This kit allows controlling the speed of a DC motor in both the forward and reverse direction.
The range of control is from fully OFF to fully ON in both directions.
Normally, switches are used to change the direction of rotation of a DC motor.
Change the polarity of the applied voltage and the motor spins the other way! However this has the disadvantage that a DPDT switch has to be added to change the polarity of the applied voltage.
Now you have two things to control the motor - a direction switch as well as the speed control.
Also, it is not a good idea to suddenly reverse the voltage on a DC motor while it is spinning.
It can cause a current surge that can burn out the speed controller.
Not to mention any mechanical stress it can cause as well.
This kit overcomes both these problems.
The direction and speed is controlled using a single potentiometer.
Turning the pot in one direction causes the motor to start spinning.
Turning the pot in the other direction causes the motor to spin in the opposite direction.
The center position on the pot is OFF, forcing the motor to slow and stop before changing direction.

SPECIFICATIONS
Voltage: The kit and motor use the same power supply.
Since the maximum operating voltage of the LM324 is 32VDC then this is also the maximum voltage available to run the motor.
Current: The IRFZ44 MOSFET can handle 49A; the IRF4905 can handle 74A.
However the PCB tracks that run from the MOSFET pins to the screw terminal block can only handle around 5A. If you do then check that the MOSFETs don't get too hot; if so then bigger heatsinks will be required.

SPEED CONTROL OF DC MOTORS
Basically, there are three ways to vary the speed of DC motors:
1. With the use of mechanical gears to achieve the desired speed.
This method is generally beyond the capability of most hobbyist home workshops.
2. Reducing the motor voltage with a series resistor.
However this is inefficient (energy wasted in resistor)and reduces torque.
The current drawn by the motor increases as the load on the motor increases.
More current means a larger voltage drop across the series resistor and therefore less voltage to the motor.
The motor now tries to draw even more current, resulting in the motor "stalling".
3. By applying the full supply voltage to the motor in bursts or pulses, eliminating the series dropping effect.
This is called pulse width modulation (PWM)and is the method used in this kit.
Short pulses means the motor runs slowly; longer pulses make the motor run faster.

Check out these related products:
DC Motor Speed Control Kit 25W
Speed Controller Kit
DC to Pulse Width Converter kit
PWM DC Motor Speed Control Module
Analog Panel Meter