Braking and regeneration
Most commonly, in most applications, a frequency converter controls the motor by providing it with energy which then drives the load. However, occasionally the energy flow is reversed, i.e. from the load via the motor back to the drive. This happens when the load is dissipating energy, e.g. B. when a crane or elevator lowers a load or when a conveyor transports material downhill. Regeneration, as it is called, also takes place when braking a high inertia load; in this case, the energy stored in the rotating mass flows back through the motor to the drive. Fans often regenerate when slowed quickly.
Of course, if there is significant friction in the system, or if there are other braking effects (such as airflow through a fan), the energy returned to the drive can be greatly reduced. However, if the regeneration is significant, the energy will flow back into the drive. In this case, continued operation of the drive will maintain a voltage across the motor so there will be magnetic flux, but the phase of the currents will change so that energy - i.e. current - will flow from the motor into the drive. The IGBTs and commutation diodes work as usual - the diodes don't act as rectifiers or anything.
As a result, current flows into the intermediate circuit and onto the intermediate circuit capacitor. Here it charges the capacitor, causing the voltage to rise. The current cannot go back to the supply (the rectifiers block this) so if regeneration continues the voltage across the capacitor will continue to rise. To avoid damage, the drive will detect this and turn off the switching of the output IGBTs. Now there is no output voltage and therefore no magnetizing current, so the flux in the motor collapses, allowing the rotor and load to spin freely with no more energy returning to the drive. The drive has tripped on overvoltage; This is a symptom of too much regeneration.
We can prevent surge trips in a number of ways. If the system regenerates during deceleration, the simplest solution is to reduce the deceleration. That is, increase the ramp-down time. Now the energy regenerated is lower for a longer period of time and this can possibly be absorbed by the losses in the system or by the drive itself. However, with high inertia loads, you will likely end up with a very long deceleration time. Another approach is to simply turn off the output of the frequency converter and let the motor and load coast to a stop. You can do this by changing the stop mode parameter P-05 (P1-05 on P2 and Eco) from 0 to 1. Now there is no ramp, the drive shuts off and the motor coasts.
The disadvantage of this solution is that there is no control over the load and the motor during the stop, so you don't know if and when everything will stop. This is not convenient in the process industry but can be ok for a fan.
None of these options will help if the load is regenerating as part of its normal operation and not when stopped. When you're constantly accelerating and decelerating, or when your crane or elevator is constantly raising and lowering, a controlled solution is needed. Some applications, like the previously mentioned downhill conveyor or an unwinder, are constantly regenerating. The energy that flows to the intermediate circuit of the drive must therefore be dissipated.
The solution is to connect a resistor across the DC to burn off the energy. The resistor is switched on and off by an IGBT built into most industrial drives and controlled by the drive software, which monitors the DC voltage and turns the IGBT on and off accordingly. This "chops" the voltage, so it's sometimes called a brake chopper. Perhaps a better description is dynamic or resistive braking. The resistor is not usually included with the drive and must be selected and purchased separately. This arrangement is shown in Figure 1.
Frequency converter with indication of brake chopper operation
Fig. 1: Frequency converter with display of brake chopper operation
With the correct braking resistor, the drive now allows typically full load current to be returned to the drive and the power dissipated in the resistor. This allows for controlled lowering of loads and unwinding, as well as rapid deceleration of high inertial loads when needed.
As mentioned, the "chopper" IGBT is usually built into the drive, but the resistor has to be chosen by the customer. Selection takes a little care. First, any resistor chosen must be able to operate at high DC voltage and be appropriately protected (i.e., fused). Second, the resistor must have a minimum ohmic value to limit the current in the IGBT. Then it must be chosen to absorb the expected power over the duty cycle of the machine. Finally, the protection of the resistor should be considered. Because of the heat they dissipate, brake resistors are often mounted outside the cab and should be out of harm's way and protected from liquid, dirt and fingers.
This solution is excellent for low and medium power and for controlling brakes that occur occasionally. However, if you're operating a container crane that continuously raises and lowers large containers, it's wasteful to burn all that energy in a drag. The solution here is a fully regenerative drive that feeds energy back into the supply. DC drives have done quite well, but frequency converters need to replace the input rectifier if a fully functional inverter is to be obtained as shown in Figure 2.
Fig. 2: Fully regenerative frequency converter
We have already established that current can flow in both directions through an inverter, so the regenerative energy now flows back along the DC link path, through the front-side inverter and back to the grid. Such an inverter rectifier also has the advantage that it can control the input harmonics in normal operation, which can be quite important with these high power levels. The additional costs for the second inverter, its control electronics and the necessary inductances are justified for drives that regenerate over 200 kW or so. They are also commonly used in variable frequency locomotives so that slowing the train pumps power back into the supply rather than wearing out the brakes.
Another trick to recovering braking energy in a multi-drive system is to tie the Dc connections of the drives together so that when one drive regenerates it simply powers another drive that is motoring. This requires a little maintenance; There is an application note that will help.
E3 Invertek drives include braking IGBTs as standard, except for the smallest unit. P2 drives also have built-in braking IGBTs (optional on frame sizes 6 and up). The elevator variant of P2 includes a braking IGBT.