Electric Drill: Motor Gets Hot

The armature is the power-producing unit of the motor. Causes of a hot-running armature are heavy dirt build up inside, cooling ports blocked (not providing enough air to keep cool), excessive sparks from the brushes (causing the commutator to overheat, transferring the heat to the armature) and 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, causing the armature to ground out. Grounding out is a common fault, which occurs when part of the winding comes in contact with the metal core of the armature.

To check and repair, remove all of the screws securing the two halves of the drill case together and open. Remove the brushes by pulling up gently on the wires connected to the holder. Remove the chuck and spindle assembly then pull the motor up and slide the armature out of the field. If there is a heavy build up of dirt, you can clean it with electrical contact cleaner and then dry it thoroughly. Next, you need to test it for proper performance.

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 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 a zero or open circuit, then the armature is not good. The last test is a bar to ground test, which is done by placing one end of the ohmmeter 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.

Install the armature in the field and secure it in the case. Reinstall the spindle and chuck assembly. Put the brushes back in and ensure the wires are out of the way. Reassemble the two halves of the drill case back together.

If a bearing becomes seized, it can cause the motor to spark excessively, generating a lot of heat. A seized bearing can also spin inside the housing, burning up the plastic housing of the drill. To inspect and replace a bearing, remove the screws securing the case together and separate. Gently pull up on the brush holder wires to remove the brushes. Remove the spindle and chuck assembly, then lift the motor up and pull the armature free of the field.

Inspect the drill casing for any burn marks, where the bearings are housed, indicating a seized bearing. Spin bearings by hand to find the seized bearing as well. Once you have located the seized bearing, place a bearing separator on the bearing, then the separator on top of a bench vice. Make sure the bench vice is open enough so it only contacts the separator. Next, use a hammer and a punch and tap the shaft to free the bearing. Lock the shaft in the vice then place the new bearing on top of the shaft and use a socket that only makes contact with the race (of the bearing) to avoid damage. Tap it in place with a hammer. Reinstall the armature in the field and reinstall the spindle and chuck assembly back in the case. Reinstall the brushes back in the case using a small screwdriver to hold them down in the holder, giving you clearance on the commutator during installation. Place all wires in their channels and re-secure the case back together.

The carbon brushes supplies electricity to the armature by rotating on the commutator. As the carbon brushes wear down and become worn, they can draw more current, causing the motor to get hot as the brushes spark up more. Also, a heavy dirt build up on, and around, the brushes can also cause a hot motor.

To inspect the brushes, remove the screws securing the two halves of the case and separate. Remove the holders from the case. With the brushes removed, inspect them for uneven wear, damage and weak springs. Any of these symptoms can contribute to the motor running hotter. Brushes getting close to 1/8 of an inch, or less, need to be replaced.

You can replace the brushes by pressing the tab down on the side of the holder to remove the brush. If the brushes are in good shape, then clean off the brushes lightly with some sandpaper. Clean any dirt out of the holders with electrical contact cleaner. Install the new, or cleaned, brushes back in the holder and make sure they lock in place. Install the brush holder in the drill case, using a small screwdriver to hold the brush down to give you clearance on the commutator. Ensure all wires are tucked back in their channels and reinstall/secure the case back together.

The field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine. Over time, this part can build up a lot of heavy dirt, or short out, causing excessive heat in the motor. To inspect the field, remove the screws securing the case of the drill together and separate. Remove the brush holders by gently pulling the wire up, connected to the brush holder. Remove the spindle and chuck assembly from the drill. Disconnect the spade connectors on the motor, the lift the motor assembly out of the case. Pull the armature out of the field to separate it from the field. Clean the field with electrical contact cleaner. Inspect the field for burn marks, indicating sparking.

You can use a multimeter to check the field for continuity to see if it has shortened. Set the multimeter to an 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 armature in the field – the field will have a notch on it, indicating the front. Place the motor back in the drill and reattach the spade connectors. Install the spindle and chuck assembly back in the case. Reinstall the brush holders, using a small-blade screwdriver to hold the brush down, allowing you clearance as you reinstall them on the commutator. Ensure all wires are tucked inside the channels and out of the way. Close the case back together and secure it tight with the fasteners.

An extension cord that is too long, or does not meet the required ratings, can cause the motor to overheat. Use an extension cord with a heavier gauge rating for power tools. When at all possible, always try and plug directly in an outlet.

The holders house the brushes and springs and hold the brushes in place on the commutator. Any physical damage to the holder, or a heavy build-up of dirt, can impact its ability to hold equal pressure on the commutator, cause more sparks, resulting in an overheating motor.

To inspect the holders, remove the screws securing the two halves of the case together and separate. Remove the holders from the case by gently pulling up on the wire attached to the holder. With the holders removed, check them for any physical damage and check that the brushes are free to move up and down. To replace, or clean, the holders, unplug the electrical connector and press the tab on the side to release the brush and spring. Clean the inside and outside of the holder, brushes and springs with electrical contact cleaner, if it is not damaged. If it is damaged, then replace.

Install the spring and brush back in the holder and make sure the tab locks in place. Reconnect the electrical connectors, then reinstall back in the drill. You may need to use a small screwdriver to hold the brush down, giving you a clearance on the commutator during installation. Make sure all wires are tucked in their channels and rejoin/secure the case back together.