March 17 2016

Applications for VFDs and energy regeneration

Guest blog by: Jonathan Bullick, KEB America

This post discusses variable frequency drive regenerative applications – that is, applications that regenerate energy and common drive options and topologies that you may use with them. Simply, a motor is an energy conversion device. We most commonly think of introducing electrical energy to a motor which converts energy to mechanical torque to do work on a load. This is called “motoring mode.” But depending on the nature of the connected load, a motor can also do the reverse. It can convert torque into electrical energy. We call this “generating mode” or regeneration.

Typical regenerative applications include machines that lift and lower, torque controlled applications, and cyclic applications. Lifts, hoists, automated storage and retrieval systems, tension unwinders, and centrifuges are all types of machines that can benefit from regeneration.

VFD parts
KEB America Typical 6-pulse VFD

Basic VFD Topology

To start, let’s review some basics. A typical 6-pulse VFD has three main parts. The first part is the rectifier stage which converts income AC power to DC power. It’s important to note that the rectifier stage is like a one-way street and allows energy to flow into the inverter but not back onto the utility.

Part two is the DC bus which consists of a capacitor bank. The capacitor bank helps smooth out the rectified power and acts as an energy buffer. Part three is the transistor or IGBT output stage. The IGBTs use pulse width modulation (PWM) to actively switch on and off the DC bus voltage to the motor, resulting in an AC current to the motor. The AC current can potentially flow both directions across the IGBT so the energy flow here is bi-directional.

When energy is in motoring mode, current flows into the VFD and out to the motor. The motor uses the energy to do work, like lifting a large load, for example. As the load is lifted potential gravitation energy in the system increases.

Gravitation energy = mass * gravity constant * height of travel

When the load is lowered the motor acts like a generator (“generating mode”) and the potential energy of the system is converted into electrical current which flows back into the VFD. Current is able to flow back across the IGBTs to the bus capacitors but is not able to pass the rectifiers. The DC bus voltage on the capacitors will continue to rise until the drive faults out with an overvoltage error. To avoid this, something needs to be down with excess energy in the drive.

Brake resistors
KEB America Braking resistors

Braking Resistors

Historically, a brake chopper circuit and a resistor have been used to dissipate excess energy in VFD systems. They work by having a brake transistor circuit, or “chopper”, connected to the DC bus capacitor stage. When the DC bus voltage exceeds a certain threshold, the brake transistor circuit is shunted and current flows from the capacitors across a large external resistor. The electrical energy in the VFD is converted to heat as the current flows through the resistor. The bigger the load and more regenerated energy you have, the more heat is created.

At KEB, we’ve designed an internal brake circuit for our drives to create compact package. Other brands of VFDs may require a separate chopper circuit which must be placed in the cabinet, requiring additional wiring. We also build chopper circuits that are on the beefy side. They are rated for 100% of the rated current so they are great for applications where substantial regen is present and for long duties. Other drives often place a duty cycle (e.g. 50%) on their chopper circuit. In some crane applications I have worked on, users must upsize the entire drive because a larger brake circuit is needed.

DC bus
KEB America DC Bus

Common DC Bus

For multi-axis applications like an automated storage retrieval crane, one good option is to use a shared DC bus topology. In this scenario, the DC bus capacitors from multiple drives are connected together. This allows the power to flow freely between the drives.

In the case where one axis is regenerating (e.g. hoist axis), the excess energy is shared with the other drives. If a second axis (e.g. trolley) is in motoring mode, it will use the hoist’s regenerated energy. If the regenerated energy is not consumed by other connected loads, then a braking resistor is still required. Typically, it is possible to get by with only one resistor rather than an individual resistor for each drive.

Another consideration for shared DC bus topologies is the input power rectification. Individually connecting each drive to AC power and connecting their DC buses might cause the all of the input power for the system to flow through one drive. This could very easily damage the input stage of the drive and cause failure. So, it is recommended that the system is charged through a single unit that is properly dimensioned. This could be done through a dedicated bridge rectifier or by using the largest drive with the largest rectifier stage to hand the system’s input power.

R6 line regen
KEB America Sample ASR machine with R6 line regen unit

Using Line Regenerative Drive

Instead of using a braking resistor, it is possible to use a line regenerative drive such as KEB’s R6. The R6 also has DC bus capacitors and is DC bus coupled to the other VFDs. It uses advanced software to continuously measure and match the utility line voltage and frequency. When the DC bus voltage on the system is above a certain threshold the R6 unit activates and it switches its IGBTs on/off to commutate current back to the line. When the DC bus voltage falls back below the threshold, the R6 unit goes into standby mode.

Another way to think of the regen drive operation is that it enables two-way energy flow in a VFD system. The regenerated energy can flow back onto the line where it is typically consumed by other electrical loads in the building or factory. The KEB regenerative drive also has the ability to act as the system rectifier. If dimensioned properly, the R6 can rectify all incoming power and can be DC bus connected to the inverters. This topology simplifies the charging sequence upon start-up as all input current flows through the R6.

Besides the net energy savings, another advantage to using a regen unit like the R6 instead of a braking resistor is that less heat is created. Braking resistors can create a lot of heat. Removing this heat could require an A/C unit, which in turn requires more energy. Applications like a cold-storage retrieval crane clearly favor regen technology and can show ROI of less than 18 months considering 24/7 crane operation.

Multi-Axis Drive

KEB’s H6 multi-axis drive combines both a shared bus and the ability to regenerate back to the line in a tightly integrated solution. The H6 has separate modules which easily connect to each other. H6 Modules include: a rectifier, active front end (allows line regen), 24VDC supply, IEC61131-3 PLC control, and single and dual output drive modules. The modules have a DC bus rail which allows for an easy connection for power sharing.

An active front end or AFE module also exists which allows for line regeneration if needed. The AFE can be coupled together with an LCL filter which provides extremely low harmonic performance.

A big advantage with the H6 is that is has been designed from the ground up as a multi-axis drive. Items like module fusing, DC bus connections and synchronizing the control of the units are all engineered to ease set-up and provide excellent performance. Another advantage is the drive footprint. For example, a drive solution for a 55kW storage retrieval crane consisting of a hoist, trolley and 4 forks can be achieved with an H6 in about 60% of the space – allowing for a smaller control cabinet to be used.

An engineer has choices when it comes to dealing with regenerative energy in VFD applications. Each solution has its advantages and disadvantages. I am interested to know which solutions you have implemented and your thoughts – leave your comments below.