01 May 2001
Small Step = Giant Leap
by Allen Chasey
Choosing a step motor can be more complex than simply matching up catalog data to system requirements.
Most often, a step motor is chosen based only on the data obtained from a manufacturer's spec sheet. The friction and acceleration torques are calculated, required pulse speeds are determined, and then the catalog hunt begins. However, how often is a step motor chosen with its inherent versatility in mind?
Step motors by nature are "dumb" devices; they have no internal electronics. Rather, they depend on the addition of a driver/amplifier circuit and pulse generator to handle the brainy chores of sequencing the motor's phases to get usable motion. Because of these traits, manufacturers wire their step motors so they can be mated to a variety of drives in multiple configurations. I'm going to primarily focus on the various ways a two-phase step motor can be wired and the corresponding performance changes and characteristics that result.
The most common types of lead wire configuration are the six-lead and eight-lead wires. Their typical wiring diagrams are as shown in Figures 1 and 2, respectively. We'll concentrate on these two lead wire configurations.
There are four different ways for a two-phase motor to be wired, depending on lead wire configuration: unipolar, bipolar half coil, bipolar series, and bipolar parallel. Each method provides different performance characteristics, even in the same motor.
Two-phase step motors used in unipolar fashion require at least six leads. The unipolar designation means current is flowing through the coil in only one direction. However, only half of the coil is being energized at any given time in each phase. The switching sequence performed by the drive determines which phase is on at what time. Running a step motor in unipolar fashion produces a lower overall torque because we're using less coil at any given instant. However, because we're using half the coil, the inductance is lower. With lower inductance, current will flow into the coils quicker, and therefore we can obtain a higher usable speed.
Bipolar Half Coil
This method is comparable to unipolar, and its resulting speed-torque curve will be similar, as only half of the coil is used. However, with a bipolar half-coil arrangement, only half of the coil will ever be used. Two of the six leads are usually cut, as they aren't needed. As bipolar implies, current flows through the coil in two directions. Whereas the unipolar method uses both halves of the coil at different times, the bipolar half-coil arrangement uses the same half of the coil every time that phase is energized but switches current flow direction.
We employ this method when low speed and high torque are the operating requirements. The "center taps" in a six-lead motor will usually be cut in bipolar series configuration, as they'll never be used. Bipolar series-wound motors have current flowing in two directions through the full coil. However, in so doing, we're increasing the winding's inductance and therefore cutting down our usable speed. What we gain is an approximate 40% increase in torque.
When using a motor in bipolar series fashion, changes occur not only in the torque and speed but also in the electrical characteristics. Because we're now using the coils in series, the resistance value increases by a factor of two. The motor's current rating will drop to approximately 70.7% of its unipolar rating. This is based on P=I2R, where P (dissipated power) doesn't change, even though our wiring has. Our new voltage rating will also change, as calculated from Ohm's law, V=IR. Inductance also changes. However, when we add two like inductors in series, we encounter a phenomenon known as mutual inductance. This mutual inductance causes the motor's inductance rating to increase by a factor of four, as shown in Example 1.
Motors running in a bipolar parallel configuration must have eight leads (unless a special winding is made at the factory, prewired with four leads). Eight-lead motors enable us to configure the windings for any method, bipolar or unipolar. In a bipolar parallel setup, the leads are "overlapped." This overlapping allows the current to flow in the same direction through both coils, which are now in parallel with each other. Bipolar parallel-wound motors give the best of both worlds; we get high torque, as we're utilizing both coils simultaneously, and we have high speed because the inductance has dropped since we added the inductors in parallel. However, this performance gain comes at the expense of high current. In driver terms, higher current means higher cost. As with the bipolar series, the motor will also have new specifications. Holding torque increases by a factor of 1.4 (again, due to P=I2R, where P must remain constant), along with the current rating. Figure 3 shows a comparison of speed-torque curves.
One of the most prevalent types of drivers in use today is the constant current "chopper," or pulse width modulation (PWM), type of drive, which can be either bipolar or unipolar. Bipolar drives are slightly more expensive because they use more switching transistors, but they have the advantage of being able to drive motors in a variety of ways. The unipolar drive, conversely, is slightly less expensive, getting by with fewer transistors. However, this is at the expense of being able to drive the motor in only one way.
