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Topic Title: DC motor circuit protection
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Created On: 25 April 2014 07:57 PM
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 25 April 2014 07:57 PM
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mossep

Posts: 344
Joined: 05 December 2010

I have been asked by a friend to try and come up with something to prevent a DC motor burning out when it gets stuck.

Its 24v DC and 1000w, they have something at the moment which has automotive blade type fuses that should blow when the current drawn becomes too high, these work sometimes, by its just as likely that they do not blow in time and the motor packs up.

Any ideas on what could be used? Im not that up on low voltage DC motor protection.

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 25 April 2014 08:13 PM
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OMS

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A standard thermal bimetallic overload module would do it - get one with a 24V coil and ideally rated for DC - although one for a 400V AC load should handle a 24V DC load

Regards

OMS

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Failure is always an option
 25 April 2014 08:23 PM
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GeoffBlackwell

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Try faster fuses, or a fast circuit breaker, or a thermal overload device .

Alternatively, if you can supply it via a series regulated power supply, you could use some form of electronic sensor to detect the overload current and activate a 'fold back' control.

A simple system would be a series control transistor (or more likely an array of transistors) in series with a low value high current resistor. A sensing transistor is connected across this resistor between base and emitter. When the volt drop across the low value resistor reaches around 0.6 volts it switches the sensing transistor on and its collector draws the drive current away from the series control transistor, thus shutting down the output to the motor .

Regards

Geoff Blackwell
 25 April 2014 08:28 PM
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Dave69

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just use a thermal oveload relay, they heat up by the current passing thorugh them and don't give a dam what voltage it is. The overload relay can be used to control a DC rated contactor. Fuses only really give protection against short circuit
 25 April 2014 08:30 PM
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mossep

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Would the inrush current of the motor not trip the thermal overload relay?

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 25 April 2014 08:32 PM
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GeoffBlackwell

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Not if an oil filled dash pot damper is used .

Regards

Geoff Blackwell
 25 April 2014 08:34 PM
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mossep

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Originally posted by: GeoffBlackwell

Try faster fuses, or a fast circuit breaker, or a thermal overload device .



Alternatively, if you can supply it via a series regulated power supply, you could use some form of electronic sensor to detect the overload current and activate a 'fold back' control.



A simple system would be a series control transistor (or more likely an array of transistors) in series with a low value high current resistor. A sensing transistor is connected across this resistor between base and emitter. When the volt drop across the low value resistor reaches around 0.6 volts it switches the sensing transistor on and its collector draws the drive current away from the series control transistor, thus shutting down the output to the motor .



Regards



Geoff Blackwell


That sounds very impressive, can it be purchased somewhere is is it something you have just thought of?

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 25 April 2014 08:41 PM
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GeoffBlackwell

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It can be made, or you can purchase general purpose regulators. However, you need caution here as matching a device to a high current motor load is not a trivial exercise. You really need a power electronics engineer to specify a working system.

Regards

Geoff Blackwell
 25 April 2014 09:04 PM
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Dave69

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Originally posted by: mossep

Would the inrush current of the motor not trip the thermal overload relay?


It depends on how long the motor takes to get upto speed. overloads are just what they say, they work on over current and will take the inrush current for a while where as fuses have to be rated high enough to take the start up current and therefore offer NO overload protection
 25 April 2014 09:17 PM
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Dave69

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or you could just buy a DC current monitoring relay off the shelf, I'm sure Notronics would do one although I don't know if they do a control voltage of 24vDC, most are 48 V
 26 April 2014 06:28 AM
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mossep

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Would either of you guys be happy to take a more detailed look at what I need to do via PM or email and suggest something? I can email some pictures of the existing setup so you can see how it could be altered. I don't wish to discuss it in to much detail on an open forum.

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 26 April 2014 06:39 AM
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GeoffBlackwell

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That is getting into detailed design - not the best thing to attempt over the web.

A lot depends on your application and its value. You could get an electronics engineer to produce a one off design but this could be expensive. Don't under estimate the problem - controlling what is a relatively high current motor that may be subject to stalling - can produce some pretty nasty back emfs that could destroy an unprotected control circuit.

If this is a relatively low value project you might, as Dave69 suggests, find an 'off the shelf' device that might work for you.

Fuses or circuit breakers would be the lowest cost option - after that you need some form of over current control - such as I outlined above.

Regards

Geoff Blackwell
 26 April 2014 08:56 AM
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John Peckham

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Probably not what I should say having an electrical bias but why not connect a clutch on the motor output.

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 26 April 2014 10:39 AM
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broadgage

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I would agree that a standard thermal overload relay is the answer.
As already posted, these detect the heating effect of the current and do not "know" if this current be AC or DC or what voltage the cicuit operates at.

The load current will be in the region of 42 amps, the exact figure depending on the actual supply voltage and tolerances.

Unless the inrush current or run up time are unusualy great, a thermal overload relay wont trip on normal starting currents.

Use the overload relay to control a standard contactor of suitable current capacity and equiped with a 24 volt DC coil.
Remote control or push button control is easily achieved with auxilary contacts, just as for AC.

Use a fuse for short circuit protection in addition, in the absence of more detailed information I would use a 100 amp BS88 or similar fuse.
I am not too keen on vehicle fuses for heavy current circuits at 24 volts DC because they have only a very limited breaking capacity of typicly 1KA which may not be sufficient if close to a large capacity low impedance battery.

Make certain that the overload relay IS IN FACT THE THERMAL sort and not some form of electronic module intended to replace a thermal overload module.
 26 April 2014 03:14 PM
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AJJewsbury

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Is there any scope for a temperature sensor on an appropriate part of the motor itself? - so you can shut it down based on the motor's actual temperature, rather than a (calibrated) guess based on the magnitude (and duration?) of the current it's drawing?
- Andy.
 27 April 2014 07:21 AM
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mossep

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Thanks everyone, some very useful information to look into.

Regarding the temperature sensor on the motor, would this act in a similar way to switch operating a contactor when a certain temperature is reached, or does it operate via another piece of equipment?

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