How does the speed controller of this cassette tape motor work?

Question

I am new to this area and for my first project I am trying to build a cassette player.

I am up to the point of figuring out the speed control of the motor.

I have been analyzing this cassette player and I can't find any speed control in the circuit. What is the explanation for this? Isn't it necessary to have a speed control? Without, it when the battery is weak the motor will spin slower and the sound will be wrong

Images of the motor board and ICs on the board:

Edit:

After tearing apart the motor, it appears that @GodJihyo is right. There is a whole circuit in the motor.

I don't understand exactly how it works (as I said I am a newbie) but I'll attach the images of the circuit:

Answer 1

The answer lies here:

The red circled part is an AN6651 motor speed control IC.

It is designed to maintain a fixed speed from a variable power source with a variable load.

The internal circuit looks like this:

It works by maintaining a fixed voltage across the motor. It varies the current as needed to maintain the voltage.

• If you grab the motor and stop it from turning, it becomes basically a short circuit - the voltage across the motor is nearly zero.
• If you turn the motor faster (by hand or whatever) then the voltage across the motor goes up.
• Between those two extremes, a given speed will have a specific voltage. If the voltage goes down, the load is heavier than expected. If the voltage goes up, the load is lighter than expected.

This is a typical circuit for the AN6651:

Answer 2

Try the motor with different voltages, eg 6v, 4.8V etc (whatever the motor normally takes and 20% less) if the speed is the same, then the speed regulator is probably inside the motor.

Perhaps there are centrifugal switches on the rotor.

A search on the motor markings suggests that this in a fixed speed motor with the speed 2400RPM set in the factory.

Answer 3

There is no external speed controller. The motor in question is designed to spin at a constant speed, 2400 rpm, over a range of voltages and loads.

I can't find any information on how this specific one accomplishes this but one way is to use a negative resistance driver circuit that cancels out the winding resistance. The circuit increases the winding voltage as the winding current goes up due to loading so the speed stays constant. That's probably how this one works.

Answer 4

There are no switches inside the motor case, centrifugal or otherwise.

Sometimes the motor is driven by a constant current circuit, so the motor gets the same current as the battery voltage decreases. Trace the connections to the motor connector, and sketch a schematic of the motor drive components.