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20th Dec 2020, 12:09 PM #31Member: Blue and white apron brigade
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Yeah keep meaning to install a contactor (I even have a couple of suitable spares in my stash!) but have only had to replace 3 relays in about 7 years. I bought a 6 pack of these relays for another project which never eventuated. Just looked up what they cost me and it was $18 for the lot so I might as well use them up. You know what its like - you're in the middle of an urgent job and its just easier to replace a plug in relay than wiring up a new contactor
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20th Dec 2020, 09:37 PM #32
Hi Nigel, Guys,
Thank you for your comments, they help tremendously.
I redrew the diagram because of the comments noted !
DC_Injection_Braking-2.jpg
Notice that I've replaced the missing neutral mains wire ! I've also deleted the unneeded wire to neutral. After studying the circuit for a few minutes, I've also deleted the wire from the negative of the capacitor and the relay contact, which basically only shorted out the Thermistor.
The current through that thermistor is very small, almost zero when charging up the capacitor. Barely enough to warm it up. But I also realised that the value of resistance there was able to control the severity of the braking effect ! Shorting it out with the relay means that the only other way of controlling the braking was to alter the capacitor value.
Nigel: Yes it should work with a three phase motor, the circuit will be a little different though. Unfortunately though I have a couple of three phase motors, but no three phase to run them with, so whilst I can design a circuit to do it I've no way of testing it. With regard to your bulb/heating element, you will see the bulb dim much more slowly than if you just killed the power. I think using a heating element is a waste of time, simply because of the time constant it has.
Right Guys, back to making the prototype !Best Regards:
Baron J.
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21st Dec 2020, 06:42 AM #33
Prototype build finished !
Hi Guys,
I've finished building the prototype circuit board for the DC injection braking !
20-12-2020-001.JPG 20-12-2020-002.jpg
20-12-2020-004.jpg 20-12-2020-003.jpg
These pictures show the assembled circuit ! Take no notice of the clamp jaw in the top left, I just used it to hold the board for photographing. There are several changes of components. The most obvious one is the Thermistor. This one is 47 Ohms cold, I've no idea what it is hot, but since the initial current through it is only the surge current into the capacitor its not particularly critical.
At some point this Thermistor will probably get replaced by a resistor of a suitable value to set the level of braking desired. The higher the value the softer the braking effect will be.
Next the Diode, the one in the other picture was short circuited, so it has been replaced by a BY127 diode. Which is a much older device but similar ratings and a bigger package.
The middle set of contacts is not used.
I've labelled mains live with a letter "L" in red and used brown coloured wires for these connections. The mains neutral connections have been wired using blue coloured wire. The motor live feed is indicated by the large red dot. Hopefully you will be able to follow the wiring and the circuit diagram.
I will be testing this device tomorrow and will post the results here.
Thanks Guys.Best Regards:
Baron J.
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21st Dec 2020, 09:04 AM #34Most Valued Member
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You keep suggesting the thermistor is charging the cap, it’s not...
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21st Dec 2020, 11:08 AM #35Member: Blue and white apron brigade
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I tried using 150W incandescent as a braking resistor on a VFD and it was savage.
There was a flash of bright light and it stopped almost instantaneously BUT the motor made a loud Thump/Grawnch, probably because the resistance changes too dramatically with the temperature.
The standard VFD braking resistor in the photo below (Al box under VFD) provides for a much smoother operation.
This one is fitted to a 2HP Radial arm saw at the mens shed.
After being switched off the saw used to coast for more than 120s with 100 or so of those seconds being almost silent and so represented a serious risk and several incidents occurred so teh Mens Shed asked me to set up a bake for it.
With the brake fitted we can comfortably stop the saw in about 4s. It can be stopped faster, eg 2s but the noise is a bit disturbing AND there is also a risk the blade may come loose.
VFDandBrake.jpg
This is a Powtran VFD and brake. The braking resistors are designed to suit the VFD rating and are dirt cheap ($10), but the shipping from China is stiff.
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21st Dec 2020, 05:10 PM #36
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22nd Dec 2020, 02:43 AM #37Most Valued Member
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Nope.. leaving it alone so there is plausible deniability in the event of something going pear shaped..
