Wikipedia:Reference desk/Archives/Science/2022 January 7

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

January 7

Doubt on armature in DC motor.

If current experience reverse direction why can't armature experience the same i.e zero degrees?

Right after armature making 90 degree turn, then direction of the current go reverse direction using split ring but why can't armature go back to 0 degrees ? Rizosome (talk) 01:22, 7 January 2022 (UTC)

See Brushed DC electric motor with its animated graphics. Philvoids (talk) 16:00, 7 January 2022 (UTC)

Your question illustrates why a normal brush-DC motor's commutator needs to include at least 3 different current flow positions. It WOULD be possible to make one with only 2, after the armature moved 180 degrees (or 180/N degrees) the current would reverse. But that motor would rotate in either direction (dependent on some rotational inertia for smooth motion), equally likely depending on which way it started.

An analogous situation concerns synchronous (AC) motors needing at least 3 (thus 4) poles fed by AC in different phases, not just 2 opposite poles with one oscillating magnetic field, so that it will run in a particular direction (but most of the power can be in one with the other just for starting it in the right direction). A notable exception is the typical synchronous motor that turns the platter in a microwave oven (a so-called "magnetron" where I live). That only has 2 opposite poles (one phase) which is why it can start in either the clockwise or counter-clockwise directions, and the food doesn't care. :-) Interferometrist (talk) 19:21, 7 January 2022 (UTC)

@Interferometrist: Your question illustrates why a normal brush-DC motor's commutator needs to include at least 3 different current flow positions. I am asking about a position of an armature not about current. Rizosome (talk) 06:01, 8 January 2022 (UTC)

If I understand your question (and I'm not sure if I do – your English is very unclear), it's because the armature is a real object with mass and therefore (angular) momentum that is already spinning in a particular direction. When the current and therefore the induced magnetic field (and therefore the force on the armature) drops to zero, the armature's angular momentum keeps it moving past the 'zero point'.

At the 'zero point' there is no force acting on the armature, so why should it change direction (i.e. velocity: see Newton's First Law of Motion)?

When the current, magnetic field and force then reverse, the armature has passed the 'zero point' and the reversed force acts upon the armature in the same rotational direction as before. The physical design of the armature and split ring ensures this.

It seems to me that the article linked by Philvoids above explains this very clearly. If you cannot understand that explanation, perhaps you should be using the Wikipedia for your own language, rather than the English Wikipedia.

(NB: Don't expect any further response from me on this query, as I have yet to be convinced that you are not feigning ignorance.) {The poster formerly known as 87.81.230.195} 90.208.89.176 (talk) 12:24, 8 January 2022 (UTC)

In this line: When the current, magnetic field and force then reverse, the armature has passed the zero point and the reversed force acts upon the armature in the same rotational direction as before.

What does this zero point mean? Rizosome (talk) 04:46, 9 January 2022 (UTC)

The "zero point" is the position of a simple armature where the windings are not in position relative to the magnetic field to generate any torque, regardless of whether any current flows in them. At this zero point the simple DC motor is incapacitated: it cannot start (or know which direction to turn) without help. Please look further into Brushed DC electric motor to understand Interferometrist's explanation of how this weakness is avoided in all but the simplest small DC motors. Philvoids (talk) 18:11, 9 January 2022 (UTC)

I got the clear idea from this line about armature: At this zero point the simple DC motor is incapacitated: it cannot start (or know which direction to turn) without help. Rizosome (talk) 01:14, 10 January 2022 (UTC)

Resolved

object moving from point A to point B with no outside force

In the new so called "Live Science" publication an article appeared yesterday titled: "Wild video shows goldfish 'driving' a water-filled car in weird experiment" [1] . I will describe the idea briefly. It is done in Israel. They made a light "vehicle" consisting of a small aquarium made of thin glass positioned on a piece of plywood with four wheels underneath. They then put a goldfish (eventually 6 goldfish, but it is not important) and using some behavior tricks enticed them to move the carriage, they call it a car, forward. They claim that goldfish showed intellect, moved the car around the obstacles, etc, but this is not what I am interested in.

I wonder what Sir Isaac Newton would have said about it. Some motion of an object can be expected when something explodes inside of it, but it is not a directional motion. Imagine there is a soccer ball on the grass and a player kicks it. The ball will fly forward. If you can measure a force applied and know a mass of the ball you can write s simple equation, calculate speed of the ball, etc. But here there is no external force. How does it work? They don't say if the fish touches the opposite wall of the aquarium with their noses or simply "swim." What is a secret here? Looking at their video one has to conclude that there is no secret. The fish swim and the car moves in a given direction! Is it a new mechanics? AboutFace 22 (talk) 19:40, 7 January 2022 (UTC)

Are you asking how the car is powered? If so, it's motorized. Cannolis (talk) 20:05, 7 January 2022 (UTC)

(ec) The car is motorised and equipped with a camera that detects the fish's position in the tank and a control system that translates those position measurements into commands to the motor. The fish doesn't actively propel the car but pilots it. I guess Isaac would have been quite puzzled because he didn't know about electric motors... --Wrongfilter (talk) 20:10, 7 January 2022 (UTC)

I missed it. Thanks. Now everything is on its place. I got scared for Sir Isaac Newton :-) AboutFace 22 (talk) 22:54, 7 January 2022 (UTC)

• Regardless of the fact that this particular case involves "cheating", the fish could have used the friction of wheels against the ground / the reaction of aquarium walls against the plywood to transmit force. Sit on a rolling chair and shake around a little bit (without falling...), you can move the chair forwards or backwards without your feet touching the ground or other objects. Tigraan^{Click here for my talk page ("private" contact)} 09:08, 11 January 2022 (UTC)