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The toy you are originally starting from comes with some method to control the motor, often through a foot pedal switch that activates the motor directly or through a relay. The motor is either on or off. Besides optional “fast” and “slow” modes, there is no way to vary the speed of the motor on the original toy. Except for the switch or relay failing, this setup is generally pretty reliable for the tiny stock motors.
Assuming you are going to replace the motor with something more powerful, and (optionally) would like more granular control over the motor's speed, you will need to find a suitable motor controller to match your chosen motor. Depending on your setup, the motor controller can be a significant part of your budget, so it requires careful consideration.
There are generally two types of motor controllers, which must match your type of motor:
In addition, the speed controller will have specifications you must work within to prevent failure:
Wikipedia has more information on the topic of electronic speed control.
Different speed controls have different features to be aware of:
Your options for obtaining your speed control are:
The Internet has many new, very inexpensive “Chinese Controllers” floating around, in both brushed an brushless forms. They are generally designed for usage in scooters or e-bikes. There are also higher-powered controllers available for golf carts, but are generally much more expensive.
It is certainly possible to get a high quality / high power controller used for much less money than new. Some folks have gotten controllers at scrap metal prices, which is hard to beat. Good sources for controllers include electric fork lifts, pallet jacks, and golf carts.
Building your own controller can be a rewarding process, but certainly involves more complexity.
There are some closed and open source motor controller projects out there to help get you inspired: