Variable frequency drives (VFDs) using pulse-width modulation (PWM) techniques are becoming the standard for controlling ac motors -and they bring complications to power cable selection. "Unless VFD system power cables are chosen carefully, high-frequency, high-current PWM wave forms can cause a number of problems that affect both motor and controller reliability," says David Cooper, applications engineer. Here are some of the issues associated with short rise-time power waveforms in VFD systems:
In this article, we'll look at common-mode currents and ground-plane noise in VFD systems, and the effect of power cable design on those problems.
At 60Hz power line frequencies, the capacitive couplings between motor stator windings, stator frame and the motor rotor pass little current. To the high-frequency components of a PWM waveform though, these couplings look like low-impedance paths to ground. These capacitive paths route high-frequency common-mode currents (CMCs) through the bearings of the motor and sometimes through the bearings of equipment connected to the motor shaft. These currents can damage motor bearings in a few months.
"You can reduce damaging CMCs flowing through motor bearings by providing a short, low-impedance path back to the inverter source," says Cooper.
This path must have lower impedance at high frequencies than the copper grounding conductors and steel motor bearings. Aluminum gives the desired low impedance at high frequencies.
Continuous corrugated aluminum armor on the power cable is the preferred ground medium for high-frequency CMCs. Continuous armor allows full 360° connections at the inverter enclosure and motor. That's important for high-frequency impedance matching. Aluminum conduit will do the job only if all joints and connections are carefully bonded. Aluminum interlocked armor (AIA) develops oxidation between the spiral layers, which increases its impedance over time.
A second problem connected with fast-switching VFD systems comes from excessive currents in the protective earth ground (PE) bus of the inverter. These currents can cause control problems, and can even damage the drive control system.
"Typical power cable designs with a single ground wire contribute to this problem," says Cooper.
When a single grounding conductor spirals among the power cable phase conductors, unbalanced magnetic fields from the phase conductors induce noise currents in the ground wire. High-frequency common mode currents as high as 15A have been measured in VFD systems. These noise currents create noise and tuning problems in the VFD inverter.
A symmetrical cable construction helps achieve complete magnetic field cancellation. Three ground conductors spaced symmetrically with the three phase conductors develop much lower levels of common-mode noise currents in the inverter's PE bus.
"Southwire's ARMOR-X power cables with continuous aluminum armor are available with construction options that reduce both motor bearing currents and ground-plane noise in VFD inverters," says Cooper. "If you're planning a VFD installation, find out about ARMOR-X."
Watch for a following Power Cable Update story that will discuss cross-talk and voltage standing waves in VFD motor control systems.