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Author Topic: Whats the difference between a 12 and a 240v motor?  (Read 3387 times)

RipSlider

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Whats the difference between a 12 and a 240v motor?
« on: August 10, 2008, 09:57:20 PM »

Hello all.


Quick question:

Barring the difference in design between a AC and a DC motor ( and I know that they are differing designs) , what is different about the motor that means it works at a specific voltage?

For e.g. if I took a motor designed for 240v, and ran 12v AC into it, what would be the result?

And WHY would this result occur?

this is a "theory of motors" questions. essentially - "Are motors made for a specific voltage, and if so, how?"


Ta

Steve
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Martin13

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Re: Whats the difference between a 12 and a 240v motor?
« Reply #1 on: August 10, 2008, 10:42:25 PM »

228 Volts ::)
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DickyD

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Re: Whats the difference between a 12 and a 240v motor?
« Reply #2 on: August 10, 2008, 10:46:25 PM »

Actually Martin it's 228 volts, but that wasn't the actual question only the topic. :angel:

You modified that as I posted.  >>:-(


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Martin13

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Re: Whats the difference between a 12 and a 240v motor?
« Reply #3 on: August 10, 2008, 10:56:48 PM »

Actually Martin it's 228 volts, but that wasn't the actual question only the topic. :angel:

You modified that as I posted.  >>:-(




It's early here and just got out of bed, noticed the error and quickly changed O0
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barry park

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Re: Whats the difference between a 12 and a 240v motor?
« Reply #4 on: August 11, 2008, 02:19:53 AM »

In case you made a 24 240 typo.....
The difference between a 12 and 24 volt DC brushed magnet motor is quite simple. A 24's armature is wound with copper of half the csa making it 4 times the resistance and twice as many turns. Same as a relay or transformer.
volt  turns     current      Cu cross sect. area   Cu length  resistance
12
24    x2          x 0.5             x 0.5                  x 2             x4
Voltage of a motor is a purely nominal figure.    Its limit is in terms of temperature...... so can run a 12 volt motor at 24 as long as you half the load and within the same temperature limit (so long as the bearings are happy)

      12 and 240 volt AC motor  (long story.....many different types )The difference between a 12 and a 240 volt AC motor is more complicated as 240 volt AC motors come in different types.

A 240 volt shaded pole motor out an extractor fan or bench top grinder will probably stall on 12 AC (no good on DC)

A 240 ac geared motor out a central heating timer (needs a direction starter) would be ideal for radars as it is synchronous to AC frequency & low power so could be driven from 12 or 24 DC through logic chips generating AC at the correct frequency. ( no good on DC )

Some 240 v motors (called universal motors) have a DC commutator winding and a rectifier to covert to DC. You would find these in washing machines and very old power drills which run on 24 v DC or 240 AC where they resemble a turbine. A small washing machine motor on 24 volts would be ideal for a 12 foot single screw model boat.

Back to the Olympics on telly
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malcolmfrary

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Re: Whats the difference between a 12 and a 240v motor?
« Reply #5 on: August 11, 2008, 10:43:09 AM »

Running a motor from a mains appliance on a lower voltage (assuming the same conditions of AC/DC/rectification) will result in a slower rotation.  There will come a point where the power generated is no longer enough to overcome the friction in the motor, when it will just sit there looking back at you.  Its just one way of getting a very heavy motor to do a job that a much smaller one could do.  The good side is that its unlikely to burn out.
There is a handbook for people who design such things that allows you to look up how many turns of what gauge of wire can be fitted into a given armature, and what the specific impedance of any particular arrangement would be, allowing calculation of likely current flow and thus power dissipation at any given voltage.  Knowing very well that I was unlikely to design motors, I dozed gently through those bits when doing principals at tech (that was the lesson first thing after lunch on a Friday......). 
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Sandy Calder

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Re: Whats the difference between a 12 and a 240v motor?
« Reply #6 on: September 05, 2008, 03:57:39 PM »

I found the difference between AC and DC motors here:- http://www.physclips.unsw.edu.au/jw/electricmotors.html
 
Sandy Calder
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sweeper

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Re: Whats the difference between a 12 and a 240v motor?
« Reply #7 on: September 06, 2008, 03:55:46 PM »

Quote
Some 240 v motors (called universal motors) have a DC commutator winding and a rectifier to covert to DC. You would find these in washing machines and very old power drills which run on 24 v DC or 240 AC

Sorry, your concept of a universal machine is wrong.
They have a commutator (never heard of a DC commutator)
They do not employ any form of rectification.

This type of motor is basically a series wound machine with virtually the same electrical characteristics. They give a high torque output at low speed (ideal for starting a load from rest) and high speed at low loads. They are invariably fitted with a cooling fan which acts as an air brake so that if the drive belt breaks the motor (being thrown into a no-load situation) will accelerate but will not reach critical destruction speed. They are commonly used in small appliances such as vacuum cleaners and spin driers. If the fan is removed, the no-load speed will increase until there is a very high risk of the armature widings "birdcaging" i.e. they are thrown outwards by centrifugal force. The windings are normally held in their slots by insulated packing to prevent this. The weak spot is often the gap between the commutator risers (the connection point for the armature windings and the commutator segments) and the armature. There is often a binder applied and varnished into place over this area.
There is only a small clearence between the armature and the pole pieces. If the two come together (at around 20k RPM) the results are "interesting".
In general, it is bad practice to belt drive with any series machine above fractional horsepower rating. Always employ a solid drive.

Regards.
 
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barry park

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Re: Whats the difference between a 12 and a 240v motor?
« Reply #8 on: September 06, 2008, 06:29:40 PM »

Quote
Some 240 v motors (called universal motors) have a DC commutator winding and a rectifier to covert to DC. You would find these in washing machines and very old power drills which run on 24 v DC or 240 AC

Sorry, your concept of a universal machine is wrong.
They have a commutator (never heard of a DC commutator)
They do not employ any form of rectification.
Hi Sweep,
My concept of series wound motors is not totally wrong as I was actually thinking of a shunt wound motor.My excuse was that my post was in the middle of the night.
Thanks to Sandy for clearing that up!
Regards
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