Easiest way to momentary push button switch a 1 amp 130V DC motor

[bigcanuknaz]

Hi all:

I have burned up 2 contactor blocks for a 22mm pushbutton. Hmmm, I think.. Something must be wrong. Lo and Behold. the contactor blocks are rated at 10 amps AC, but only .5 amps DC. (3000 ops/hr)

We are using this switch to help speed control a cart unloading soil in an organic tomato greenhouse. For safety we press the pushbutton to move, and release to stop. We often do this as often as 20 times per minute (or 1800/hr). The motor is 1 amp, 130 V DC. We are switching the output of a Bodine Electric Company WPM - 2109E1 DC Motor speed control.

Bodine Electric Company WPM - 2109E1 DC Motor Speed Control

What is the best device to use. I have looked for 120VDC (or 130) 1 amp pushbuttons, and not found any (This might be the easiest solution).

I guess I could also use some kind of electronic relay (triac or scr???) and then trigger that relay with a switch. Not sure at all how to do this. I could also use a larger derated ac contactor, but that seems silly. A specific DC rated switch seems smarter.

I could also take apart the speed controller, and change the on/off switch on it (or wire in series) a push button. But I am not sure if powering and unpowering the speed controller that often will break it too...

Thanks for your help.

Norm

 

[dougy83]

Hi Norm,

I take it you have connected the pushbutton in line with the motor... I wouldn't do that myself for a couple of reasons (motor EMF may arc on switch contacts - would need a snubber, high currents through switch, speed controller doesn't get to do its accelerate/decelerate action).

No, it's not a good idea to insert the switch inline with the AC as it may put stress on the controller (e.g. high inrush currents).

There is a connection point on the terminal block inside the case for an 'Inhibit' switch - have you tried using it? It's apparently a low voltage signal connection, so you can use a small switch - only catch is that it needs to be a normally-closed switch (or have a normally-closed contact anyway). I'd suggest testing it out to see if the acceleration/deceleration is ok for the job before worrying about getting the switch.

While I'd think using the inhibit connection would do the job, an alternate option would be to tap into the pot on the front panel to decrease the speed to 0 when the switch is released - but it's more effort than the above.

Regards,
Doug.

 

[KeepItSimpleStupid]

You should be using H1 and H2 or the inhibit connection. Ufortunately, there is no specs on the signal and the controller "coasts" to a stop.

Anything that compromises the safety of an operator, you should consider such items as light curtains and safety relays. Any work on the mechanism, MUST use the ON/OFF switch to remove power completely.

 

[bigcanuknaz]

Thanks for the replies.

I like the idea of the inhibit to do the speed pulsing at the low end. I will try that. Thanks, dougy83.

And yes, safety is a concern too. I will add some separate locking e-stops. Keepitsimple, if I understand you right, these should go on the 120VAC *supply* to the speed controller, is that right? Dougy83, if it locks, then either the speed can be turned down, or even if started under speed, it would not happen very often, so as to cause inrush overheating damage to the controller.

Thanks,

Norm

 

[bigcanuknaz]

Thanks,

Works like a charm!

I just wired the power directly to the motor, and used my starting 22mm pushbutton with a NC block on the inhibit.

And the "coast" down happens real fast, in our application, so the e-stops will only be used (hopefully never) in real "e" situations.

We have been modifying this machine extensively, but you would think the initial builder would have used the "inhibit H1-H2 contact" rather than the power output to the motor from the get go. I am sure he was unaware of it as well. I will try to find him to let him know the right way to do it as well.

Thanks again,

Norm

edit: readability

 

[dougy83]

Dougy83, if it locks, then either the speed can be turned down, or even if started under speed, it would not happen very often, so as to cause inrush overheating damage to the controller.

I meant the inrush through the controller's rectifier and bulk/filter capacitor which happens every time you apply AC power to the controller & not the motor stall current (which the controller _should_ limit/control). Anyway, it's not an issue when using the inhibit as you are now.

Works like a charm!

I'm very glad to hear that

 

[KeepItSimpleStupid]

Yes, you safety or emergency stop should interrupt the AC line voltage.

Alhough this https://www.electro-tech-online.com/custompdfs/2011/01/Interfacing_Safety_Relays_Rev04.pdf pertains to AC drives, it does illustrate the principles involved when safety is involved.

You really need to do a "risk assessment" to determine what might happen if, for instance:
1. The motor controller fails to stop with the inhibit circuit
2. Is a breaker for the AC power sufficient for an E-stop
3. What SIL level is required.
4. Remember that if you use a relay to interrupt AC power that's spring returned, the contacts can weld together.
5. What happens in a power fail response. A proper Safety Instrumented system would say use a Start/Stop station and actually monitor that the contacts did actually change state.
6. Electrically tripping the AC breaker (shunt trip) with the Estop might be an option for you. These are special breakers. Usually you connect the switched power to the trip terminal on the breaker. The breaker will trip and remove the source of power to the trip mechanism.

Sometimes you don't want the safeties to fail. I know. I designed a system that worked properly when the lab blew up from a Hydrogen explosion. There was no fire, per say. The actual failure was a excess flow valve for the Hydrogen delivery system that made the operator think that the tank was empty. So 1500 PSI of Hydrogen was released into a line rated for vacuum. This caused a total re-design of the gas delivery system for all gasses and even the times that cylinders were changed had to occur when the building was empty. Some gasses required the use of an SCBA (self contained breathing apparatus).

 

[bigcanuknaz]

Thanks, Dougy83.

Thanks KeepItSimple:

This "soil unloading cart" is self contained, with ac drive conveyors and the single dc travel drive motor running off of a 4000watt generator. I was planning on using the e-stops to kill the generator.

Good thought on making sure it is safe even with a multiple contact fusing failure. E-stops are hard to fail this way, but it can happen.... Both the "start" contact for the generator, and the 110VAC for the drive are low amperage, so able to feed through the e-stop easily.

With your advise, KeepItSimple, I will do a dual contact on the e-stops killing the generator, and interrupting the 110V to the dc drive as well. Everything is well guarded and protected, and the operator is protected and excluded, but you never know when you might get a mechanical failure which could cause safety problems too.

Health and Safety is an area that needs constant attention and improvement. We haven't had a lost time injury in over 5 years, and I intend to keep it that way. (We are saving over $2,000/year on our worker's comp insurance, so we try to use this up doing safety projects. We can only get back up to 2,000, but it can cost us 10 or 15 times this if we have a poor record!!) This looks like a good project to apply to all our equipment. Make it all at least dual failure proof...

Thanks!!

Norm