By Jenny Karnacewicz, Application Engineer, ElectroCraft Corp. Dover, New Hampshire
Experts pretty much agree that motor current is proportional to torque output, and voltage is proportional to motor speed, but do you know when this is might not be true?
A typical ElectroCraft stepper motorand SA4510 bipolar drive intended for numerous motion control applications where precision rotational displacement and speed control are required.
Bipolar drives supply stepper-motor windings with current waveforms that look like square waves when fully stepping or sine waves when micro-stepping at a high resolution. The waveforms shown in the oscilloscope displays illustrate the drive current in a half-stepping mode, which is two discrete steps for every pulse delivered to the stepper motor over a speed range of 500 to 4000 half-steps/s. Unfortunately, as the step rate increases, less voltage is available to supply the required winding current. This is because the BEMF (back electro-magnetic force) voltage increases, which, in turn subtracts from the supply voltage and limits the average (RMS) current during the ON time of the phase winding energized by the drive. At a critical step rate, the current fails to reach the set-point value, and the system behaves like a stepper motor powered by a voltage drive. The series of oscilloscope images show how the waveform continues to deteriorate at faster speeds: its amplitude decreases and progressively less current reaches the drive.
The nominal current provided by the bipolar stepping drive is only available up to a certain speed (or step rate) of the stepper motor. After this, the motor’s inductance and BEMF limit the drive to a voltage drive. The BEMF (back electro-magnetic force) of the motor is voltage generated by the motor due to flux from the rotating magnetic field. Inductance limits the rate at which current builds in the winding and depends on the stack length and number of turns of copper wire used in the windings.
The motor’s output torque is proportional to the current provided by the drive at speeds from below the critical step rate to the saturation point of the motor’s magnetic field. The motor’s windings and permanent magnet properties, along with the motor’s construction, determine the flux limits. For instance, for the sample motor discussed here, the LH1719-M100 Stepper System, the requirements change at about 3000 half-steps/s. The stepper motor is unable to receive rated current from the drive. Adding more current to a stepper system that has reached its critical step rate cannot increase the output torque. This is illustrated in the Torque/Speed curve for the sample motor system where increasing the current does not produce more torque after the critical step rate of 3000 half-steps/s.
One solution to increase the step-motor torque is to increase the bus voltage to the drive. The current remains proportional to torque, but when the bipolar drive can no longer provide rated nominal current to the stepper motor, more voltage is needed to exceed the BEMF of the motor. Adding more supply voltage to the drive is the only way to increase the critical step rate and supply more current to the stepper motor, so more torque develops at higher speeds. This is illustrated in the Torque/Speed curve for the LH1719-M100 Stepper System where more voltage, not current, increases the torque above the critical step rate.
Series vs. parallel wiring
Because the inductance of the motor windings directly affects the critical step rate of the system, controlling this property can optimize high-speed operation. For example, an eight-wire stepper motor can be connected in either series or parallel.
Step-motor phases wired in series use all the ampere-turns to reach full inductance, so the motor provides higher torque at lower speeds. In contrast, parallel windings use less ampere-turns, which decrease the inductance of the motor. This increases the torque at higher speeds but decreases it at lower speeds. The effect of series vs. parallel wiring is illustrated in the torque/speed curve for these wiring alternatives using the same nominal current from the drive. The drive ceased to be a constant-current drive at 6200 half-steps/s wired in parallel and at 3000 half steps/s wired in series.
Correctly sizing a stepper motor for an application should include a review of the input power as well as the voltage and drive current supplied to the motor for high-speed operation. The amount of heat the motor can dissipate determines the maximum current rating. For this reason, published torque/speed curves for stepper motors are provided at a specific bus voltage. Use the ratio between the published voltage and the actual available or required bus voltage (called the over-voltage ratio) to predict high-speed operation of a step system. For example, a 48-V bus voltage provides twice the maximum motor step rate as a 24-V bus voltage supply. The published torque/speed curve can be adjusted by moving the X-intercept of speed in a direct ratio to the over-voltage ratio to generate a new theoretical torque/speed curve for the system as shown in the last figure.