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ATX Power Supply shutting down

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c-funk22

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Hey guys,

I am writing to ask if you would know how I could solve an issue I am having with my current setup for a project. I am using a Raspberry Pi to turn on an air pump through a relay board. Previously, the air pump was setup such that power was supplied from:

230V (AC) -> Computer ATX Power Supply (Switchmode AC to DC) 12V -> Air Pump DC Motor.

The first air pump was rated at 12V 8A. The power supply can output 36A at 12V. Although,I had issues with this air pump as it would trip the power supply over-current protection. To overcome this problem I am using this circuit (found on this forum: https://www.electro-tech-online.com/threads/cheap-and-cheerful-dc-motor-soft-start.30314/) to “soft start” the motor to reduce the inrush current when starting the motor from idle as I believe the initial current spike is shutting down the power supply.

So initially, the current is limited by the 0R5(10W wire wound resistor) resistor for a short amount of time, when the capacitor is charged the MOSFET(IRF540N) allows the current to flow through it instead. This effectively reduces the current to the motor for a short amount of time at the start.

I changed my air pump recently as it wasn’t delivering enough air flow. The new pump is a higher current pump, it requires 14A at 12V. I thought the previous slow start circuit would work but it appears the MOSFET is now overheating when turned on in the first few seconds. Previously I could hear the motor was running at a slower rate then spread up, now the motor just starts and runs without the lower voltage initially for 0.5sec then being fully powered after. Do you think this is due to the motor it self? (I measured a resistance close to 0.5 ohm across the terminals, and the previous motor had a resistance of 0.7 ohm).

KFbdFdz.png


What do you think I should do to solve the problem?

I was thinking of:
  1. Trying to make the circuit again with new calculated R,C values
  2. Add high inductance in series to the motor terminals to slow down the rate of current change (I got a 470uH ferrite core inductor but it gets warm and I am not sure about the usual temperature range, so I haven’t explored this option further)
  3. Add a 12V 1.3 Ah lead acid battery in parallel to the +12V line from the power supply, which will be charged by the the computer ATX power supply, this should provide the required current to start the motor if it too demanding for the power supply.
  4. Starting capacitor?




Image of Air Pump:
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Image of soft starter circuit:
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Image of Relay:
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Image of ATX power supply:
PtFPVLZ.jpg

SMkjKC6.jpg


I would really appreciate any help with this issue, thank you!
 
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If the new pump is rated at 14 A, I would expect it to take somewhere around 5 - 10 times that on startup. I also think that your measurement of resistance isn't correct, because I would expect at most 0.2 Ω for a motor like that. 0.2 Ω will result in 40 W of heat being generated in the windings at 14 A.
Resistances below 1 Ω are difficult to measure, and ordinary multimeters are generally very inaccurate for resistances that low. Also the motor brushes may not be linear, so the resistance measured at low currents may not be the same as that when 12 V is applied.

I was thinking of:
Trying to make the circuit again with new calculated R,C values
You could try that. The first thing to find out is what resistance do you need to get the motor to start. If the motor doesn't move with the resistor you have , the circuit isn't going to work as intended.
You might also need a bigger MOSFET / heatsink as you will get quite a lot of heat generated during starting.
Add high inductance in series to the motor terminals to slow down the rate of current change (I got a 470uH ferrite core inductor but it gets warm and I am not sure about the usual temperature range, so I haven’t explored this option further)
That won't work. You would need a vast inductor to do what you want. It would need keep the current from changing at more than about 20 A per second with a 12 V supply, so I would think that you would need around 0.5 H, which if rated at 15 A would be well over 10 kg.
Add a 12V 1.3 Ah lead acid battery in parallel to the +12V line from the power supply, which will be charged by the the computer ATX power supply, this should provide the required current to start the motor if it too demanding for the power supply.
That would help, but with a starting current that will be close to 100 A, I would suggest a battery that is above about 10 Ah. If you look at https://www.farnell.com/datasheets/612602.pdf you will see that a 10 Ah battery will drop to 11.75 V when 30 A is taken, so even that would be marginal.
Starting capacitor?
Those are used on AC motors.