One of the chopper drive's advantages is its ability to keep a fairly constant current flowing through the motor's coils. Using PWM, we can use a driver with a higher voltage than the motor's rating. It's very common to see a 24-, 48-, or even 60-volts DC driver being used with a rated motor. We do this for one fundamental reason: speed.
Every motor has a unique time constant, te, that can also be expressed as L/R, or inductance divided by resistance (measured in millihenries and ohms, respectively). Consequently, we measure this time constant in milliseconds. As many of us may remember, our college physics professors showed it takes roughly five time constants to reach a steady-state condition in an RL circuit. Therefore, if we know the motor's time constant, we can determine how long it takes the motor to reach steady-state current when energized with applied voltage. When the coils reach this rated current, we achieve rated torque, as torque output is proportional to current.
When we start driving the motor at faster shaft speeds, we're decreasing the amount of time a given phase can be energized. Consequently, we eventually reach a point where the phase's coil is no longer energized sufficiently to produce rated torque, and the torque drops. This is why a motor's speed-torque curve resembles a mirror image of a charging RC circuit curve.
So, back to our question: Why will a higher-voltage drive obtain us a higher usable speed? To answer this, let's use a "ball on a hill" analogy. In Figure 4, we see a ball perched atop a hill. For simplicity's sake, the height (h) of the hill corresponds to driver voltage, the length of the hill (d) is our motor's coil resistance, and the slope of the hill (h/d, or s) is the current flow through the coil. As we can see in the example, the ball will roll down the hill at a speed that relates to the steepness, or slope, of the hill.
Now let's "push" that hill up to a height four times that of our original example, keeping d the same, as we want to use the same motor (Figure 5). Our new height corresponds to a higher drive voltage with the same motor resistance. There's a considerable change in s and, correspondingly, in our current flow. In essence, then, the higher voltage is "pushing" the current faster through the coils. This becomes necessary when we're switching phases on and off at a blinding rate to achieve high shaft speeds.
Please note, however, that we gain only so much speed by increasing voltage. In the example with our ball rolling down the hill, eventually things such as friction and wind resistance limit how fast the ball rolls, no matter how high the hill is. In our motor example, things such as the number of turns of wire, inductance, wire diameter, and temperature eventually limit the motor's shaft speed, no matter how high a voltage we apply. Figure 6 compares varying drive voltages for the same motor.
In many cases, a different motor may be selected if we properly address all parameters at the start. For example, if our system requires a higher torque at lower speeds, with a very low system inertia, we might choose a typical NEMA size 17 single-stack motor wound for bipolar series, instead of a half-stack NEMA size 23 motor wound for bipolar half-coil operation. Using the same drive, our first option would save us space and possibly money. Of course, duty cycle, driver current limitations, and resonance also come into play when considering an alternate motor. Especially with today's new high-torque variety of hybrid steppers, having several motor sizes to choose from is certainly an option worth examining.
Choosing a step motor can be more complex than simply matching up catalog data to system requirements. Knowing a motor's catalog rating is only half the battle. We've also got to consider drive methods, voltages, wiring types, and, of course, system cost if we want to make that shiny new step motor a successful part of our design. MC
Figures and Graphics
- Example 1
- Example 2
- Figure 1: A typical six-lead step motor wiring configuration.
- Figure 2: The classic eight-lead step motor wiring configuration.
- Figure 3: Comparative torque-speed curves for unipolar and bipolar series step motors.
- Figure 4: "Ball on a hill" analogy: The ball's speed is a function of slope (s).
- Figure 5: Increasing height (h) while maintaining distance (d) has a drastic effect on slope (s) and, by extension, speed.
- Figure 6: Comparative drive voltages for the same motor, varying the number of turns of wire, inductance, wire diameter, temperature, etc.
Allen Chasey is a district sales manager for Oriental Motor USA Corp. He received his B.S. in applied physics from Jacksonville University in Jacksonville, Fla. Allen has seven years' combined sales and applications engineering experience with fans and AC and DC motors. Contact him at 135 Brandywine Blvd., Suite B, Fayetteville, GA 30214; tel: (770) 716-2800; fax: (770) 719-8515.