I've never worked with or studied electro-mechanical components, but my understanding is that relays and motors are basically just inductors. If they're inductors, wouldn't they act as a short under DC and need the current controlled by an external resistor or transistor?
\$\begingroup\$
\$\endgroup\$
5
-
13\$\begingroup\$ inductors made with really, really long, relatively thin wire \$\endgroup\$– jsotolaCommented Jun 13, 2021 at 0:35
-
\$\begingroup\$ Each core is rated for max current before saturation, so yes the current must be limited somehow. They draw less current when moving due to back EMF \$\endgroup\$– D.A.S.Commented Jun 13, 2021 at 1:00
-
\$\begingroup\$ A motor is not an inductor. \$\endgroup\$– ChuCommented Jun 13, 2021 at 7:10
-
10\$\begingroup\$ Re, "...are just inductors." What they are is coils of wire. What we call them depends on which aspect of their behavior we find most interesting. We call them "electromagnets" when we're talking about how they actuate mechanical components. We call them "inductors" when we're designing the driver circuit that needs a flyback diode to protect the output transistor from the inductive "kick," etc. We could even call them "resistors" under some circumstances, though at that point, it might be smart to think of it as two components; a "resistor" and an "inductor" in series. \$\endgroup\$– Solomon SlowCommented Jun 13, 2021 at 14:35
-
\$\begingroup\$ Ideal inductors would indeed act the way you describe: if you stall an ideal DC motor, it will produce infinite torque and consume infinite current. If you're talking about real-world components, then "short" is only defined in a context: in some schematics, 1 kOhm is a short, while in other 1mOhm is a significant resistance. \$\endgroup\$– Dmitry GrigoryevCommented Jun 14, 2021 at 11:50
Add a comment
|
5 Answers
\$\begingroup\$
\$\endgroup\$
5
No component is ideal, and inductors tend to be the least ideal of the passive components--and relays and solenoids are even less ideal than your average inductor, because they aren't designed to be close to ideal.
All materials (other than superconductors in their superconducting phase) have some positive resistivity. For copper, this is a very low resistivity, but it's still nonzero. Solenoids and relays try to maximize the magnetic force produced by current through a coil, so they use many, many turns of very thin wire--and "long and thin" is exactly the recipe for a high resistance. So there's a significant real resistance in the coil in addition to its inductance, and this resistance sets the steady-state current.
-
12\$\begingroup\$ Also, while relays and solenoids might operate on a static magnetic field, motors operate on a dynamic magnetic field and the DC current is commutated across different coils inside the motor so it's not really DC inside the motor anymore, nor is it just an inductor in a motor...there's back EMF produced which opposes the voltage on the outside. \$\endgroup\$– DKNguyenCommented Jun 13, 2021 at 0:42
-
\$\begingroup\$ I completely missed that the question was asking about motors, too! \$\endgroup\$– HearthCommented Jun 13, 2021 at 0:43
-
\$\begingroup\$ How long and thin are we talking? I've tried to search for this but I can't find typical values for coil wire gauge, or number of turns for typical signal relays anywhere. 100mA @12V gets you ~80 Ohms which seems way way above the resistance of even really long data cables. \$\endgroup\$– ShredderCommented Jun 13, 2021 at 4:10
-
1\$\begingroup\$ Data cables aren't nearly as thin as the wire used in relays, and I expect the winding is several kilometers of wire in some of the higher-voltage relays. \$\endgroup\$– HearthCommented Jun 13, 2021 at 4:12
-
\$\begingroup\$ I think this answers the question. I figured out the equivalent resistance, and the length of wire and thought that was impossibly high. I can't find any numbers for reference of how many turns of wire are needed in a typical relay. \$\endgroup\$– ShredderCommented Jun 14, 2021 at 14:13
\$\begingroup\$
\$\endgroup\$
3
All inductors have series resistance. Well, unless they are made with super-conducting wire, I guess. But coils designed for DC operation have sufficient DC resistance to limit the DC current to a reasonable level for continuous operation. This is convenient so that the user of the coil does not have to figure out a way to limit the current. It is like having a coil plus a current limiting resistor combined in a single unit.
