Author Topic: Newbie: windings relationship to voltage  (Read 1530 times)

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Offline rthorntnTopic starter

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Newbie: windings relationship to voltage
« on: September 02, 2018, 12:24:49 am »
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

 

Offline Brumby

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Re: Newbie: windings relationship to voltage
« Reply #1 on: September 02, 2018, 12:49:34 am »
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?
Yes

Quote
- they both use windings?
Yes.  Windings are what create an electromagnet.

Quote
- they both use inductors?
Inductance 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.

Quote
So what governs the optimal voltage to be put through a motor or solenoid?
Heat.  A simplistic answer I know, but heat is often a limiting factor where voltage and/or current considerations look ok.

The basic effectiveness 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.
« Last Edit: September 02, 2018, 12:54:59 am by Brumby »
 
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Online IanB

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Re: Newbie: windings relationship to voltage
« Reply #2 on: September 02, 2018, 12:51:51 am »
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.
 
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Offline rthorntnTopic starter

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Re: Newbie: windings relationship to voltage
« Reply #3 on: September 02, 2018, 02:30:24 am »
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?
« Last Edit: September 02, 2018, 03:56:39 am by rthorntn »
 

Online IanB

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Re: Newbie: windings relationship to voltage
« Reply #4 on: September 02, 2018, 03:34:08 am »
Thanks, so for motors, current determines the torque, voltage determines the speed and the watts it can dissipate as heat is determined by its design?

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 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 it will also have to be able to handle the extra 7W of heat?

So if you want a small 48v motor with low torque and high speed, to power a fan blade, it needs to be very low current, otherwise it will generate a lot of unnecessary torque and heat?

Well, 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.
 
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Offline rthorntnTopic starter

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Re: Newbie: windings relationship to voltage
« Reply #5 on: September 02, 2018, 03:44:08 am »
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.

« Last Edit: September 02, 2018, 04:20:08 am by rthorntn »
 

Offline Connoiseur

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Re: Newbie: windings relationship to voltage
« Reply #6 on: September 02, 2018, 04:16:08 am »
Thanks, so for motors, current determines the torque?

No! It's the other way round. Torque determines how much current your machine draws.



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 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 it will also have to be able to handle the extra 7W of heat?


No! 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).

So if you want a small 48v motor with low torque and high speed, to power a fan blade, it needs to be very low current, otherwise it will generate a lot of unnecessary torque and heat?


Just 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)  :horse:
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.

 
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Offline Zero999

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Re: Newbie: windings relationship to voltage
« Reply #7 on: September 03, 2018, 10:50:58 am »
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?
No, because increasing the voltage will increase the speed, unless the load is also increased to compensate.

Quote
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?
If 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.

Quote
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.
Yes that's true.
 

Online IanB

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Re: Newbie: windings relationship to voltage
« Reply #8 on: September 03, 2018, 04:08:15 pm »
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
 


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