Either the water mentioned above or else too small a motor for the load. I used to use commercial dough mixers to mix epoxy and found that it was cheaper to buy the biggest honking motor I could find.
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"Well, I've wrestled with reality for 35 years, Doctor, and I'm happy to state I finally won out over it." Elwood P. Dowd
Did it just start doing this after previously running properly? Did something in the process change that would cause an overload?
Check the amperage draw of the motor When it is slowing down. Are all three legs (if it is three phase) roughly equal? How does it compare to the stated FLA of the motor.
Are any of the stator windings showing too little resistance?
I would also check to see that the input voltage is as expected. If it is three phase they should all be about the same. Sometimes there are problems with the supply voltage.
If this checks correct I suspect there is a mechanical problem as mentioned. Such as the mixer is being overloaded or it is too small.
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If is a Hobart M-802 or a V-1401 ... your clutch disks are worn and need to be replaced....if it is Univex.... probably a glazed drive belt.....
The make and model information is needed to give you any assistance at all.... also as noted previously... if it is not 3 phase..it is not a "work horse"...
"Brushes" indicates a DC drive motor to me.......
More information needed.........
Please check the DC motor specifications, is the duty cycle/overload cycle matching the load. probably as the dough thickens and becomes more viscous, the shaft may not be able to provide the requisite torque.