Next I want to introduce some terms which are relevant to describe the stepper motors and then present one of usual mistakes people make when selecting a stepper motor for an application.
Nominal Current:
This is the maximum current for which the cable that go through the coils we described earlier is made and therefor the motor can withstand without getting to hot and break.
Nominal Tension:
This is the tension for which the stepper motor is having the nominal current flow through its coil.
If we apply this 2 values to the law of Ohm we get the resistance I mentioned earlier can be measured while identifying the cables coming out of the stepper motor.
R = U / I
Holding torque:
This is the torque provided by the stepper motor and which prevents the stepper motor to rotate.
Power:
Expressed in Watt and can be computed by multiplying a current in Ampere with a tension in Volt:
P = U * I
End of the terms and lets dwelf into interesting matters!
Lets compare 2 stepper motors, one with a nominal voltage of 12 VDC and one with 1 VDC, lets assume both motors have a nominal current of 1A:
The nominal power of the first motor, which translates into torque:
P = U * I = 12 * 1 = 12 W
The nominal power of the second motor, which translates into torque:
P = U * I = 1 * 12 = 12 W
The first impression is, the two motors are of identical power and this would fully comply with the perception we have from brushed or brushless motors! Let me tell you that of the 2 I prefer the second motor! Surprising, isn't this true?
Well, lets explain this! The control logic to operate a stepper motor includes a function that is called PWM, or Pulse Width Modulation. This function uses a switch that switches the current ON and OFF a couple of thousand times per second. The relationship between the ON and OFF times is called duty cycle and basically says how much time the switch is ON allowing current to flow and how much time the switch is OFF and the flow of the current is interrupted. This technique is also used to control the speed of a brushed motor i.e.! Well in stepper motor controllers this technique is used to limit the amount of current flowing through the coils normally to the nominal value of the current. Lets explain this on the example of my own sail boat model and the operation of the stepper motor there! I promise that later i will present movies loaded onto YouTube to explain this in a visual manner!
I do have 12 battery cells LiFePO4, each with a capacity of 16AH and with a tension, fully loaded of 3.65 VDC and fully unloaded, 2.0 VDC. That means the battery pack made of the 12 cells will supply a tension that will be below the max value of:
12 * 3.65 = 43.8 VDC
and above the min value of:
12 * 2.0 VDC = 24 VDC
I will use the tension provided by this battery pack to feed the 2 stepper motors I use to control the main and the headsail. lets apply this tension value to the 2 stepper motors introduced above:
First motor:
P = U * I = 12 * 1 = 12 W (nominal values)
P = U * I = 43.8 * 1 = 43.8 W
You see that the power of the stepper motor can be about 3.5 times the nominal value!
Second motor:
P = U * I = 1 * 12 = 12 W (nominal values)
P = U * I = 43.8 * 12 = 525.6 W
You see that the power of the stepper motor can be about 3.5 * 12 = 42 times the nominal value!
You see the second motor has a dramatically higher power than the first motor and it explains why I do prefer the second motor. The lesson i am trying to pass is as follows:
When you have the choice between two stepper motors with the same nominal current, always prefer that one that has a lower nominal tension specified! This says that the second motor is the one of much higher value and better implementation! More generically. Ignore stepper motors with a nominal tension value that is high and choose the one with the lowest possible nominal tension value, but still offering the torque you need and want!
This higher tension applied to the stepper motor has another important effect. It allows the stepper motor to run at much higher step rates and as a consequence at much higher speed loosing less torque as it gets faster! Advanced motors often do specify up to which tensión the stepper motor can be operated, this value being always much bigger than the nominal tension value. I hope you understand now the difference. The manufacturer of the stepper motor is saying with this that due to the way he has designed and manufactured his stepper motor a tension applied to it not higher than this value will not ruin the motor. Remember, the coils are made by winding slim cable and this cable as an isolation coating. if the tension is too high, combined with the nominal current the motor could break somewhere in its design!
Lets explain why a stepper motor can turn at higher step rates without loosing all his torque! By changing the flow direction of the current through the coils in the stepper motor by inversing the tension applied, we explained this above, the coil behaves like one to which an alternate power supply is being applied, as we know from a transformator. Here we have, saying it simple, 2 coils with different number of windings. The AC tension applied to the primary side, the input to the transformator, induces, that is how the effect is called, a tension in the second coil which is higher or lower depending uüpon if the number of windings on the secondary side, the output side, is higher or lower.
Well in a stepper motor the higher the step rate is, the higher the frequency of the change of polarity is and the higher the induced tension is! This tension has the opposite polarity as the applied tension. Saying it in an equation:
U
Resulting = U
Applied - U
Induced In numbers:
U = 43.8 V - 33.8 V = 10 V
As a result the effective tension responsible for the torque has been reduced to less than
1/
4 so the power in the stepper motor and the torque available is also reduced to less than1[size=78%]/[/size]
4This can get to the point where the stepper motor does not have the torque available to even do a step with no load! So applying the higher tension allows the stepper motor to tolerate a higher induced tension before becoming inoperable!I have been learning a lot while trying to understand the operation of stepper motors and why my stepper motor, with just 12 VDC applied, a nominal tension of 1.6 VDC, did not move. The next issue I want to present to you is a bit complex and better understandable once I have made the videos to explain. Basically the issue is about what parameters do exist that influence the operation of a stepper motor, its degree of vibration, its degree of noise, the amount of torque versus step rate, the connection of the coils in parallel or in series, I have already explained physically what this means, the impact of the microstepping, the impact of the characteristics of the microstepping, the acceleration and deceleration rate of the stepper motor. For all this there is a GUI as part of the IDE supplied for free from
www.trinamic.com and a board called StepRocker, which allows to play with this parameters by changing them on the GUI (Graphical User Interface), which is running on a PC to which the StepRocker board is connected via USB. It might sound intimidating, but it is really just getting a rough understanding and starting to play with it!
http://youtu.be/X4X_EUxqKEoHere the link to a short video that shows how using the right functions to operate a stepper motor can make a big difference! I have used this S-Ramp function, which is just clicking a box in the GUI, being able to accelerate the stepper motor even more. For me, where my stepper motors will be turning a drum for the sheet, this function will make the difference between being able to operate the sails during a turn of the sail boat in fraction of a second, to have it last a couple of seconds, as i get to see it on other implementations!