Comparison of Drive Methods for a 2-Phase Motor

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Comparison of Drive Methods for a 2-Phase Motor

Postby om_tech_support_JT » Wed Nov 20, 2013 7:52 pm

Q: I am looking for a driver and stepper motor combination for my application, but there are different wiring configuration schemes available. For example, there are bipolar series, bipolar parallel, and unipolar connections. I am not sure exactly what the differences are between these configurations. Could you please explain the advantage and disadvantages of each type?

A: For a 2 phase stepping motor, there are numerous wiring configurations as well as driver types available. The number of possible wiring configurations depends on the number of wires on the motor. The ideal driver type and wiring configuration would depend on the performance requirements for the motor. A unipolar driver only allows a unipolar connection with a 6-wire motor. A bipolar driver can be used with either 4, 6, and 8 wire motors, and accepts multiple wiring configurations such as bipolar series and bipolar parallel. In this article, we will go over each wiring configuration in detail and explain how they would alter the motor's performance.

For a unipolar 6-wire configuration, here is a simplified version of the circuit:

Unipolar vs bipolar - unipolar motor winding and driver circuit.jpg
Unipolar vs bipolar - unipolar motor winding and driver circuit.jpg (35.78 KiB) Viewed 3608 times

With unipolar connection, only one transistor turns on at a time to allow current flow through that winding. Current always flows in one direction in each winding following the A-A’-B-B’ sequence. The winding current saturation time is determined by the formula t= L/R, thus a smaller inductance yields a faster energizing time in the windings, which results better torque performance at high speed range. However, since only half of a coil of windings is energized at a time, the magnetic flux density is less compared to bipolar connection, thus the magnetic force becomes smaller between the stator and the rotor. Therefore, a unipolar configuration generally provides less torque when motor is rotating at low speed compared to bipolar configurations.

Now let’s look at the bipolar series configuration:

Unipolar vs bipolar - bipolar series motor winding and driver circuit.jpg
Unipolar vs bipolar - bipolar series motor winding and driver circuit.jpg (43.9 KiB) Viewed 3608 times

A bipolar series connection allows current flow in both directions in the same winding. A bipolar series circuit usually has 2 “H” bridge configuration for each set of windings. Two transistors turn on at a time to allow current flow in one direction. Then the other two transistors turn on so current will flow in the reverse direction. Switching current flow direction will create opposite magnetic polarity in the stator winding. By switching the ON/OFF state of the transistors following an energizing sequence of A-A’-B-B’, motor rotation is achieved. With a bipolar series configuration, since a full coil of windings is energized at a time compared to half coil for unipolar, it will create more torque due to a stronger magnetic force. However, the energizing time constant t is larger since t=2L/R. Therefore a bipolar series configuration is not as good as unipolar configuration at high speeds.

Here is an internal circuit diagram for the bipolar parallel configuration:

Unipolar vs bipolar - bipolar parallel motor winding and driver circuit.jpg
Unipolar vs bipolar - bipolar parallel motor winding and driver circuit.jpg (23.05 KiB) Viewed 3608 times

Compare to unipolar and bipolar series configurations, the bipolar parallel configuration offers the best torque performance in both high speed and low speed range. The drawback is that it requires more driving current. Bipolar parallel configuration connects 2 coils of windings in parallel, therefore, both inductance and resistance is reduced to half, which yields a time constant t=L/R. This allows the windings to charge/discharge at a faster rate, thus high speed torque performance is improved. Also, since a full coil of windings is energized at a time compared to the half coil from a unipolar configuration, the bipolar parallel configuration also produces higher flux density for more low speed torque. Therefore, it will provide better performance in both high speed and low speed region.

The image below shows the different torque-speed curves generated with different wiring configurations for our PK266-E2.0 motor.
Unipolar vs bipolar - PK266-E2.0 speed torque performance comparison.jpg
Unipolar vs bipolar - PK266-E2.0 speed torque performance comparison.jpg (56.72 KiB) Viewed 3608 times


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