The neat way of starting that is with a variable PWM controller, but that is quite a complicated device to get to work correctly.
 
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Driver 300, thank you so much for that informative reply. I ran a few simulations with LTSpice and I can see what you mean with the inductance, way too big.

If the new pump is rated at 14 A, I would expect it to take somewhere around 5 - 10 times that on startup. I also think that your measurement of resistance isn't correct, because I would expect at most 0.2 Ω for a motor like that. 0.2 Ω will result in 40 W of heat being generated in the windings at 14 A.
Resistances below 1 Ω are difficult to measure, and ordinary multimeters are generally very inaccurate for resistances that low. Also the motor brushes may not be linear, so the resistance measured at low currents may not be the same as that when 12 V is applied.

That makes sense, given the low resistance I have a Fluke 115 multimeter, I should keep in mind it won't be as accurate at the lower resistances.

You could try that. The first thing to find out is what resistance do you need to get the motor to start. If the motor doesn't move with the resistor you have , the circuit isn't going to work as intended.
You might also need a bigger MOSFET / heatsink as you will get quite a lot of heat generated during starting.

The odd this is the motor does start, it seems that the resistors had no effect in limiting the current and the MOSFET goes straight to conducting current, albeit getting very hot which is my primary concern. I am trying to figure out what exactly I should change today I went from a IRF540N (http://www.irf.com/product-info/datasheets/data/irf540n.pdf) to a
STP60NE06 (http://media.digikey.com/pdf/Data Sheets/ST Microelectronics PDFS/STP60NE06-16(FP).pdf). Would you know or recommend a better MOSFET to suit the application? I also ran a simulation on LTSpice I think I need around 1 ohm instead of 0.5 ohm but high power handling resistors. I think I am going to follow an online tutorial on how to setup LTSpice and import the correct MOSFET libraries so I can see what is happening better.

That would help, but with a starting current that will be close to 100 A, I would suggest a battery that is above about 10 Ah. If you look at http://www.farnell.com/datasheets/612602.pdf you will see that a 10 Ah battery will drop to 11.75 V when 30 A is taken, so even that would be marginal.

The battery option is what I started with due to being limited on time. Is there a way of measuring the starting current? I believe my multimeter has a 10A fuse which means I cannot measure the current by putting it in series with the motor. What I was thinking is of using the 1.3Ah battery to provide the initial starting current then the ATX power supply will charge the battery back up and also supply current for the motor. This is done using the microcontroller which turns on relay 1 which supplies power from the battery to the motor then shortly relay 2 activates power from the power supply to the motor and battery. So it only needs to be on for a couple of seconds...do you think I still need to get a bigger battery in this case?

The neat way of starting that is with a variable PWM controller, but that is quite a complicated device to get to work correctly.

Due to not being able to design a fully working PWM controller in a short period, I was looking at these motor controllers. Sorry to ask you so many questions, do you have any experience with any of these controllers or would be able to recommend any, if not that is fine you have already been a great help!

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It looks like the battery option would only last for a little while. I will need to redesign the circuit or go for the PWM option. Thanks again for your time and putting me on the right track!
 
This is done using the microcontroller which turns on relay 1 which supplies power from the battery to the motor then shortly relay 2 activates power from the power supply to the motor and battery.
Are you sure those relays can handle the start-up/running current? I can't make out their DC current rating from the pic.
 
That is a good point, thank you for the heads up I was considering this, but forgot to buy a relay yesterday. If the inrush current is going to be 5-10 times 14A, thats even more reason to go away from the relay setup, I probably need to get a PWM controller or implement the soft starter again. Does anyone have any recommendations on the motor controllers:

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or recommend a better MOSFET for the soft starter circuit to suit the application?
 
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