Brake Choppers Demystified
In some motor applications the inertia profile can be such that once power is removed the motor effectively becomes a generator, feeding power back into the control system at a gradually decreasing magnitude until motor speed decreases. For adjustable speed drives (ASDs) this presents a primary problem – the risk of DC bus over-voltage as the capacitors charge in response to the “reverse” power flow. Modern ASDs resolve this problem differently depending on the configuration of their power components (typically uncontrolled or controlled rectifier arrays referred to as “bridges”). Regenerative drives (often referred to as 4-quad), made for handling reverse-torque applications such as elevators, utilize anti-parallel bridges – one set conducts from drive to motor, and the other from motor to drive – to dump the generated power back into the supply line. Non-regenerative ASDs (sometimes referred to as single-quad) do not have this capability and so must use other means of dissipating the back-fed power, namely a brake chopper circuit. So as to avoid confusion with injection braking technology, let’s examine the brake chopper in more detail. Much of what follows is taken from ABB’s “Technical Guide no. 8: Electrical Braking”, which can be found in the […]