Stepper Motor Drives

Unipolar Drive

The stepper motor system consists of a translator circuit that receives a signal that includes the number of steps and the direction. The translator circuit sends four individual control signals to the switch set circuit and the switch set circuit sends power signals to each of the two phases (windings) in the stepper motor. Figure 11-63 shows a diagram of the typical stepper motor system. You should notice that the signal for the number of steps is included with this diagram and it is a series of square wave pulses (one for each step the rotor should move). The signal for the direction is a constant-voltage signal that is either positive or negative.

The translator typically receives its signal from a programmable logic controller (PLC) or other type of microprocessor controller. In some systems the controller is specifically designed to provide motion control or sequential control. The controller sends a command signal that consists of the number of steps the rotor should turn, and the direction signal indicates the direction. The step signals can be detected with LEDs or with an oscilloscope as they are sent to the translator. This means that if you are troubleshooting the translator and you want to know if it is receiving the pulses that represent the steps, you can use an LED indicator or scope to see these pulses. The direction of rotation signal can be detected with a voltmeter to determine if the signal is a positive voltage or negative voltage.

The simplest type of drive amplifier for a stepper motor is the unipolar drive. From the electrical diagram in Fig. 11-64 you can see the circuit consists of two transistors that are connected to each winding set (phase). You should also notice that the two windings that make up each set have a terminal connection at the point where the two windings are connected. The positive voltage supply is con-nected at this point. Each winding has a transistor emitter-collector circuit connected in series with it, and the negative terminal of the voltage supply is connected to the emitter of each transistor.

This type of amplifier is called a unipolar drive because current can only flow in one direction at any one time. The motor must be a bipolar type so that current can be reversed in the second segment of the winding to get the motor to ran in the reverse direction.

The chopper drive amplifier provides a means to control the current in the stepper windings to provide better torque control. Figure 11-65 shows an electrical diagram of this type of drive amplifier. You can see that each winding segment has four transistors connected to it. The transistors are identified as TR1 through TR4. The circuit also has two diodes and small resistor (approximately 0.1 ß1). Current is supplied to the winding by energizing TR1 and TR4 or by energizing TR2 and TR3. The direction of current flow will determine the polarity of the stator pole.

In the chopper circuit the combination of transistors allows the current to be recirculated through the winding if the current requirement to provide the motor torque at any instant is reached. This means that no current is wasted, and the drive amplifier and motor are more efficient. In a typical resistance-limited (R-L) drive the motor draws maximum current when the rotor is not turning which results in wasting up to 90% of current to the motor. In the chopper amplifier the chopper circuit allows the transistors to chop the current so that it can control the recirculation through the windings so that little current is wasted.


One of the problems with permanent magnet motors is that they will generate a current anytime the rotor is turning. This generated current may cause a problem when the motor starts to decelerate and the current will become stronger than the supplied current. If this occurs, the excess current will damage the switching circuit components.

One method to control this problem is to include a power dumping circuit with the switching transistors. Figure 11-66 shows an example of the location of the power dump circuit. In this diagram the power dump circuit is represented as a block.

Figure 11-67 shows the detailed electronic circuit for the power dump circuit. The power dumping circuit has a detector to check the current threshold and turn off all transistors if the threshold is exceeded. When all of the transistors are turned off, the current will be isolated from the circuit components, and a capacitor will allow a path for the current to circulate through the winding until it dissipates below the threshold and becomes harmless. This type of circuit also provides a means to increase the efficiency of the drive slightly because the excess current that is built up during deceleration is regenerated instead of wasted.

The components in the power dump circuit use the rectifier and capacitors to establish a reference voltage from the AC applied voltage. The AC applied voltage is separate from the DC voltage the circuit will see from the motor windings. When the motor decelerates, the voltage that is generated by the stepper motor rotor will exceed the reference voltage, which will turn on transistor TR2. When TR2 is turned on, it will provide a path between the two potentials (HV and 0V) through the 33-0 resistor. This path allows the excess current to be regenerated, which will cause it to dissipate. When the current has dissipated below the threshold, the transistor is turned off again and the circuit wait for the threshold to be exceeded again.

Purchase Industrial Electronics from Prentice Hall.

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