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Stepper Motors
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What is stall torque?
Stall torque is also called dynamic torque, and it is the maximum torque that can be applied to a motor before it stalls or loses synchronization. This is the torque represented on a torque-speed curve.
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Stepper Motor Basics
Stepper motors are brushless, synchronous electric motors that convert digital pulses into mechanical shaft rotations. The rotation of a stepper motor is divided into a set number of steps, sometimes as many as 200. Each step must be sent to the stepper motor as a separate pulse. A stepper motor can only receive one pulse and take one step at a time, with each step having the same length. Since each pulse causes the motor to rotate at a precise angle - typically 1.8 degrees - you can precisely control the position of the stepper motor without any feedback mechanism being used. As the frequency of the control pulses increases, the stepping movement converts into a continuous rotation with the velocity of the rotation directly proportional to the frequency of the control pulses. Stepper motors are widely used because of their low cost, high reliability, and high torque at low speeds. Their rugged construction enables you to use stepper motors in a wide environmental range.
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Advantages of Using Stepper Motors
As the speed of a step motor is proportional to the frequency of the input pulses from the controller, a wide range of rotational speeds can be achieved. Stepper motors are capable of precise open-loop positional control without any feedback mechanism. Using a load that is directly coupled to the shaft of the stepper motor, a very low-speed rotation is possible. There are no contact brushes in a stepper motor, which makes it quite reliable. In general, a stepper motor's life is determined by its bearing. Stepper motors are very effective at starting, stopping, and reversing. Stepper motors provide precise positioning and repeatability of movement. An energized stepper motor maintains full torque at a standstill position.
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Types of Stepper Motors
There are three types of step motors: permanent magnet, hybrid, and variable reluctance. The hybrid stepper motor offers the greatest versatility and combines the best characteristics of both variable reluctance and permanent magnet stepper motors. A hybrid stepper motor consists of a multi-toothed stator pole and a permanent magnet rotor. In a hybrid stepper motor, the rotor has 200 teeth and rotates 1.8 degrees per revolution. The hybrid stepper motors provide high static and dynamic torque as well as a high step rate. Computer disk drives and CD players are among the applications for hybrid stepper motors. Hybrid stepper motors are also widely used in industrial and scientific applications. Hybrid step motors are used in robotics, motion control, automated wire cutting, and even in high-speed fluid dispensers.
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"1.8 degrees or 0.9 degrees? "
Step accuracy is the primary characteristic of a step motor.
Without step accuracy, the motor is useless.
Based on motor manufacturing capability, step accuracy is rated at +/- 5% of the full step.
That means a 1.8-degree motor would have a step error of +/- 5.4 arc minutes, while a 0.9-degree motor would have a step error at +/- 2.7 arc minutes.
Because the motor step accuracy is determined by the torque stiffness, which is determined by the maximum holding torque and the number of teeth on the rotor.
Motor torque function: T(θ) = To*Sin(Nθ)
Torque stiffness: dT(θ)/d = N*To*Cos(Nθ)
(where to=maximum holding torque, N=number of rotor teeth, θ=rotor displacement)
A 1.8-degree motor has a 50-tooth rotor and a 0.9-degree motor has a 100-tooth rotor. With the same manufacturing capability, a 0.9-degree motor will have twice the step accuracy of a 1.8-degree motor.
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What factors affect motor noise?
Our motors are designed for quiet operation. This is achieved through our proprietary motor design techniques and material selection. There are many factors to consider, such as how the motor is mounted in the application and the load and speed at which the motor will operate. Another consideration is the type of bearings selected for the motor and/or gearmotor. The motor/gearmotor being considered should be thoroughly tested for noise levels in the design before making a final decision.
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What is the difference between a closed-loop stepper motor controller and an open-loop stepper motor controller?
Closed-loop controllers provide feedback on the motor response after a command has been issued, whereas open-loop controllers issue commands without any input from the motor.
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Amps per coil or Amp per phase?
Yes, a traditional six-lead motor can be used as a bipolar motor depending on your requirements.
A bipolar driver will have four outputs for the phase wires. Only four lead wires from the six lead wired motor will be used.
Depending on your motor performance requirements, you can connect the motor to run in "half coil" or "series". Series can be achieved by connecting A to Abar then B to Bbar from the motor circuit to your bipolar driver.
When running in a series connection, the rated current of the unipolar motor must be converted to series rated current by multiplying the unipolar rated current by a factor of 0.7
Series Rated Current = Unipolar Rated Current * 0.7.
For half coil connection; A to ANC on Phase A will be used to replace A and Abar, and then B to BNC on Phase B to replace B and Bbar; Current input will remain the same.
