In 1938, General Electric began production of a 2-phase synchronous induction motor which, at 60 Hz, ran at 75 rpm. The low speed resulted from there being a different number of rotor to stator poles or teeth, which made the motor a good bidirectional control motor. This motor was used by Superior Electric Company of Bristol, CT for running power-driven autotransformers used to dim lights in auditoriums and similar applications. General Electric ceased production on this motor in the mid 1950s because sales were not high enough to support production.
Superior Electric engineers redesigned the motor to run at 72 rpm on 60 Hz and began to consider using a 4-step dc voltage sequence to have the motor move in increments of 1.8 deg 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.
The Principle of the Stepping Motor
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.
Figure 1 illustrates a typical step sequence for a 2-phase motor. In Step 1, phase A of a 2-phase stator is energized. This magnetically locks the rotor in the position shown, since unlike poles attract. In step 2, phase A is turned off and phase B is turned on. The rotor rotates 90 deg clockwise. In Step 3, phase B is turned off and phase A is turned on but with the polarity reversed from Step 1. This causes another 90 deg rotation. In Step 4, phase A is turned off and phase B is turned on, with polarity reversed from Step 2. Repeating this sequence causes the rotor to rotate clockwise in 90 deg steps. See Figure 2 for an electrical representation of the current flow in the two phases.
An electronic drive sends the electrical pulses to the two phases at the appropriate time to create rotation. In this example, the rotation is 90 deg per pulse or step. In practice, the motors have more poles to create smaller step angles such as 15, 7.5, or 1.8 deg 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. However, 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 2-phase alternating current creates a 4-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.
Every once in a while you hear that this motor is better than that motor. There is no such thing as the “best” motor. If this were true, there would only be one motor on the market. When you have an application, you have to determine which is the best motor for that application. You have to consider such things as cost, dependability, life, and many other factors.
Brush-type dc motors have several coils wound on a rotor or armature. These coils are connected to a commutator. The commutator consists of copper segments. (See Figure 4) In this example shown, there are twelve coils so one step is 360/12 or 30 deg. 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 deg apart, connect to the appropriate coil, which rotates the rotor 30 deg. (See Figure 5)
This rotation causes the brushes to connect to the next coil, which causes another 30 deg rotation. This, in effect, causes continuous rotation. This motor is called a brush-type dc motor. 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 cells and a magnetic ring magnetized with many poles. (See Figure 6)
This is called a brushless dc motor, shown at right.
One of the disadvantages of a stepping motor is the ringing of the rotor when you take a single step. The rotor does not stop instantly but oscillates about its final position for several milliseconds. If a second step is introduced while the rotor is moving in a negative direction, you have a conflict and a loss of torque. In extreme cases, you can cause a resonance and the rotor will vibrate or stop.
There are ways to dampen this oscillation but they increase the price. In a brush-type or brushless motor, the switching is done at exactly the right time and this problem is minimized.
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. (See Figure 7)
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 deg. In step 3, poles 3 and 6 are energized and so on.
Induction and Squirrel Cage Motors
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.
In the 1930s and 1940s, Haydon Manufacturing Company was making clock motors and timing motors. During World War II, the company was making dc motors used in aircraft, tanks, submarines, and a variety of portable military equipment. In the 1960s Haydon began to make stepping motors. Today, Haydon Switch & Instrument, Inc. is a worldwide leader in permanent magnet and hybrid stepping motors and linear actuators, holding several patents related to stepping motors.