Proper Wiring Practices for Variable Frequency Drives

Variable Frequency Drives (VFDs) control the frequency and voltage of the power delivered to a 3-phase electric motor. They increase energy efficiency and reduce stress on the motor by providing a soft start, as well as improve process control and maintain consistent levels of torque to match the needs of their respective loads. VFDs are used in a variety of applications, which include motion control systems, ventilation, machining and waste water processing. While efficient and effective, VFDs require specific wiring considerations to maximize efficiency and prevent Electro Magnetic Interference with other machinery.

Electromagnetic Interference

• Electromagnetic Interference (EMI) is a common concern with VFDs that utilize Insulated-Gate Bipolar Transistors to provide the Pulse Width Modulated output necessary to control the motor. Because VFDs rapidly switch the fixed DC bus voltage to achieve a variable frequency, the variable pulse created can cause several problems for both the VFD-controlled motor and nearby electronics. Unwanted fluctuations in motor speed, communication errors and CPU exception faults are all possible. Nearby equipment may also undergo signal distortion within systems that control pressure, flow and temperature signal, as well as control system faults and interference in transmission on television and radio frequencies, telephone lines and data networks.

Cable Selection

• Much of the EMI protection in a VFD-operated system relies on proper cable selection. For signal cables, standard twisted-pair cables will provide some protection. However, shielded twisted-pair cables that are specifically designed to reduce interference are preferred. Use a clamp around exposed cable shield to terminate it at one end, but do not terminate the shield through the pigtail connection as this will increase high frequency impedance and nullify the effects of the shielding.

  Motor cables may employ single conductor wires routed separately from the signal and AC wiring, but shielded power cable is a more effective guard against EMI problems. The motor cable's shielding will direct noise current back to the VFD and prevent it from taking unpredictable paths, which may intersect and interfere with sensitive machinery. Motor cable shielding is terminated at both ends. Three phase conductors plus ground in a conduit will provide some protection against EMI if shielded motor cable is unavailable, but the conduit's points of contact with other equipment may still conduct some interference.

  Use twisted-pair cables and route them away from power conductors for serial communications. Some serial communication systems call for shielded cables, but these generally reduce the length of cable before it begins to diminish data rates. In addition to twisted-pair and shielded twisted-pair cables, some proprietary serial communications systems employ their own cable designs.

Grounding

• Grounding to the equipment frame is important to minimize the high frequency impedance and prevent high frequency currents from disbursing throughout the power network. A specific ground connector between the VFD and the driven motor is optimal, as is a direct point of contact between metal surfaces. If two painted surfaces are connected, the paint will form a barrier against an effective ground connection.

Cable Routing

• In general, cables that perform different functions should not be routed parallel to each other. Motor wiring, signal wiring and power wiring will all tend to interfere with each other, even through shielding, if given significant exposure and proximity. In systems where parallel routing cannot be avoided, maintain as much distance between cables as possible and separate them with a grounded conductive partition.