on the motor.
Lift the motor up and remove the other brush holder. Inspect both brush holders for damage as well as wear on the brushes and replace as needed. If they are in physically good shape, then clean thoroughly. Reinstall the one brush holder back in the housing and make sure it is in all of the way. If the brush holders are not in all of the way, or positioned a little out of alignment, it can cause excessive sparking as well. Now, reinstall the transmission screws and put the end cap on.
On some models, in order to access the brush and brush holders, all you have to do is remove the end cap on the drill. You can now remove the brush and brush holders and perform any replacement or cleaning of these parts without taking the entire drill apart. When completed, reinstall the brushes and the end cap.