Question Motor and power supply question?

Thanks a lot noorbeast - i was trying to cheap out on the psu - didn't realise about the spikes for the motor due to direction change.

While building my pc, i had this issue with a generic psu - my rtx 3080 was spiking with MSFS 2020 and this led to shutdowns. I had to change the psu (with a good quality unit that could absorb these spikes). Never had this issue since.

Thanks again for the advice.

You can connect multiple drives to one power supply. Each unit draws as much current as it needs. The problem with switch mode power supplies is that they shutdown immediately when overloaded opposed to transformer+recifier types which can handle some overload for short durations.
One possible solution would be to connect a car battery in parallel which can handle large peak currents. This way, the PSU only has to supply the average current which is a lot lower. The battery also can handle reverse current. Reverse current (motors acting as generators when braking) can blow your PSU and your drives because the capacitors only have little capacity and the voltage rises quickly when reverse charged.
But be careful with car batteries. They don't shutdown but can deliver multiple 100A for several minutes. So it's a good idea to insert fuses so in the case something fails it doesn't catch fire.

Thank you, very interesting oversight. Indeed, there will need to be braking and rotor slippage with the change in direction.
This will get me to re-think my compact design

Handling reverses and spikes are made with ultra-fast flyback diodes (((and bunch of caps calculated not so much as per capacitance, as per pick current used. i.m. bunch of 220uF caps with ~1A ripple current is better vs 2000uf or whatever with ~150mA ripple current. In order they can handle ripple currents and work long)))

The path where the flyback stuff from motor goes, depends on topology of H-bridge control also. It can go back to PSU, or flywheel and decline within motor.
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Bay the way I managed to use 180W line in 350W PC PSU without any changes to it, so it does not goes to protection. The thing also was not to drop wires. Switch on carefully and you can control motor with no problem.
But obviously should not work with all PSU as is.

Freewheeling diodes and some 220uF caps won't help you much. The energy has to go somewhere. Say, if you lift 100kg by 0.2m that's roughly 200J of stored energy. If that mass falls back down and is decellerated in 0.1s that makes 2kW. Multiplied with the efficiency of the motor (say 80%) this is still 1.6kW. If your PSU has 2200uF caps the voltage will raise from 12V to theoretically 380V which they surely can't take. They will blow up immediately. Even with ten times the capacity (22,000uF) the peak voltage will still be 120V.
With worm gears and slower motion the numbers are not that extreme. If the efficiency is only 40% and the decelleration lasts 0.5s the power to dissipate is reduced to 160W but it's still 80J of energy.

Larger VFDs use bleed resistors to get rid of the brake energy. But for a 12V system this is difficult. The resistor has to be in the 0.1 Ohm range and has to handle large currents. I think a car battery is still the easiest solution.

VFD and BLDC controllers are another thing. To put resistor via mosfet to active path on resistor it is simple schematic.

Freewheel diodes are made for this things. They eliminate the voltage spike, don't let it to emerge, giving constant power supply to source with no interruptions.

Protects the mosfets. So you can use low voltage mosfets with them. Caps. There will be no 100+ volts spikes or whatsoever.

Dissipates the back EMF from motor. It doesn't matter what kind of power source you are using including the battery.

Resistor can't do this. Including the fact that PWM is on itself is power interruptions already.

The battery is not the best solution in this terms on the contrary.

You misunderstand the capacity. It is a bunch of caps with lower capacity but with better ripple current characteristics. So this way the capacity is enough and ripple current handling is as required.

The control strategy is totally different, but the power stage works exactly the same. DC motors have two terminals so 4 transistors for the H-bridge. BLDC and induction motors have 3 terminals so 6 transistors.

There are two different things that must not be mixed up. First, there is a short duration spike every time one leg of the H-bridge switches. This lasts several microseconds. So total energy is low. This energy can recirculate in the motor path (slow decay) or go back into the DC bus caps (fast decay). The energy is stored in the motor winding. If no new power is added by switching DC bus voltage to the motor the voltage spike will quickly decay.

