I have an existing system where a motor is driven by an ECU. In the current system the motor used is a 48 V brushed DC. But the ECU has the capability of running a 3phase brushless motor. The ECU itself runs on 12V supply from the vehicle, with a 48V supply feeding in to the ECU just for the motor circuit.

I'm introducing a protection circuit to protect the device being driven. I need to be able to ensure the motor current does not exceed a predefined limit. Actually there are to be two different limits, one for high current mode, and one for low current mode. The switching between these has to be done using a purely HW circuit (No embedded SW or micro allowed). This is a secondary circuit in case the control Micro/application goes wrong. If the current draw exceeds whichever limit is in place, then the circuit needs to disconnect the 48V supply.

The anticipated current limits are Low = 2A, High = 20A.

The thing I need advice on is this;
I need to measure the current draw of the motor circuit, accurately and regardless of the motor type and drive direction.
The measurement needs to be accurate enough to ensure it trips if the current exceeds the threshold. But does not false trip. Filtering of the measured current is OK over a short period (about 50ms).

What measurement devices would you suggest for this application? And what would the circuit look like.

At this stage I'm not particularly looking for the method of detecting over current and disconnecting. More the method of measuring the current draw itself.

But if you do have suggestions for how to detect the over current condition and a suitable device for disconnecting the Motor supply, then I'm happy to hear those.

 

The most simple solution is a resistor in series with the motor. The resistor would have to be low enough not to effect the motor operation, but large enough to detect the 2A current. A 0.05 ohm resistor would develop 0.1V for 2 A and 1V for 20A. These values would be reasonable to detect without too much circuitry.
As to switching the motor supply, I would favour a relay. Otherwise a MOSFET would do the job just as well.

 

You can get simple current monitor boards, cheap ($2.00) that uses the ACS712 IC.