Inertial loads, and/or those in processes requiring rapid deceleration, present special challenges for variable speed drives (VSDs) and the motors they control. As power is removed or frequency reduced at the VSD output, a load with high inertia will prevent its motor from slowing down as quickly as it would under light- or no-load conditions. In such cases, the motor will change from its normal operation as a current “consumer” and become a current generator, wherein the motor shaft’s mechanical movement produces a negative current flow before the motor’s flux degrades. During this process, called regeneration, the power flow is back-fed into the DC bus of the VSD. The rectifier bridge upstream of the DC bus blocks the flow of this current, and unless the drive is properly equipped the DC link risks over-heating and damage.
So how does a VSD compensate? There are several techniques used depending on process requirements, capital cost, and other factors. Of these, two of the more common are brake choppers and the use of rectifiers with regenerative diode bridges. Brake choppers are essentially switches which close when the DC bus voltage reaches a defined limit and divert the back-flowing current through a resistor, dissipating the energy as heat. Depending on current-carrying requirements, brake chopper circuitry can consist of simple transistors or higher-rated circuits with separately installed resistor packages. Some drives include brake choppers as standard; many others require that they be specified as options. The rating (and expense) of the required components depends not only on amperage but on how frequently the regeneration will occur, commonly referred to as duty cycle. Since both the chopper circuit and the resistor (by definition) are resistive in nature, they heat up when carrying current; the more often they are conducting, and the longer they are doing so, the more robust they need to be to operate safely. Total operational time must not exceed the resistor’s heat dissipation capacity over its rated duration. All this can mean significant expense. For example, a braking resistor for a 110 kW/150 HP, 400V drive with a 10% stopping duty is rated for safe operation for 3 seconds on/27 seconds off. Such a unit in a protected enclosure might be 28″ x 10″ x 10″ and list for $1500, whereas the same drive under a higher 33% stopping cycle might need a 30″ x 18″ x 24″ unit listing for $5000. As you can see, it is important that stopping power be carefully defined to ensure a cost-efficient selection.
Drives with regenerative capability are designed with rectifier sections with diode bridges installed anti-parallel. One bridge handles current flowing in the forward direction (from supply to drive to motor) and one set accepts current flowing in the reverse direction (from motor to drive to supply). The drive controller continuously monitors power flow and switches from bridge to bridge as needed. In this way, power produced by regeneration can be fed back into the supply line, at times reducing overall energy costs.
A different but analogous situation occurs in a process requiring rapid deceleration of the load. The energy needed to overcome friction losses and rotating inertia and force the motor to ramp down more quickly can come from the drive, within limits. If sized properly, and the stopping requirements are not too severe, the drive can control the motor flux and force a programmed ramp-down. Often the losses stemming from the mechanical load connections (e.g. couplings) and friction supplement the drive’s deceleration energy, so the drive is not stressed. In more severe cases, some type of supplemental braking is needed. This is often accomplished by DC injection braking, wherein a DC voltage is applied to the motor to produce a braking torque in the rotor. Smaller drives, say less than 3.7 kW/5 HP, sometimes come standard with injection braking capabilities, whereas larger drives require an external unit. Generally, the heat produced by DC injection is dissipated in the mass of the rotor, so an external chopper/resistor are not needed.
I trust this entry has shed some light on the impacts of motor braking on VSD application and costs. Please feel free to let us know of your experiences in our Comments section If you would like to discuss your own application further, please contact us at email@example.com, or visit our websites, www.joliettech.com/blog and www.joliettech.com. Thanks for reading, and stay tuned later this month for another column.
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