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Chatting about stepper motors and their control

Hellmut1956

Aug 11, 2014
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Hi friends, I have fallen in love with stepper motors and this goes together with studying technical detail about those motors, their mechanical, electrical and magnetic behaviour. recently I listened to a 3 day seminar about motors offered by the "digikey university", if remember it correctly. I was surprised how outdated and in consequence some information was plainly wrong as it was geared towards giving engineers an initial understanding of brushed, brushless and stepper motors.

What I have learned and verified through own experiments is coming from the company "Trinamic". To start for this chat with the most important aspect for somebody considering to use a stepper motor, is to be able to get a first strike understanding of what is the best stepper motor to choose for an application requiring a certain power in Watts!

If you compare 2 stepper motors with same power in Watt, what would be the better motor two choose of the following 2?

Motor 1:
Nominal tension 12 VDC, nominal current 1 A gives the nominal power of 12 W

Motor 2:
Nominal tension 3 VDC, nominal current 4 A gives the nominal power of 12 W

Which motor would you choose and why?
 

Hellmut1956

Aug 11, 2014
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Great my dear friend Minder! In light of all that wisdom, how do you respond to my question and why?
 

Old Steve

Jul 23, 2015
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If I was running a 12V system, I'd go with motor1, and if I was using a 3V supply, I'd go with motor 2.

The 12V motor would be better if the only consideration was current.
 

Hellmut1956

Aug 11, 2014
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Thanks for your response "Old Steve". it proves that a very important aspect of stepper motors is unknown to most, and it is extreme important to know this as it is key to have the best performance available. I will explain why!

Both stepper motors offer a performance of 12 W, right? But the second motor is of better quality as its coils are able to allow 4 A of current to flow through them! Why is this important? Because you would, if you knew about stepper motors you would choose motor 2! Control circuits for stepper motors allow to define what the maximum value of current is the stepper motor can use! The control logic uses PWM to limit the flow of current to the nominal value, her in case of motor 2, 4A!

Now it is evident the relation between current and tension applied to a circuit, here one of the 2 phases of a bipolar stepper motor, follows the Ohm equation! This applies obviously only in the static case where the stepper motor is just holding its position! Now lets have a look of thge equation for performance:

P [W] = U[V] * I[A]

Using the nominal values for current and tension the result for both motors is 12 W! Now lets play the game using not the nominal value for the tension, but lets assume we apply 36 VDC to both stepper motors. remember that applying the Ohms law to motor 1 a current of 3A would flow:

P = 36 V * 3 A = 108 W

The motor would be killed!

Now lets apply the fact that electronic circuits to control a bipolar stepper motor use PWM to limit the current flowing through the coils to 1 A. Now lets apply the equation for performance to motor is with 36 VDC applied to the motor and the PWM limiting the flow of current to 1A:

P = 36 V * 1 A = 36 W

Suddenly motor 1 is capable to deliver triple performance compared to its nominal values!

Now lets do the same exercise with motor 2, starting with applying the Ohms law computed value for current as we did for motor 1:

P = 36 V * (4A * 12 = 36 V * 48 A = 1728 W

The motor is killed!

Now lets compute the performance, but taking into account that the control circuit limits the current flowing through the coils to 4 A!

P = 36 V * 4 A = 144 W

The motor 2 is capable to deliver 12 times the performance!

So, motor 2 / motor 1 = 144 / 36 = 4

Motor to is capable to deliver 4 times the performance of motor 1 applying the same 36 VDC to both motors, possible due to the current limitation achieved through the PWM used by the control circuit to limit the current to the value defined as its nominal current value!

So the result are:

1. Choose when having to select which motor to use the one with the lower nominal tension value!
The cables in his coils allow more current to flow through them making it the better choice.

2. Try to have the tension applied to your stepper motor as high as possible.

Good documented stepper motor have as a parameter in their datasheet the maximum value the stepper motor is guaranteed no to be damaged! is this result not surprising to you, my dear readers! And it is not the only piece of information about stepper motor properties important to their use! I will give you more data as soon as you have digested this piece!

You are welcome to ask, criticize or comment my reply!
 

Old Steve

Jul 23, 2015
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Thanks for your response "Old Steve". it proves that a very important aspect of stepper motors is unknown to most, and it is extreme important to know this as it is key to have the best performance available. I will explain why!

