The basic function of a motor driver is to provide the rated current to the motor windings in the shortest possible time. Driver voltage plays a large part in a step motor’s performance. Higher voltage forces current into the motor windings faster, helping to maintain high speed torque.
Two of the most commonly used drivers for step motors types of step motor driver are the following:
Constant current drivers are also known as PWM (pulse width modulated) or chopper drives. In this type of driver, the motor current is regulated by switching voltage to the motor on and off to achieve an average level of current. These drivers operate using a high voltage supply, generating a high driver voltage to motor voltage ratio, giving the motor improved high speed performance.
Constant voltage drivers are also known as, L/R or resistance limited (RL) drivers. In this type of driver, the amount of current a step motor receives is limited only by the resistance/impedance of its windings. For this reason, it is important to match the motor’s rated voltage to the voltage of the driver. Constant voltage drivers work best in low speed and low current applications. They become inefficient at high speeds and high current levels. In certain situations, resistors may be placed in series with the motor’s windings to allow the motor to be operated using a driver voltage larger than the motor’s rated voltage to increase performance at higher speeds.
Driver & Winding Configurations
Step motor drivers can be divided into two types, unipolar and bipolar. All 6 and 8 lead wire motors manufactured by NMB can be configured to be driven by either a bipolar or a unipolar driver, however, 4 lead wire motors can only be run by bipolar drivers.
When trying to decide between a bipolar motor driver and a unipolar motor driver, it’s important to have a basic sense of the distinction between each type of motor. In a bipolar stepper motor, there are usually four leads, two pairs of two, with a coil powered by each pair. In a unipolar stepper motor, you normally have two sets of three leads, with each set of three powering a coil with a center tap. In some cases you will find five leads, where the two common center taps are combined. Bipolar stepper motors tend to be less expensive, but unipolar motors perform better at high speeds. Therefore, which type of motor driver and motor you should use will depend largely on the job you need performed. In fact, some of the applications in your project may be better suited to unipolar configurations while others will work best with bipolar considerations. For maximum efficiency, you will want to know all the major distinctions between bipolar and unipolar configurations before choosing a motor and motor driver. Here are some more of the most important differences you will want to consider when selecting your driver and motor.
Unipolar driverscan send current through a motor’s windings in only one direction. Unipolar drivers tend to achieve better high-speed performance.
Bipolar drivers 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 unipolar winding configuration, only half the coils of each winding are used at a time. Energizing half of the coils is beneficial because it reduces the winding’s inductance. Inductance is an electrical property that fights changes in current flow, particularly at higher speeds. The unipolar winding configuration tends to give better high-speed performance. The disadvantage of this type of configuration is that at lower speeds it tends to give less torque than configurations that use the entire winding. See Fig. 4a.
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. See Fig. 4b.
A bipolar half coil winding configuration can be used to achieve unipolar performance with a bipolar drive. In this configuration, the motor’s inductance and low speed torque are less than those in the bipolar series configuration. As in the unipolar configuration, the half coil configuration tends to give better performance at higher speeds. Both 6 and 8 lead wire motors can be connected in the bipolar half coil configuration. See Fig. 4c.
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. See Fig. 4d.