In certain applications, alternating-current (AC) variable speed drives (VSDs) can offer several advantages over traditional methods of process control, including energy savings, reliability and process improvement. Petrochemical drive applications, however, often require the motor to be located long distances from the drive. In some cases, this distance can lead to premature motor failure and negate these advantages.
Conventional 480-volt (V) pulse width modulated (PWM) AC drives switch 650V direct-current (DC) power to make a variable voltage, variable frequency AC output. The resulting output is a two-level waveform.
Figure 1 (top). Conventional two-level PWM output. Figure 2 (center). Voltage overshoot due to long cables. Figure 3 (bottom). NPC three-level control PWM output (Courtesy of Yaskawa America Inc.)Two-level PWM is acceptable when output lead length is small. But when distance begins to increase, peak output voltage will also increase because of voltage overshoot. This overshoot is caused by capacitive coupling in the motor cables and reflective waves. Peak voltages up to three times the original DC switching voltage have been observed in some applications. If this peak output voltage exceeds the motor's insulation withstand capability, motor winding insulation damage will result.
Over the years, many solutions to this problem have been created and used. While the most common solution is to specify inverter-duty motors, use of this equipment does not guarantee a problem-free application. National Electrical Manufacturers Association (NEMA) MG1 Part 31 specifies that "definite purpose inverter-fed" motors should be capable of withstanding 3.1 times the rated voltage.
This means that 460V motors must be able to withstand 1,426V. As previously stated, conventional PWM drives can deliver two to three times the DC switching voltage, or up to 1,950V peak in long-cable applications.
Reactors, long lead filters or sine wave filters can be added to the drive output. When used correctly, these devices can reduce the peak output voltage to within the motor's insulation capability. However, they often occupy valuable real estate, decrease reliability and increase power losses.
An alternative 480V drive topology that addresses these challenges is the neutral-point clamp (NPC) three-level control PWM drive. This drive topology switches 325V DC instead of 650V DC like in conventional PWM drives, resulting in a more sinusoidal waveform with lower peak voltages.
The three-level drive never exceeds the insulation limits of NEMA MG1 Part 31 motors, regardless of output lead distance.
In petrochemical plants, drives are generally located inside electrical equipment rooms, and motors are located in the process area. For applications where the drive and motor are separated by distances of up to 2,000 feet, some drives can eliminate the need for additional equipment such as output filters and keep the total voltage level at the motor terminals well within the range of standard inverter-duty motor insulation. The elimination of output filters reduces the total system space significantly and reduces installation costs, especially for applications where installation space is limited.
Petrochemical plants use AC drives for energy savings and process control improvements. When applied correctly, AC drives are more reliable than the process control devices they replace. However, typical installations require the drives to be located in electrical rooms separate from the driven motor. Often the distance between the drives and motors is 800 feet or more. Three-level drives can reliably power motors at these distances with no degradation of motor insulation.