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Peak current into water pump?

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Flyback

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Hello,
We have to design a universal input offline Flyback SMPS to supply the "Whale Gulper 220" water Pump.
Our SMPS will have a settable output voltage, either 12V or 24V.
The datasheet for the pump just says "24V, 2A" or "12V, 4A".
Though we do not know if this has high current peaks or not?
Do you know what the current waveform will be like?....will it have high peaks, and what will the peaks be? What will the frequency of the drawn current be?
The customer gave us a sample waveform, which shows the current into a 24V pump. -It actually shows a scope shot of
the voltage across a 100milliohm sense resistor in series with this pump....the waveform was a 2Hz (sort of) sine wave, (with a DC offset, so not negative going) which had a 5 Amp peak
bit which lasts for about 100 milliseconds.
We have no idea if the scope used was calibrated or if it was accurate.
I am of the impression that scope voltages are generally not accurate...so we have no idea if the peak current on this waveform is accurate or not.
Do you know what the current waveform into this pump will be like?

Whale Gulper pump datasheet:
https://www.defender.com/pdf/503200_Whale_Gulper220.pdf
Whale gulper 320 pump details
https://www.defender.com/product.jsp?path=-1|51|2234226|2234234&id=1777934
 
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The numbers you saw seem logical to me. A scope is about +/- 3% on the vertical of older scopes, newer scopes with cursors can be quite accurate. If you really want to know the inrush current when the pump motor starts either run some test of your own or contact the people who manufactured the pump. For 12 volts they recommend a 5 Amp automotive type fuse and for 24 volts a 2.5 amp automotive fuse which makes sense. The supply should be capable of exceeding those currents with some overhead. Anyway, it sounds like they want a power supply to power the pump off shore power when a boat is not underway? I would just ask for a pump and run my own test. Also, look at the actual specifications for the automotive fuses they suggest, such as time/current.

Ron
 
"Dry Running Current 2.4 /1.2 amps"
Specify Voltage, Max. Current Draw: 3.5 Amps @ 12 Volt DC

The pump motor will have startup surge currents up to 8x rated current due to coil resistance.
So plan on driving 30A for surges.
 
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Thankyou, the fact that the motor will literally be like a short circuit on startup is a problem for us, as we are using the L6566B PWM controller, and this has internal short circuit and overload protection, which will unfortunately be tripped by the startup "short circuit" of the motor.
I believe that we should use a simple controller like UCC38C43B which has no short circuit or overload protection, and we should add these things in ourselves with external circuitry. Of course, as you say, we need to get a motor and actually see what the startup short circuit really is like in terms of magnitude and duration, and also , measure the motor winding resistance.

L6566B PWM controller
https://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00167474.pdf
 
1) current limit is a good idea.
2)You could build the power supply so it can only deliver 1.5 or 2x what the motor needs to run. (will not start) On the output put a very large capacitor that can store enough power to start the motor. The motor start current is coming from the capacitor. Or at least 1/2 from the cap.
3)You could build a supply that can deliver stall current for 1/2 second.
4)Many off the power line supplies use "boot strapping". For those that do not speak Americana English. This is where you bend over and grab your boot strings and pull up very hard. If you pick up you boots, while you are in your boots, you can lift your self off the ground. Most off line supplies, when in current limit, can not support their PWAM IC and the internal supply drops, resetting the IC. You need to build a supply that can go into current limit, Vout=20%, and keep the IC alive and pumping.
5)Stall current is only for a short time so you will not need larger heat sinks. You will need large inductors, diodes and MOSFETs.
6)If you limit the stall current it will take longer for the motor to get started.
 
Thanks, I think you are right and using the bigger output capacitor is a good idea too. Though the PSU we are replacing is a small , cheap mains transformer, which as you know, has absolutely no trouble in sourcing the surge current that the motor wants at startup......its bringing it home to me that mains transformers have this head start (over smps's) when supplying motors.
 
I believe that we should use a simple controller like UCC38C43B which has no short circuit or overload protection, and we should add these things in ourselves with external circuitry. Of course, as you say, we need to get a motor and actually see what the startup short circuit really is like in terms of magnitude and duration, and also , measure the motor winding resistance.

I agree, things with motors like stall or locked rotor current is why God gave us fuses. :) The current will increase well exceeding the fuse limits. I would do as Ron suggest above using a large capacitor on the output for the motors inrush current. The beauty of having a motor to work with is when you test it you can lock the rotor or stall it and watch an automotive fuse open. You can measure all of your times and currents in a plot.

Ron
 
The following automotive fuse datasheet might help us to very roughly work out what is the magnitude/duration of the motors startup surge current.....I mean, the motor datasheet says it has a 2.5A fuse......so this could mean that the motor startup current is 15 Amps for 20ms, or say 8 Amps for 40ms.

Our Flyback SMPS will have 400mT in its transformer if its supplying 10 Amps to its output....so we are hoping that 10 Amps will be enough to start the motor. In fact, we might even just charge up the flyback's 1.4mF output capacitor first, and then switch this on to the motor so as to get a bit of a headstart in starting the motor off.....hopefully the charge in the output c ap will supply enough of a transient current to the motor to get it moving....then the flyback can do the rest.