Both stepper motors offer a performance of 12 W, right? But the second motor is of better quality as its coils are able to allow 4 A of current to flow through them! Why is this important? Because you would, if you knew about stepper motors you would choose motor 2! Control circuits for stepper motors allow to define what the maximum value of current is the stepper motor can use! The control logic uses PWM to limit the flow of current to the nominal value, her in case of motor 2, 4A!

Now it is evident the relation between current and tension applied to a circuit, here one of the 2 phases of a bipolar stepper motor, follows the Ohm equation! This applies obviously only in the static case where the stepper motor is just holding its position! Now lets have a look of thge equation for performance:

P [W] = U[V] * I[A]

Using the nominal values for current and tension the result for both motors is 12 W! Now lets play the game using not the nominal value for the tension, but lets assume we apply 36 VDC to both stepper motors. remember that applying the Ohms law to motor 1 a current of 3A would flow:

P = 36 V * 3 A = 108 W

The motor would be killed!

Now lets apply the fact that electronic circuits to control a bipolar stepper motor use PWM to limit the current flowing through the coils to 1 A. Now lets apply the equation for performance to motor is with 36 VDC applied to the motor and the PWM limiting the flow of current to 1A:

P = 36 V * 1 A = 36 W

Suddenly motor 1 is capable to deliver triple performance compared to its nominal values!

Now lets do the same exercise with motor 2, starting with applying the Ohms law computed value for current as we did for motor 1:

P = 36 V * (4A * 12 = 36 V * 48 A = 1728 W

The motor is killed!

Now lets compute the performance, but taking into account that the control circuit limits the current flowing through the coils to 4 A!

P = 36 V * 4 A = 144 W

The motor 2 is capable to deliver 12 times the performance!

So, motor 2 / motor 1 = 144 / 36 = 4

Motor to is capable to deliver 4 times the performance of motor 1 applying the same 36 VDC to both motors, possible due to the current limitation achieved through the PWM used by the control circuit to limit the current to the value defined as its nominal current value!

So the result are:

1. Choose when having to select which motor to use the one with the lower nominal tension value!
The cables in his coils allow more current to flow through them making it the better choice.

2. Try to have the tension applied to your stepper motor as high as possible.

Good documented stepper motor have as a parameter in their datasheet the maximum value the stepper motor is guaranteed no to be damaged! is this result not surprising to you, my dear readers! And it is not the only piece of information about stepper motor properties important to their use! I will give you more data as soon as you have digested this piece!

You are welcome to ask, criticize or comment my reply!
Your condescending attitude is quite offensive, Hellmut.
I am already very familiar with stepper motors and don't need lessons in power dissipation etc. I have used them many times over many years.
PWM is not always used in their operation as you claim. Often, a fixed, constant voltage is applied. All I meant regarding current was that the supporting circuitry and wiring could be simpler if the current through it was lower. No more and no less.

And assuming that I did not want to use PWM, if I had a 12V supply, does it not follow that I would choose a 12V motor? Otherwise I would need a good regulator capable of high current to drop the voltage to 3V to use motor 2.

You say, "the second motor is of better quality as its coils are able to allow 4 A of current to flow through them".

I say, "a motor that uses a higher current is not necessarily of better quality".

I can see that this is going to develop into an argument, and I'm not interested, so will not respond here again.
 
Last edited:

Nicholas Schlensky

Oct 7, 2015
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If you used to 3V motor you could current control the motor drive signals and operate with a higher voltage 12V. You can get faster stepping by this method because the speed to reach Imax is quicker with the higher voltage.
 

Old Steve

Jul 23, 2015
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If you used to 3V motor you could current control the motor drive signals and operate with a higher voltage 12V. You can get faster stepping by this method because the speed to reach Imax is quicker with the higher voltage.
Quite true, and when it's called for I do exactly that, but sometimes only a simple circuit is wanted, due to expense or when there's no need for fast pulse rates. It just depends on what's being built. And it doesn't change the fact that PWM is not 'necessary' to run a stepper.

And although Hellmut 'likes' bipolar motors, there's a good reason for using unipolar motors too. Bipolar steppers require a H-bridge for each winding, whereas a unipolar motor only needs transistors pulling each end of a centre-tapped winding to ground, with the centre-tap connected to +Ve. Much more cost-effective.

