This lesson covers the basic parts and operation of an Adjustable Frequency Drive (AFD). These parts are divided into 4 sections: Rectifier, Intermediate circuit (DC Link), Inverter and Control & Regulation.
AFD in a Larger System
This section covers the parts and operation of the Adjustable Frequency Drive (AFD). It is important to keep in mind that the Driveis just one part of a system. In the diagram above, notice the disconnect switch, fuses, bypass switch, thermal overloads, PLC etc. all play an important part in making an application work correctly. Inside the AFD there are 4 major sections: rectifier, intermediate circuit (DC Link), inverter and control/regulation. This fourth section, control and regulation, interfaces with the other 3 sections.In very general terms the operation of the drive is as follows. Power first goes into the rectifier, where the 3-phase AC is converted into a rippling DC voltage. The intermediate circuits then smoothes and holds the DC Voltage at a constant level or energy source for the inverter. The last section, the inverter, uses the DC voltage to pulse the motor with varying levels of voltage and currentdepending upon the control circuit. The pattern of the pulses going to the motor makes it appear similar to AC sinusoidal waveform voltage. Each one of these sections is reviewed in some detail in the pages that follow.
To understand the parts of an AFD better, an example of a 400kW (500Hp) drive is used.
In the picture above notice the fuses and disconnect switch.
Aseach part is explained pictures of these parts on a 400kW (500Hp) drive are displayed. This large drive is used in this lesson for the size of the parts are easy to identify. One of the options for these large drives 225 – 400kW (300 – 500Hp) is to have fuses and a disconnect switch mounted inside the drive. With smaller size drives fuses and a disconnect are separate but are still part of theoverall system as described on the previous page.
2) Rectifier Section
Its function is to change 3-phase AC into DC.
The 3-phase AC voltage goes into the rectifier section of the AFD. In the rectifier section there is a group of gated diodes (silicon rectifiers or SCRs). In the vast majority of AFDs, these diodes are in a group of 6 as diagramed above. One AFD manufacturerhas stressed that there should be more sets of diodes, 12, 18, even 24. The reasons for and against this are covered in another lesson. Diodes (D1 through D6) allow current to flow only in one direction when enabled by the gate signal. In this diagram, the AC power on L1 goes into Diodes D1 and D2. Because of the position of these diodes, current flow can only go up. The D1 diode conducts whenthe AC is positive and D2 conducts when the AC goes negative. This drives the top line (+) more positive and the bottom line (-) more negative. Diodes D3 and D4 convert L2 power to DC and Diodes D5 and D6 convert L3. A volt ohmmeter or VOM can be used to measure this DC voltage. The positive or red lead is placed on the plus (+) and the negative or black lead is placed on the minus (-). In this typeof circuit, the DC voltage is 1.35 times the AC line voltage. If 240 Vac is coming in, 324 Vdc is generated. If 380 Vac is coming in, 513 Vdc is generated. If 460 Vac is the line voltage, 621 Vdc is generated. If 575 Vac is the line voltage, 776 Vdc is generated. Because of line (power coming in) and load (power to the motor) changes, the DC Voltage level is constantly moving above and above thisexpected value.
2) Rectifier Section
SCR Heatsinks Incoming Power
The Rectifier section contains terminals for incoming power, silicon rectifiers (SCR) and heatsinks
In the picture shown above, the rectifier part is indicated. Six SCRs are used to change the incoming power from AC to DC. This rectification can generate a considerable amount of heat, so the SCRs are mounted onto a...