Automotive fuse
https://www.littelfuse.com/~/media/...-fuses/littelfuse_fks_ato_32v_blade_fuses.pdf
 
then the flyback can do the rest
Some timers I get confused as to if you "flyback" are doing the work.
OR
The power supply "flyback" is doing the work.
OR
Maybe you are a robot, and you "flyback" are a power supply "flyback".
OR
Your cat is named "flyback" and I am really lost.
Our Flyback SMPS
"Our engineer Mr. Simpson PHD."
Our SMPS will have a settable output
<joke removed about set-able output>
 
Some additional considerations (if the pump is to be used as advertised, i.e., a shipboard "grey" water pump):

This is a diaphragm pump (that explains the ≈2Hz waveform). It also means that the initial in-rush current will be somewhat repeated with each cycle of the diaphragm (as it switches from suction to discharge duty). And total current might periodically exceed that if a plug of hair/soap/food is encountered.

I think that it would also be prudent to emulate whatever "head" (suction and discharge) will be encountered in the actual installation.

And, if it were me (and, of course, it's not), I'd tack on an extra 30=50% increase in short term SMPS current capacity simply because it's a marine application: at sea there's only so many fuses you can have on board.
 
thanks, so even though the motor is moving continuously you are saying there will be inrush?...

33 seconds onwards shows motor smoothly moving action...
https://www.youtube.com/watch?v=zb-cE_oq_do

I am wondering if a normal mains rectifier bridge and smoothing capacitor responds well to eg rough mains inverter outputs ("stair step mains")?
 
The surge current is also called the locked rotor current from DC voltage over Rs coil resistance.

The dynamic water load occurs in 2Hz cycles will be less than above.

Once the motor starts turning the variable reluctance increases the effective impedance until it reaches full speed of 2Hz after gear reduction ... to the the dry load current unless it is primed with water.

Does the manual call out a regular or slow blow fuse 5A?
I would expect a time delay fuse.
 
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thanks, so even though the motor is moving continuously you are saying there will be inrush?... ...
Only at first. What I meant was that with a variable load the normal operating current will occasionally "peak".

... 33 seconds onwards shows motor smoothly moving action...
https://www.youtube.com/watch?v=zb-cE_oq_do
...

Again, varying as above.
... I am wondering if a normal mains rectifier bridge and smoothing capacitor responds well to eg rough mains inverter outputs ("stair step mains")?

I don't think there would be a problem. So long as the filtering cap is sufficiently large (say, 1000uF), the DC output with be quite smooth. It will take a few hundred mSec to to get to its peak value.
 
Only at first. What I meant was that with a variable load the normal operating current will occasionally "peak".


Again, varying as above.

I don't think there would be a problem. So long as the filtering cap is sufficiently large (say, 1000uF), the DC output with be quite smooth. It will take a few hundred mSec to to get to its peak value.

A rated load of 4A @12V the average impedance is effectively 3 Ω with peaks above and below this.
The dry load is 2.4A@12V this is an effective average of 5 Ω

For the rectifier cap, 10% ripple on voltage the current will be 10x average current at peaks with 10% duty cycle and thus be effectively load of 0.3 Ω on 4A average to the cap.
with a 2 Hz ripple consider RC= 500ms thus you need 500ms/0.3Ω = 5/3 Farad Cap, not 1k uF but 16k uF Then the cap effectively shorts out the supply until charged up.

I suggest you use a 12V SLA for the cap and use any battery charger.
This has many Farads and can handle the surge currents easily. Then use your offline battery charger.
 
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Hello, this is a new question , but again concerning the same pump as the above...

Supposing we supply the whale gulper 220 pump with 6VDC, then how do we know that it won't stall? I mean, it doesn't stall in the lab, but as the pump ages, surely there will be a certain DC voltage at which the pump will stall?
Do you know what is this DC voltage?

Also, presumably, when operated at low voltage, the pump has more chance of stalling when also operated with the maximum discharge head and suction lift distance ?(ie 4 metres & with the narrower 3/4 inch piping). Do you agree?

We don't want the pump to stall, as then it has no back EMF, and just looks like a 1 ohm load to the SMPS...this represents an overload and will make the SMPS shut down on its overload protection.

Whale Gulper 220 pump datasheet:
https://www.defender.com/pdf/503200_Whale_Gulper220.pdf
Whale gulper 220 pump details
https://www.defender.com/product.jsp?...234&id=1777934
 
then how do we know that it won't stall?
You don't ;). IMO it's inevitable that, unless it dies first from an electrical fault, the pump will at some point stall as a result of aging or gunk accumulation. Low voltage drive may prolong its electrical life but will result in lower torque and hence a greater tendency to stall for mechanical reasons.
 
Thanks Alec_t, regarding stalling, I presume that a motor would be equally as likely to stall when driven with 6V continuous DC, as when driven with 24V DC pwm'd with a 0.25 duty cycle.?
--I mean, both are 6V on average, but something just tells me that the pwm'd one would be less likely to stall?
(please bear in mind its a DC motor)

(The actual nominal value of voltage for the pump is 24VDC, but as discussed, one can drive it at lower voltages to save energy when one doesn't need such a high flow rate..the problem being, which is the lowest voltage at which the pump can be driven , and not result in regular stalling?)
 
The grid has a similar challenge on fault detection after reclosure but uses time surge response curves for clearing, detecting a true fault or ignoring all the motor start surges.

If the PFC switch and magnetics cannot handle the surge, then one must try something different.


This is why I initially suggested a small battery and charger is better to handle the 8x to 10x starting surge currents.
 
Good grief!
This is a pump, a mechanical thing which moves gunky sticky gray water and bildge water around.
"If in doubt build it stout, using stuff you know about." Not an original thought of mine, but it fits this situation.

My crystal ball predicts that in a few years time our member 2PAC Mafia will be posting on here that he has some crazy pump power supply to repair with some SMPS kit in there.
He will also be asking "Why could they just not put a good old mains transformer in there?"

JimB
 
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