Two Basic Types of Stepper Motor Drivers

Stepper motors and stepper-based linear actuators are often selected for open-loop motion control devices and equipment. These can be found in a wide range of products and systems such as: laboratory equipment, medical devices, vision systems, analytical equipment, office products, semiconductor equipment, aerospace, communications systems and light industrial equipment.

Two Basic Types of Stepper Motor Drivers

The two major types of drives for stepper motors and stepper-based linear actuators are the L/R driver and the chopper driver.

Some of the criteria for choosing the drive type include:

  • Cost of the drive
  • Physical size and configuration of the drive
  • Variable power source
  • Rated output current of the drive
  • Motion duty cycle
  • Total loading on the motor
  • Required speed range of the motor

The L/R Drive

Think of this type as a “constant voltage” drive. For continuous duty motor operation in a room temperature environment you essentially match up the available power source voltage for the L/R Drive to the rated coil voltage of the motor. Regarding the name L/R Drive – the “L” is the electrical symbol for Inductance and the “R” is the electrical symbol for resistance. Since the stepper motor torque is proportional to ampere-turns it is the current through the motor windings that determines the output performance at any speed including zero.

At standstill the maximum“Holding Current” current through the windings is limited by the coil resistance. As the stepping rate (motor speed) increases, the coil inductance becomes a major current limiting factor (limiting the rate of change of coil current) along with the Back- emf. Back emf is a generated voltage proportional to the speed that is produced within the motor windings during rotation which works against the source voltage, because every motor is also a generator. The motors operated with an stepper motor driver will have a relatively limited performance range when compared to using a chopper drive. The source-voltage-to-motor-voltage ratio with the UR Drives is basically 1:1 whereas with chopper drives it can be many multiples such as 2:1, 4:1, 8:1, or more.

Some of the reasons for selecting an L/R drive instead of a chopper drive might be a lower cost of the drive, smaller physical size of the drive, a relatively slow motor speed range, use of a unipolar motor or the limitations of using a battery power source. A good example of a product utilizing many of these previously listed reasons for using an UR drive with a small stepper-based linear actuator is a handheld electronic pipette.

Typically, LUR driven performance curves published by stepper motor and stepper based linear actuator manufacturers were developed with the full rated motor voltage available at the motor’s lead wires at zero steps per second. If there are any voltage drops through the drive circuitry then this DC power supply voltage would be set slightly higher to compensate for the total voltage loss in the drive.

The Chopper Drive

Think of this type as a “constant current” drive. For continuous duty motor operation in a room temperature environment you set the output RMS (Root Mean Square) current of the chopper drive to the rated RMS coil current of the motor. Regarding the name chopper drive, this technique for maintaining the proper motor phase current levels throughout a usable speed range is to rapidly turn on and off (i.e., ‘chopping”) a relatively high source voltage via a proportional duty cycle while circuity monitors the current levels in the motor windings. Chopper drives can be separate ‘stand- alone’ units or integrated with the motor. For one example of a compact ‘stand-alone’ chopper drive.

If the application has a fairly short duty cycle (.e., the ‘full powered’ ON or ‘Run’ times relative to the 0FF or lower-current zero motion ‘hold’ times) in a moderate temperature environment, then a higher magnitude of ‘Run’ current can be used to increase the motion performance of the motor. However, care must be taken when using this higher than rated ‘Run’ current. The current levels and ON times versus ‘Hold’ or 0FF times, as well as the ambient temperature, and any motor cooling methods (conduction, convection, etc.,) will determine the internal coil temperatures. It is recommended to consult the motor manufacturer if significantly high phase currents are necessary.
The additional circuitry within chopper drives sense the magnitude of the phase currents, and to control the voltage ‘chopping’ may increase their price (compared to an LR drive), but it can help to maintain a high level of motor torque or force throughout a relatively wider speed range. The power supply voltage to a chopper drive is typically much higher than the rated voltage of the motor. As discussed in the UR drive section above the source- voltage- to-motor voltage ratio for a chopper drive is usually significantly higher than 1:1 and is typically 8:1 or even higher. Therefore the relative performance range can be greatly improved.

The inductance of relatively low voltage stepper motors and actuators is significantly less than their mechanically equivalent motors of higher rated coil voltages. For very good motor performance over a wider speed range, a low voltage motor operated with a chopper drive at a relatively high source voltage is selected. The relatively low inductance and lower Back -emf characteristics of a low voltage motor in conjunction with a high source voltage chopper drive can provide excellent performance results. The major requirement with these low voltage motor configurations is that the drive has to be capable of providing higher levels of phase current.

As a cautionary note, some chopper drive manufacturers advertise their product’s output phase current levels as a peak value, using larger values is typically a marketing tactic. However, the continuous duty phase currents for stepper motors and stepper-based linear actuators are typically rated as RMS (Root Mean Square) values. The conversion: RMS = Peak x 0.707.

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