Using sense resistors in motor control
The use of current sense resistors is part of a trend in motor control system design that benefits from adopting new digital isolation technologies. These technologies offer higher reliability levels to designers based on the introduction of the component level standard IEC 60747-17, which specifies the performance, test, and certification requirements for capacitive and magnetically coupled isolators. Digital isolation offers other benefits such as faster loop responses, allowing for integrated overcurrent protection, as well as narrower dead times. This enables smoother output voltages that, in turn, provide better control of torque. This article presents a summary of the differences in standards between traditional optocoupler-based technologies and inductive and capacitive technologies for reinforced isolation. It also provides an overview of a system using digital control of a motor drive that incorporates current sense resistors for sensing winding current. The article will also offer recommendations for selecting the best current sense resistor for this application.
Update on Standards for Isolation as Applied to Motor Drives
Designers of motor drives are most likely aware of the need to comply with international standards for isolation. Isolation is necessary for a number of reasons:
It prevents electrical noise from the ground connection of a high power circuit being induced onto a low power signal line.
It provides electrical safety for end users by preventing dangerous voltages and currents from transferring to a benign, low voltage environment.
The IEC 61010-1 Edition 3 standard specifies that the system-level designer must be aware of the distances between conductors, through air (clearance) and over surfaces (creepage). It also stipulates they must know the separation between conductors and metallic parts in potting, moulding compounds, and thin film insulation. A designer should ensure that the chosen components guarantee a certain level of safety if they are being used on systems compliant to IEC61010-1. According to the standard IEC 60747-17, the reinforced isolation is tested using the industry accepted time dependent dielectric breakdown (TDDB) analysis, which then helps to extrapolate the device’s lifespan and continuous working voltage (VIORM).
While IEC 60747-17 (DIN V VDE V 0884-11) was adopted to specifically define insulation using inductive and capacitive technologies, the well-established IEC 60747-5-5 standard was used to define the insulation using optocoupler technologies. However, IEC 60747-5-5 does not specify the TDDB analysis to determine the continuous working voltage or lifetime. It relies on the partial discharge voltage test to establish the working voltage, but does not define the working lifetime of the device. Hence, inductive and capacitive technologies have a minimum rated lifetime of 37.5 years, while there is no definition for optocoupler-based isolators.
Table 1 summarizes the key differences between optocoupler and non optocoupler-based standards. The conclusion is that non-optocoupler-based standards will gain more acceptance over time as they offer greater security to design engineers and longer operating lifespans.
Table 1: Key Differences Between Optocoupler and Non-optocoupler-Based Isolation
Figure 1: Block diagram of three phase motor drive with digital isolation and sense resistors.
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