What I'm trying to say is that there is more than one way to skin a cat. (39, according to the robot in "Lost in Space". :) )
 

Hellmut1956

Aug 11, 2014
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All I was writing is what to many is not obvious that any modern stepper motor control system you automátically have the PWM limiting the current to a value to be selected by the user and this he is doing depending which motor he wants to use. The answer of Old Steve did apparently shown he didn't know what I was targeting too. Sadly many feel that when new knowledge about a topic is presented they feel offended personally considering that they know everything there is to know they already know! The fact is that opposed to what dear Old Steve is writing, many do not know what the PWM is for and that it is available even in now pretty outdated controller circuits like the pair L297 and L298. I did clearly point to the fact I was taking about bipolar motors only not denying that in certain applications there are good reasons to decide for a different solution!

Otherwise I would need a good regulator capable of high current to drop the voltage to 3V to use motor 2.

This sentence my dear Old Steve is not correct. Basically any stepper motor control circuit does include the PWM to limit the current flowing through the coils. You do not need any external device but a resistance connected to one of the pins of a component to define to what current the PWM should limit the current. When you have to choose between to motors like the number "1" and "2" I took as an example it makes more sense to choose the number "2" because this motor does not need 4 A, it can live with 4A of current! In the L297 datasheet, really an outdated device Pin 15 sets the Vref value. I state what is written on page 3 of the L297 datasheet:

Reference voltage for chopper circuit. A voltage applied to this pin determines the peak load current

So this is just general basic knowledge of today but often unknown. I recently attended a webinar from EEDesign about motors and the presenter did not address this issue at all!

The fact is that speaking with expert users one made a nice comment about stepper motors. he said that he knew what stepper motors are because he had hurt his arms frequently when dealing with them when touching such a motor accidentally! That gets me to another fact I would like to present, knowing that certain experts here do know the topic! I am just now going to talk about about power and torque made available by the stepper motor comparing motors "1" and "2" again just as examples. Heat dissipation is coming from different sources within a stepper motor but it is valid to write that a coil using a cable capable of handling a current of 4 A has a smaller internal resistance then a motor where just 1 A of current is possible. So the heat dissipation due to the internal resistance of the cables in the coils is smaller in motor 2!

Lets now go on by looking at a diagram that shows the torque of a certain stepper motor against the stepping frequency:

14217496655_6d5d4b093d_z.jpg


The 4 curves in this diagram are to my personal opinion a good orientation to address the aspects of a stepper motor I would like to address in this chat, again welcoming critics, questions and additional inputs!

9568215543_6acd82a91a_z.jpg


Lets go by parts! A simple bipolar stepper motor has its 2 phases and just 1 coil per phase. But if you take the third picture to the right of the first row and you compare it with the second picture on that same row you can see that the second picture is just a motor where the 2 coils in series are connected in series and that point brought out of the motor as a "center point" connection. To explain the meaning of the 4 curves never the less the third picture is more useful! If you now look on the third picture of the second row you can see that the 2 coils have been connected in series, while on the single image in the third row the two coils of a phase have been connected parallel! So 2 of the curves display information about the technical data of the same motor with the coils of a single phase connected parallel, while the other 2 show the information regarding with the coils connected in series. The diagram looks at the data connecting the motor once to 24 VDC and once to 48 VDC!

The diagram shows the torque available at the stepper motor for all 4 configurations and at both voltages against speed, which is equivalent to frequency of stepping of the stepper motor! Again risking to be seen as condescending and by no means being this my intention here my second question:

1. Why is the torque available from the motor getting smaller while the stepping frequency is increased?
2. Why are the 4 curves showing different behaviour and why are the torque over speed variation of the same stepper motor nearly identical when comparing the first curve with 4.2 A at 24 VDC identical to the curve of this same motor but with only half the current but at 48 VDC?

Just a short justification from me. I am presenting this as a chat and not as a tutorial to promote talking over the topic. Hopefully dear Old Steve will overcome its offense feeling and I would like to state what an old greek philosopher said about having knowledge: I know more then all the others, because I know I do not know! I am always searching to find areas of knowledge I do not have so that I can learn and to be advised when I am wrong, because only this way I can correct my wrong knowledge!
 

Old Steve

Jul 23, 2015
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Don't be concerned about my reply, Hellmut. You won't be hearing from me again.

