BLDC pump suddenly runs dry...what happens next?

[Flyback]

Its just to pump water to flowers....we don't want extra bits like flow sensors....are you saying we cant do it without flow sensors?.....surely theres some way that we can do it without flow sensors.......ie, just using the back emf signals to represent speed?

 

[4pyros]

@ 4pyros
Can you point to an example, where the motor is a BLDC type driven at a fixed frequency?

Here is the 1st hit to come up after a google search of "bldc pump motor control"
**broken link removed**
While I am no expert there seems to be a lot about the subject on the web.

 

[Reloadron]

Its just to pump water to flowers....we don't want extra bits like flow sensors....are you saying we cant do it without flow sensors?.....surely theres some way that we can do it without flow sensors.......ie, just using the back emf signals to represent speed?

Then you can take a look at the solutions 4pyros has been discussing. You will need to take a good look at the features offered by the controller you are using. I merely pointed out how I would have gone about it.

Ron

 

[tcmtech]

Seriously it's in a water tank and you can't come up with a way to float something in there that would sense if the water level got too low and turn the pump off?

I will consult with the 4th graders on my bus route engineers that I chofer around town tomorrow and see what they have to say.

 

[shortbus=]

Is there some way that an led could sense the water level?

 

[alec_t]

surely theres some way that we can do it without flow sensors.......ie, just using the back emf signals to represent speed?

?? But won't the back-emf signal be constant if, as you say in post #1,

when the pump suddenly runs dry...after all, the motor speed does not increase

 

[Flyback]

when it suddenly runs dry I think it will try and increase (speed) , but the rotor will just slingshot round wildly, and the back emf sensing circuitry will not be able to keep up,and will pulse the coils at the wrong times, and so it wont speed up, but will just spin very roughly....not necessarily going faster at all?

 

[ronv]

Hard to tell, but I would think the current would drop when the load (water) is removed.

 

[Flyback]

in a brushed dc motor that is correct...it would drop..but in a sensorless controlled brushless DC...that's one whole diferent story

 

[ronv]

So your running it at it's maximum frequency?

 

[Flyback]

The ML4425 3 ph BLDC controller datasheet doesn't say what the max possible frequency is.....

 

[Reloadron]

Since the pump is submersible and in a tank why not just use a float switch in the tank and when the water level drops to a set point (about empty) turn off the pump.

The controller has a /BRAKE input that is internally held high through a internal 4 K Ohm resistor. Placing pin 25 low (GND.)

Braking
When the BRAKE pin is pulled below 1.4V, the low side
output drivers LA, LB, and LC are turned on and the high
side output drivers HA, HB, HC are turned off. Braking
causes rapid deceleration of the motor and current limiting is
de-activated, and care should be taken when using the
BRAKE pin. BRAKE is has an internal 4kΩ pull-up as
shown in Figure 10, and can be driven by a switch to ground,
an open collector or drain logic signal, or a TTL logic signal.

The speed set input voltage range is 0V to Vref. and as to max frequency have you even read the data sheet on the controller chip?

Voltage Controlled Oscillator
Frequency Range RVCO = 5V, SPEED FB = 6V 1.5 1.85 2.2 kHz

 

[tcmtech]

Ever put a amp meter on it to see if the input amp draw changes when the motor unloads? Odds are it does.

 

[skimask87]

Ever put a amp meter on it to see if the input amp draw changes when the motor unloads? Odds are it does.

What did those "engineers" have to say?

 

[tcmtech]

Bigger priority to deal with today. One of them dropped his cookies blueprints and had a meltdown and I had to take him back to the school the office so a teacher HR could have a talk with him.

 

[BobW]

If the current doesn't change significantly when the pump runs dry, then it must be a horribly inefficient motor.

 

[Flyback]

we are feeding the motor with a constant current source SMPS....the inverter does not control the motor current....so how will the constant current source smps "know" that it must reduce its current....?...I say that the constant current supplying smps will not know to supply less current unless its speed feedback input shows an increase in speed.?

..if its a dc brushed motor with a supply which has a big dc link capacitor, then yes, the dc brushed motor just knows to draw less current.....not so the current source fed BLDC..surely?

 

[BobW]

I have no idea why you would want try to operate a motor from a constant current supply. That is obviously the root of your problem. A motor (BLDC or otherwise) is normally run from a fixed voltage supply (possibly with current limiting to prevent overtorque), and it will draw whatever current it needs to drive the mechanical load.

I notice on edaboard (https://www.edaboard.com/thread313736.html), where you have a parallel discussion going on, you mention that you are using an ML4425 BLDC controller. Yes, it has built in current limiting, but the purpose of that is to limit maximum current, as I mentioned above to prevent overtorque situations. It is certainly not intended for operating the motor with constant current. If you've somehow managed to set it up so that it is forcing a constant current through the motor, then it's anyone's guess as to what may happen.

You need to rethink the fundamental principle of operation. Feed the motor from a voltage source, not a current source. Then it should be fairly straightforward to sense the actual mechanical load from the current draw.

 

[tcmtech]

If you are using a constant current source then watch for the voltage change when the motor unloads.

 

[Flyback]

ML4425 Datasheet
http://pdf.datasheetcatalog.com/datasheet/MicroLinearCorporation/mXuqrrw.pdf

We do have overvoltage protection for the case of open load.
The following states why you should run a BLDC from a current source....................
http://www.ecnmag.com/articles/2012/...less-dc-motors
..just above the waveform graph, says that BLDC inverters run cooler (less switching losses) if driven at 100% duty cycle and with the DC link voltage regulated to give the required speed.

You can increase system efficiency even further by not using PWM at all," according to Dave Wilson, technical lead for Texas Instruments Motor Solutions Group. "Instead, use 100-percent duty cycle all the time and only switch the inverter transistors at the commutation boundaries. To control the motor’s speed, simply change the DC voltage driving the inverter transistors. This approach reduces the switching losses to almost zero on all phases, and it mitigates high frequency losses inside the motor."

...they do say regulate the voltage, but that surely is the same as regulating the current?

Also, with high power drives, you don't want a current sense resistor connected to the bottom of the motor coils, as you have to wire to it, and thus its too noisy.....you must simply switch the motor coils in successively and control the current from the upstream switched mode current regulator....so you do not PWM the motor coils.....but just switch the coils in for the whole of the coil "On" time, then go to the nxt coil, like as normal in a way