Step Motor Basics

Section 1: Motor Theory A stepper motor is a constant output power transducer, where power is defined as torque multiplied by speed. This means motor torque is the inverse of motor speed. To help understand why a step motor’s power is independent of speed, we need to construct (figuratively) an…

Section 2: Speed-Torque Curve Basics In the previous section, it was shown that motor torque varies inversely with speed. This then is the motor’s natural speed-torque curve. Below a certain speed, called the corner speed, the current would rise above the motor’s rated current, ultimately to…

Section 3: Mid-Band Instability A stepper motor is highly resonant because it is a mass-spring system. The “mass” portion is the rotor and load inertia while the “spring” portion is the restoring torque of the magnetic field that drags the rotor along. Because of this, velocity lags torque by 90…

Section 4: Motor Power Basics The motor power output (speed times torque) is determined by the power supply voltage and the motor’s inductance. The motor’s output power is proportional to the power supply voltage divided by the square root of the motor inductance. If one changes the power supply…

Section 5: Motor Connections Stepper motors have four, six, or eight wires; older motors may have five wires, but they will not be covered here. Four-wire motors are the simplest to connect and offer no connection options. Simply connect one winding to the terminals labeled “Phase A” and “Phase…

Section 6: Power Supplies The choice of a power supply is determined by the voltage, current, and power supply type (i.e. switching versus linear, regulated versus unregulated, etc.). By far the most problematic and complicated factor is voltage, which will be discussed last. The easiest factor in…

Section 7: Motor Heating and Power Supply Voltage There are two major causes of motor heating: copper losses and iron losses. Copper losses are the easiest to understand; this is the heat generated by current passing through a resistance, as in the current passing through the motor’s winding…

Section 8: Accuracy and Resolution Step motors by and large are used in open loop positioning and velocity applications. There is not feed-back transducer to set the ultimate accuracy of the system. Consequently it falls on the motor and the drive’s precision and behavior to determine the accuracy…

Section 9: Choosing a Step Motor and Power Supply Voltage The choice of a stepper motor and power supply voltage is entirely application dependent. Ideally, the motor should deliver sufficient at the highest speed the application requires and no more. Any torque capability in excess of what the…