# Three phase reversal detection and correction

#### vishweshgm

##### Newbie level 6
Hi,

I want to design a system to detect phase reversal (in a 3phase systeM) and correct it. 3 phase line is fed to a 3phase 10HP motor (Input AC Line voltage is rms 110v/50Hz) and hence all 3 phase sequence must be maintained even if someone connects L1,L2,L3 incorrectly. I looked online for ideas but I could find ideas for detection but not correction. I guess, we need a set of contact relays which needs to be controlled using micrcontroller based on detection of phase in each line fed-in. So here is the unit block diagram in my mind:

I have some ideas for phase monitor using micro controller (like using 3 opto couplers to detect zero crossing time and taking the time difference between all 3 to find if the phase is in sequence. I am yet to finalize logic though). But I need your help to see if correction unit can be implemented practically. Kindly suggest ideas/links if anyone already implemented it.

#### FvM

##### Super Moderator
Staff member
You'll interchange two phases. Two contactors required, one for forward and one for reversed connection. Quite similar to a regular reversing motor contactor circuit.
--- Updated ---

vishweshgm

### vishweshgm

Points: 2

#### KlausST

##### Super Moderator
Staff member
Hi,

for phase reversal/correction: just two phases need to be switched. So a simple relay should work.

But why does it need to change automatically. Doesn´t the motor have fixed wiring?

Klaus

#### FvM

##### Super Moderator
Staff member
The dashed line symbolizes a mechanical interlock, not necessary required but provides fail-safe protection.

#### Warpspeed

You can do it with two opto isolators and a single D type flip flop.

Take any phase, call it "A"
When "A" goes through its zero crossing in one particular direction, it can clock a D type flip flop.
At that instant, phases "B" and "C" one will always be positive and the other always negative.
That relationship reveres if the two phases are reversed.
You only need monitor for example phase "B" to see if its on its positive or negative half cycle to know if phase rotation is correct or reversed.
If two phases follow on in correct sequence, the third must be correct.

The state of the flip flop then determines if a relay is operated or released.
The relay contacts just transpose phases "B" and "C" to achieve correct phase rotation.
Simple and cheap to implement.

As this is most likely safety related, a simple mechanical device such as a sprague clutch might be a good idea.
The clutch could either lock the motor shaft if it tries to reverse, tripping the motor overload circuit breaker, or decouple the load from the motor if the motor ever runs in reverse. Either would be a pretty fail safe backup.

Last edited:
vishweshgm

### vishweshgm

Points: 2

#### Easy peasy

@ Klaus, having a proper mechanically interlocked 3 phase contactor system is a lot safer in the field than using relays to switch only 2 phases.

I imagine OP is thinking about shipping product to various places and having mains input wiring done by others - thus to guarantee motor direction right from the get go - phase direction must be established and set.

#### KlausST

##### Super Moderator
Staff member
Hi

Let's imagine it's a simple motor (initially without electronics and relay):
Then the effort is rather big:
* power supply to operate relays and electronics
* 1 or two power relays
* control electronics
... means a lot of cost and the risk of malfunction of any of these devices.
(One has to consider how often (many times per minute?) it is switched ON/OFF and how long (24/7?) the device is ON. Also mind the voltage peak when switching OFF an inductive motor ... that all energy goes to the power supply / optocouplers... if no additional effort and cost is taken for overvoltage protection.
It's all a benefit for a (dumb) customer while all the risk and cost is on the manufacturer.

******

But if it's a device that already includes some control logic and power relays:
Then I'd add a rather cheap, low power (capacitive supply) unit that just disables the activation of existing power relays. The motor does not work when wrongly wired. A LED may say "reverse direction".

*******

Also consuder specil cases:
* loss of one phase
* connected to a VFD
* and situations one can't consider now

******
I don't say it doesn't make sense at all. If it's an expensive device or safety related device .. or portable... it may make sense.

Klaus

#### Warpspeed

I was thinking of some kind of potentially dangerous machinery, that might be portable, or mobile, or even rented.
The type of thing that might be plugged in just about anywhere by people with little or no training.

#### FvM

##### Super Moderator
Staff member
Three-phase monitoring relays are standard products in industrial control, see e.g.

It's a matter of design objectives if you prevent machine start in case of phase reversal (usual) or perform automatic correction (possible). Phase loss would block operation in any case.

Two points about contactor usage. It's not safe to use a single 2xNO/2xNC contactor to switch phases, it likely causes a phase short due to delayed extinction of contact arc. A mechanical interlock provides additional safety against control signal failure and welded contacts.

#### vishweshgm

##### Newbie level 6
I imagine OP is thinking about shipping product to various places and having mains input wiring done by others - thus to guarantee motor direction right from the get go - phase direction must be established and set.
Yes this was my intention.

(One has to consider how often (many times per minute?) it is switched ON/OFF and how long (24/7?) the device is ON. Also mind the voltage peak when switching OFF an inductive motor ... that all energy goes to the power supply / optocouplers... if no additional effort and cost is taken for overvoltage protection.
It's all a benefit for a (dumb) customer while all the risk and cost is on the manufacturer.
Hmm. All I am thinking as "protection" is just to use varistor across all 3 hot wires and ground.

Two points about contactor usage. It's not safe to use a single 2xNO/2xNC contactor to switch phases, it likely causes a phase short due to delayed extinction of contact arc.
I never used interlock before. I googled and understood how it works. So if I have to use interlock then, I guess I need 2 units of 4pole (one pole for holding contact) contactor?

Thank you all for insightful comments. Considering diagnostics and indication point of view and all your inputs I rigged up a quick - schema to get clarity. I want to go with microcontroller approach but will use logic suggested by @Warpspeed. Also as @KlausST mentioned I can take care of loss of phase diagnostic too if required. VFD case is very good point. I am not sure if the same logic works or not. Since this forum doesnot encourage micrcontroller discussion, I won't discuss variable frequency detection in SW here(But I am attaching the logic I plan to do in controller)

I have 3 phase available with me here in office. I'm gonna try this next week with light resistive load that I have. Thanks again for the support.

#### Attachments

• QPS_0012_Threephase.pdf
677 KB · Views: 10
• THree Phase Flowchart.drawio.png
73.8 KB · Views: 8

#### KlausST

##### Super Moderator
Staff member
Hi
Hmm. All I am thinking as "protection" is just to use varistor across all 3 hot wires and ground.
Sounds simple.
I still recommend to design carefully.
* Connected t the input or at the motor
* expectable energy
* expectable current
* expactable voltage
* expectable duration

Just to avoid that the whole added circuitry causes more trouble than without it.

Klaus