Due to your attitude here in this thread, a PM from a member yesterday insisting that I help sort out a program problem that he was having, an argument today which ended in personal abuse directed at me, then a follow-up in a PM by that same member, listing all that he felt I'd done wrong and informing me that he'd posted selected parts of the (now closed) thread on another forum to illustrate what sort of people are here, I don't feel comfortable on these forums any more and will not be returning. I've had enough.

You should take a good look at your attitude toward other people, or you will completely alienate yourself and receive no replies to your posts whatsoever.
 

Minder

Apr 24, 2015
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I am always searching to find areas of knowledge I do not have so that I can learn and to be advised when I am wrong, because only this way I can correct my wrong knowledge!
In a general sense I would have picked motor 2, this is from the some of the knowledge picked up by practical applications from such as the largest forum of stepper motor users out there, namely the CNCzone, where many posters are represented by manufacturers such as Gecko drives etc.
As a personal preference I much prefer DC or BLDC servo's, for several reasons, not the least is the tendency for steppers to be noisy in operation and also dissipate the rated wattage (heat) when stationary.
M.
 

duke37

Jan 9, 2011
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Hi Helmut
I have never used a stepping motor but do not see how the principles of physics should be different for them.
Question 1
The torque drops off as speed rises due to the inductance limiting the rate of rise of the current.
Question 2
If you have a motor with one turn on each pole, it will need a certain voltage and current to give a certain torque and will dissipate a certain amount of heat.
If you take the same basic motor but wind it with two turns per pole with wire of half the area and drive it with double the voltage, you will get the same current, double the torque and double the dissipation. The inductance will be four times the single turned motor.

It seems to me to be logical that the motor is run on a voltage less than the specified maximum. In the graphs you show, the torque never exceeds 3Nm presumably due to iron saturation so blasting a 4V motor with 12V is not likely to help you much.
 

Hellmut1956

Aug 11, 2014
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Hi Minder, let me start explaining the responses to my questions.Every time the control circuit changes the inputs to the motor to have it step to the next step, yo may have on both, or on just one of the phases a change of polarity when dealing with full step motion. This change of motion represents a change of the flow of current through the coil and so an tension of opposed voltage is generated by induction. The faster this changes of polarity happens, as you increase the step frequency, the higher is the induced tension of opposite polarity. Those two tension, the one applied to the phase and the induced tension of opposite polarity add to a smaller resulting effective tension. So there will be a frequency, were the induced opposite polarity tension equals the one you apply to the motor. As the torque is proportional to the effective tension to the phases moves towards a value closer to "0 VDC", so does the available torque move towards "0" too as the diagram displays.

Now that explains why applying a higher tension to the stepper motor it will take a higher step frequency until the induced tension reduces the effective tension to the same value. So a right possible answer to question 1 is making the reference to the induced tension in the coils!

So a possible answer to question 2 is that doubling the applied tension makes serially connected coils in a phase generate the same torque at any frequency as having the coils connected in parallel, as the current splits in 2 equal paths flowing through the parallel connected coils! So if you want to achieve the highest possible torque, connect the 2 coils of a phase in parallel. The reason is that the full applied tension is effective over each of the coils, that means 2 torque generators adding their torque. Do you put the 2 coils serially that at the same speed on each of the coils you have only 50% of the tension!

I hope that gives the explanation. Now let's continue to a more abstract topic of the operating noise! Let me give you a link to an source of information that makes noiseless operation of stepper motors possible at low speeds:

https://www.google.de/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=stealthChop

I would leave the explanation to the webpage I have supplied the link for!

Another interesting aspect is the benefit of using microstepping opposed to full or half steps. let me share with you a story from the early days I did deal with stepper motors! I did use a stepper motor which I operated using a controller with the L297/L298 components applying 12 VDC. If I properly remember the nominal voltage of the stepper motor was 4 VDC. All I got experimenting with this setup was a stepper motor vibrating but not stepping at being noisy! Much later I got my hands on a controller board from the company Trinamic called "stepRocker" and found out that having 16 or more microsteps per full step made the stepper motor do its steps as desired. Why is this?

9483371177_daf1f68bf0_z.jpg


In full step operation each step represents a mayor change of the applied voltage level. Now in real life the voltage does swing and so might look like the red curve and not like the green one desired. The faster and the higher the change of the voltage level applied to the coils of a stepper motor is the stronger the amplitude of the swings becomes. Together with it, as far as I understand it, there are other parasitic effects which result in that at a certain frequency the stepper motor can not step as the fluctuation of the applied tension results in an undefined pattern.

