Newbie: windings relationship to voltage - Page 1
Forum section: electronics / beginners

author: rthorntn
author_rank: Frequent Contributor
timestamp: September 02, 2018, 12:24:49 am
content: Hi,So say I'm building a device, it must use 48v DC.Among other things the device will have a motor and direct acting solenoid, say i'm building them myself, I'm trying to figure out what makes a motor and/or solenoid a certain voltage, is the following correct:- they both use electromagnets?- they both use windings?- they both use inductors?So what governs the optimal voltage to be put through a motor or solenoid?Thanks.Richard

author: Brumby
author_rank: Unknown
timestamp: September 02, 2018, 12:49:34 am
content: Quote from: rthorntn on September 02, 2018, 12:24:49 amYesQuoteYes. Windings are what create an electromagnet.QuoteInductance is a property arising from having a current flow through a conductor. Windings produce greater inductance because of the multiple turns of wire grouped together.QuoteHeat. A simplistic answer I know, but heat is often a limiting factor where voltage and/or current considerations look ok.Thebasiceffectiveness of a motor or solenoid relate to the current flowing and the number of turns in each winding - but the geometry and materials used are also important. The actual calculations for designing your own motor and solenoid are something I will leave to the experts. We are bound to have someone here with a better handle on that than myself.However, if I want something particular, I'll just use my requirements and look up specifications of products already available.

author: IanB
author_rank: Super Contributor
timestamp: September 02, 2018, 12:51:51 am
content: First you figure out how much current the device needs to do its job. Then you arrange that the resistance/impedance/reactance of the device is such as to limit the current to what is desired. For the solenoid it is just resistance, for the motor it will depend on load and speed. Also, most DC motors need some kind of control or regulation over their speed otherwise they will not run in the manner desired. You can't just stick a voltage on them and hope for the best. Even for solenoids, you might want to control them to vary between the pull-in force and the holding force so the coil doesn't overheat.

author: rthorntn
author_rank: Frequent Contributor
timestamp: September 02, 2018, 02:30:24 am
content: Thanks, so for motors, current determines the torque, voltage determines the speed and the maximum voltage is therefore about efficiency and reliability?So in the same motor, provide it with either 5V or 12V and it will draw the same amps?If so, let's say the 12v motors current is 1A, so at 5V it will consume 5W and turn at "slow" RPM, at 12V it will consume 12W and turn at "fast" RPM (the torque will stay the same) but at 24v the extra speed will probably cause reliability issues?

author: IanB
author_rank: Super Contributor
timestamp: September 02, 2018, 03:34:08 am
content: Quote from: rthorntn on September 02, 2018, 02:30:24 amWell, no, I didn't say any of that.Also, an ideal motor is not supposed to dissipate any power as heat. Any heat is wasted power and represents a loss of efficiency. Of course real motors cannot be 100% efficient, but even so most of the electrical input power is supposed to leave as mechanical output power. Very little is supposed to be turned into heat.

author: rthorntn
author_rank: Frequent Contributor
timestamp: September 02, 2018, 03:44:08 am
content: Thanks, I reread my post the wording isn't right.But a motor once up to speed with a consistent load will always use the same amps regardless of voltage?What if I have a motor rated for 24v and I put 48v through it, if it works for long enough it will spin a lot faster but I will potentially shorten it's life, what potentially fails, is it a mechanical failure because it can't handle the speed, it gets past a certain speed and excess heat builds up?From googling it sounds like good motors should specify their peak current, speed, temperature and that excessive voltage breaks down the insulator inside the motor.So monitoring of motor current, speed and temperature makes sense.

author: Connoiseur
author_rank: Regular Contributor
timestamp: September 02, 2018, 04:16:08 am
content: Quote from: rthorntn on September 02, 2018, 02:30:24 amNo! It's the other way round. Torque determines how much current your machine draws.Quote from: rthorntn on September 02, 2018, 02:30:24 amNo! It will draw higher current if you supply higher voltage as windage losses, vibration losses etc. depend on speed. Consequently, they load the armature more and more as the speed increases.In DC machines, the major causes of heat are Joule heating in armature coils, brush contact voltage drop, iron losses (hysteresis & eddy current loss in armature core) and in the case of field excited machines, field winding copper loss (not applicable to permanent magnet machines).Quote from: rthorntn on September 02, 2018, 02:30:24 amJust as stated above no machine can generate unnecessary torque. It's up to the user to extract it (of course only till the current is within the safe operating region for the given ambient temperature)For a given power, the product of torque and speed is constant. You can either have a lot of torque and low speed or vice versa.The speed of any electrical machine depends inversely on the number of magnetic poles. i.e. more the poles; less the speed; more the TORQUE PROVIDING CAPACITY.

author: Zero999
author_rank: Super Contributor
timestamp: September 03, 2018, 10:50:58 am
content: Quote from: rthorntn on September 02, 2018, 03:44:08 amNo, because increasing the voltage will increase the speed, unless the load is also increased to compensate.QuoteIf the load is light, so the current through the windings is not exceeded, then the bearings and brushes will fail first, as they will be operating at double their design speed.QuoteYes that's true.

author: IanB
author_rank: Super Contributor
timestamp: September 03, 2018, 04:08:15 pm
content: Here's an illustrative video of a DC motor running up to speed. You can see the current through the motor in real time, and you can also see something of how the motor is controlled to keep it operating in the desired manner. The current is not at all constant, but varies quite a lot.If you look at the motor current indicator you can also see a "red band" showing the region where the motor would overheat from exceeding the maximum allowable current.https://youtu.be/tJNqpcsvlqI