2- Unipolar vs. Bipolar
Unipolar - A unipolar driver's output current direction cannot be changed. There are two sets of the coils for each phase in a motor. Only one set of the 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 the coils in each phase can be connected either in series or in parallel to become one set of a coil. Current direction changed 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 that reason, proper heat dissipation important with bipolar drivers.
3- Mechanical Phase
The number of mechanical phases is defined as the pitch of the rotor tooth in degrees divided by the full step angle.
4- Electrical Phase
The number of electrical phases is defined as the number of independent winding coils being used.
5- Mechanical Angle & 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 mode 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 mode of a 1.8° motor, the electrical angle is 9º.
6- Amps per coil vs. Amp per phase
If two sets of the coil are wound in each phase, and the motor is driven by a bipolar drive, the Amps per coil (Ic) & the Amps per phase (Ip) will be different, depending on the type of connection. Ip equals 1/  of Ic when the two sets of the coil are connected in series. Ip equals of Ic when the two sets of the coil are connected in parallel. (See motor rating) Since only one set of the coil can be energized at a time with an unipolar drive, there are no differences between Amps per coil and Amps per phase. Also, if there is only one set of coils being wound in each phase, there are no differences between Amps per coil and Amps per phase.
7- 40% more torque
The number of turns is doubled in bipolar mode and Ip equals 1/ of Ic when two coils are connected in series. The torque is approximately proportional to the Amps times the turns. If the NI represents unipolar drive torque, then the 2N*(1/ ) I (= NI) will represent the bipolar drive when coils connected in series. are approximately 40% more than 1. The number of turns is the same in bipolar mode and Ip equals of Ic when two coils are connected in parallel. If the NI represents unipolar drive torque, then the N* I (= NI) will represent the bipolar drive when coils connected in parallel. are approximately 40% more than 1.
8- What voltage should I use?
In order to get the maximum output from a motor for a given application, we have to maximize the torque at the operating speed. Therefore, selecting the right speed for the application is very important operating speed. Over 1000 pps full step is not desirable if the power supply voltage is less than 12V. High power supply voltage (> 24V) would be necessary if operating speed is selected over 4000 pps full step is necessary.
Lin Engineering will design the right motor for your application if the operating speed, power supply voltage, and available current from a driver are provided.
9- Micro-stepping
Micro-stepping is use to increase a motor's step resolution. This is achieved by controlling the motors phase current ratio. It should be noted that micro-stepping does not increase step accuracy.
Micro-stepping will allow a motor to run smoother and with less noise. The degree of the improvement depends on the step accuracy of the motor.
10- L/R Drives or Voltage Drives
Lin Engineering does not recommend L/R drives or voltage drives, as such drives provide constant voltage. Step motors generate heat during operation, This heat will increases the motor's resistance. Any change or variation in the resistance of the motor will change the current supplied in a constant voltage drive system.
Lin Engineering strongly recommend the use of constant current driver for all applications.
11- 1.8 degree or 0.9 degree
Step accuracy is the primary character 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 step error of +/- 5.4 arc minutes, while 0.9-degree motor would have step error at +/- 2.7 arc minutes. this is because the motor step accuracy is determined by the torque stiffness, and the torque stiffness is determined by maximum holding torque and the number of rotor teeth.
Motor torque function: T(  ) = To*Sin(N )
Torque stiffness: dT( )/d = N*To*Cos(N )
(where To=maximum holding torque, N=number of rotor teeth, =rotor displacement)
A 1.8-degree motor has a 50-tooth rotor and 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.
12- Number of Teeth and Available Step Angle
N t = 360º / (S ´ N p )
Or
S = 360º / (N t ´ N p )
N = Number of rotor teeth (must be an integer)
S = Full step angle in degree
N p = Number of mechanical phases (must be an integer)
= Number of full steps to repeat the same mechanical line
up between the stator tooth and a rotor tooth
N p = 4 for 2-phase bipolar motor
= 10 for 5-phase bipolar motor
= 3 for 3-phase unipolor motor
13- Holding Torque vs. Dynamic Torque
Holding torque is the maximum restoring torque developed by the rotor when one or more phases of the motor are energized. The dynamic torque is called running torque or pullout torque. It varies at different speed by different driver technologies and power input. As a rule of thumb, the maximum dynamic torque is about 70% of the holding torque.
14- Step Accuracy & Microstepping
Step accuracy is inherent in a motor mechanical design and is controlled by the torque stiffness. Micro-stepping increases the step resolution but not step accuracy . Micro-stepping a motor without good step accuracy can not provide the smoothest motion.
No step motor can maintain the same accuracy of a full step when micro-stepping. Our lab tests show step errors will double or quadruple from full stepping to 1/64 micro-stepping (12,800 SPR) with IM483 driver . Therefore, a motor with good step accuracy at full/half-stepping does not automatically provide good step accuracy at micro-stepping.
Lin's size 17, 0.9-degree motor, step error changed from ±1 arc-minute to ± 1.5 arc-minute. Our competitor's motor step erro changed from ± 1 arc-minute to ± 3 arc-minutes for 0.9 degree motor or ± 3 arc-minutes to ± 12 arc-minutes for 1.8 dgree motor during micro-stepping. This is one of the reasons that Lin's 0.9 degree motors are the most poputlar in precision motion applications.
15- System Resonance
The resonance of a motor is determined by square root of (torque stiffness divided by total inertia). We cannot completely eliminate the motor resonant frequency. But we can change the resonant frequency by changing the rotor inertia or system inertia or change the torque stiffness.
16- Gear Coupling vs. Belt Coupling vs. Direct Drive
Theoretically, in order to maximize the system efficiency, the load inertia reflected to the motor should be the same as rotor inertia. Therefore, a reduction is necessary to improve the system efficiency. Gear coupling creates noise and backlash, while belt coupling is limited on the load carrying capacity but does not generate much noise or backlash. Direct drive is preferred if the motor step size, torque and inertia are acceptable.
17- Heat Dissipation
Step motor current rating is based on the heat dissipation (current I^2 times resistance R) of the motor. A larger motor can dissipate more heat than a small motor. So, the larger motor current rating can be higher with the same phase resistance. A motor mounted in a very good heat conductor can dissipate more heat than a motor mounted on the non-conductor. So the current rating can be higher with a better heat Dissipation.
18- Full Stepping & Half-stepping
1. Full stepping -
· Peaking Holding Torque: T ab = Vector sum ( T a + T b) = 1.414 T a = 1.414 T b
· theoretical average torque: 90.07 % of max. holding torque
2. Half stepping -
· Peaking Holding Torque: T ab = ( T ab + T a)/2 = ( T ab + T b)/2 = 1.207 T a = 1.207 T b
· theoretical average torque: 97.45% of max. holding torque
Full stepping torque/ Half stepping torque = (1.414/1.207) ´ (90.03/97.45) =1.08
In General, full stepping will gain 8% more torque than half-stepping. However, the 29.3% full stepping torque-ripple causes noise and vibration at low speed operation.
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