A motor controller converts the battery DC-voltage to a form that can be used by the motor.

24V36V 350W Electric Brush Motor Speed Controller for e-bike conversion kit (24V350W)
24V36V 350W Electric Brush Motor Speed Controller for e-bike conversion kit (24V350W)
$18.99
Amazon.com
Sunwin 48V 1000W Electric Bicycle Brushless Speed Motor Controller For E-bike & Scooter
Sunwin 48V 1000W Electric Bicycle Brushless Speed Motor Controller For E-bike & Scooter
$38.25
Amazon.com
48V/72V 2000-3000W Brushless Sine Wave Controller ,E-bike Hub Motor Controller for Electric Bicycle Kit.
48V/72V 2000-3000W Brushless Sine Wave Controller ,E-bike Hub Motor Controller for Electric Bicycle Kit.
$119.98
Amazon.com
Super power SABVOTON 96V 100A 5000W Programable sine wave electric bike controller
Super power SABVOTON 96V 100A 5000W Programable sine wave electric bike controller
Amazon.com
72v 100A 5000W Ebike sabvoton sine wave programmable controller
72v 100A 5000W Ebike sabvoton sine wave programmable controller
$375.00
Amazon.com
72V 150A Sabvoton Programable Sine-wave Motor Controller for 5000W Electric Bicycle Kit/ Ebike Kit.
72V 150A Sabvoton Programable Sine-wave Motor Controller for 5000W Electric Bicycle Kit/ Ebike Kit.
Amazon.com
72v 100A 5000W Ebike sabvoton sine wave programmable controller
72v 100A 5000W Ebike sabvoton sine wave programmable controller
$399.00
Amazon.com
Sabvoton SVMC72100 3kw Controller for Ebike Electric Bicycle Brushless DC Motor
Sabvoton SVMC72100 3kw Controller for Ebike Electric Bicycle Brushless DC Motor
$369.95
Amazon.com
48V 60V 72V 96V 5000W Sabvoton Sine Wave Programmable Controller + LED voltage display Twist Throttle with Electric Switch +Brake Lever+ Torque arm for 48-96V 5000W Electric Bicycle Motor/E-bike Kit
48V 60V 72V 96V 5000W Sabvoton Sine Wave Programmable Controller + LED voltage display Twist Throttle with Electric Switch +Brake Lever+ Torque arm for...
Amazon.com
Amazon price updated: February 7, 2018 7:20 PM

The ebike controller is the brain of the e-bikes. It is connected to all the other electronic parts such as the battery, motor and the throttle if exist. It takes all the inputs from all the other components and determine what should be signaled to them in return.