BLDC Water pump operates with no water?

If a three phase BLDC and its voltage source inverter is pumping water and spinning at 8000rpm, , at maximum duty cycle, with a 34V DC link, and taking 20 Amps, then what happens if the water suddenly goes and the pump is operating dry?

Does the pump suddenly speed up?

The BLDC Inverter operates always at maximum duty cycle, and the motor current is limited by a current source SMPS upstream of the inverter........presumably, the current source would need to suddenly reduce its current if the water ran dry?

As a springboard, we look at a DC brushed motor (which we are more familiar with)...

It will automatically speed up with no load. It also draws less current.

The danger is that it will wear out its bearings prematurely. Therefore some kind of sensor is needed, to turn off the motor and/or alert the operator.

And you do not want to slow it down. At slow speeds a brushed motor draws overmuch current.

As for brushless type...

My understanding is you continually need to sense the position of the armature, and turn on the next coil at just the right moment, to pull a magnet toward it.

It cannot sense if there is a load or not. It will not speed up if there is no load.
Your control circuity will make it run at the normal speed. It will use the normal amount of current, even though it is not pumping water.

Therefore you need to sense whether the pump has gone dry, so as to turn it off, and/or send an alert.

I could be wrong.

I agree the brushless motor's speed will remain almost constant, subject to the the controller. However, in my experience, the current draw is dependent on the load.

You do not describe the pump. Some pumps do not do well, if run dry. Also, if it is a centrifugal pump, it may not to be self priming.

John

My thoughts are that the current will increase as the load increases. This is because the current pulse through the phase coils has to last longer to get the armature into the correct phase with the coils before the current is switched off. or put it another way, if the motor is drawing X amps at full load producing some measurable output power, gallons of water an hour, if the water disappears and the pump in still drawing the same power, where is the excess energy going to, because its not moving water!
Some pumps rely on the water to lubricate their seals.
Frank