What Goes Into Making a Variable Field Magnetic Motor?

Variable Field magnetic Motor can mean different things depending on the context. What I saw in the video is completely different to what I thought when I read the title of the question.

The first thing that sprung to mind was Field Oriented Control (or Vector Control). FOC is a type of control that modifies the magnetic flux (thus Variable Magnetic Field), in order to achieve maximum torque and variable rpms.

Basically the way it works ( in one of the implementations ), is that the current in each winding is controlled in such a way so that the magnetic flux in the system is such that the rotor rotates in the desired rpm and torque.

This type of control can be used to different types of motors:

• DC Brush,
• Brushless DC (BLDC) and
• step motors
• AC induction motors (sensorless field control)

So (at least the way I understand it), you can't really build a Variable Field Magnetic Motor. There are many hardware implementations of motors, that can be controlled with a varying magnetic field.

PS: I am not an expert in the field.

Although the previous answers address the OP's question, I am adding an answer since they do not explain the mechanism of the variable field magnetic motor.

The variable field magnetic motor is usually built as a brushless DC motor (BLDC). The mechanism works by moving the rotor out of line with the stator so the effective magnet strength is lower. This increases velocity constant $K_v$ because the motor has to run faster to produce enough back-EMF to counter-match the supply voltage. It is the permanent magnet equivalent of varying field current in a shunt-wound motor. Indeed, motor speed could be raised by simply increasing voltage, but this also increases material (iron) losses due to hysteresis and eddy currents. Field weakening reduces the magnetic field strength as it raises rpm, reducing the loss at light loading. However it also reduces torque and output power, so it is more suited to applications which require high efficiency at both low rpm & high torque and high rpm & low torque.

Here is a video showing the increasing speed of the rotor with decreasing cross sectional interface between the curved surfaces of the rotor and stator.