I need to find a safe and reliable way to power a DC hobby motor off mains supply. The motor is large (i.e. 500W) 14.4V. Therefore, the current requirements are large i.e. 35A for a 14.4V supply.
I can't find such a power supply to purchase and my budget is small. Therefore, I thought I could use an ATX power supply from a computer. They typically have a high current 12V line that can deliver the required current, 42A in this case.
I will be using a power MOSFET to regulate the power going to the motor by switching it on and off using pulse width modulation PWM. There should be some huge inductive spikes created. I need a way to protect the power supply from these.
I was thinking of using several layers of protection just to be safe as long as they are compatible with each other. There location in the order that I mention them is arranged starting closest to the motor and moving towards the supply. The first line of defense is a freewheeling diode put in parallel with the motor forward biased from 0 to the positive supply direction. In addition, perhaps an RC snubber also in parallel. Then a 12V standoff TVS also in parallel in case the other two fail. However, this will short the power supply if it is triggered. If the supply doesn't have over current protection it will kill the supply. I need some sort of fast acting fuse or polyswtich to at as a current limiter in series with the supply. I've noticed that a the polyswitches are very slow. like 9 seconds to "disconnect" the line and I don't want to use a fuse that I have to replace when this happens. Is there a good solution to this problem, or is this all too much? That is, should I just leave the TVS and polyswitch out of the design?
Answer
I have bad experience of similar attempt. The cause is that contemporary switching supplies are too smart for dynamic inductive load with regenerative an other switching spikes. The only workaround is to use raw power supply (transformer, bridge, zero active components).
What you will possibly see if you try direct connection:
- Motor will accelerate OK on low torgue
- Motor will trip supply when acceleration is high on start from zero velocity
- Motor will accelerate OK with high acceleration but will trip supply protection when decelerated
- Motor will never burn, but supply very likely will
- Nothing will will break and will work ok, but you will trip mains 20A breaker once in a while.
- Trying to put ferrite chokes on positive wire will make no difference, supply will keep tripping
- Overall currents will never be near 50% of rated by supply, but peak values will keep protection tripping even if the motion is smooth
- The protection diode (overvoltage 50V surge diode in parallel to 48V supply) can help for 90% of motions, but supply will be ever tripping unpredictably over long run
- etc.
So answer is probably the thing we did not try, but which can help:
- Calculate the energy budget for motions and double it and put super large capacitor at the output. With surge diode with voltage rating 1-2V greater than nominal voltage and a bit lower than overvoltage protection of supply.
Other aspects involved:
- The breakdown of high power semiconductors is caused not by voltage, but by speed of voltage increase (about 5000V/microsecond typically). Rule of thumb "Everything is thyristor"
- The TVS is a must. They survive at 49..50V 400W loads with no sweat.
- The regenerative energy absorbtion (large capacitor and/or power resistor with few diodes) is a must if voltage is over 100V. Can be skipped for low voltage setup unless you need 24/7 run in hard conditions.
- Custom FET switching is cause of trouble, simply because you cause mechanical shock with infinite torgue on start, unless you use PWM with clever S-curve acceleration.
- Safety is also not last aspect if you deal with near horsepower motion.
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