As far as motors go, they have back EMF. That is to say there is a magnetic field in motion with respect to a coil. So a voltage develops across the coil.
The wire typically used for solenoids and similar is called "magnet wire." This wire has a very thin insulation coating often referred to as "varnish" or "enamel" even though it may actually be a more modern polymer coating such as polyurethane. It is readily available in very small diameters. For example 30 AWG magnet wire has a diameter of 0.28 mm. Its resistance is around 330 Ohms per km. The weight of this wire is under 500 grams per kilometer. The coating on this wire is designed to withstand high temperatures. Operating temperatures of 150 C or even higher are possible.
Smaller diameter magnet wire is also available.
-
\$\begingroup\$ Sure, but it would have to be a ridiculously long wire to have resistances of 100 or 200 Ohms right? I've been googling but I can't seem to find any typical values for standard gauges or number of turns for common place relays, I don't have a frame of reference for what's implemented. \$\endgroup\$– ShredderCommented Jun 13, 2021 at 4:08
-
12\$\begingroup\$ Get a cheapy relay and pull it apart. Have fun unwinding the coil and counting the turns. You’ll be there a while…. \$\endgroup\$– KartmanCommented Jun 13, 2021 at 4:21
-
\$\begingroup\$ but it would have to be a ridiculously long wire to have resistances of 100 or 200 Ohms right Not at all. I routinely use SMT relays with about 0.75cm^3 volume that have 2kOhm coil resistance. They are wound using a ridiculously thin wire, but not super long - their coil is tiny. \$\endgroup\$ Commented Jun 14, 2021 at 16:08
\$\begingroup\$
\$\endgroup\$
When you have two resistive elements in series, like a coil and resistor in a DC circuit, then each one consumes power proportional to its resistance.
That is why you don't put a resistor in series with a relay or motor. You want to deliver power to the relay or motor instead of wasting it in a resistor.
The magnetic force produced by a relay or motor coil is proportional to current * turns. For a given coil material and a given space for the coil, you need to fill that space in order to maximize how much force you get for any energy input.
For constant power, you could fill that space by increasing the thickness of your wire, which would increase the current (decreasing voltage). Alternatively, you could use more turns (increasing voltage and coil resistance).
Regardless of which way you choose, the maximum force you get is the same, so once you determine how much power you want to spend in your coil, the wire thickness is specifically chosen to produce the desired resistance, when the coil space is filled with turns of wire.
\$\begingroup\$
\$\endgroup\$
1
The correct DC voltage across a solenoid or relay is important to ensure it actuates and doesn't overheat. A resistor in series with a low voltage solenoid may be useful to allow it to be operated from a higher voltage. For example the KEMET EC2-5NU is a 5V relay with a coil resistance of 178 ohms. To run it off 12V a series resistor of about 250 ohms would be suitable. This will be inefficient as both will have a 28mA current flowing through them.
As it is likely that a relay is being switched by a micro controller a better (more efficient) way is to pulse width modulate the driving transistor, in this case with a duty cycle of 5/12 or 42%. The average current will then be less, about 12mA.
As DC motors draw different current as their back EMF changes the PWM approach is the only suitable approach to use.
-
1\$\begingroup\$ Welcome to EE.SE, Ken. I think you may have missed the point of the question. You can edit to improve your answer. \$\endgroup\$ Commented Jun 13, 2021 at 21:07
\$\begingroup\$
\$\endgroup\$
Ideal inductors would indeed act the way you describe: if you stall an ideal DC motor, or apply DC to a solenoid, they would produce infinite force and consume infinite current.
If you're talking about real-world components, then "short" is only defined in a context: in some schematics, 1 kOhm is a short, while in other 1 mOhm is a significant resistance. In case of relays, the coil resistance is high compared to the other parts of the circuit, so the current is limited. The same is true for stepper motors which are designed to hold a position indefinitely while being powered by DC. Power motors, by contrast, have a low coil resistance and are not designed to be stalled. This is why one could say that applying DC to a power motor coil is a short.
Speaking of power motors, they can be powered by DC as a whole, but individual coils are supplied by AC, produced either by a mechanical commutator or by an invertor circuit in case of a BLDC.
answered Jun 14, 2021 at 11:58