Welcome to the May installment of Drive Time. This column features topics related to drives, motors, and processes of interest to you. You can contribute to this column by asking questions, sending comments, and participating in the discussion. To talk to us, use the "Letters" link in the left column. This month's question is about some definitions for types of power converters and the terminology used to describe them.
What is a 6-pulse converter? What makes it 6-pulse as opposed to 12-pulse or 18-pulse?
One way to think about the "pulse number" of a rectifier circuit is to look at the pulses of the power frequency voltage that show up in the output voltage of the rectifier. Therefore, the pulse number of the rectifier is related to the circuit arrangement of the rectifier devices and the connections to the power system with transformers, if applicable.
Rectifiers can have thyristors or diodes for rectifying elements. Thyristors allow controlled voltage output, but need gating circuits and controls to work. Diodes are more simple and rugged, and require no control, but the output voltage depends on the input voltage. Ac drives that do not require regeneration use diodes as source converters because they are low cost and the fixed output voltage is perfect for the dc link of a Pulse Width Modulated (PWM) inverter.
The history of rectifiers is filled with interesting circuit types using 3-pulse midpoint connections, interphase transformers, and 6-phase sources. However, the rectifier practice has been refined to the point where the 3-phase, 6-pulse bridge rectifier is the most commonly used connection. It is also the building block for other higher order connections. A simple diagram of the 6-pulse bridge is shown in Figure 1.
Figure 1. Basic 3-phase 6-pulse diode rectifier circuit
With a 3-phase input voltage, the output voltage of the 6-pulse bridge rectifier has 6 pulses of power frequency voltage in one period of the power system frequency. Or, the output ripple voltage of the 3-phase rectifier is 6 x the power frequency. If the input frequency is 60 Hz, the dc ripple frequency is 6 x 60 = 360 Hz. Higher order harmonics also exist, but are outside the scope of this discussion. The ripple effect can be seen in Figure 2, where a 3-phase voltage waveform is shown with the negative voltage pulses rectified (above the x-axis). The envelope of the unfiltered dc ripple voltage, shown in black, is emphasized to show how the peaks of the voltage waveform are picked off by the diodes. The six pulses of the peaks form the output voltage of the 6-pulse rectifier.
Figure 2. Three Rectified Sine Waves
Higher Pulse-number Converters
Higher order pulse-number converters can be formed using the 6-pulse bridge as a building block. The product of 6 times the number of 6-pulse bridges gives the overall pulse number of the converter system. For example, 6 times 2 bridges gives 12-pulse; 6 times 3 bridges gives 18-pulse, etc. Of course, more than just additional converters are needed; the proper phase shift must be applied to the voltages at the input to the converters to obtain true 12 or 18-pulse output. Based on the definition given earlier, the 12-pulse rectifier has 12 pulses in the output voltage for one period of the power system frequency. An 18-pulse rectifier has 18 pulses in the output voltage for one period. What happens in a 12-pulse rectifier is a second set of 6-pulse waves is added to the output, with a 30 electrical degree offset to the first set that fills in the valleys between the 6-pulse peaks.
So, what's the reason for using higher pulse numbers? Most often, the motivation is reduction of harmonic currents the rectifier injects into the ac power system. For example, a 12-pulse rectifier may have about 13% current total harmonic distortion (THD), while a 6-pulse converter can have 35% current THD - a big difference for a large rectifier. Another benefit is less ripple in the output voltage, but because this is usually internal to the ac drive, it is not as visible as the harmonic currents at the input.
As usual, your feedback is welcome. Use the link at the left of this column to respond or ask questions.
Barry Dick is a senior application engineer with TM GE Automation Systems in Salem, Virginia.
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