When a load becomes overhauling — pulls a motor faster than its synchronous speed — the motor acts as an alternator, converting mechanical power to electrical power. If we introduce an adjustable-frequency controller (AFC) into the equation, the synchronous speed can be at any operating speed. The synchronous speed of an ac motor is proportional to the applied frequency. For example, a four-pole motor operating on 60-Hz power has a synchronous speed of 1,800 rpm. At 30 Hz, it’s 900 rpm.
A typical example of a regenerative application is an elevator. The motor is loaded as it lifts the elevator car and its contents. When traveling downward, however, the motor “holds back” on its descent (negative torque) and must have some place to deposit this regenerating energy. In reality, most elevators are sufficiently counterweighted to minimize both motoring and regenerative loading, but both still exist.
Examples of regenerative applications include: Winders and unwinders Machine tools Dynamometers Elevators Centrifuges Cutter knives Tension control Test stands Hoists and cranes Downhill conveyors Press feeders Run-out tables
Many seemingly innocuous applications can actually turn regenerative during certain parts of their operation. When the applied frequency is suddenly decreased to a large centrifugal fan, it becomes a high-inertia flywheel looking for a place to deposit its energy. Similarly, a press may have an eccentric counterweight for balance, causing a regenerative stroke during its downward cycle.
Every load should be examined to determine what an overhauling load can be during any part of the operating cycle. If it is overhauling, the user has several options for dissipating excess power:
• De-tune high-inertia loads by lengthening deceleration time.
• Add a resistive snubber across the dc bus to handle short-term, intermittent regenerative loads. In these units, sensors measure the dc bus voltage. When it exceeds specific limits, a power circuit connects a resistor across the dc bus to dissipate the excess energy.
• Use a line-regenerative AFC or addon regenerative module for more severe regenerative situations. AFCs equipped with line regeneration will handle both intermittent and continuous overhauling loads.
Non-regenerative AF drives
Most adjustable-frequency ac drives share a common operational configuration — vary both the frequency and voltage applied to an ac motor. The most effective way to accomplish this is to first rectify the incoming ac power into dc, filter it, then feed it to a dc-to-ac inverter. The power semiconductors in the inverter modulate this dc into variable-voltage, variable-frequency ac to power the induction motor, Figure 1. Today’s most common technique for synthesizing this 3-phase ac motor voltage is known as pulse width modulation (PWM).
Typically, an AFC has a one-way street — the rectifier that can only deliver power to the dc bus. No provision exists for transferring energy from the inverter section back to the power line. Reason: most AFCs use unidirectional ac-to-dc rectifiers, typically consisting of a diode or SCR bridge rectifier. This makes economic sense for the majority of applications, which are motoring only.
When an overhauling load is encountered, these AFCs have no where to deposit large amounts of power. Compounding the problem is the fact that the dc-to-ac inverter section is bi-directional, enabling motor counter voltage (CEMF) to increase the dc bus voltage. However, the typical rectifier section can’t return the power to the line. Should an overhauling load condition persist, the drive may turn itself off, because the dc bus voltage exceeds allowable limits.
Reasons for using regenerative AFCs
There are several key reasons to use regenerative AFCs:
Fast or controlled decelerating loads, such as large rolls or centrifuges.
Energy recovery in high-duty-cycle applications and applications with continuous regeneration, such as unwinders.
Generating power with induction generators driven by wind power. Key benefits for this application include:
• Power generation over a range of wind speeds.
• Induction generator simplicity, low cost, and maintenance.
• High power factor.
Three recent technological advances have brought the regenerative AFC into vogue:
Better power semiconductors such as insulated gate bipolar transistors (IGBTs), which are hybrid MOSFET/BJT devices. These can switch on and off hundreds of amperes, thousands of times per second, even when controlled by digital signals (1s and 0s).
Faster microprocessor control by using 32-bit microprocessors at high clock speeds. This enables a regenerative regulator to process the complex mathematical algorithms necessary for optimal control of current, voltage, total harmonic distortion, motor torque, power, and power factor.
PWM vector AFCs use speed feedback from the motor to control motoring and regenerative torque in the linear torquecurrent region for many drives. Others use complex control algorithms to calculate the motor load and speed, thereby eliminating the need for motor speed feedback. However, these generally have lower response than designs with the feedback.
Types of regenerative AF controllers The table summarizes the characteristics of the following four types of regenerative adjustable-frequency controllers.
