In 1938, General Electric began producing a two-phase synchronous induction motor which, at 60 Hz, ran at 75 RPM. The low speed resulted from using a different number of rotors to stator poles or teeth, which made the motor a good bi-directional control motor. This motor was used by Superior Electric Company of Bristol, CT, to run power driven autotransformers used to dim lights in auditoriums and similar applications. General Electric ceased producing this motor in the mid 1950’s due to slow sales.
Superior Electric engineers then redesigned the motor to run at 72 RPM on 60 Hz and began to consider using a four-step DC voltage sequence to have the motor move in increments of 1.8 degrees per step or 200 steps per revolution. Microswitches, magnetic reed switches, and relays were used to demonstrate the basic operation at engineering conferences and seminars. In the early 1960s, transistors made operating the stepping motor practical and applications quickly developed. The hybrid stepping motor came alive.
Haydon began making stepper motors in the 1960s. Now called Haydon Kerk Motion Solutions, the company manufactures stepper motor-based linear actuators, rotary motors, lead screw assemblies, and linear rail and guide systems used in niche market applications.
Stepping Motor Principles
If you create two magnetic fields of opposite polarities, they will attract each other – creating motion. If one of the magnetic fields is fixed on a shaft, you have rotary motion of some angle. Now, to continue this rotary motion, you have to create a new magnetic field at a different position.
There is an electronic drive that sends the electrical pulses to the two phases at the appropriate time to create rotation. In this example, the rotation is 90 degrees per pulse or step. In practice, the motors have more poles to create smaller step angles, such as 15 degrees, 7.5 degrees or 1.8 degrees per step. The term “stepping motor” traditionally refers to a motor that runs from pulses from an electronic drive. By sending the motor a specific number of pulses, you would know the rotor position at any time. The movement created by each pulse is precise and repeatable, which is why stepper motors are so effective for positioning applications.
All motors are really stepping motors. The only difference is the size of the step angle and how the switching of the pulses is created. Many AC motors can be run as stepping motors. The two-phase alternating current creates a four-step sequence just like the electronic drive, except that it is a sine wave instead of a square wave.
The limitation is that you are limited to only one speed, the 60 hertz of the incoming power. In this case, since the motor runs synchronously with the incoming frequency, it is now called a synchronous motor – not a stepping motor. However, it is the same motor. Nothing has changed. The motor can also be made to run on a single phase AC current. A phase shifting circuit is used to split the single phase into two phases.
Brush type DC motors (see Fig. 2.) have several coils wound on a rotor or armature. These coils are connected to a commutator. The commutator comprises copper segments. The switching is done mechanically instead of electronically, by means of a commutator with 24 copper segments. The coils are connected to the copper segments. Two brushes, 180 degrees apart, connect to the appropriate coil which rotates the rotor 30 degrees.
This rotation causes the brushes to connect to the next coil, which causes another 30 degree rotation. This, in effect, causes continuous rotation. The field can be either permanent magnets or a wound field. If it is a wound field, it is called a universal motor, because it can run on alternating current or direct current. Another version of this motor replaces the commutator and brushes with Hall-effect cells and a magnetic ring magnetized with many poles. This is called a brushless DC motor, as shown in Figure 3.
Variable Reluctance Motors
A variable reluctance motor is a stepping motor that does not use a permanent magnet. A step is achieved by the principle that the rotor will rotate to minimize the reluctance path of a magnetic circuit.
In the first step, pole 1 is magnetized north and pole 4 is magnetized south. In step 2, poles 2 and 5 are energized and poles 1 and 4 are turned off. The rotor rotates 60 degrees. In step 3, poles 3 and 6 are energized and so on.
Induction and squirrel cage motors have but a single turn electrical conductor in the rotor. They generally use copper bars or cast aluminum. The electric current is induced into the rotor from the stator field. These are AC motors and stepping is derived from the line frequency. There are many other specialized motors. However, the end result is still the same: to obtain rotary motion, you have to create a rotating magnetic field.