9486170248_1656370fbf_z.jpg


basically what happens from microstep to microstep is that the polarity applied to the phases of the stepper motor is not switched in one step, but the voltage applied follows the values resulting from a sinus curve for which a lookup table is stored in the controlling IC. Now, the changes of the voltage applied to the phases of the stepper motor from microstep to microstep is much, much lower and in consequence the swinging is massively reduced, what also affects the other parasitic causes of "noise"! So with 16 microsteps per each of its 200 full steps of a full 360° turn my stepper motor did perfectly operate with the 12 VDC I did apply from a external source. So using finer and finer microsteps results in a much cleaner operation of the stepper motor and as a result it can move faster
 

Hellmut1956

Aug 11, 2014
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The reason it is a 3Nm torque specified motor I guess. You are right, the laws of physics fully apply to a stepper motor as well. What probably because I am not an english native speaker I miss in what you write and my explanations are poor and may be the cultural differences generate the impression I am being condescending as good Old Steve wrote. Nothing is more distant to my intention. I choose the "chat" methodology for this thread because I am looking for people to discuss topics and so I do place questions to see if the point was known! let me try to repeat what I tried to explain:

1. The higher the speed a stepper motor is rotating the higher the frequency is at which it steps through the turn! Now I am totally confident that you know that an induced tension is related to the rate of change of the tension in a coil. So faster frequency of stepping means a faster rate of change of the tension at the coil and as a consequence the induced tension which is always opposed polarity is higher in absolute numbers. Now the induced tension adds to the applied tension so that the stepper motor just "see" the resulting tension! So es an explicit response to your question #1. What you write is correct, as the amount of current flowing through the cables in the coil is proportional to the effective tension. As the effective tension, the sum of the applied tension + induced tension = a smaller tension as the induced tension has the opposite polarity changing the equation from "+" to "-". I hope I have been able to explain this.

The focus to the issue I mostly see this aspect spoken about using the tension values, as the current is a result of it.

2. Here I am certain ywhat you say is right. Just happens in the practical world that you deal with existing motors, so the number of turns of the cable and the saturation are aspects the designer of the motor has handled. Most of the users of stepper motors just look at to what power in watt they need and what value of the tension is available in the system. Here I have like my found that users not familiar with stepper motors choose a stepper motor with the nominal voltage being the tension available in the system as you would do with other motor technologies. And this is not a smart way to choose the right motor out of available alternatives. Lets say a naval modelist, like I am one, wants to use a stepper motor for what ever reason for his model and he has a 12 VDC lead battery in the model. So he goes to ebay and looks for stepper motors of the desired power, lets stay with my examples, he wants 12 W. So most user not familiar with stepper motors tend to choose one with a nominal 12 VDC value and not one with lets say just 3 VDC nominal value. Here he has 2 choices, at least. One is to have available the maximum power and as consequence torque a stepper motor can offer. The he should choose the one with 3 VDC nominal tension specification. He would apply his 12 VDC and as a result this same nominal 12 W motor will offer 4 times the performance/torque and will be able to support higher stepping frequencies and´as a consequence higher turn rates!

Of course, before controller ICs for bipolar stepper motors became widely available and the prices dropped to very low level, unipolar stepper motors were chosen as the control of them is much easier and you can use cheap transistors. Cheap printers often still use unipolar motors due to this reason. That is the nice thing about electronics and engineering. Many solutions are possible to adapt to the wide variety of application parameters. I do remember at the beginning of my career I was Field Application Engineer for National Semiconductor, the DP8500 product family. This product was aimed to offer high graphics performance. One day I had to deal with a project called Minitel from Alcatel France as a lab manager for graphic terminals. The Minitel was a predecessor of the Internet in France. France Telecom offered their customer the choice of either continuing to receive the phone books, or to get for free a Minitel phone that had a small screen and made it possible to get any telephone number. Something trivial these days, but before the Internet it was different. by the way, the large success of the Minitels was due to red district applications! The graphic controller from a french company they used was optimized to make possible as cheap as possible Minitel phones. My project was to compete against the established solution by using an NS32CG16 that had hardware on board to generate data communication by modem. So I learnt to respect the challenges not just in high end systems, but also in low end ones. The design team at Alcatel received a years salary as bonus if they reduced the cost of a Minitel by 1 USD!
 
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