When a motor is in an overhauling condition—that is, the load is moving faster than the designated motor speed—the motor acts as a generator and produces electrical energy from mechanical energy. This electrical energy, however, needs somewhere to go, and the most common way of dealing with its release is through dynamic braking.
How dynamic braking works
With dynamic braking, the electrical energy generated during stopping is released as heat through a voltage regulated transistor and resistor. There are actually two versions of a dynamic braking circuit: a “chopper” and a “dynamic brake.” The dynamic brake circuit includes the switching device (an insulated-gate bipolar transistor, or IGBT), the control circuit, and the resistor.
A chopper, on the other hand, only includes the regulatory circuit and switching device, with the resistors being separate components. This allows the resistors to be appropriately sized and mounted remotely, which can be important, since the resistors generate a significant amount of heat. The combination of switching device and control circuit is generally referred to as the “chopper module,” while the resistor is referred to as a “dynamic brake resistor.” Dynamic brakes are typically rated for duty cycles in the 20 percent range, and choppers are often used in heavier-duty applications.
Simplified schematic of a dynamic braking circuit.
Image credit: Rockwell Automation, Inc.
There are two types of control for dynamic braking: hysteresis control and PWM (pulse-width modulation) control. With hysteresis control, the control circuit keeps track of the DC bus voltage level and turns the transistor on when the voltage reaches a predetermined level, in order to avoid an overvoltage fault in the drive. When current is flowing to the resistor, the energy is turned into heat, which causes the DC voltage to decrease. As the voltage drops to a preset “low” level, the transistor is turned off.
Where hysteresis control turns on the transistor and leaves it on until the voltage drops to a predetermined level, PWM control turns the resistor on and off according to the level of the DC bus voltage. In general, hysteresis and PWM control methods are equivalent in function, but PWM control is preferred for applications with a common DC bus because it helps avoid a situation where one drive does a disproportionate share of the dynamic braking work.
Linking the dc bus together between
two VFDs through fused connections makes
one simple form of a common bus. An oversized
drive then supplies ac-to-dc rectification.
Image credit: Yaskawa America
In a common dc bus, a single rectifier supplies power to the DC bus for all the DC-AC inverters, rather than an individual rectifier in each AC drive.
Dynamic braking or regeneration?
Dynamic braking is used when energy needs to be dissipated periodically, and regeneration is generally preferred when the motor is frequently acting as a generator. From an application standpoint, overhauling loads (a condition where the load is moving faster than the designated motor speed), such as conveyors and cranes, cause energy to be generated continuously and make recovery and reuse more cost-effective. But applications where the deceleration speed varies, such as fans, are suitable for dynamic braking. While regeneration lowers energy usage, dynamic braking reduces wear on braking components that rely on friction. And although energy is wasted as heat in dynamic braking, its upfront cost is significantly less than that of regenerative drives.
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