Second, there is regenerative energy that comes from the kinetic and potential energy (weight*height) of the mechanics. This has nothing to do with the PWM switching. The motors act as generators and transform the mechanical energy to electrical energy. This lasts several milliseconds to seconds. Power is up to around half of the motors power draw (efficiency squared) and energy is power * time. Because of the longer time of the mecanical deceleration and the much higher storage capacity this does not go away within microseconds. The energy has to be either stored in a (very large) capacitor or battery or dissipated with an electical load (bleed resistor).

If your system is very inefficient (stepper motors, worm gear, lots of friction) and your decelleration is not very aggressive there are chances that your drives survive without energy dump. Good drives have internal protection and simply cut the motor current when the DC bus voltage gets too high. But that does mean that torque is also cut and your motor coasts to stop instead of braking actively.

I will not already trust someone much after this narrative, when the point was that right flybacks and caps are already in the system).

You talk as if there is nothing in between the psu and motor that is directly connected to it. Post above is also in this direction.

Assuming that BTS or similar drivers are used, you already have stuff alike within this driver. (But it will not outperform the good ultrafast high voltage huge amps freewheelers and can be used additionally to make the life of a BTS driver easier. Or even simply possible sometimes)

How does the conventional PC power supply, with 180W 12V line handles the whole scope of work of FFB wheels then (long stalls, fast rotations, fast reversal on full speed etc) of a powerful motor? When it “shuts down from a glance on it”?

No problem with PC PSU, Led switching PSU 12v12A, another one 12V 35A whatsoever. I use them perfectly for a long time and now power up the RF power amplifier.

According to yours it should already be broken. Or instantly.
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How to collect back emf?

The back EMF energy is made available through the reversing of the polarity of the magnetized pole pieces thus collapsing the field around the coils and reversing the flow of energy to the recovery diodes, which is capturing the back EMF(c)



“Flyback or resistor(c)“
Or resistor. Yes let it be a battery to psu but a bulky way though.
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Example.

Relay. For change of directions instead of H-bridge.
-Capacitors 30 400uF 35,5A regarding ripple current:
One 40-50V mosfet (or whatever number of them) for PWM on negative side. (irlb3034pbf, later one used irf3205)
-Flybacks 30U60DN -600V30A. Connected as a bridge for both directions of motor rotations.

The back EMF decays there.

All works perfectly.

Simple led power supply. All the same with PC PSU. And it's a tricky one as we know any way with their protections. It's just enough of moving or dropping cables and it goes to shut down protection. Obvious. I have recordings of this test b.w.

No flyback. No caps: THE WHOLE LOTTA sparks in relay contacts during that time it had before mosfet's right suppressors (just left there for there protection and to see what will be with them here), just teared to pieces.

192A mosfet with limitation according to its package.(300A+ on the datasheet)
And don’t forget about the snubber used on the motor that time.
I will remind you that it's a MY1025 motor, 12v power supply.

Suppressors - Teared to pieces.
“wheelcheck” step test was turned on. That was not even completed due to the above. One change of direction is enough.

Another one.
All the same but only mosfet 40-50v are left. (No matter how many)
It's enough of just a soft lock bump, with the motor slowly turned to and touching this position.
What is does? Mosfet instant burnt-out.





It's just to say that for experience of using switching PSU for this applications as per above . All is working don't know, and still working good.

You miss the point. As I already said, the problem I talk about has nothing to do with flyback diodes and voltage peaks that come from the energy stored in the motor coils (inductance). We both agree that the driver power stage wouldn't survive without the the diodes. They are already there so everybody is happy.

The problem I talk about is the kinetic and potential energy stored in the mechanics of the rig. If you move your rig upward against gravity or accelerate the motors have to provide power. The energy goes from the electrical system to the mechanics. If you move downward or decellerate this energy (or part of it depending on efficiency) comes back.

It doesn't matter if you trust me or not. This are the laws of physics, called "Law of conservation of energy". If your rig is lightweight and the efficiency of you motors and gear is poor you might be lucky and very little energy comes back so nothing blows up. The bigger the mass, velocity and efficiency are the more likely you will get in trouble and then flyback diodes won't help. The energy can easily be much bigger than the capacitors in your electronics can take.

No need to argue. If everything works you don't have to worry about it. I just wanted to mention it as somethingg you have to consider in some cases.