I need some basic education on cordless tool motor

Thread Starter

Emare

Joined Dec 8, 2017
19
I have an 18V Rigid angle grinder ( model R86040 ) that doesn't work so I dismantled it and do not understand what I found. The motor appears to have 4 brushes and the magnets in the stator are so strong that it was difficult to remove the rotor. Have things changed so much since I dismantled my last electric tool? Are 4 brushes common and is it normal to have such strong magnets in the stator?
 

MaxHeadRoom

Joined Jul 18, 2013
19,973
Yes, you need to be very careful when removing a rotor, otherwise you can chip or crack a field magnet.
DC brushed motor do come with four brushes as well as the two brush variety.
Test the motor with an automotive battery, it is often the internal multi cell battery block that fails.
Max.
 

crutschow

Joined Mar 14, 2008
24,302
Don't know how common it is but the motor is apparently a permanent-magnet DC motor with a brushed commutator rotor.
It's quite common to use strong permanent magnets in cordless tools since they make a smaller motor for a given power, and are more efficient than universal type brushed motors.
 

MaxHeadRoom

Joined Jul 18, 2013
19,973
Just about all cordless hand tools are DC brushed motors.
At one time it was necessary to slide in a steel 'keeper' when removing the armature in order to retain the magnet strength, with modern magnetic rare earth materials it is not deemed necessary anymore.
For some reason, steppers seemed more prone to this.
Max.
 
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Thread Starter

Emare

Joined Dec 8, 2017
19
Thanks, I had another battery which was functional so was able test that. I thought their was something wrong with the bearings since it was so hard to turn. Bearings great, just the magnets
Yes, you need to be very careful when removing a rotor, otherwise you can chip or crack a field magnet.
DC brushed motor do come with four brushes as well as the two brush variety.
Test the motor with an automotive battery, it is often the internal multi cell battery block that fails.
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
19,973
Although most cordless hand tools are DC brushed motors, the AC/corded versions use series field brushed motors.
Advantages of a Series Wound Motor is that it develops a large torque and can be operated at low speed, it is well-suited for starting heavy loads at low RPM. An example Automotive starters.
If the motor is overloaded, the speed drops and the torque increases accordingly.
The series motor draws less current and power from the source compared to a shunt or compound DC motor.
The down side it has high revs with no load and essentially operates in a runaway condition, but can be used on AC or DC.
Max.
 

Thread Starter

Emare

Joined Dec 8, 2017
19
Although most cordless hand tools are DC brushed motors, the AC/corded versions use series field brushed motors.
Advantages of a Series Wound Motor is that it develops a large torque and can be operated at low speed, it is well-suited for starting heavy loads at low RPM. An example Automotive starters.
If the motor is overloaded, the speed drops and the torque increases accordingly.
The series motor draws less current and power from the source compared to a shunt or compound DC motor.
The down side it has high revs with no load and essentially operates in a runaway condition, but can be used on AC or DC.
Max.
 

Thread Starter

Emare

Joined Dec 8, 2017
19
Thanks for the quick lesson. I have checked the switch, the bearings and the brushes. There is an 'electronic control' module on the negative power input and one of the terminals is a bit discolored. Perhaps the module is shot. Any other suggestions for trouble shooting this motor?
 

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MaxHeadRoom

Joined Jul 18, 2013
19,973
Then it would point to the motor, can you remove the brushes and make sure they are not in contact with the comm?
Another test, is if you place a voltmeter on the leads and rotate the armature as fast as possible, it should generate.
Max.
 
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MaxHeadRoom

Joined Jul 18, 2013
19,973
The exploded view shows a simple right angle direct drive bevel gear, no reduction.
You should be able to turn the output shaft in order to turn the motor.
Max.
 

Thread Starter

Emare

Joined Dec 8, 2017
19
The exploded view shows a simple right angle direct drive bevel gear, no reduction.
You should be able to turn the output shaft in order to turn the motor.
Max.
I have removed the right angle drive and the motor does turn, although not freely due to the powerful magnets in the stator.
 

MaxHeadRoom

Joined Jul 18, 2013
19,973
Normally the rotor should be able to revolve freely, shorting the motor leads will cause it to be resistant to spinning freely.o_O
Max.
 
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