Self-developed Variable Speed Drive Inverter 380V 3 Phase 220kw For Food Processing Industry

VFD in practice

The power electronics usually come in a single package with a mounting plate – a power module. As these components dissipate heat, they need to be fixed to a heatsink, usually an aluminium extrusion or casting. The heatsink will have a fan or fans to cool it, except for the smallest units. It’s an advantage if the fan only switches on if the heatsink is getting hot, as the fans can wear out over time, as well as blow dirt, dust and fluids into the VFD.

There will usually be at least one ’power’ printed circuit board close to the power module to carry the circuits associated with the IGBTs. There will be gate drive circuits, a power supply, current measurement circuits and an inrush circuit. All these will interface to the control circuit – more of this later.

Each IGBT requires a gate drive circuit. These turn the IGBT on and off as needed, and are connected directly to the IGBT and therefore to the power part of the variable frequency drive. The gate drives will also need a power supply, so actually four separate supplies are needed, one for each ‘upper’ IGBT and a common one for the three ‘lower’ IGBTs. As the power supply needs to supply the control electronics, the fans, and everything else, it becomes rather complicated. Usually a small IGBT supplies a high frequency transformer from the DC link, the secondaries of which feed the various supply regulators.

Current measurement is important and difficult in a variable frequency drive. Important because we need to monitor the output currents (to give good motor control and protection) and possible short circuit currents to protect the IGBTs. Difficult because the measurement points are in the output (or DC link) of the VFD and need power and isolation to get the signal back to the control circuitry. In practice this means using several Hall effect sensors to measure the current in the output, and one or two fast sensors or resistive shunts in the DC link.

The DC link capacitor will be mounted on a power board as well, and when the variable frequency drive is first switched on it will be uncharged, so a circuit consisting of a relay and resistor is used to limit the inrush current. The capacitor charges through the resistor and the relay shorts the resistor after a couple of seconds for normal operation.

So that’s the power pcbs. On the largest variable frequency drives they will be connected to the IGBTs and capacitors by hard wiring or bus bars; on smaller VFDs everything will be mounted onto one or more power boards.

The control electronics will usually be split between the power pcbs and the control board. A special integrated circuit on a power board will generate the on and off signals for the gate circuits. This will be controlled from the main processor on a separate control board; communication between the two will use a fast serial link. This makes isolation of the control board easier. It is important to remember that isolation here (using optocouplers) is necessary as the power pcb is connected to the DC link, and hence the mains.

FPR500A Product specifications