I’m going to leave you with this thought, you don’t need rectified mains to achieve your end, around 20V of flat DC will give you a locked rotor condition on a small motor.. A relay and a power supply from a laptop will achieve your aim much more simply and safely...
Leave you to it..
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22nd Dec 2020, 07:38 AM #38
DC braking Injection !
Hi Guys,
I've had a play around with the circuit board and a 1 HP single phase induction motor, actually the one on my 6X4 bandsaw. With the components that I've used it stops the motor almost dead ! Now I know that there is not a lot of inertia on a bandsaw, but it did surprise me a little just how violently it stopped. Great for an emergency stop on that machine.
I need to try another motor and see what values I need to change to soften the braking effect. I'll have to obtain another motor, Ideally a larger one and test with that.
Right ! Now to reveal my cunning plan !
********************************************
DC_Injection_Braking_Thyristor.png
In this circuit I've replaced the relay contacts with a Thyristor and used the relay contacts to trigger the Thyristor. Contact arcing and burning now gone ! The BT151 device I've used is a quite old junk box one, but any device capable of switching 10 or more amps at 400 volts will work here. The gate current for this device is a mere 20 milliamps, so solving the contact damage issue.
Thanks Guys.Best Regards:
Baron J.
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22nd Dec 2020, 08:43 AM #39Most Valued Member
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- melbourne australia
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22nd Dec 2020, 08:47 AM #40Most Valued Member
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22nd Dec 2020, 02:00 PM #41Diamond Member
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- Aug 2019
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- Revesby - Sydney Australia
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In the original circuit, I'm not sure that is what the resistor does?
1) In the braking position, the capacitor is direct connect to the motor. Resistor bypassed/shorted.
2) In the active position, the resistor limits the charging current into the capacitor.
No resistor means a lot more work by the diode and cap when charging, in addition to the work when it does the braking. I suspect that will mean early cap failures.
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22nd Dec 2020, 07:37 PM #42
Hi Chris, Guys,
Thank you for your comments.
Not long ! The back EMF from the motor would kill the low voltage electronics in the output side.
If you look at the 50 Hz waveform across an induction motor, at switch off, depending upon at what point of the wave the supply voltage is removed, you will see a spike of about twice the input voltage. This is simply due to the motor back emf and the coincidence of the supply voltage peak occurring together.
The same will occur in the braking circuit, but its duration is only for a very short time, a fraction of a half cycle.
It would be more likely to damage or kill the Thyristor than anything else.Best Regards:
Baron J.
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22nd Dec 2020, 07:52 PM #43
Hi Nigel, Guys,
Thank you for your comments.
The resistor/thermistor is there purely to limit the surge current into the capacitor, mainly protecting the rectifier.
If you look at the diagram in post 32, you will see that I removed the relay connection shorting the thermistor out.
This also has the effect of limiting the discharge current into the motor when the power is turned off and the relay connects the capacitor + to the motor.
It also means that by altering the resistance at that point you can control the discharge current into the motor, thus adjusting the braking effect.
I doubt that the capacitor life would be much affected by discharging into what is basically the impedance of the motor inductance. Ignoring motor back emf, it wouldn't be as damaging as putting a dead short across the capacitor.Best Regards:
Baron J.
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23rd Dec 2020, 08:54 PM #44
Hi Guys,
I've been doing some playing about with the 1 HP (750 Watt) motors that I have and noticed that there seems to be a marked difference in braking behaviour between the two that I've tested with. Both motors have the same ratings and are both two pole, 2800 rpm. Neither have anything mounted on the spindles.
The motors are of greatly differing diameters ! The rotor on one is about 4" inches in diameter and the other about 2" inches in diameter. One brakes quite hard and the other brakes and then just spins freely for a few turns. The motors have different DC winding resistances as well.
It appears that the larger motor with the slightly higher winding resistance doesn't discharge the capacitor quite as rapidly as the smaller motor with its lower winding resistance.
My conclusion is that it is going to take a fair amount of experimentation to arrive at a suitable set of values to suit most motors. I did increase the capacitance to 440 uf with very little change in behaviour between the two motors, they just braked a little harder.
Unless I can come up with an easily adjustable set of parameters, each application will be trial and error !
I welcome comments on these findings.
Thanks Guys.Best Regards:
Baron J.
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23rd Dec 2020, 09:02 PM #45Most Valued Member
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