I've been building a circut to run a motor forwards and backwards for a project I'm working on. I'm not sure if I need diodes to protect the transistors, because I have no idea how to add them as the current alternates direction. I'm not sure if having no diode will damage the transistors.
Do i need a diode? and how would i put one accross my motor?
The feasibility of putting a diode across a motor depends upon whether the current in the motor only flows in one direction.
i have no idea how to add them as the current alternates direction.
So, in your case, since the current alternates direction, you cannot put a diode across the motor.
You can, however, place diodes within an H-bridge in a way that protects the transistors.
The diodes in the circuit above will protect the bipolar transistors from over-voltage caused by back EMF from motor, provided that the power supply can accept reverse current.
[D2 protects the collector of Q1 from exceeding the supply voltage by more than one diode voltage drop. D4 provides the same protection for Q3. D1 and D3 protect the collectors of Q2 and Q4 from going below ground more than one diode voltage drop.]
It is not clear from your image what components you are using to drive your motor. It appears that there are only 3 transistors, so you could not have a full H-bridge. Furthermore, it is not clear whether the transistors are bipolar or MOSFET. If Mosfet, it is almost certainly the case that the transistors have an internal "body diode" connected between source and drain. These body diodes can serve the same purpose as the diodes added to the BJT H-bridge above. You can still use external Schottky diodes if the body diodes are not adequate for some reason.
motors are mostly capacitive, They have a high current when the voltage changes, but only a small voltage step when the current changes.
But if you still want a diode snubber use a bridge rectifier (or diodes organised that way) with the motor connected to the AC terminals and the supply connected to the DC terminals.