A fundamental rule when selecting and applying variable frequency drives is "load is king." That is, the load is the controlling variable in most applications. The drive and motor react to what the load is doing, not the other way around. And in situations where factors cause the load to overpower the motor, energy regeneration issues exist. If these issues are not properly addressed, equipment damage, poor process control or downtime can result.
This article will explain what regeneration is and why it happens, while suggesting techniques to mitigate it, including dynamic braking. Drive users need to examine their applications to properly select the regenerative device that optimizes drive and motor performance and, ultimately, maximizes productivity.
What is regeneration?
Any time a motor turns faster than the controlling drive is commanding it to turn, it''s no longer a motor but a generator. Instead of taking electrical energy from the drive and converting it to mechanical energy, the motor is converting the load''s mechanical energy back to electrical energy and returning that energy to the drive. This process is called regeneration, and it typically occurs when:
Gravity contributes to downhill movement
Anytime a conveyor is going downhill and gravity moves the load faster than commanded motor speed, the motor will regenerate energy. This frequently occurs when a conveyor changes altitude, either from one floor to another, or from the top of a coal mine to the bottom.
Load inertia overpowers the motor
For example, a punch press has a spinning flywheel driving a crank that raises and lowers a die; during half of the cycle (raising the die) the load is motoring and during the other half of each cycle (lowering the die), the motor is regenerating.
Load speeds are stopped or changed quickly
When users command a decrease in speed, but the load is unable to slow down immediately because of high inertia, the load overhauls the motor and the motor becomes a generator. Using the same punch press example, if an operator needed to stop the press for a new setup, allowing the press to coast to a stop would lower productivity. Instead, the press must be stopped quickly, causing considerable regeneration.
Dealing with regeneration
When energy regenerates back to the drive, it is first absorbed into the DC bus capacitors, raising the DC bus voltage. The drive monitors its own bus voltage, and when that voltage exceeds the recommended safe level for the bus capacitors, the drive will shut itself off, eliminating the path for regenerated energy and protecting its internal components. Once this occurs, the motor and the load are no longer under drive control, and they begin coasting to a stop, halting the application and causing expensive downtime.
To optimize performance and reliability of a drive application, manufacturers can use flux braking, bus regulation, shared or common bus, line regeneration, or dynamic braking techniques to efficiently manage energy regeneration.
Flux braking is a software feature that can adjust the electrical output to the motor to temporarily cause the motor to be extremely inefficient. The regenerated energy is dissipated within the motor itself in the form of heat. This is an effective way to deal with low amounts of regenerated energy. However, if duty cycles are high, meaning the load pushes the motor into a generator state for an extended time or too frequently, users should seek alternative regenerative methods that avoid excessive motor heating and possible motor failure.
Because regeneration occurs when motor speed is greater than commanded speed, one way to deal with the resultant regenerated energy is to eliminate it completely by increasing the commanded speed so that it is equal to or greater than the actual motor speed. Bus regulation is a software feature that monitors DC bus voltage and adjusts output frequency to regulate the voltage, controlling the regenerative state.
In the punch press application example, when the flywheel inertia overhauls the motor, the drive will sense the increasing DC bus voltage and increase the output frequency. When the cycle is motoring, the drive will decrease the frequency back to the commanded value. Although bus regulation may be constantly adjusting the drive''s output, the manufacturing process is usually unaffected because the substantial amount of inertia present will not allow the motor to actually change speed. Bus regulation is a good option for situations where the amount of regeneration is low, but the occurrence is frequent.
In some instances, users can connect DC buses of two drives together. Instead of returning regenerated energy back to the AC line or trying to stop energy from coming back to the drive, the energy is shared between the drives. When one process regenerates, energy is sent to the second drive via a common bus, rather than being wasted as heat. Systems built with many coordinated drives, such as a paper winding machine or a steel processing line, are typically designed as a common bus system. However, some issues may exist for connecting standard drives, particularly if they have radically different ratings.
Instead of preventing or absorbing regenerated energy, line regeneration puts the energy back onto the AC power line. Drives that have line regenerative capability contain a second inverter section that converts the regenerated energy back to AC and puts it back on the AC line. Because this solution requires additional hardware with the second inverter, it does raise the initial cost of the drive. However, line regeneration allows the highest performance of all of the solutions and it conserves energy in applications requiring higher horsepower (above 100 hp) or high-duty cycle.
The most common method of dealing with regeneration is dynamic braking. It is a more cost-effective solution than line regeneration or common bus and offers higher performance than flux braking or bus regulation. A dynamic brake dissipates regenerated energy into a resistor or resistor bank, converting the excess energy into heat. By switching the energy onto the resistor when the dc bus voltage rises, the drive''s internal components are protected and the process will continue.
Dynamic braking is generally cost effective at lower horsepowers and where duty cycles, or how frequently the brake is used, are low. Dynamic braking can be used at higher ratings and duty cycles, but the size of the required resistors greatly reduces cost effectiveness and energy efficiency. Applications that require occasional quick stops or speed changes are ideal candidates for dynamic braking.
Breaking down the dynamic brake
A dynamic brake consists of three basic parts: the voltage sensing circuit, the brake IGBT and the brake resistor. The voltage sensing circuit and brake IGBT together are called the chopper. The chopper module monitors the drive''s bus voltage. When energy regenerates from the motor, the bus voltage rises. The chopper module and its associated circuitry sense the rising bus voltage, and when the voltage exceeds a set, safe level, the circuitry will turn on the chopper''s transistor in order to divert the excess energy into the resistor. The module is called a chopper because to control energy flow to the resistor, the transistor is quickly turned off and on in a pattern similar to the pulse-width modulation (PWM) output of the drive itself, "chopping" up the electrical waveform that it sends to the resistor. As the energy is "bled off" to the resistor, the dc bus voltage begins to fall back to normal levels and the chopper transistor is turned off. The resistor simply converts the electrical energy into thermal energy.
Selecting a dynamic brake
The process of choosing the correct dynamic brake chopper and resistors requires some knowledge of the electrical (motor) and mechanical (load) characteristics. The amount of energy present, speed changes required, system inertia present, the performance requirements and other factors must all be considered. Duty cycle or the ratio of "on " time to "off" time can play a critical role in resistor selection. Resistors have fixed thermal characteristics, which dictate energy-versus-time trade-offs. The typical resistor can dissipate a relatively large amount of energy for a short time or a low amount of energy for a long time; the current vs. time value remains relatively constant. Therefore, a high duty cycle (frequent braking) requires either a very large resistor bank or a limit on the amount of braking energy. A high amount of braking requires either a very large resistor or very infrequent use (low duty cycle).