Over current protection

[adrianvon]

Hi all,

I am doing a project where i am going to use an AC motor. Now i want to do a circuit which will warn to the user when the motor is stuck. Since the motor use more current when stuck, is there a circuit that i can use when there is over current??

i already did research but i only found circuits for DC motors...

All kind of help is appreciated

Thanks in advance

 

[Reloadron]

You don't mention motor size but anyway the condition you mention is a "locked rotor". Yes, there are current sensors to sense AC current that could be used in this situation. The greater the load on the motor the higher the current. So we could monitor the motor current using an AC current sensor. When the motor current exceeds a preset level the motor can be shut down. A Google of AC Current Sensors will bring up dozens of hits. Thy can be as simple as a current transformer or a more complex hall effect sensor. They can be a turn key packaged solution or you can roll your own and build it.

However, in the case of AC motors the motor starter circuit can also be set with thermal overloads so if the motor current exceeds a set current the motor contactor drops out. This is the easier way to go about protecting the motor.

How you go about protecting the motor is a matter of your choice. How many HP motor and what current?

Ron

 

[adrianvon]

Thanks for your reply Ron.
Im not sure yet about which motor to use. I still have to do same research, but it will not be a powerful motor since i am going to use it to move a shutter. Is this AC current sensor good ?? : http://electro-circuit.com/sensorstranducer/ac-current-sensor-circuit/
Since this is a school project i have to build the sensor, so i cannot buy an off-the-shelf AC current sensor.

By the way, in a separate project i am using milli volts as the Vref of a voltage comparator. In the same box there will be a transformer and the main AC supply (230VAC 50Hz). Can in any way this affect the milli voltage ?? maybe increasing it (distortion) ??

Thx in advance

 

[Reloadron]

OK, the AC current sensor you linked to is an example of one I mentioned. It is a "CT" (Current Transformer). A current is passed through the center which is the primary and the secondary produces a current output proportional to the primary current. These units are sometimes called donuts because they look like donuts.

They can be large enough to measure thousands of amps or small enough to measure a few amps. They are also low in cost. While not quite as accurate as more complex hall effect systems what they lack in accuracy they make up in cost. Attached is an image of a larger 200:5 CT. When a line carrying 200 amps runs through it, it outputs 5 amps. Thus the 200:5 ratio.

This is a link to some smaller versions. As can be seen some are easily board mountable. If you look at the small circuit in your link the CT is capacitive coupled to a resistor to ground. That resistor provides a voltage drop to the operational amplifier input.

When designing and building a circuit like this there are some considerations. For example when we flip a switch and turn on a motor the motor has a large inrush current. While this steep current is only momentary we don't want our circuit responding to it. Just a little potential "gotcha" that can ruin the day.

Once you choose a motor and the normal current and locked rotor current are known then the project can move along. What you want to do is derive a DC voltage from your sensor proportional to motor current. That voltage can then be passed along to a Comparator circuit. If you are not familiar with comparators you can read about them here.

All in all you have a good start.

Ron

 

[adrianvon]

Thanks Ron for your detailed reply. I learn something new with every reply you leave

By the way, can you help me in this please:

In a separate project i am using milli volts as the Vref of a voltage comparator. In the same box there will be a transformer and the main AC supply (230VAC 50Hz). Can in any way this affect the milli voltage ?? maybe increasing it (distortion) ??

or is it better to use an amplifier with both Vref and anf the input to eliminate low voltages (milli volts) ???

 

[4pyros]

but it will not be a powerful motor since i am going to use it to move a shutter.

As in a camera?

 

[Reloadron]

I share the same question as to the "Shutter". I just assumed it was not a camera shutter but sometimes assumptions are a bad thing.

As to this:

In a separate project i am using milli volts as the Vref of a voltage comparator. In the same box there will be a transformer and the main AC supply (230VAC 50Hz). Can in any way this affect the milli voltage ?? maybe increasing it (distortion) ??

Yeah, my bad as I forgot to cover that.

Whenever possible this is the sort of practice that good design practice begs to avoid. However, that is not always possible. When I have to run small signal voltages in the same box as mains AC power my first choice is to amplify the millivolt signals to more usable voltages. The problem here is if the mV signals have noise on them and we make them bigger, we also make the noise bigger. This is especially true with low frequency noise like 50-60 Hz. Low pass filtering can sometimes help.

However, before we go getting complicated the simple option is to use good quality shielded cable grounding the shield. For example Alpha Wire and Cable 2412C two conductor AWG 20 cable. Grounding only at a single point to avoid ground loops. Start simple, see what you have and work from there.

