To obtain high torque at high speeds requires a large drive voltage with a low resistance and low inductance. With an L/R drive it is possible to control a low voltage resistive motor with a higher voltage drive simply by adding an external resistor in series with each winding.
Explanation: Step angle is defined as =(Ns-Nr)/(Ns+Nr)*360 where Ns is the number of stator poles and Nr is the number of rotor poles.
Closed loop-capable stepper motors merge the benefits of stepper and servo motor technology. They run more smoothly and have a lower resonance than stepper motors, provide position feedback and control, feature short settling times, and exhibit no step loss at all.
There are three main types of stepper motors:
- Permanent Magnet Stepper. PM steppers have rotors that are constructed with permanent magnets, which interact with the electromagnets of the stator to create rotation and torque.
- Variable Reluctance Stepper.
- Hybrid Syncronous Stepper.
One feature of stepper motors that differentiates them from other motor types — particularly servo motors — is that they exhibit holding torque. This means that when the windings are energized but the rotor is stationary, the motor can hold the load in place.
The servo motor is a closed-loop mechanism that incorporates positional feedback in order to control the rotational or linear speed and position. The motor is controlled with an electric signal, either analog or digital, which determines the amount of movement which represents the final command position for the shaft.
Applications of Stepper Motors
- 3D printing equipment.
- Textile machines.
- Printing presses.
- Gaming machines.
- Medical imaging machinery.
- Small robotics.
- CNC milling machines.
- Welding equipment.
One of the main advantages of the Hybrid stepper motor is that, if the excitation of the motor is removed the rotor continues to remain locked in the same position as before the removal of the excitation. This is because of the Detent Torque produced by the permanent magnet.
In two-phase on - full step, Fig2, the motor is operated with both phases energized at the same time. This mode provides improved torque and speed performance. Half step excitation mode is a combination of one phase on and two phase on full step modes.
Hybrid stepper motors are constructed with a rotor made of two sections, or cups, with a permanent magnet between them. The surfaces of the rotor cups have precisely-ground teeth (typically 50 or 100 teeth per cup), and the cups are aligned with an offset of ½ tooth pitch between the two sets of teeth.
NEMA 17 stepper motor is a stepper motor type specified by NEMA. NEMA 17 steppers have a 1.7 x 1.7 inch faceplate. NEMA 17 steppers are commonly used in 3D-printers, CNC router, Linear actuators and mid-torque precise movement applications.
The basic working principle of the stepper motor is the following: By energizing one or more of the stator phases, a magnetic field is generated by the current flowing in the coil and the rotor aligns with this field.
The Permanent Magnet Stepper Motor has a stator construction similar to that of the single stack variable reluctance motor. The rotor consists of permanent magnet poles of high retentivity steel and is cylindrical in shape. The rotor poles align with the stator teeth depending on the excitation of the winding.
At higher speed increases, torque output from stepper motors diminishes. No wonder then that most stepper motors aren't recommended for continuous running at high speeds under such conditions. One solution is to increase supply voltage … but respect the system voltage-supply limits and the dangers of overcurrent.
A unipolar stepper motor is one where each phase has a center tap. phases is connected to ground via a MOSFET transistor (Fig. 2). As such, the Unipolar winding only requires 4 transistors to run the motor.
Therefore, a hybrid type stepper motor uses both the permanent magnet rotor as well as a toothed rotor and stator in their design. With strong permanent magnets, toothed rotor, toothed stator, and a tiny air gap, a hybrid type stepper motor is able to focus the magnetic flux better than the other two types.
Stepper motors also tend to be compact and inexpensive. This makes these motors a good fit for medical, biotech, security and defense, and semiconductor manufacturing applications. Servo motors are a better choice for systems requiring high speed, high acceleration, and high accuracy.
Stepper motors are DC motors that move in discrete steps. They have multiple coils that are organized in groups called "phases". By energizing each phase in sequence, the motor will rotate, one step at a time. With a computer controlled stepping you can achieve very precise positioning and/or speed control.
The third pin of the servo connector carries the control signal, used to tell the motor where to go. This control signal is a specific type of pulse train.
So the servo tends to be faster, more precise accuracy and centering and draw less current doing it all. The company has some quality control program in effect (quality control is expensive). They usually offer warranty program, and have spare parts available too.
A Stepper Motor Driver is the driver circuit that enables the stepper motor to function the way it does. For example, stepper motors require sufficient and controlled energy for phases in a precise sequence. Due to this, stepper motors are considered more advanced than the typical DC motor.
The position of the servo motor is set by the length of a pulse. The end points of the servo can vary and many servos only turn through about 170 degrees. You can also buy 'continuous' servos that can rotate through the full 360 degrees.
The main difference between the two motors is their source of power. AC servo motors rely on an electric outlet, rather than batteries like DC servo motors. While DC servo motor performance is dependent only on voltage, AC servo motors are dependent on both frequency and voltage.
The top Servo Motor disadvantages are:
- Servos Motors requires tuning to stabilize the feedback loop.
- Servo Motor will become unpredictable when something breaks.
- Complex controller requires encoder and electronic support.
- Peak torque is limited to a 1% duty cycle.
Generally speaking the top speed of a stepper motor is approximately 1000rpm. If you need to go above this speed but still require intelligent control of the motor you should consider a brushless DC motor and controller (brushless ESC).
