Replacing a variable resistor (rheostat) with MOSFET

I have a separately excited DC electric motor. I control the speed by varying the field with a rheostat. I would like to substitute 2 n-channel power mosfet to this old and cumbersome rheostat taken from the lab. Once the motor has reached 1200 rpm, the bypass resistor is cut off and the armature is fed directly from the battery until it reaches 2000 rpm. Then the required variation of speed is between 2000 and 3000 rpm only. Of course, a limited variation of speed is possible by field weakening but this is enough for our application.

The motor is a ES-84-12 from D&D Motor Systems. At start, current is shunted around through a large 1 ohm (10 kw) protection resistor. The field is set at 6.25A for 14.8 ohms (13.6 from the rheostat + 1.2 from the field winding). Progressively the resistance is brought up to 19.2 ohms (18 + 1.2) to increase the current in the armature. At this point (1200 rpm) the motor reaches enough BackEMF to plug it directly to the pack.

So, your advice on substituting 2 (or 3 for a smoother start) SK1019 n-channel power mosfet would be appreciated. I include a sketch at: http://carrier-denis.net/11-39A_MOSFET_as_var_res.jpg

I have a separately excited DC electric motor. I control the speed by varying the field with a rheostat. I would like to substitute 2 n-channel power mosfet to this old and cumbersome rheostat taken from the lab. Once the motor has reached 1200 rpm, the bypass resistor is cut off and the armature is fed directly from the battery until it reaches 2000 rpm. Then the required variation of speed is between 2000 and 3000 rpm only. Of course, a limited variation of speed is possible by field weakening but this is enough for our application.

The motor is a ES-84-12 from D&D Motor Systems. At start, current is shunted around through a large 1 ohm (10 kw) protection resistor. The field is set at 6.25A for 14.8 ohms (13.6 from the rheostat + 1.2 from the field winding). Progressively the resistance is brought up to 19.2 ohms (18 + 1.2) to increase the current in the armature. At this point (1200 rpm) the motor reaches enough BackEMF to plug it directly to the pack.

So, your advice on substituting 2 (or 3 for a smoother start) SK1019 n-channel power mosfet would be appreciated. I include a sketch at: http://carrier-denis.net/11-39A_MOSFET_as_var_res.jpg

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