Electronic – Why does the potentiometer burn up in the setup

Given that you've said R7 is set to 0 ohms, the two fixed terminals of R6 are shorted together. When the wiper is at either extreme, its resistance to either fixed terminal will also be 0 ohms. When the resistance is 50%, both halves will be in parallel. Given that this appears to be a 50 k pot, we'd expect

$$R_{eq}=\frac{R_{max}}{2}||\frac{R_{max}}{2}=\frac{R_{max}}{4}=\frac{50000}{4}=12500$$

just as you measured.

What you have will work, but has some issues. I don't know what kind of "SCR Controller" you have, which is a big black hole. In the simplest terms, you could use a variac feeding your full wave bridge. That would give speed control. With that, you should have a variac that has a zero-output interlock switch. Only energize the motor at zero volts. If you energize a standard permanent magnet 90V DC motor with full voltage, it will appear as a dead short to the supply, since if it isn't rotating, there is no counter EMF to limit the current. You might be able to substitute the SCR controller in place of the variac, but with the same caveat, start at 0 Volts. The best solution, which should be pretty inexpensive for an under 2HP motor, is to just purchase a DC motor controller. A quick look shows a bunch available on Amazon for under $40. A DC Motor controller will generally have an inherent acceleration/deceleration ramp setting, current limiting, and possibly even a setting called "IR Comp", which when properly set will allow the motor to run at a virtually constant speed independent of the load. EDIT TO ADD: The simplest DC Motor controllers in the 90VDC class are basic phase controlled SCR's (with additional intelligence for current limit, ramps, etc). Generally, these are also the most reliable. There are other PWM controllers for 90/180VDC motors which also work well, and they are also surprisingly inexpensive.