CR Circuit for driving a motor 3

[gavie14]

Hi guys, I want to design a CR circuit such that when a switch is closed a motor powers up slowly (as opposed to full torque instantaneously) and when the switch is opened the motor stops straight away. The motor has to rotate in both directions so the ciruit has to have a SPCO switch in it.

I guess that a diode has to be incorporated into the circuit to stop the motor from rotating in the opposite direction when the capacitor is discharging. Circuit design is not my strong point so if someone could offer any advice, it'd be very much appreciated. The circuit should be as simple as possible.

Thanks very much.

 

[ScottyUK]

What kind of motor and what is the load? What you're proposing is very simplistic and of dubious provenance - no offence intended!


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[MacGyverS2000]

The motor has to rotate in both directions so the ciruit has to have a SPCO switch in it.

I guess that a diode has to be incorporated into the circuit to stop the motor from rotating in the opposite direction when the capacitor is discharging.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[gavie14]

Scotty, yes, it probably is very simplistic to you, but my electrical engineering knowledge stops at a module I did back in my undergrad years at uni. I do know what you mean by "dubious provenance", explain please..?

MacGuyver, I mean I hope that when the user presses the switch in direction 1, the motor rotates clockwise, but when he lets go (switch in centre position) the motor stops. Conversely, when the user presses the switch in direction 2, the motor rotates anti-clockwise.

I have an idea in my head, I'm just waiting to gain access to some software to test it virtually.

 

[Noway2]

I assume that you want a "soft start" to prevent the 6x(+) across the line current draw. Use a switch to apply voltage to the motor through a series resistor. Then use a timer relay to close a contactor across the resistor so many seconds after motor start. When the switch is opened, the motor stops and the bypass contactor drops out.

 

[gavie14]

Norway2, that's exactly what I wish to do. However, is it possible to do so with capacitors and resistors and not a timer relay? The reason I ask this is that I googled timer relays and, to be honest, it seems a bit beyond my skill set.

I also need the circuit to be as small and simple as possible. I'll hopefully be able to test my circuit tomorrow and put this debate to rest (fingers crossed!), but thanks for the help so far.

 

[waross]

Scotty may have been trying to suggest that knowing the motor type, voltage, Hp and whether AC or DC may help you get some reasonable replies.
On the other hand if a timer relay is beyond your skill set, you may need more than free internet help to solve your problem.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[gavie14]

DC circuit and a maxon motor as in the following link:-

http://shop.maxonmotor.com/ishop/article/article/118400.xml

I had another look at timer relays and I guess I could make one, but I'd rather not. I'm going to test my circuit tomorrow on MultiSim and hopefully everything will be copasetic. If not, then timer relay it is.

I get that this problem probably seems simple to you guys, but I can't stress enough how little experience I have in this department, especially developing something independently.

 

[Henneshoe]

Depending on the size of your motor, NTC thermistors will limit the inrush current. When the thermistor is cool they have a high resistance; when the switch is closed current flows through the thermistor, heats it, and the resistance drops to near 0.

One problem to be aware of is if the switch is opened then rapidly closed the thermistor will still be hot, its resistance will be near 0, and the motor will pull the full inrush current.

 

[Skogsgurra]

You can forget about inrush current and switching voltages. A motor of this size doesn't have any problems with that.

Concentrate on what you want to do instead of on possible secondary effects. You obviously want the motor to start slowly and ramp up in speed when you activate a lever (SPCO) in one direction. It shall then run until something happens (lever back to zero, I think) whereupon it shall stop as fast as possible.

The same thing shall happen in reverse when the lever is activated in the other direction.

If so, what shall the ramp-up time be? And what stopping time is acceptable (cannot be zero milliseconds). Fast stopping can be achieved by short-circuiting the armature (moderatletly fast) or plug braking (real fast) with reverse polarity until speed is zero.

I doubt if a SPCO will do the work. You need at least a DPCO if you want to reverse the motor (both leads need to be switched). BTW, the correct terminology used to be (and still is, in some quarters)SPDT (Single Pole Double Throw for the first switch type and DPDT for the second type).

Also, a CR circuit will make the opposite of what you want. An RC circuit will make the acceleration soft. Not the best solution from an efficiency point of view, but that is probably not very important in this case.

The capacitor needs to be an electrolytic one and that means that you have to be careful with the polarity. May be a problem when reversing the motor.

The inclusion of a diode: Yes, perhaps. Where do you want it?



Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[gavie14]

Damn it, I thought I replied to this already. Anyway, Skogsgurra you're exactly right in detailing what I'm after, the part in parentheses about the motor stopping when the lever is at zero is correct also. The ramp-up time should be around 5 seconds. The stopping time should be as short as possible while still being safe.

What do you mean by an RC circuit being unsuitable as the acceleration is soft..? As for the diode, that was my idea, I know that a diode only allows current flow in one direction so I thought that should a CR circuit be used, then the diode be placed after the capacitor but before the motor (relative to the current flow).

Argh! I didn't think it would be this tough (or I would I be this bad).

 

[Skogsgurra]

Please find attached a standard circuit that can be used if you do not want to involve eletronics.

"F" and "R" are two-pole cange-over switches for Forward and Reverse.

There are two parts. One for the reversing and braking and one for the ramping/soft start.

The discharge circuit has one error ("automatic drawing"): Remove the horizontal line going from D.R. to lower left connection of "R". If you keep it in circuit, you will burn out the D.R. resistor.



Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[gavie14]

Wow! Amazing, Skogsgurra, I'll test it virtually asap, but it looks good. Thanks so much!

 

[gavie14]

It works! I can't thank you enough, Skogsgurra. You've saved me a lot of time.

 

[gavie14]

Damn it, spoke too soon. I know I'm being a pain in the ass now Skogsgurra, but when a simulation with a voltmeter placed across the motor, it peaks at around 2.76 V with a 12 V source. Is there anyway for the motor to eventually end up with 12 V across it?

I'm guessing the answer is pathetically simple, but I think I've shown how lacking my knowledge in this field is already...

 

[Skogsgurra]

Try it IRL!

If your switch model has normal resistance (milliohms) and normal leakage (nA). And if you have ohms instead of any other unit for the resistors, if your capacitor doesn't have too much leakage and if it doesn't break down below 12 V and if you have the polarity right and if your motor model draws between a few milliamps and around 200 mA, then you should get something between 10 and 12 volts. Depending on how much load you have on the motor shaft.

Check also the current limit of your DC supply.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

BTW: You did read the last sentence in my 8 Jun 10 3:51 post?

That about removing the wire to the discharge resistor.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[gavie14]

I think I've removed the correct wire, you can see what I've built in the attachment. As for building in real life, I'm still to acquire the components I'm afraid.

Thanks for all your help again, you've decreased my hatred of electronics from a 10 to a 9 (it's no small feat)...

 

[Skogsgurra]

And then you haven't really done any electronics yet..

(Yes, correct wire removed)

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[gavie14]

Right, at least I know everything is fine circuit wise. Cheers.