DC Motor Control Question

[Industrial Toys]

Building a small Welding turntable for a friend with fractional HP, DC Motor and Drive.

I have built hundreds of DC Polarity Reversing circuits using a DPDT switch, but I have an industrial panel switch with three positions, and four contacts. Nothing is connected in the center position and the contacts are either in the other two positions. I was sure I could substitute this contact arrangement for my normal DPDT switch, but I can't make it work without getting an unintended short as I am trying to reverse the wires in one position.

Any ideas. I did not draw it out on account of laziness, but for the time I have spent on it, would have been well worth it.

I don't really need reversing, but I only had this more complex switch to go in an existing hole. So I thought, I would offer that feature to my friend.

ALSO. Is there any danger in disconnecting a DC motor from a DC motor control circuit. Ptentially, when there is still some residual power in the drive?

 

[Roadworthy]

I'm not sure I understand the question. If you turn off the power disconnecting the DC motor poses no problem. The DC motor itself will drain residual power when switched off. As far as your switch I haven't a clue what you're dealing with.

 

[Industrial Toys]

Thanks. It's hard to describe.

Does a DC motor control mind having it's load removed, I guess is the question number two.

For Reversing. Switch has three positions. Four Contacts. All open in center (B) position. Two Closed in lets say the (A) position and the other two closed in the (C) position. Can this contact arrangement be used to reverse the polrity to the motor?

 

[3Ts]

I see that you normally use a DPDT switch (which has 6 tabs for the wire connections), this switch makes no sense as a DPDT switch unless the terminals are cross wired internally. With having only 4 leads, you're going to have to figure out which terminals are connected together in each of the switch positions. An ohm meter will tell you which are connected together.

On a second reading of your post, it sounds like one pair of terminals is connected with the toggle to the left, and the other is open. Then toggle to the right and the opposite situation occurs, i.e. 2nd pair of terminals are connected and the 1st are open. With toggle in center both sets are open. This really means you have 2 SPST switches in the same body. I do not see how you can make a reversing switch out of it, you need a DP type switch.

 

[Mendonsy]

Thanks. It's hard to describe.

Does a DC motor control mind having it's load removed, I guess is the question number two.

For Reversing. Switch has three positions. Four Contacts. All open in center (B) position. Two Closed in lets say the (A) position and the other two closed in the (C) position. Can this contact arrangement be used to reverse the polrity to the motor?

That switch will work. See drawing.

 

[kenmbz]

I wonder if that switch has a make before break setup. Would explain the short while switching?
Otherwise Mendonsy's diagram will work if it's a true open on switching between poles.

 

[3Ts]

Here's my understanding of his switch:

 

[PILOON]

If this link works:
Gmail

This uses a DPDT, spring loaded, center off style switch.
I use this to control my 2000 lb ATV winch to rotate my snow blower chute.
Has been flawless for 5-6 seasons for me.

 

[bearthebruce]

Thanks. It's hard to describe.

Does a DC motor control mind having it's load removed, I guess is the question number two.

For Reversing. Switch has three positions. Four Contacts. All open in center (B) position. Two Closed in lets say the (A) position and the other two closed in the (C) position. Can this contact arrangement be used to reverse the polarity to the motor?

An electronic controller does not like it. A motor is an inductive device. Once current starts flowing, it wants that current to keep flowing. When the line to the motor is opened, the voltage at the terminals of the motor flies to a high voltage for a instant owing to the stored energy in the coils of the motor.

You didn't mention if this was a relay controller or electronic controller. Relays are far more forgiving. However, contacts have been known to be eroded by the voltage spike that occurs when opening the relay. If the relay controller is designed for the rated horsepower, the controller should have no problem switching.

Remember too that relays and switches do not break the circuit cleanly... there is a time for the motion of opening to occur. During that time, there is the high voltage and at the same time, a higher resistance contact because the whole contact surface is no longer engaged..

As to your switch idea and arrangement, yes, that switch should reverse the motor. Wish I had some drawing tools here... I would sketch out how I would wire the circuit. Oh wait, Mendonsey did it for us.

 

[3Ts]

An electronic controller does not like it. A motor is an inductive device. Once current starts flowing, it wants that current to keep flowing. When the line to the motor is opened, the voltage at the terminals of the motor flies to a high voltage for a instant owing to the stored energy in the coils of the motor.

You didn't mention if this was a relay controller or electronic controller. Relays are far more forgiving. However, contacts have been known to be eroded by the voltage spike that occurs when opening the relay. If the relay controller is designed for the rated horsepower, the controller should have no problem switching.

Remember too that relays and switches do not break the circuit cleanly... there is a time for the motion of opening to occur. During that time, there is the high voltage and at the same time, a higher resistance contact because the whole contact surface is no longer engaged..

As to your switch idea and arrangement, yes, that switch should reverse the motor. Wish I had some drawing tools here... I would sketch out how I would wire the circuit. Oh wait, Mendonsey did it for us.

And when the contacts close, they bounce a little so there are multiple make/break cycles. If you are using a contactor, it needs to be rated for DC. An AC one may or may not work. The reason is because as the contacts open there is an arc that develops between the 2 contact points. The length of the arc is voltage dependent, an AC relay has it's voltage go to zero at some point and will self extinguish the arc. A DC circuit voltage remains the same and is dependent on the relay contacts opening far enough to extinguish the arc (although there are ones that use magnets to increase the arc length).