Motor Controller Current Limiting

[azhillman]

I was wondering what others have done to control the current of there motor controller.
There are many threads on motor shields that have built in current limiting, but I am talking about when you use the pwm to drive a bank of mosfets.
In my own project I'm going to use a Lem Hal 300-s and monitor the current and when it is exceeded it will reduce the duty until it is under the set limit.
It will also turn off the enable of the mosfet driver and set the duty to 0 if the max limit is reached, until it is reset.
I plan on having hardware do the max current lock out, as a back up to the software limiting.

[semicolo]

I may be mistaken but this sensor seems to handle 300A, are you using a 15KW motor?

Else you probably won't get enough accuracy unless using a high quality ADC.

Current regulators on shields usually have a shunt resistor and just measure the voltage accross this resistor.

[azhillman]

The sensor is rated for a max of 900 amps, but has a target reading of 300 amps. 
And yes, the brushed permanent magnet motor I'm controlling is rated for 8 kw cont and 15 kw peak at 48 volts.

I realize most people are just using a shield because they only need to provide a small current in contrast to what I need.
The Lem Hal 300-s has an output of +/- 4 volts at 300 amps, and the controller I'm building will have a max of 300 amps, so at 375 amps it is 5 volts.

[semicolo]

Man that's rather heavy stuff, no wonder you need that sensor.

You can't really compare that to the motors those shields are supposed to drive.

[MarkT]

At those power levels you'll want a hardware over-current fault detection circuit, it only takes a fraction
of a millisecond to turn MOSFETs to vapour at kilowatt dissipations...  You can output the current
sensor output to a comparator for fault detection as well as ADC for control and feedback purposes.

You'll need good layout practices to reduce the amount of electromagnetic interference from your
high-current paths.

375A H-bridge is no mean feat BTW....

[azhillman]

That is what I planned on doing, having a hardware max current shut of the mosfet driver and stop the pwm, and use the ADC to manage the current under cirtain conditions to help the behavior of the motor controller.  This means that when with in limits the software using ADC manages the current and if that fails or the current exceeds the limit, it shuts the controller down.  I will probably need to seek advice on a forum related to the purpose of the controller, as most peoples projects do not require such a large amount of power.

[MarkT]

Have you seen the OSMC project? http://tech.groups.yahoo.com/group/osmc/

[azhillman]

No, do you need to be a member to view any of it?

[sonnyyu]

http://www.robotpower.com/osmc_info/

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