DC motor pump always transports in same direction, regardless of polarity

Question

Okay, this hurts my head: I have a vacuum pump which I salvaged and it seems to be this one: https://www.yingyidz.com/product/yyp370-xk-mirco-vacuum-pump/

I intended to get a H-Bridge motor driver, so I can make it suck and blow, controlled by a microcontroller. My thinking, is that as it's just a DC motor connected to some kind of plastic funnel airflow network, all I need to do is reverse the polarity to alter which end sucks and which end blows.

To my surprise, when hooking up to a bench supply, it doesn't matter which way I connect the terminals, it still sucks from one hole and blows from the other. To be clear: Which hole sucks and which hole blows does not change when I change polarity!

So... How is this possible? I see no rectifier or any circuitry. I guess what I want to do is not possible? Maybe there are just one way valves inside it?

Another stange thing is that there is a red dot on one terminal, indicating it is polarity sensitive, when it doesn't appear to matter.

Answer 1

Some quick googling reveals that this is a membrane pump. The actual pumping action is performed by a membrane that moves up and down. The direction of the motor does not change the direction of flow in such a device.

Answer 2

The motor is reversing direction. But the air flow is not set by the direction of the motor, it is set by the one-way valves in the membrane. As the motor rotates (regardless of its rotation direction), it moves the membrane in and out.

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More generally, there are motors that will not change direction when you reverse the voltage.

A shunt DC motor will rotate in the same direction regardless of the polarity at the terminals. That's because, when you reverse the polarity at the terminals, you are reversing the polarity of both the armature winding and the field winding. As you're reversing both magnetic fields, the torque remains in the same direction.

This is true also of a series DC motor.

I am not saying that that pump necessarily uses a shunt or series DC motor. (In fact, I am sure it uses a PM DC motor.) I am just saying that behavior does not violate any laws of physics.

Answer 3

There's multiple misconceptions here.

1. A pump's direction of transport doesn't necessarily have to do with the direction that the motor powering it turns; there's many counterexamples (pumps using centrifugal forces, membrane pumps, the classical reciprocating "hand-cranked well pump" style pump attached to an excenter on a motor. So, maybe next time you wonder about the physics of a pump, start with wikipedia!
2. A motor's direction doesn't necessarily have to do with the direction of current flowing through it. Davide mentions multiple motor types where that's true. More recently, we do see more and more "Brush-Less DC motors", which actually are AC motors with a controller that inverts the applied DC voltage as it sees fit. If designed to work with arbitrary input polarities, these can do whichever direction they're programmed to run in, for example. (Your motor doesn't look like a BLDC motor though)
3. Because something is not like you expect it to be, it doesn't break the laws of physics. (You know that, I know that, everyone else here knows that as well. Engineers hate these pseudo-surprised clickbaity headlines – it's exactly the counterfactual things they have to fight at work. I changed your question's title to solve that issue.)

Answer 4

There are two questions in one thread.

The first one was already answered by Unimportant and more extensively by Davide Andrea.

Some pumps have one-way flow regardless of power input polarity (diaphragm, piston, diffusion), some can be reversible by changing polarity (Francis turbine) some can be severely damaged or destroyed by incorrect polarity (rotary vane pump).

In some cases the input polarity or torque direction may be dictated not by means of opperation but by the design of the assembly. There might be a threaded joint between the engine axle and the pump excentric/crankshaft. One polarity cause the thread to self-tighten, the other cause it to self-loosen.

In other words: If there is recommended polarity, there is usually a good reason to recommend it and even better reason to follow such recommendation.

Answer 5

I think I have solved how to make this work:

I have 2 of these pumps, so I can hook up a second one to blow just by putting the input on the output pipe, connected to the same airline.

I think I then need to add 2 solenoids (which I also have) and programme the micro controller to activate one or the other, so airflow shuts off one way while the motor is working on the other side. No H-Bridge needed.