Let's leave 5-phase steppers out of the mix right now as they are a rather unique item manufactured by only a scant few companies...
OK, Here goes... The vast majority of stepper motors have two windings in them. If the windings are NOT center tapped, this is a bipolar stepper motor. If they are, it's a unipolar stepper motor.
Bipolar stepper motors are also known as two-phase motors. They have the advantages of:
1) Ability to be microstepped.
2) More torque output for a given size because all the available copper (the windings) are being used at once.
They have the disadvantage of more complicated drive electronics as each wire needs to be either driven (+) or (-)
Unipolar motors, also known as 4-phase motors also have two windings, but they include a center tap. Each center tap is either brought out separately (a six wire motor) or they are tied together internally (a 5 wire motor). Either way, they are generally driven the same. If a 6-wire case, the two center taps are usually connected together externally.
These have one big advantage... Ease of driving... The center taps are connected to the (+) supply. The remaining wires (connected to the ends of each winding) are connected to (-) in sequence to move the motor. ABCD is CW, DCBA is CCW.
Of course the polarity could be reversed with the (-) connected to the center taps but using (+) common makes for an easier output stage as open collector transistors or open drain FETs can be used without any any level translation circuit. OK. I'm getting too complicated...
With any stepper motor, the holding torque with power applied substantially exceeds the running torque. When in a detent with thr rotor not moving the stepper will "hold" its rated torque. In other words, it will resist being backdriven until the rated torque is reached.
Running torque is another matter... The maximum attainable running torque is generally 0.63 times the rated holding torque (if anyone is curious why I can explain). The available torque falling off with increasing speed.
OK, so why use stepper motors?
Let's say you have a simple brush or brushless DC servio motor and you want to move 7.2 degrees... You ramp up the current, the motor starts to move. As you approach your destination the motor slows down and you get where you need to be. Does this happen by itself? Nope. You need some sort of position measuring device (encoder, resolver, pot, etc) to tell the controller you're there. You also need a control algorithm. Now what happens if the load tries to backdrive you to another position? Well, torque needs to be throttled to maintain position. This can yield an efficient solutuon since current (torque) is only taken when it's needed. Also, basically infinite resolution can be obtained...
Enter steppers... You need to move 7.2 degrees... Let's say you have a 1.8 degree stepper motor (200 detents). Just step it 4 times (4 x 1.8 = 7.2 deg) and you've moved 7.2 degrees. No fuss no muss. If you keep current going after you get to 7.2 degrees the motor will resist being backdriving by its rated torque... The downside? You're drawing current to maintain position even if the motor is delivering no torque to counter being backdriven.
A good start?
John
