Re: [CAD_CAM_EDM_DRO] Synchronous motors Vs. Stepper Motors

Hi,

The explanation below is very accurate but probably doesn't go far enough
for someone that asked what was the difference between a stepper and a
synchronous motor.

Synchronous Motor :

These motors are usually mains powered by AC and designed to run at a
continuous speed dependent on the frequency of the AC. In the UK we run at
50hz @ 240volts in the States it is 60hz @ 110volts. The synch speed is also
dependent on the number of poles. This is usually 2, 4 or 8 but other pole
configurations are possible. The power is normally measured in hp or
fractions of a hp. With 1 hp = 746 watts. These motors can be designed to
run on single and three phase power but without some form of phase converter
are not interchangeable. It is possible to run a three phase on single phase
power with a phase converter but there is a penalty in that they run hotter
and not as smoothly as the motor designed for the job. I am aware of the
resent improvements with electronic controls but they are very expensive.

At 50hz and the most common a 4 pole motor the synch speed is about 1450rpm
with an 8 pole at 960rpm and a 2 pole at about 2850rpm. These speeds are a
little higher in the States because of the higher frequency of the mains.

Most synchronous motors consist of a run winding and a start winding. In
order to get the thing turning an error must be induced called a phase
shift. The motor must be converted to two phase working for starting. There
are a number of ways to achieve this :

Split-phase in which the necessary phase-shift to the start winding is
obtained by a combination of inductive and resistive characteristics in the
winding.
Capacitor Start/Induction Run where a capacitor provides the phase-shift at
starting, disconnected when running.
Capacitor Start and Run in which capacitance is used both in starting and
running. In some cases a centrifugal switch may be used to connect extra
capacitance at starting, to improve the starting torque. The centrifugal
switch usually opens at about 75% of full speed, and re-closes somewhat
lower.

These motors are also rated in the number of starts per hour as with every
start a lot of heat is generated and it needs time to cool or the motor will
be destroyed. They are designed to run at a continuous speed with few
stop-starts in one direction only. It is possible to chance the direction by
reversing the wires or either the start winding or the run winding but this
can only be done with the motor stopped. Speed control and either DC
injection or mechanical braking is also possible but usually only found on
larger machines, like radial-arm drills, as a safety feature.

Stepper Motor :

These motors are designed for positional control and they can have a few
poles to many hundreds of poles. The more poles the smaller the step angle.
In practice this is usually 2 or 4 pole but the rotor has teeth on it which
makes it seem that there are hundreds or poles. We use mainly 200 steps per
rev which is a step angle of 1.8 degrees. The faster a stepper motor turns
the less torque it has and the torque is usually measured as holding torque,
in the US it is measured in oz. inch or for larger ones lbs. foot. These
motors are not designed to run continuously and work by giving the pole in
front of the rotor a DC pulse long enough to attract the rotor. If you want
to keep the motor going in that direction a further pulse is given, again to
the next pole in front of the rotor. If it is required to stop the rotor on
this pole the pulse is continued on this pole and once the rotor gets to
this position it will stop and stay there. All this requires a driver
control circuit and a means of telling the driver which pole to energize.
This is where the computer comes in with the software necessary for CNC.

I hope this explains more than confuses.

Regards,

Terry

> Ejay Hire wrote:
>
> > Synchronous motors Vs. Stepper Motors
> >
> > What is the Difference?
>
> In one sense, they are quite similar. If you are asking about
> the Superior Electric steppers vs. their "Slo-Syn" motors,
> the only difference is that the Slo-Syn motors usually are wound
> for 120 V AC drive, and have very high inductance compared
> to similar stepper motors. This limits their top speed.
>
> In a more general sense, the biggest difference is that stepper
> motors are optimized for moving in small increments and having
> high holding torque, while synchronous motors are optimized
> for moving at constant speed, as for clocks and tape recorders.
> Otherwise, the stepper motor usually has a lot of poles, and
> the synchronous motor usually has from 2 to 8 poles. But, there
> are synchronous motors with a high pole count.
>
> Jon