Motor Control. Part 2

[Bobster]

http://www.electriciansforums.co.uk...inees-only/96152-what-you-doing-trainees.html

Following on from this thread, seems a few are interested in having a go.

Now following from the answer ElectroChem produced:

Ok here's my go.

Edit: Can't seem to quote the picture that goes here. It's post 23 in the above thread.

And the bits and pieces mentioned on how this could be improved.

I would like the same again please, only this time I want a forward and reverse pushbutton, with a common stop button.

Motor size 5.5KW no power factor to worry about, but the motor has an efficiency of 98%.

Note, once running in one direction the motor should not be able to go in the other direction without being stopped first.

I'll leave you to do the power drawing this time, however if people are struggling/need pointers. Post up and I'll help.

 

[ElectroChem]

I'll whack out an answer tomorrow morning, I've got an exam on fuses/breakers/protection which should take all of an hour or so and then it'll be free time waiting for everyone else to finish. I'll hold off on posting till someone else has had a try though.

edit: When you said motor has to stop before reversing, did you mean physically stop rotating or just be unpowered? If the former I'll have to hit up my lecturer for some advice, I've not used rotational interlocks on motor controls before.

 

[Bobster]

For simplicity's sake the motor just has to be unpowered.

Although feel free to have a crack with an interlock if you wish. You will need a rotation monitor, with a monitoring relay. If you do decide this route, please stick up the parts you plan on using, that way I can look at the connection details etc... Or I can just give you the parts I've used before for this job.

 

[pennychew]

I've got half an idea about this one, I'll get it drawn up tonight

 

[pennychew]

Ive got the power side sorted and just need to draw up the control side now. was going to do it while the Man Citeh game was on but we didnt get humped so i got drawn into that instead!

 

[ElectroChem]

Drew these up a few days ago but been absolutely swamped with work, crash project to finish a robot before client shows up and wants to see it running.

 

[pennychew]

Drew these up a few days ago but been absolutely swamped with work, crash project to finish a robot before client shows up and wants to see it running.

View attachment 26905

View attachment 26906

Thats the same set up i was going for on the power side with the forward reverse contractors to swap the phases. Still never got round to drawing the control side up yet as I've got serious man flu and work is just piling up :-(

One of the downsides of working for yourself, i no longer have the luxury of time off no matter how rough i am

 

[Bobster]

Well I don't have the information on what kind of rotation monitor your using. However there is something wrong with how you've applied it. I'll let you have a think before I explain.

The rest of the circuit is correct though. Well done.

 

[ElectroChem]

Well I don't have the information on what kind of rotation monitor you're using

Neither do I, it's a hypothetical part.

However there is something wrong with how you've applied it.

All I can bring to mind is perhaps such sensors are usually set to give signal at 0rpm then cut out at speed? In which case R1.1 becomes NO.

 

[Bobster]

Neither do I, it's a hypothetical part.

No problem.

All I can bring to mind is perhaps such sensors are usually set to give signal at 0rpm then cut out at speed? In which case R1.1 becomes NO.

So looking at where you have put the contact, regardless of it being N/O or N/C. What happens when you start the motor?

 

[ElectroChem]

What happens when you start the motor?

Doh! Nice one Steve...:6:

OK, take out R1 in the stop line, add R1.1 and R1.2 as contacts in the pushbutton branches of the start circuit.

 

[happysteve]

Hi,

Heavily influenced by ElectroChem's answer, all credit for this to him; this is just a tidy-up, and to check my understanding:



So, I've assumed that the rotation sensor outputs a voltage allowing R1 to energise and R1.1, R1.2 to be closed when stopped, and R1.1 and R1.2 to be open circuit when the motor is moving.

K1.4 and K2.4 are normally open, and become closed when the respective relay is energised (maintaining current to the coil).

K1.5 and K2.5 are normally closed, such that when (for example) K1 is energised, K1.5 will be open circuit. You could in theory do without these, but then if your rotation sensor failed and became jammed shut (erroneously reporting "motor stopped" when in fact it is actually moving) then by pressing the button for the opposite direction you would short out two of the phases.

I forgot to label the thermal overload contact (above the stop button).

Am I on the right lines?

Like I say, ElectroChem did all the hard work, this was inspired by his diagram and I just want to check my understanding.

Cheers

 

[happysteve]

PS: Ah, I see now why you need K1.5 and K2.5. More importantly than a fail-safe for the rotation sensor, it's what prevents you shorting out two phases by pressing "FWD" and "REV" at (almost) the same time.

 

[ElectroChem]

PS: Ah, I see now why you need K1.5 and K2.5. More importantly than a fail-safe for the rotation sensor, it's what prevents you shorting out two phases by pressing "FWD" and "REV" at (almost) the same time.

Almost right. The electrical interlock of K1.5/2.5 is itself a layer of redundancy. Look on the 415V drawing, the diamond and dottet line between the contactkrs indicates a mechanical interlock. When one contactor is pulled in you can't close the other, even if you opened the cabinet and physically pushed on the armature.

 

[happysteve]

Almost right. The electrical interlock of K1.5/2.5 is itself a layer of redundancy. Look on the 415V drawing, the diamond and dottet line between the contactkrs indicates a mechanical interlock. When one contactor is pulled in you can't close the other, even if you opened the cabinet and physically pushed on the armature.

Well well! Thanks for that, I've learnt summat new today. Cheers!