The electrical compatibility between the motor and the driver is one of the most critical factors in a stepper motor system design. Some general guidelines in the selection of these components re:
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TORQUE - Torque is proportional to the winding current and the number of turns of wire. To increase the torque by 20% increase the current by 20%. To decrease the torque by 50%, reduce the current by 50%. Because of magnet saturation, there is no advantage to increasing the current to more than 2 times the rated current. Doing so may damage the motor.
Why does a step motors torque decline as the speed increases? As mentioned above, torque is directly proportional to the current flowing through the motor coils. The current must ramp up to +100%, then -100% 50 times per revolution (200 quadrature states). The motors' coil inductance opposes changes in current. At higher speeds the current does not have enough time to ramp up to maximum. Before it reaches the maximum, it must reverse and go in the other direction. The maximum rate of current change is inversely proportional to the square of the inductance and directly proportional to the power supply voltage. Maximum speed with minimum torque loss is obtained by minimizing the coil inductance and maximizing the power supply voltage. Since bipolar parallel mode has one fourth the inductance, of bipolar series mode, the torque is much geater at higher speeds.
SPEED / TORQUE CURVES - It is important to have a speed/torque curve because it describes what the motor can and cannot do. It is also important to keep in mind that a speed/torque curve is for a given motor and a given driver. Torque is dependent on the driver type and voltage. The same motor can have a very different speed/torque curve when used with a different driver. The same motor with a similar drive, similar voltage and similar current should give similar performance. Speed/torque curves can also be used to roughly estimate the torque produced using different drivers at varying voltages and current.
Generally, the driver output voltage should be rated higher than the motor voltage rating.
MOTOR INDUCTANCE - Inductance is the property of a winding that resists changes in current flow. Stepper motors are available with different windings and a varying degree of inductance. A high inductance motor will provide more torque at low speeds and similarly the reverse is true.
SERIES vs PARALLEL CONNECTION - There are two ways to connect a stepper motor; in series or in parallel. A series connection provides a high inductance and therefore greater performance at low speeds. A parallel connection will lower the inductance but increase the torque at faster speeds.
4 lead motors can only be run by bipolar drives
6 and 8 lead motors can be configured to be driven by either a bipolar or unipolar drive.
Unipolar drives can send current through a motor's winding in one direction only. Energizing half of the coils is beneficial because it reduces the windings inductance. As mentioned above inductance is the electrical property that fights change in current flow, espeically at higher speeds. Unipolar drivers tend to achieve better high speed performance. The disadvantage is that at lower speeds unipolar drives tend to give less torque than bipolar drives.
Bipolar drives can send current through a motor's windings in both directions. Step motors can be connected to these drives in several different ways to get different motor performance, making a bipolar drive much more flexible than a unipolar drive.
In a bipolar series winding configuration, both halves of the phase are connected in series. Since the full coil is used, the same motor will produce 40% more torque in the low to mid speed range. Unfortunately, this configuration has four times the inductance of the same motor operated in the unipolar configuration. Although the motor has good low speed torque, the torque will drop off rapidly at high speeds.
A bipolar parallel winding configuration can only be achieved using an 8 lead wire motor or by internal wiring. In a parallel configuration, one half of the winding phase is placed in parallel with the other half. This allows the full winding to be used while keeping the inductance low. This combination allows the bipolar parallel configuration to produce 40% more torque than the unipolar winding configuration while still performing well across a wide range of speeds. However, due to the parallel configuration, the winding resistance is halved and the motor will require 40% more current than the same motor run in a unipolar configuration to produce this increased torque.
AVAILABILITY - Motors listed below that have the GOLD GEAR next to the part number are typically available from stock.
