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11th Jan 2021, 06:44 AM #61
New Circuit !
Hi Chris, Guys,
As promised a new updated circuit !
I've made several changes including a snubber capacitor across the Thyristor and a protection diode for the gate terminal. The snubber capacitor is there to slow down the rate of voltage rise caused by the motors back EMF. Also the Thyristor should have a minimum voltage rating of 800 volts. This is because depending upon the point on the incoming mains at which the braking is activated, there could be nearly 700 volts peak across the Thyristor. The gate protection diode will prevent the gate from seeing a negative voltage large enough to destroy the gate junction. As little as -5 volts will cause damage to the Thyristor.
At this moment I don't know what the RC time constant needs to be to ensure that the gate current is enough to keep the Thyristor turned on for however many mains cycles that it will take for the motor to stop rotating. So the trigger capacitor and gate resistor values are to be determined.
The zener diode I have used is a 22 volt 5 watt one and has a series resistor of 3.3 K at 10 watts rating fed from a 1N4007 rectifier diode. This gives an available charging current for the capacitor of about 30 ma.
DC_Injection_Braking_Thyristor-3.png
As soon as I can get my hands on a suitable Thyristor I will build and test this circuit.
Watch this space.
Thanks for looking !Best Regards:
Baron J.
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11th Jan 2021, 02:08 PM #62Most Valued Member
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That looks better John. Good luck with the thyristor.
Chris
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12th Jan 2021, 10:59 PM #63
Testing !
Hi Guys,
I've been playing around a little with feeding the 1 Hp motor I have on the bench, with DC using just a 1N5406 3 amp rectifier in place of the Thyristor. Depending on the value of "R limit" I can cause the motor to stop violently enough to throw itself off the bench. Unfortunately I don't have any suitable resistors to hand to make up a value of more than 50 ohns. I'm guessing that I'm probably going to need four or more times bigger than I have.
I'm currently collecting some high powered resistors to try and get an idea of the range of control that injecting DC into a motor rotating at 2800 rpm gives.
Thanks Guys, your thoughts are appreciated.Best Regards:
Baron J.
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13th Jan 2021, 09:25 AM #64Most Valued Member
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John,
a 240V kettle element should be around 20-30 ohms.Chris
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21st Jan 2021, 06:12 PM #65
Hi Guys,
Whilst waiting for the Thyristors that I've ordered to turn up, I have been doing a lot of testing and analysis of circuit variations and the few different motors that I have available to play with.
I'm finding that motors of different configurations behave differently under direct current injection. It appears that motors with centrifugal switches are more heavily braked than capacitor start ones. Though the amount of load/drag on the motor and the size/diameter of the rotor make a fair difference. I've tried to make comparisons under similar conditions for all motors.
Note that universal motors are mostly unaffected by using DC injection and using low value resistors or simply a short circuit directly across the motor is the most effective way of slowing these motors down.
I'll post more later.
Thanks for your patience !Best Regards:
Baron J.
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23rd Jan 2021, 07:06 PM #66Most Valued Member
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27th Jan 2021, 10:51 PM #67
Hi Guys,
I have just received the Thyristors that I ordered this morning. Originally supposed to be coming from the foundry in Italy, but they were actually dispatched from Hong Kong on the 16th and have just turned up via DHL. So I make that 11 days in transit.
So I can now get on with doing some definitive development.
Watch this space.Best Regards:
Baron J.
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1st Feb 2021, 06:04 PM #68
DC Injection braking !
Hi Guys,
As you know I've been playing about with this idea for a little while now ! A have come up against an issue which is in the original circuit, once the braking circuit has been activated the DC braking power is automatically switched off once the capacitor has discharged.
The original circuit has a number of problems, one of which the capacitor value has to be extremely large. This creates a problem with relay contact welding ! Easily overcome with the use of a Thyristor switch. However this produces another problem, a Thyristor used on DC cannot be easily turned off without introducing much more circuit complexity, and I'm trying to keep things a simple as possible.
The new circuit has no way of turning the braking DC off and as it stands requires that the mains supply is broken immediately the motor has stopped turning !
I can do this automatically but it introduces a short time delay before the machine can be restarted.
My question is, would say a 15 or 20 second delay period be acceptable ?
The normal stop button can still be pressed to stop the machine and would reset the braking circuit.
Note: the motors used in testing stops in much less time than this.
Your thoughts please before I go down this path.
Thanks Guys.Best Regards:
Baron J.
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2nd Feb 2021, 08:55 PM #69Most Valued Member
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Depends on the machine I guess, it would likely drive me nuts on a lathe, other machines not so much.
But then I'm not even sure I fully understand the question
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2nd Feb 2021, 11:11 PM #70
Hi Stuart,
Thank you for your post.
In the original circuit, once the braking capacitor had discharged, that was it, the circuit had effectively turned off. In the latest incarnation the braking is from a DC power source which is permanently powered when the brake switch is pressed. It doesn't get deactivated until the mains power is removed. So needing to find some way of shutting the DC power off, I thought about using a thermal switch ! But the ones that I've tried so far can take a number of seconds to cool down and reset. This prevents the motor from being restarted and rapidly braked again.
Sorry if this seems confusing. I'm trying to keep this circuitry as simple as possible to make it easy for people to build for themselves. For those that are into electronics and programming the whole thing could be done with a MCU and a couple of relays.
I'll post a new circuit when I've got it all figured out.Best Regards:
Baron J.
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3rd Feb 2021, 07:09 PM #71Most Valued Member
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Hi John,
My interest in this circuit is for my table saw. I don't think a 20 second gap between restarts would be a problem. But, would it simplify the circuit if it had a manually operated pushbutton braking switch with an interlock to prevent it being applied when the motor is running? I wouldn't mind holding the brake button closed for a few seconds. Depending on the cut, I often wait for the blade to stop anyway before removing the piece of timber. That takes at least 20 seconds on my saw.Chris
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3rd Feb 2021, 08:03 PM #72
Hi Chris,
Thanks for your post.
Yes I do use a push button switch to activate the braking, the motor power also gets disconnected at the same time. The issue is that the DC braking current is still present even after the motor has stopped. In order to avoid having the DC still flowing into the motor after it has stopped and the machine stop not being activated, I am trying a thermal switch !
The basic problem is that you cannot easily turn off a Thyristor in a DC circuit, you have to stop current flowing through it. Thus the idea of a thermal switch to break the current flow.Best Regards:
Baron J.
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3rd Feb 2021, 08:36 PM #73Most Valued Member
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I might misunderstand your circuit John, but doesn’t your push button switch cause the contactor to latch? So you don’t need to keep the button pressed? If so, I’m proposing you don’t latch the contactor, but instead the operator keeps the push button pressed until the motor stops. The operator then releases the button, thereby disconnecting the DC braking voltage from the motor. I haven’t sketched this out, so maybe it’s bollocks!
Chris
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3rd Feb 2021, 08:50 PM #74
Hi Chris,
I've basically used the relay to remove motor power when latched. The other set of contacts are used to turn on the Thyristor. A push button is used to cause the relay to operate and lock on.
Test-02.png
This shows the relay circuit and the proposed thermal switch. The DC comes in through the Thyristor. The thermal switch removes the power after a few seconds giving the motor time to stop. I've tried this on my 2 HP table saw and the blade comes to a fairly quick stop.
If you press the saw stop button just as the blade stops it removes the DC power.
Your thoughts are appreciated.Best Regards:
Baron J.
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3rd Feb 2021, 09:01 PM #75Most Valued Member
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I think the RHS of the circuit diagram is cut off John.
Chris
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