I have an old sewing machine motor with a foot pedal that I would like to replace my rod drying motor with. I could make a clamp for the foot pedal to control the motor speed but I thought in my oldc electronics class that operating an electrical AC on less than 110 volts was bad for the motor. However, ceiling fans run a various slow speeds so there must be something different with the way it's wound. And since my sewing machine motor came with a variable speed foot pedal, I assume I can run this motor at a low rpm for several hours without burning the house down . . . ? Thanks.

I'm not, but my Dad is. I picked up a few (very few) things along the way. Most variable speed motors are DC, not AC. Most that run on AC power convert the current to DC before it reaches the motor. Someone more adept than myself will have to fill in the details. I think Emory Harry could help you here.

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Tom is correct about the DC motor. That is definately the way to go...completely variable speed. You can see if your sewing machine motor is AC or DC by using a voltmeter to test the wires that go directly into the motor. If it turns out to be DC, then you are in luck. You can yank the motor out and set it up the way you want. You can make a device to change the RPM's by getting a bridge rectifier (turns AC into DC) and a big rheostat. (A rheostat is a variable resistor with a knob that will let you increase or decrease the voltage.) Just be sure that the rectifer and rheostat are 'big' enough to handle the 120 VAC and the amount of current drawn by the motor. I hope this helps.

Any electrical motor that has brushes will run on either AC or DC voltage, (Note: the motor does not convert AC to DC), and the speed of these motors can be controlled by the amount of the voltage applied to the motor. The speed of sewing machine motors is controlled by the foot pedal, which is a high powered rheostat (aka: variable resistance). Lamp Dimmer switches are another method of controlling the speed of AC/DC motors.

You can run a sewing machine motor at a reduced speed for a short time without burning the motor up. However, running the motor at low speeds for long periods of time will cause the motor to over heat and ultimately burn out. This over heating is due to the following principals of operation of any motor.

1. The motor offers very little resistance to current flow when the power is first turned on - this results in a relatively large surge (starting) current . As the motor speed increases, a counter electromotive force (CEMF), proportional to rpm, starts to build up in the motor's armature, which is in opposition to the applied voltage, this CEMF buildup continues until the CEMF reaches it's designed value,. At this point, there will be a difference between the Applied Voltage and the CEMF. This difference voltage is the actual amount of voltage that the motor is designed to operate at - and it is always much less than the applied voltage.

2. Since the CEMF is proportional to rpm, then this voltage will be low for low rpms, and higher for high rpms.
Which in turn causes a much higher difference voltage to be applied to the motor when operating at reduced rpms. Keep in mind that this difference voltage is what causes the motor to operate and is what causes power dissipation. At some point, (as a function of low rpm) the difference voltage exceeds the power dissapation ability of the motor and it will burn out.

3. To prove the forgoing - prevent the motor shaft from turning (zero rpm) and the motor will instantly get hot and ultimately burn out. - Why? Because you prevented the motor from generating the CEMF and so the difference voltage is the full applied voltage.

Confusing but factual,

John Mantele BSEE 1951

I am afraid that I am not going to be able to add much to what has already been said. I remember studying motors in school but that was a heck of a long time ago and I have had zero experience with them since. Plus there are a heck of a lot of different types of motors, parallel, series and compound DC motors: universal, induction and syncronous AC motors all of which have different characteristics none of which I remember.
I suspect that Tom and Brian are correct that it is a DC motor.

Tim,
I think that what I would do is run the sewing machine motor at the slower rpm but check it fairly often just by putting your hand on it and then on the AC wire coming into the motor. If it is drawing too much current the motor case or the AC input wire will get warm and you should be able to feel it in just a few minutes. If it does not get hot I would not worry about it. I do vaguely remember that some types of AC motors draw more current at low rpms than at higher rpms.
The little motors intended for drying are very cheap why don't you just get one and make a seperate dryer.

Ask me how the meter that Brian suggests works. I can tell you exactly how that works in more detail than you probably care.

John,
Thanks, In reading your post it is all starting to come back to me. Many motors are brushless. Do all of the sewing machine type motors have brushes?

I just dug out that sewing machine motor and the plate on it says "110-120 volts, .53 amp, 25-75 cycles, AC & DC". In the junction box, the AC cord comes in, one side attaches to one motor lead, the other connects to one lead of the foot pedel cord, the other foot pedal lead hooks to the other motor lead, completing the circuit. It looks the same as installing a light switch for a light. I put the Fluke meter leads across the two foot pedal leads and the "faster the motor went, the AC voltage reading decreased"! This isn't eactly how I thought it would work and I'm sure Emory is chuckling to himself waiting to see how I did. I ran the motor slow for a couple of minutes and it didn't warm up at all.

Saving money wasn't the reason for trying this motor, I was thinking of having the versatility of a variable speed motor - then again I wonder why I would want that!

Tim,
I'm not chuckling. I didn't know any better than you did. John is the only one that knew what he was talking about. The reason you saw the voltage decrease if you put the meter leads across the foot pedal is I think you were measuring the voltage across the rheostat not across the motor. The rehostat is just a variable resistor that must be in series with the motor. As the voltage across it decreases the voltage across the motor will increase.

Tim,

Sounds like you might have a universal motor. It will run on AC or DC. The speed is varied by adjusting the input voltage. Check for brushes. These types of motors are common in mixers, hand held power tools and sewing machines. There will be a lower limit to the speed that the motor will run. This is a function of the motor's construction. As far as over heating, this will be related to the speed that the motor is running at and the amount of fan cooling built into the motor. Usually there is an fan built inside the motor. Run the motor at full speed and feel for air circulation around the motor. If your idea was to run this motor at a very low speed, as in drying, I wouldn't recommend that. It would be better to get a dedicated drying motor. They are geared to run at low speed all day.

Ed

Don't forget that by gearing down the motor using either gears or pulleys, you can allow the motor to run faster than if it was on a 1:1 direct drive to your wrapper or dryer. If you're worried about long hours of drying at low speeds, you can always use the sewing machine motor for wrapping and something slow like a rotisserie motor for drying. On my home made rig, I have both types of motors mounted so either one can drive the blank, depending on where I put the drive belt.



Edited 1 time(s). Last edit at 03/02/2005 06:34PM by Rich Levy.

Tim, it would be much eaiser to go to Fishsticks4u website click on wrappers scroll down to Forecast click on wrappers and parts and look at part WARP06 price $6.22.