Dynamic braking is one of the braking methods used for AC Induction Motors Braking. In this article Electrical Engineering XYZ shares the fundamentals of DC Injection braking.
If a powered AC induction motor spins at a speed faster than its rotating magnetic field, it acts as a generator: supplying power back to the voltage source, transferring kinetic energy from the spinning rotor and machinery back into electrical power. This makes for an interesting experiment: take an internal combustion engine, steam turbine, water turbine, or some other mechanical prime mover and mechanically force a powered induction motor to spin faster than its synchronous speed (i.e. force it to achieve a negative slip speed). If a power meter is connected between this motor and the AC line power grid, the meter will register negative power (i.e. power flowing from the motor to the grid, rather than from the grid to the motor).
This principle holds true for an induction motor powered by a VFD as well: if the rotor is spun faster than the speed of the rotating magnetic field produced by the VFD, it will act as a generator, sending back more power to the VFD than it receives from the VFD. Since the magnetic field’s rotational speed is variable – thanks to the VFD’s ability to synthesize virtually any desired frequency – it means an induction motor may be made to operate as a generator at almost any speed we desire.
When acting as an electrical generator, an induction motor requires an input of mechanical
energy. That is, it will require mechanical effort to keep the rotor spinning faster than synchronous speed, since the motor naturally “wants” to spin at synchronous speed or slower. This means a generating motor acts as a brake, attempting to slow down whatever is keeping it spinning faster than synchronous speed. This braking effect is in direct proportion to how much the generated energy is used or dissipated by an electrical load. If we build a VFD to dissipate this energy in a controlled manner, the motor will have the ability to act as a dynamic brake.
In a VFD circuit, the “reverse” power flow received from the motor takes the form of currents
traveling through the reverse-protection diodes placed in parallel with the output transistors. This in turn causes the DC bus filter capacitor to charge, resulting in a raised DC bus voltage:
Generating currents through reverse-protection diodes
Without a place for this energy to dissipate, however, there will be little braking effort, and the capacitor will be quickly destroyed by the excessive DC bus voltage. Therefore, in order for dynamic braking to work, the VFD must be equipped with a braking resistor to dissipate the received energy. A special transistor rapidly switched on and off to regulate DC bus voltage ensures the capacitor will not be harmed, and that the braking is effective.
This next schematic diagram shows how a braking resistor and its accompanying transistor could be added to the simple VFD circuit. Once again, the switching circuitry used to turn the braking transistor rapidly on and off has been omitted for simplicity:
The braking transistor switches on in direct proportion to the DC bus voltage. The higher the DC bus voltage, the greater the duty cycle (on time versus total time) of the braking transistor. Thus, the transistor functions as a shunt voltage regulator, placing a controlled load on the DC bus in direct proportion to its degree of over-voltage. This transistor never turns on when the DC bus voltage is within normal (motoring) operating range. It only turns on to clamp DC bus voltage to reasonable levels when the motor spins faster than synchronous speed.
With this braking circuit in place, the only action a VFD must take to dynamically brake an AC induction motor is simply slow down the applied AC frequency to the motor until that frequency is less than the equivalent rotor speed (i.e. create a condition of negative slip speed).
As with DC injection braking, the braking torque created by dynamic braking is a function
of magnetic field strength and rotor speed. More precisely, it is a function of the Volts/Hz ratio applied by the VFD to the motor, and the magnitude of the negative slip speed. Braking torque is primarily limited by the braking resistor’s power rating and also the power rating of the VFD. Since the kinetic energy dissipation occurs outside the motor, there is little rotor heating as is the case with DC injection braking.
Summary:
Dynamic Braking is one of the braking methods used for braking AC motors. List of all braking methods:
- DC injection braking
- Dynamic braking of AC Induction Motors
- Regenerative braking of AC Induction Motors
- Braking using Plugging
References/Further reading:
- AC Motor Braking methods PDF Handbook extracted from Instrument Handbook authored by Tony R Kuphaldt under CC4.0L