Industrial automation and energy efficiency efforts are increasing the use of variable frequency drives (VFDs) in motor systems such as conveyors, pumps, and industrial robots. Cable selection for this type of motor is far more complicated than determining wire gauge based on load current and insulation level based on operating voltage.
Modern VFD motor systems use switching mode power electronics to produce a pulse width modulation (PWM) drive signal with extremely fast edges. These fast transients increase the signal reflections caused by impedance mismatches between the cable and the motor terminals, creating standing waves that increase the voltage stress across the cable. In addition, the line-to-line and line-to-ground capacitances of the cable affect driver performance and increase the charging current. Since the VFD PWM signal contains a large number of high frequency harmonics, the motor cables must be effectively shielded to reduce electromagnetic interference (EMI).
This paper briefly describes the VFD and discusses the challenges faced by designers in selecting VFD motor cables to ensure the functionality, reliability, and safety required for proper operation. The VFD cables of LAPP are then presented and demonstrate how they can be used to provide stable power and control signals while reducing EMI radiation and susceptibility to harsh environments.
VFD Introduction
Industrial automation requires that the motor operates reliably and efficiently, and can operate in any direction within the full speed range. The VFD, sometimes referred to as a governor driver, is a motor controller that regulates the speed and torque of an AC induction motor (ACIM) by varying the power input frequency, voltage, and duty cycle of the motor. The working principle of VFD is to use AC rectification input and DC output to generate PWM signal to drive the motor. By adjusting the frequency, width and amplitude of these pulse signals, the motor speed and output torque can be controlled in various motor drive systems.
To realize its function, the VFD consists of three main components (Fig. 1): a rectifier that converts AC to DC, an inverter that converts DC to PWM flow, and a VFD controller.
VFD rectifies AC input and generates PWM signal using DC (click on amplification)
Figure 1: VFD rectifies AC input and uses DC to generate PWM signal to control motor speed and output torque. Picture source: Art Pini)
The controller monitors the motor operation through a variety of sensors to control critical motor parameters. These sensors include rotary transformer/encoder feedback, tachometer, and temperature and vibration sensors.
This rectifier uses conventional diodes followed by filters. The inverter adopts power field effect transistors (FET) or insulated gate bipolar transistors (IGBT). These transistors are driven by an isolated high voltage gate driver, which is centrally controlled by a VFD controller.
The VFD differs from conventional three-phase AC operation in that the signal of the drive motor is not a sine wave, but a PWM pulse (Fig. 2).
PWM pulse of VFD generates sinusoidal current response
Figure 2: PWM pulse of VFD generates sinusoidal current response in motor winding. Picture source: LAPP)
PWM signal frequency is generally 2 kHz to 20 kHz. The inverter alternately connects the motor to the positive and negative poles of the AC bus and to the DC common voltage. DC bus voltage is close to peak AC bus voltage. The VFD PWM waveform used produces a sinusoidal current response to control the motor speed and torque.
Due to the characteristics of the PWM wave, special cables are required to connect the VFD to the motor. This waveform is a rectangular pulse with wide spectrum and rich in harmonic. The VFD cable is specially designed to reduce the radiation of these high frequency signals. In addition, in order to minimize the switching loss of inverter switching devices and maximize the system efficiency, the pulse hopping speed needs to be set as fast as possible. This results in a very high voltage change rate (dV/dt) at the pulse edge. These features, combined with fast edges and high frequency spectral components, result in high levels of electromagnetic interference. Fast edges also produce transmission line reflections where cable impedance changes. This reflection creates a standing wave in the cable, which increases the voltage on the cable and requires the VFD cable to have a higher voltage rating.
Cable capacitance between metallic conductors is another concern. When the inverter switch connects the cable to the DC bus, a current surge is generated which charges the capacitance of the cable. This increases the instantaneous current level and may damage the cable. This common mode current may flow between phases or from a phase to earth. This current may also enter the ground loop through the motor frame and pass through the motor bearings. The current flowing through the bearing causes pitting on the bearing surface, which shortens the life of the motor. These problems typically occur in VFD systems with high voltage, high motor rating (HP), and long cable runs.
As with all wires and cables, current flows through the DC resistance of the cable causing power loss. In addition, due to the wide spectral bandwidth of PWM signals, the cable resistance may increase due to skin effect. These resistance effects vary with cable length.