Synchronous rectifier drives
These units contain two complete IGBT bridges, both PWM controlled, Figure 2. The input ac power currents are nearly sinusoidal and devoid of the 5th and 7th harmonics produced by AFCs that use diode bridge rectifiers.
In the motoring mode, the IGBT rectifier section works with a resonant-tuned input line reactor and dc bus capacitor to create dc bus voltage. A complex switching pattern enables a higher dc bus voltage than that created by a conventional diode bridge rectifier. This ability to regulate dc voltage can be beneficial during brown-out conditions.
During regeneration, the IGBT bridge feeds pulses of the excess dc bus voltage into the ac power line, minimizing harmonic distortion and maximizing input power factor.
Presently used on engineered drive systems, this design will be available next year for standard drive applications.
The key benefits and features include:
• Bidirectional power flow to and from the ac power line.
• Controllable input power factor.
• Lowest ac line harmonics.
• Ability to create higher motor output voltages than the input voltage.
• Adaptable to common dc bus designs.
• Power-dip ride-through when line voltage sags.
Bi-directional transistor rectifier
Designed for use as an add-on module to PWM AFCs, this backward-conducting bridge is similar to the basic power bridge in a synchronous rectifier bridge. Bi-directional modules can be used independently on stand-alone, single-section PWM AFCs; or they can be used to augment a multiple- AFC, common-bus system.
Self-contained modules monitor the dc bus voltage. When a predetermined threshold is exceeded, the IGBT bridge switches on at the peak of the input power sine wave. Because power returns to the line at the exact center of the sine wave cycle, there is little angular displacement between current and voltage (the definition of displacement power factor).
Current Source Inverter (CSI)
Based on mature SCR technology, current source inverters (CSI) are unique among AFCs. CSIs can actually transpose the ac line source and load during regeneration. As its name implies, this AFC is a current source that regulates motor terminal voltage. At the heart of the CSI is a large dc choke, Figure 3. Its inductance is 10 times the motor inductance. (An inductance impedes changes in current.) During regeneration, the voltage across the dc bus inverts polarity. This action enables the unidirectional SCR converter to put power back into the ac line.
CSIs are best suited for stand-alone applications that require slow dynamic response. Typical examples include centrifuges, induced and forced draft fans, and centrifugal pumps.
Key benefits and features include:
• Robust SCR converter and inverter sections.
• Inherently regenerative.
• Relatively low manufacturing costs.
• Commercially viable for applications of 100 hp and larger.
PWM with regenerative sixpulse SCR rectifier
Typically referred to as an S-6R, the six-pulse controlled-bridge rectifier on this PWM unit features a second reverseconnected rectifier, Figure 4. A common practice with regenerative dc drives is to connect both rectifier bridges in parallel to the ac line. On an AFC, the rectifier must be protected from inverting faults. These faults occur when the dc bus voltage exceeds the peak of the sine-wave, ac line voltage. This voltage difference will usually cause one of the SCRs in the reverse- bridge to remain in conduction while a forward-bridge SCR also conducts. This dual conduction produces a phase-to-phase short circuit, turning expensive drives into smoke generators.
To avoid this problem, connect the reverse- rectifier to a higher voltage source (usually a transformer with a higher secondary voltage), Figure 5.
Regenerative PWM inverters with S-6R rectifiers offer:
• Reliable SCR power semiconductors.
• Reverse six-pulse power circuit that is adaptable to common-bus applications.
• Power dip ride-through using phaseback, phase-forward control.
Common dc bus configuration
To conserve energy and reduce the required capacity of the input ac-to-dc rectifier, multiple AFCs can share a common dc bus, Figure 5. This enables drives that are regenerating to supply energy to those drives that are motoring.
If the net power is always away from the ac line (more drives motoring than regenerating), a regenerative converter isn’t needed. In this case, the regenerated power is immediately re-used by the motoring drives.
One major advantage with multiple AFCs on common dc bus is that you can use a single regenerative module, sized for worst case energy return. Thus, the regenerative module must have the capacity to handle only the surplus energy that is unused by the motoring AFCs.
The engineer designing to a common dc bus must coordinate individual AFC protection. Most systems use many combinations of fusing, circuit breakers, and reactors. Plus, electrolytic capacitors in the dc bus store tremendous amounts of energy. Therefore, it is essential to prevent an isolated AFC failure from causing a catastrophic fault to the entire system.
Norman C. Lindner is a senior product specialist for the Reliance Electric Co., Cleveland.