Non Electrical person needs help!!

• posted

Hello All,

I wonder if someone would be kind enough to help me. I am a designer with limited electrical experience, and I am makeing a prototype which I need to have a switch do some dynamic braking.

What I would live to have is a rocker switch for UP/DOWN and when in the center BRAKE.

If this is at all possible I surely would love the help, however if I'm totally nuts (and I accecpt that I might be), please let me know.

TIA, Muggs

• posted

Center brake would be just another set of contacts on your switch. Closed when in the center position. As for the braking your going to have to provide that else where in your design. I have seen brakes mounted externally on motors like elevator motors.

• posted

You're not nuts at all. It's a simple circuit to do with relays. Choose relays whose contacts are rated to carry the current & voltage that your Up, Down, and Brake loads require. See the first three diagrams and descriptions below. (There are more diagrams after the second, but you do not need them if you use double pole relays, and it's simpler to understand.)

Switch wiring diagram:

Switch Ground---A_RelayCoil----A C B-----B_RelayCoil---Ground | Positive-------------------+

Relay Point wiring diagram:

A B Ground----UP---0 0-----+----0 0----Down----Ground | Positive---------------+

The rocker switch is double throw, center off. I've drawn it above with an A side and a B side. Polarity does not matter - I show it as ground (negative) connected directly to the relays, and plus connected to the relays via the switch. You could also use AC instead of DC, if you chose AC relays. The center (common) contact goes to positive at the voltage source.

The relay wiring diagram shows the open and the common points of relay A and B contacts as 0 0 When the corresponding relay is energized there will be a path between the contacts, like this: 0-0 See the diagram below for the following description: A second set of contacts on each relay is used for the brake. The closed point of the A side relay is wired in series with the closed point of the B side relay and then to the BRAKE, like this:

A B Ground----Brake----0-0----------0-0-----Positive

As shown, both relays are de-energized, and there is a path for positive to the Brake.

When either the A relay or the B relay is energized, there is no connection through the closed point, which is shown as 0-0 above. When the relay is energized, the diagram looks like this 0 0 - there is no connection from the common to the closed point.

For the sake of completeness, I'll draw diagrams below that show using only 1 set of points at each relay.

A_Relay B_Relay Ground------Up---0 N--------C 0--Down---Ground / \ Positive----------C N---Brake---Ground

The contacts consist of a Common point C, Open point O, and a closed point N. The line to point C can connect either N to C or O to C, on either relay. The circuit is shown with both relays de-energized - you can see the path for positive to get to the Brake. The next diagram shows the A relay energized.

A_Relay B_Relay Ground------Up---0 N--------C 0--Down---Ground \ \ Positive-----------C N---Brake---Ground

Note that with the A relay energized, there is a path for positive to get to Up, but it has no path to get to Down or Brake. The final diagram below shows the B relay energized.

A_Relay B_Relay Ground------Up---0 N--------C--0--Down---Ground / Positive----------C N---Brake---Ground

Note that with the B relay energized, there is a path for positive to Down, but there is no path for positive to Brake. Note also that the switch makes it impossible for both relay A and B to be energized at the same time, but that is shown in the first diagram, not this one.

• posted

I think you guys missed the "dynamic braking" part mentioned by the OP Dynamic braking is where the motor itself is used as a brake using back EMF. We would need to know more about the type of motor used in order to give any advice about dynamic braking circuits. As an interesting aside, AMF 8230/70/90/XL bowling pinspotters all use squirel-cage induction motors and dynamic braking. BTW, this kind of braking will stop a large motor dead real fast, but has no holding power once stopped. I posted a schematic of such on alt.binaries.schematic.electronics if interested.

Also, I've seen other motors that have a separate winding that gets DC injected for braking.

• posted

Thanks guys,

Yes Nukie, that's exactly what I'm talking about, just like your Makita, DeWalt "chop" saw, when you release the trigger the motor comes to screaching halt.

What I should have probably mentioned is I will be using a 24V DC motor, and from what I understand, I can brake this by removing power to the motor and shorting the... Armature?

What I want to use is a simple DPDT rocker switch, so if someone could explain to me how to wire up the switch I would be very greatful.

Muggs

"Nukie Poo @verizon.net>"

8230/70/90/XL
• posted

Yes, and if the motor is not fastened down real good it will jump right off its mountings...

• posted

"Nukie Poo

I sure did!

• posted

Assuming a simple DC motor, directly connected to the supply (no electronics H drive or whatever) :

C--------x--Motor-y--+ / | Pos------0 0------Neg | | Neg------0 0------Pos | \ | C--------------------+

That's a DPDT switch, giving you direction reversal, but no dynamic braking. Without additional components, you can't get direction reversal and dynamic braking with a DPDT switch. To get dynamic braking, you need to provide a path across the motor - points z and y. You can add a relay to do that. The coil of the relay is wired to the points labeled C. You wire a closed relay point and its operating point to the points labeled x and y.

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