A new approach to system protection for industrial motor drives

You’re an industrial drive systems designer tasked with figuring out an approach to power protection for your system’s motor. Whatever solution you decide on must account for over-voltage, under-voltage and over-current conditions in the event of abnormalities in your system. If you cannot properly design for these conditions, the system’s motor will be damaged when things like faults occur due to over-current, or if the motor overheats from over/under-voltage. Over time, this will lead to potential system-wide failure, meaning down time and lost revenue.

In this case, you might decide to add an external power-protection circuit to monitor over-voltage or over-current conditions, and send a notification to cut power to the motor if needed. This expands the system’s bill of materials (BOM), adding cost and taking up space.

 The C2000™ Delfino™ F28377x and Piccolo™ F28004x MCU series provide a solution to these power design problems in the form of up to eight on-chip comparator subsystems (CMPSS). Each CMPSS contains two analog comparators, comparator H (high) and comparator L (low), with 12-bit integrated digital-to-analog converters (DACs). Using each comparator to provide a distinct voltage or current threshold, the CMPSS output can be used to trigger a trip zone, which forces the pulse-width modulation (PWM) to one of four possible states: high, low, high-impedance or no action. The on-chip comparators are asynchronous: regardless of the system’s state, when the inputs to the comparators change, they can react immediately to events such as a trip condition. This means that the whole process of an abnormal event occurring, and the PWM output changing takes only about 55ns. Figure 1 below shows the total amount of time between an abnormal event and a trip zone being triggered.

Figure 1: Time between a detected abnormal event by the CMPSS to a trip being triggered

Depending on what condition occurs, a range of actions are possible through the trip-zone submodule, the location to where trip signals are sent. In the event of a short circuit or over-current condition, the trip-zone submodule allows for a one-shot trip (OSHT). Cycle-by-cycle (CBC) tripping is also possible to limit the current or voltage when it goes beyond certain thresholds.

Finally, the F2837x and F28004x enable design flexibility since the trip signals can be used in more ways beyond changing the PWM state. These devices utilize a series of crossbars (X-BARs), which are essentially multiplexers, to enable the connection of many signals to a single location. This provides extremely flexible signal routing. The enhanced pulse width modulation (ePWM) X-BAR allows the CMPSS output to be directly sent to the trip zone submodule, which enables low-latency tripping. However, the usefulness of the X-BAR system is further enhanced through the output X-BAR, which enables the external routing of trip signals to a general-purpose input/output (GPIO) pin. This is useful when other parts of the system need notification about abnormal events. One such use case in the industrial drives space would be routing trip signals to a safe-torque-off (STO) circuit, which when activated prevents further motor-shaft rotation.

Figure 2: An integrated approach to protection with the CMPSS and X-BARs

In this post, I showed how to use the comparator subsystems in conjunction with X-BARs to trigger trip zones for protection purposes. This is simply one application of the comparator subsystems and X-BARs, however. The flexibility of the CMPSS and X-BARs on C2000™ devices, allow them to be used for a variety of different use cases.

No longer must you scramble to create additional subsystem blocks for cases of faults or other abnormal events. With the on-chip comparator subsystem and X-BARs in the F2837x and F28004x system-wide protection can be implemented easily and on-chip.

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