Ron

 

[adrianvon]

As in a camera?

A window shutter

Thanks a lot for your help. I think the best thing to do is to use an amplifier near the temperature sensor (which will supply the mV) to eliminate long wires which will act as an antenna and therefore eliminating noise distortion.

 

[Reloadron]

A window shutter

Thanks a lot for your help. I think the best thing to do is to use an amplifier near the temperature sensor (which will supply the mV) to eliminate long wires which will act as an antenna and therefore eliminating noise distortion.

Strongly agree. Temperature sensor signals do not take kindly to noise. When using temperature sensors I like my signal conditioning as close as possible to the sensor in high noise environments.

Ron

 

[4pyros]

I think the best thing to do is to use an amplifier near the temperature sensor

Even better to convert it to digital at the source.

 

[MrAl]

Hi,

I've used those current transformers for a while now and i know they work really well. Thanks again Ron!

 

[adrianvon]

will an RF choke be good for reducing distortion ??

 

[Reloadron]

It's not really distortion. Distortion is more a matter of harmonics, hum and noise. Well in my opinion anyway. No, I would not be to quick to shove a choke in there. For DC maybe a .01 uF cap but I wouldn't worry about it unless there is a problem.

Hi Ya MrAl & pleased it worked. Yeah, I remember that.

Ron

 

[Reloadron]

Even better to convert it to digital at the source.

I don't know? I like using either a voltage or current loop from the source. Then I worry about A to D in data acquisition systems. Maybe just a me thing.

Ron

 

[adrianvon]

Which one from these is the best to use for my application:

Hall Effect Sensor: https://www.electro-tech-online.com/custompdfs/2011/10/0900766b800b6bc2.pdf
or
Current Transducer: https://www.electro-tech-online.com/custompdfs/2011/10/0900766b80290071.pdf

and what are there equivalent output voltage (for instance 0.5V for 1A) since it is not mentioned in the datasheet?

 

[Reloadron]

Have you come up with a motor with specifications yet?

The Honeywell link is a hall sensor for measuring rotational speed, so that won't work. The LTS 6 sensor is a 6 Amp sensor. The output for 0 amps in the primary is 2.5 volts. My guess as I am not sure is the output will vary between 2.5 volts plus .625 volt and 2.5 volts minus .625 volt. That is to say that +/- 6 amps primary will give 3.125 volts down to 1.875 volts. Remember using a sensor like this you have an AC current going above and below a 0 amp baseline. Think sine wave here. That is why the 0 amp output is 2.5 volts with sensors like this. THis is some guessing and speculation on my part as I generally use more expensive versions that provide a nice either 4 to 20 mA DC loop or analog DC voltage out like 0 to 5 or 0 to 10 volts. I use these type units. For me in a commercial application where time and money come into play I would rather spend about $95 USD for a turn key solution.

The person in these forums who is really, really good at designing operational amplifier circuits for transducers like you are looking at is Eric Gibbs. Hopefully he will see this thread and comment. I just don't want to give you bad information or too much guess work.

Ron

 

[adrianvon]

Hi,

Would two car wiper motors be powerful enough to move the shutter that i have mentioned previously?? the shutter will be approximately 75" by 75" so it will be quite big.

Below please find attached a rough planning of how the shutter will be.

all kind of help is appreciated...

Thanks in advance

EDIT: by the way, the AC motor was eliminated since it can be a hazard for an application like this (which will be outside), and AC motors are difficult to control their speed.

 

[4pyros]

Would two car wiper motors be powerful enough to move the shutter

That would depend on how fast you what it to move and how much gearing you use.
You should not use two motors tied together like that, if its one long shaft, one motor will always out work the other.
You need one long drive shaft and one motor on one end or the other.

 

[adrianvon]

That would depend on how fast you what it to move and how much gearing you use.
You should not use two motors tied together like that, if its one long shaft, one motor will always out work the other.
You need one long drive shaft and one motor on one end or the other.

Thanks for your reply.

Speed is not an important concern for this project and the least gears i use the better.

Will it work with just 2 gears (to reduce speed and increase torque) ??

and if i use one motor i think it will struggle to rotate a 75" long shaft...

 

[MrAl]

Hi,

That's interesting using two motors. If they are the same model it might be ok. I would think that they would be able to turn with about twice as much force unless they are not perfectly matched, then one will provide more force than the other but i dont see that being a problem unless it draws too much current and overheats. Current regulation might be possible too.
I've never actually used two motors like that though but only one motor that could handle the full load.