Recall that the basic equation for electric power is P = VI, or, power = voltage multiplied by current. This means that for a given power level, voltage and current are inversely proportional. In other words, the higher the supply voltage, the lower the current draw will be. For AC motors that are used in high-power applications, operating at a low voltage causes the motor to draw very high current, resulting in higher energy usage, lower efficiency, and higher cost. As you can see from the power equation, current draw can be reduced by increasing the voltage supplied by the drive.
The full equation for three-phase power is:
P = V * I * PF * 1.732
P = power (watts)
V = voltage (volts)
I = current (amps)
PF = power factor (supplied by the manufacturer)
The power factor is the ratio of working power to apparent power—essentially, how effectively the electricity is being used.
The constant 1.732 is the square root of 3, and is used for 3-phase power to account for the fact that all three phases do not produce the same amount of power at the same time.
AC drives are generally classified as low voltage, medium voltage, and high voltage, although the classifications for each type vary by manufacturer and even by standard. The ANSI C84.1 standard defines low voltage as 240 to 600 VAC, while medium voltage is typically 2300 or 4160 VAC (3300 or 6600 VAC in Europe and most of the rest of the world), but it can be as high as 69,000 VAC. High voltage, according to ANSI C84.1, is 115,000 to 230,000 VAC.
Based on the three-phase power equation above, here’s an example of the difference in current draw between a low voltage and a medium voltage drive:
If a 1000 hp (746 kW) motor operates at 480 V, the current draw will be:
746,000 W = 480 V * I * 0.87 * 1.732
I = 1031 A
The same 1000 hp (746 kW) motor, operating at 4160 V, will have a current draw of:
746,000 W = 4160 V * I * 0.87
I = 206 A
Medium voltage drives can be either current source inverter or voltage source inverter types, although VSI types are more popular due to high reliability and low harmonic distortion. AC drives in this configuration are referred to as medium voltage multi-level voltage source PWM drives.
Recall that inverters convert DC power back to AC power at the required frequency and voltage. The term “multi-level” refers to the output of the inverter, and PWM is pulse-width-modulation – which switches the DC voltages in order to create the AC power. In the multi-level design, the inverter uses multiple DC voltages (rather than just two voltages, as with a typical 2-level inverter) to synthesize the AC waveform, making it nearly sinusoidal in nature. This allows the PWM switching frequency to be reduced and reduces the dV/dt, which reduces harmonics. Most medium voltage AC drives also include a multi-phase transformer on the front end, which works in conjunction with the multi-level inverter to reduce harmonics. And for applications with highly dynamic loads, an active front end may also be used. The active front end reduces harmonics on the line (input) side by monitoring the harmonic level and actively filtering the waveform to provide dampening.
The term dV/dt refers to the change in voltage over time. In PWM applications, it describes the rapid voltage rise at each pulse of the PWM waveform.
Medium voltage AC drives generally benefit applications that require motors from 500 to 20,000 hp, where the difference in current draw between low voltage and medium voltage becomes substantial. Motors in this range are often used to operate large compressors pumps, and fans in power stations, petrochemical plants, water/wastewater treatment facilities, and mines.