We bought a new sewing machine that is "practically perfect in every way" but one.
It is nearly impossible to start up very slowly, the speed control has no finesse at all. Once rolling, you can back it down to a nice slow pace but that does not help much. When easing on the foot control, it just growls and then jerks into motion.
I tried opening it to see what kind of motor it has but after removing about 10 screws, I still could not get it apart without possibly braking the casing.
I imagine all low end machines use the same type of motor and have the same sort of problem but I am wondering if there is a fix. I think the speed control is in the foot pedal and if getting something better would help, that would be nice but there may be no alternative way to control the speed.
If the pedal speed control is just a variable power resistor (rheostat) in series with the motor, which was very common on sewing machines, the motor is probably a universal motor. You will get better low speed control by replacing the pedal with a triac-based phase control motor controller (light dimmer style, but designed specifically for a motor load). The phase control will provide more torque (and therefore more control) at low speed.
My mother would use one hand to "kick start" the sewing machine; she would depress the pedal and get the growl and then give a little shove on the big wheel of the machine to get it started. Not an electronic fix, but certainly a low tech solution.
Jack, From your description, it appears that the problem is not the motor itself, but either the controller, or the internal mechanics of the machine.
Lets talk about the Controller First. Disconnect it from the machine or, if it cannot be disconnected, unplug the machine from the wall. Hold it in your hand and squeeze the part you put your foot on. Initially the thing should not move easily, but should require an increased pressure to overcome the spring preload. (your foot does not have the sensitive feedback your hand has and it is somewhat stronger then your hand) Once the button starts moving, the movement should be nice and uniform with increasing force with your fingers. if the movement is not smooth but jerks kind of like your description of the motor operation, then you may have something not alligned properly inside the controller. Also check with the pressure not applied centrally on the button, Maybe the foot pressure is off to one side, causing the button to bind on the controller housing. If the movement is not smooth, Then you have no hope of smooth machine operation.
Now to the motor and it's load.
I would get your machine operator ( you or your spouse) to set up with the type of sewing that is causing the problem and once you have the problem situation, Again turn off the power to the machine and try to turn the machine by hand. (Assuming, of course, that your machine has the wheel you can turn by hand) Is the thing easy to turn ? or does it have that hard, Tight, bit at the start ?
It is possible that in certain sewing situations can have a hard starting point, but it is more probable that if it is hard to start then there could be something wrong in the machine mechanism.
These couple of tests should get you down the right path to identifying the problem. There is the possibility that both these tests could show up nothing wrong, in which case I would be looking at how the motor is coupled to the sewing mechanism, Maybe something there is not alligned right, and that is causing an initial binding.
Of course, before Proceeding to dismantle the machine, if it is new and covered by a warranty, then I would persue that before all else.
I have two Necchi sewing machines, and I know what the OP is having a problem with. One machine has a ribbed drive belt directly from the motor to the crank shaft of the machine, and the controller is a phase control system foot pedal. This machine acts just like the OP had described: there is not much power at the lower end of the controller and the system will lurch when more power is applied.
The other Necchi machine that I have is a swing-arm machine, and the controller is also a phase control, but the motor drive is double-linked to the main crankshaft: there is a drive belt to a pulley system that reduces the drive ratio, and then another drive belt to the crankshaft. This gives much better control over the lower end of the drive spectrum for smooth starting. It is not as fast in the upper spectrum as the other Necchi model, but I don't need speed. I'm not in that much of a hurry.
I am going to look into the speed control that was posted here that describes a high torque at the low end.
I can adapt this item to the pedal setup as a replacement for the original controller.
As Tom and Roy mentioned, it is worth checking that everything mechanical is easy to move before looking for an electronic solution.
As I mentioned, I use/have used this controller and can recommend it - it is far more complicated than the simple CR delay + diac used which does give it a much wider range of phase control - the extra complication is worth it.
However, it certainly doesn't equal the performance of a pwm system for low speed operation - which will always be much superior.
Pwm combined with velocity feedback, as John mentioned, is close to ideal. This isn't as difficult as it may sound - as optically divided disks are now freely available - depnding on the drive shaft diameter, one from a mouse or one from a cheap inkjet printer will do nicely. Simply take the pulses from the optically coupled pair to a simple diode+C+R integrator and use that as the control voltage feedback to the pwm controller. I haven't seen kits for these but have made up countless number of the things. If the disk has a zero position reference (if not, you can make one easily enough- eg by filling in one slot to give a wide pulse) then you can use that to give positional feedback (e.g ensure that "on" pwm pulse coincides with the needle about to enter the material). Add a second integrator and you can feedback acceleration as well as velocity and position - so the velocity won't overshoot as the needle leaves the material. Not that that probably matters, but I couldn't say - sewing machines and I live in different Worlds ...
This combination of speed control and motor has been around since forever. You can spend a lot, experiment a lot, and perhaps derive something that will work better. But millions of seamstresses have learned the technique of goosing the controller just enough to break the motor loose and get it turning, and then backing off to control the speed. Takes a little finesse, but my Mom and two sisters have done it for years.