0 down vote favorite Full disclosure: Not an electrician, mostly clueless.
Setting: Home Brewing Beer
Short: 120v DC variable drive motor running full tilt when 240v AC 5500w element is connected to mash tun.
Long: I have a 50 gallon brew kettle with a 240v 5500w AC heating element. The element has variable power supply. Recently I added a 120v DC variable speed motor to agitate the mash (stirring 50 gallons of mash by hand gets really old). The motor work appropriately when the heating element is not attached to a power supply; I can adjust the speed of the motor.
When the heating element is attached to its power supply, the agitator motor runs at full speed, and cannot be adjusted. I spoke with the manufacturer and they suggested that I have an electrician come out.
My house was built in the 20's and has lots of "character". Many of the circuits are not grounded. That being said, I am (fairly) certain the circuit that the agitator is running on is grounded, as well as the heating element.
Thoughts? Places to start trouble-shooting?