3 Phase Motor - 3 Individual Circuit Breakers - Detect if 1 Breaker Tripped And Shut Down Motor

[hevans1944]

it's probably too complicated to be practical.

Yep. That seems to be the gist of this entire thread. Why do something simple and safe, when with a lot of effort you can complicate the hell out of it?

In addition to my "simple" video solution, several other "solutions" also come to mind, although probably none of them are practical, acceptable to the OP, or inexpensive to implement.

For example, if the motor is connected to the three circuit breakers, and the three-phase windings are intact and functional, it is a trivial matter to measure the phase currents with three Alegro Hall-effect current sensors. These interface nicely to Microchip PIC micro-controllers. If a circuit breaker is open, there will be no current in that phase. The PIC micros are so cheap and so small that you can dedicate one to each Allegro sensor and use a simple go/no-go threshold detection scheme to decide whether a phase circuit breaker is open or closed.

 

[Frankchie]

Mahonroy,
Why is it necessary to kill the other two phases if one phase dies?
Seems to me the other phases might draw more current, but judicious fusing on those phases should prevent damage. Okay, the motor might run slow, but if that is a serious problem maybe a tachometer alarm is a better solution?

 

[Tha fios agaibh]

I'm with Minder.
Use a contactor with an overload relay connected below it.
It's a requirement to have thermal protect on most motors anyway.
When the motor single phases after losing any phase it will trip the ols, which in turn will turn off the contactor/relay.

Monitoring voltage isn't feasible.
It's possible to use a coil on each phase (ct loop) and monitor the current but I'd advise against it.

 

[Tha fios agaibh]

Mahonroy,
Why is it necessary to kill the other two phases if one phase dies?
Seems to me the other phases might draw more current, but judicious fusing on those phases should prevent damage.

No. Motors are different than most devices because they require much more current to start. Often more than 600 percent of full load current. Therefore they have to be big enough to start the motor without blowing.

Fuses and breakers are primarily just for the protection of the branch circuit conductors from a short circuit, whereas overload protection is designed for stalled or overloaded motors.
Two different animals.

 

[hevans1944]

It's possible to use a coil on each phase (ct loop) and monitor the current but I'd advise against it.

Current transformers are not necessary, and they are expensive and bulky too. The Alegro Hall-effect current sensor is inexpensive, small, and responds to AC as well as DC. It is the natural choice for making a go/no-go decision on whether a circuit breaker has tripped.

For this application, AC currents will appear as positive and negative deviations about a baseline output voltage that is one half the DC supply voltage (nominally 5 V) applied to operate the Hall-effect sensor and its signal conditioning electronics inside the Alegro package.

The current to be measured is applied through two pairs of pins on the device that are connected to a low-resistance conductor loop. The loop creates the magnetic field the Hall-effect sensor detects. It is galvanicaly isolated (insulated) from the other pins on the package. Since the power supply is uni-polar, to accommodate AC signals the output will be half the supply voltage for zero current. The output then swings more positive (towards +5 V) or more negative (towards 0 V or power supply common) as the current goes through its AC cycle.

This output swing will then be capacitively coupled to a rectifier and filtered to remove the AC ripple. The resulting filtered DC signal will be proportional to the AC motor current in that phase and can be used as a go/no-go indication that the circuit breaker is open, provided there is current present on at least one of the other two phases. However, if there is no current on any of the three phases, this could indicate either all three circuit breakers are open, or there is no voltage available on any of the three phases.

To prevent these two "false negative" results, the OP should add phase-voltage measurements on the input side of the three circuit breakers. I recommend this, along with some means (a push-button switch?) to allow the motor contactor coil to be energized by one of the PICs when all of the Alegro current sensors are reporting zero current.

 

[bertus]

Hello,

Perhaps the attached PDF might give you an idea on the use of the hall sensors that @hevans1944 mentioned.

Bertus

 

[Frankchie]

I like the hall sensor idea, but I'm not sure I see a need for a Pic, or multiple Pics. Just use the three filtered DC signals, amplified if necessary, to drive an AND gate that controls the Op's 3 pole relay. Use a latching arrangement, maybe a fourth pole on the Op's relay, to make sure momentary phase interruptions are captured. Of course, you need a push button switch to bypass or reset the latching circuit for starting. I don't understand the need for "phase-voltage measurements" unless it's related to some kind of automatic reset. But my understanding is probably not important at this point.

One caution, I designed a a hall effect current measuring circuit a few years back and noticed a significant drift related to ambient temperature. So any design should ensure plenty of headroom for threshold levels. Also, a small delay may be necessary to avoid triggering on sort transients. This a reminder that really the best way to go is a commercial overload device as there are probably many gotchas lurking in a DIY design.. I might design such a circuit or my own personnel use, but I would not do so for somebody else.

 

[TCSC47]

For goodness sake -- keep it simple. Anything mains, go for the tried and tested commercial solution. A 3 phase mechanically linked circuit breaker. If the box is too small fit a bigger one. If the cost is too great (in your opinion), consider the cost of being responsible for someone's electrocution or property fire.

 

[Minder]

It may also contravene N.A. NFPA70/NFPA79, EU and BSI regulation compliance .
Max.