Difference between Unipolar and bipolar stepper motor

Unipolar stepper motors

A unipolar stepper motor has two windings per phase, one for each direction of magnetic field. Since in this arrangement a magnetic pole can be reversed without switching the direction of current, the commutation circuit can be made very simple (eg. a single transistor) for each winding. Typically, given a phase, one end of each winding is made common: giving three leads per phase and six leads for a typical two phase motor. Often, these two phase commons are internally joined, so the motor has only five leads.

Unipolar stepper motor operates with one winding with a center tap per phase. Each section of the winding is switched on for each direction of the magnetic field. Each winding is made relatively simple with the commutation circuit, this is done since the arrangement has a magnetic pole which can be reversed without switching the direction of the current. In most cases, given a phase, the common center tap for each winding is the following; three leads per phase and six leads for a regular two-phase stepper motor.

You will usually see that both these phases are often joined internally, this makes the stepper motor only have five leads. Often a stepper motor controller will be used to activate the drive transistors in the proper order. Since it is quite easy to operate these stepper motors, they are often very popular among hobbyists and are usually the cheapest way to get precise angular movement

A microcontroller or stepper motor controller can be used to activate the drive transistors in the right order, and this ease of operation makes unipolar motors popular with hobbyists; they are probably the cheapest way to get precise angular movements.

(For the experimenter, one way to distinguish common wire from a coil-end wire is by measuring the resistance. Resistance between common wire and coil-end wire is always half of what it is between coil-end and coil-end wires. This is due to the fact that there is actually twice the length of coil between the ends and only half from center (common wire) to the end.) A quick way to determine if the stepper motor is working is to short circuit every two pairs and try turning the shaft, whenever a higher than normal resistance is felt, it indicates that the circuit to the particular winding is closed and that the phase is working.



Bipolar stepper motors

Bipolar motors have a single winding per phase. The current in a winding needs to be reversed in order to reverse a magnetic pole, so the driving circuit must be more complicated, typically with an H-bridge arrangement (however there are several off the shelf driver chips available to make this a simple affair). There are two leads per phase, none are common.

Because windings are better utilized, they are more powerful than a unipolar motor of the same weight. This is due to the physical space occupied by the windings. A unipolar motor has twice the amount of wire in the same space, but only half used at any point in time, hence is 50% efficient (or approximately 70% of the torque output available). Though bipolar is more complicated to drive, the abundance of driver chip means this is much less difficult to achieve. An 8-lead stepper is wound like a unipolar stepper, but the leads are not joined to common internally to the motor. This kind of motor can be wired in several configurations.

bipolar stepper motors, there is only a single winding per phase. The driving circuit needs to be more complicated to reverse the magnetic pole, this is done to reverse the current in the winding. This is done with an H-bridge arrangement, however, there are several driver chips that can be purchased to make this a more simple task. Unlike the unipolar stepper motor, the bipolar stepper motor has two leads per phase, neither of which are common. Static friction effects do happen with an H-bridge with certain drive topologies, however, this can be reduced by dithering the stepper motor signal at a higher frequency.Bipolar motors are generally better than unipolar motors. They have more torque and are more efficient

Some Major Comparison

A stepper motor may have any number of coils. But these are connected in groups called “phases”. All the coils in a phase are energized together.

  • Unipolardrivers, always energize the phases in the same way. One lead, the “common” lead, will always be negative. The other lead will always be positive. Unipolar drivers can be implemented with simple transistor circuitry. The disadvantage is that there is less available torque because only half of the coils can be energized at a time.
  • Bipolar drivers use H-bridge circuitry to actually reverse the current flow through the phases. By energizing the phases with alternating the polarity, all the coils can be put to work turning the motor.
  • A two phase bipolar motor has 2 groups of coils. A 4 phase unipolar motor has 4. A 2-phase bipolar motor will have 4 wires – 2 for each phase. Some motors come with flexible wiring that allows you to run the motor as either bipolar or unipolar.
  • However, they are more complicated to drive because they need reverse current.
  • In terms of construction, bipolar motors have multiple (at least two) independent windings. A wire comes out of each of the winding’s ends, so you get two wires per winding.
  • Unipolar motors also have multiple windings, however, in addition to the ends of each winding being connected to wires, the middle is also connected to a third wire.
  • The absence of this third wire means that bipolar motors are slightly simpler to make.
  • When it comes to driving these motors, however, the simpler bipolar motor requires a more complex driver. This is because, to precisely control its motion, we need to be able to drive current in each winding in both directions.
  • On the other hand, in a unipolar motor, we can get away with current that flows only in a single direction. This means that the driver electronics can be made simpler. The tradeoff is that we use only half of each winding coil at a given time, and this translates to lower torque and efficiency.
  • However today, with easy access to motor drivers like H-bridges, it is easy to drive bipolar motors with alternating current. Unipolar motors advantage of not needing the reverse current is not a big deal anymore.