Advisory: Whether a half coil or series connection is used, it is recommended to take proper precautions to insulate the other two lead wires that are not being used.
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My stepper motor is running hot to the touch. Is there something wrong?
Stepper motors are rated to withstand a case temperature of 80 degrees Celsius, which is hot to the touch but does not harm the motor.
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How much torque safety margin should I use when sizing a motor?
We recommend a torque safety margin of at least 30%.
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What is a Step Motor?
A stepper motor is an electromechanical device that converts electrical impulses directly into mechanical motions. The sequence, frequency, and quantity of electrical impulses applied to the motor coils can be controlled to control stepper motors' steering, speed, and rotation angles. We can achieve precise position and speed control without closed-loop feedback control with a position sensing system, using a stepper motor and related supporting driver to composite a simple, low-cost open-loop control system.
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What is mechanical angle or electrical angle?
Mechanical angle represents the step angle of the step.
In the full-step mode of a 1.8-degree motor, the mechanical angle is 1.8°.
In the 10 micro-stepping modes of a 1.8° motor, the mechanical angle is 0.18º.
An electrical angle is defined as 360° divided by the number of mechanical phases and the number of micro-stepping.
In the full-step mode of a 1.8° motor, the electrical angle is 90°.
In the 10 micro-stepping modes of a 1.8° motor, the electrical angle is 9º.
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In what ways can an encoder be used to determine position and speed?
Step motor size typically corresponds to the amount of torque that the motor can produce, as a larger motor can accommodate more turns per coil, resulting in a greater amount of torque. Moreover, a larger motor generally has a greater amount of inertia, which allows it to be matched to loads with greater inertia.
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What are the advantages of using step motors?
1. Speed can be easily determined and controlled by remembering that speed equals steps per revolution divided by pulse rate.
2. A step motor can make fine incremental moves.
3. A step motor doesn’t require encoder feedback (Open-loop).
4. Non-cumulative positioning error.
5. Excellent low speed/high torque characteristics without gear reduction.
6. Holding torque of the step motor can be used to hold loads in a stationary position without overheating.
7. Ability to operate on a wide speed range.
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How fast can I run my step motor?
Most stepper motors are designed for low-speed (3000 rpm or less) operation. Once you get to higher speeds, servo motors are typically used.
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What is the difference between Unipolar and Bipolar motors?
Unipolar – A unipolar driver's output current direction cannot be changed.
There are two sets of coils for each phase in a motor.
Only one set of coils can be energized at a time.
Each coil represents one phase.
Therefore, only 50% of the winding is utilized in the unipolar drive. The number of mechanical phases equals the number of electrical phases.
Due to the fact unipolar drivers only use 50% of the windings, the performance ranges from low to moderate.
The benefit of this is that it doesn’t generate too much heat.
Bipolar – A bipolar driver's output current direction can be changed. 100% of the winding is utilized in the bipolar drive.
That means the two sets of coils in each phase can be connected either in series or in parallel to become one set of a coil.
The current direction change from the driver creates another mechanical phase.
The number of mechanical phases is always twice the number of electrical phases.
Bipolar drivers provide 40% more holding torque than unipolar drivers but typically run at higher temperatures.
For this reason, proper heat dissipation is important with bipolar drivers.
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How can 4-wire, 6-wire and 8-wire motors be connected?
First of all two drivers exist Unipolar and Bipolar, Unipolar drives output to 6 leads of a step motor, and Bipolar output to 4 leads of a step motor.
So a 4-lead motor can only be connected to a Bipolar driver.
A 6-lead and 8-lead motor can be connected to a Unipolar driver or a Bipolar driver.
A wiring diagram shows the possible connections.
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How do brakes work on a stepper motor?
The brakes hold the motor shaft in place by locking their position around the shaft. A brake cannot be used to slow a motor shaft.
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What is the difference between a round and a square stepper motor?
Round motors have a lower inertia and are more efficient at high speeds. We offer square motors with higher inertia, more torque, and excellent performance at low to medium speeds.
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What is the difference between holding torque and dynamic torque?
When one or more phases of the motor are energized, the holding torque is the maximum restoring torque developed by the rotor.
The dynamic torque is called running torque or pullout torque.
Depending on the driver technology and power input, it varies at different speeds.
As a rule of thumb, the maximum dynamic torque is about 70% of the holding torque.
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As one of the largest integrated manufacturers of motion control and intelligent lighting products within China, MOONS’ has professional teams of research and development, manufacturing, sales and technical support all over the world. Our website (www.moonsindustries.com) is the first professional E-business platform in this industry, on which an extremely wide range of high performance motion control and intelligent lighting products are supplied to customers around the world.
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