Variable Frequency Drive Types

In order to maintain proper power factor and reduce excessive heating of the motor, the name plate volts per hertz ratio must be maintained. This is the main function of the variable frequency drive (VFD). The four main components that make up AC variable frequency drives (VFD's) are the Converter, Inverter, the DC circuit which links the two, and a control unit, shown in Figure 2. The converter contains a rectifier and other circuitry which converts the fixed frequency

Figure 1. Typical power consumption of various fan control systems. (Source: Moses et. al., 1989)

AC to DC. The inverter converts the DC to an adjustable frequency, adjustable voltage AC (both must be adjustable to maintain a constant volts to hertz ratio). The DC circuit filters the DC and conducts the DC to the inverter. The control unit controls the output voltage and frequency based upon feedback from the process (e.g. pressure sensor). The three main types of inverter designs are voltage source inverters, current source inverters, and pulse width modulation inverters. Each will be briefly discussed in the next section.

Inverter Types

The voltage source inverters (VSI) use a silicon controlled rectifier (SCR) to rebuild a pseudosine wave form for delivery to the motor. This is accomplished with a six-step voltage inverter with a voltage source converter and a variable voltage DC bus. As with any SCR system, troublesome harmonics are reflected to the power source. Also the six-step wave form sends current in pulses which can cause the motor to cog at low frequencies, which can damage keyways, couplings, pump impellers, etc. [Phipps, 1994].

The current source inverters (CSI) also use an SCR input from the power source but control the current to the motor rather than the voltage. This is accomplished with a six-step current inverter with a voltage source converter and a variable voltage DC bus. The CSI systems have the same problems with cogging and harmonics as the VSI systems. Many manufacturers only offer VSI or CSI VFD's for larger horsepower sizes (over 300 HP).

The pulse width modulation (PWM) VFD's have become the state of the art concept in the past several years, starting with the smaller hp sizes, and available up to 1500 hp from at least one manufacturer [Phipps, 1994]. PWM uses a simple diode bridge rectifier for power input to a constant voltage DC bus. The PWM inverter develops the output voltage by producing pulses of varying widths which combine to synthesize the desired wave form. The diode bridge significantly reduces the harmonics from the power source. The PWM system produces a current wave form that more closely matches the power line wave form, which reduces adverse heating. The PWM drives also have the advantage of virtually constant power factor at all speeds. Depending on size, it is possible to have power factors over 95% [Phipps, 1994]. Another advantage of the PWM VFD's is that sufficient current frequency (~200+ Hz) is available to operate multiple motors on a single drive, which would be advantageous for the printing press example discussed earlier. The next

Figure 2. Typical VFD system. (Source: Lobodovsky, 1996)

section will discuss the types of loads that require adjustable speeds that may be controlled by variable frequency drives.

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