# Thread: Doubt regarding stepper motor current rating

1. ## Doubt regarding stepper motor current rating

Hi all,

If a stepper motor has 1.5A written on it, is it the motors maximum current required or current per phase. And what exactly is this current per phase and how do we calculate the motors maximum current. I am confused, please help!!!

2. I believe that is generally the maximum current per phase. It's never the "required" current because you can always /under/ drive a motor. If there is a voltage rating, then you can measure the coil resistance and use ohms law to calculate the current, but that doesn't really solve the "per phase" vs "total" question.

When it comes right down to it, the amount of power you can jam into a motor is limited by two things: Heat and insulation breakdown in the windings. The insulation is only affected by max voltage... well... until it melts! The heat is affected by MANY things: Mostly by motor current times voltage which is the power the motor is accepting, but also by how much cooling you can apply, how many eddy currents are caused in the motor windings by AC components in the drive signals (choppers cause more eddy current heating) and so on. There is a limit to the torque produced by a given drive current and past the rated current you won't get more torque because the magnetic field can't help turn it anymore, but that doesn't make the motor blow up or anything.

So don't exceed the max voltage rating or the windings might short and don't let the motor get too hot, but other than that, driving more than rated current really isn't that big a deal. I've driving 2 amps through a 1 amp rated motor without any problems... and I get a /little/ more torque... maybe half again as much... as I did when driving 1 amp at the same voltage. My power supply only put out a fraction of the motors rated max voltage so I wasn't in any danger of damaging it.

Other will correct me if I'm wrong or have their own opinions, but the point is: Don't get terribly hung up on knowing the motors exact rated current. If you motor is 1.5 total and you drive it at 1.5 per phase (3 total) and just keep a finger on it to make sure it isn't overheating, you should be just fine.

There is also a lot more info on how to estimate what you need in the way of motor and drive at techref.massmind.org/techref/io/steppers.htm#Estimating

3. Hi James,

If i am not wrong, you are the founder of Linistepper. And its an honor that you took time and explained it to me, thank you so much. I have built many open source driver circuit for my projects and to be honest nothing like Linistepper and still using your circuit for my CNC router.

Now coming back to the question, If i have 3 stepper motors and if i add up their current it comes up to 5A do i have to provide a 24V 5A power supply. Is is safe to provide so much of current and voltage on Linistepper.

4. My math comes up to 9 amps, not 5. Assuming full step, unipolar or bipolar windings, 1.5 amps per phase, there will be two windings turned on all the time for each motor. That will be 3 amps per motor at the rated motor voltage. Times three motors is 9 amps.
If you do not know the voltage, Just use a high enough voltage and a limiting resistor, picked to maintain the 1.5 amps per phase or winding. Then measure the voltage across an energized motor winding, and now you know.
I think for half stepping a unipolar motor, you may have to add a little more current because you will be powering an additional winding (the other winding of the winding pair) for part of the time, during steps. It is beyond me as to why anyone wants to half step a stepper at the expense of lower torque. If you use a 10 pitch leadscrew and a 200 step motor (1.8 degree/step), you only advance .0005" per step. That is fairly smooth motion to me. Even a two start 10 pitch leadscrew nets a .001" per step. Unless your CNC machine is made like my 2500 lb Bridgeport mill, it probably flexes worse than that. Joe

5. ## Thanks

Thank you so much, now i get it if a motor is specified 0.8A/phase that means the total current comes up to 1.6A that is full stepping mode i can select a power supply of 2A for single motor for Half stepping mode. please correct if i am wrong.

Can you suggest any Interface board for linistepper.

Warm Regard's
Naveen Govindaraj
Machine Design Engineer
India

6. Originally Posted by naveen.a019
Hi James,

If i am not wrong, you are the founder of Linistepper. And its an honor that you took time and explained it to me, thank you so much. I have built many open source driver circuit for my projects and to be honest nothing like Linistepper and still using your circuit for my CNC router.
Thanks, but actually, you are wrong: Roman Black is the founder (designer, publisher) of the Linistepper. I just sell the kits, answer questions, and document his knowledge. But I do spend a lot of time answering questions and so I very much appreciate your recognition of that.

Originally Posted by naveen.a019
Now coming back to the question, If i have 3 stepper motors and if i add up their current it comes up to 5A do i have to provide a 24V 5A power supply. Is is safe to provide so much of current and voltage on Linistepper.
You actually need to provide something like 6 amps, because there is a need for current to drive the Linistepper circuit itself in addition to the motors. The linistepper is rated for 1.5 amps at 35 volts in microstepping mode with a normal (large) heatsink. The voltage is a pretty hard limit... 36 is probably ok, but no more. The current * voltage or wattage is limited by the heat the power transistors can dissipate. So with a good CPU cooler type heatsink (with a fan) you can safely run 2 amps (I do 2 amps at 24 volts with no problems) and perhaps even 3 amps, but at that point the microstepping modes above half step will not be very accurate in terms of the between step positioning. More and better information about tuning for higher currents is well documented at:
techref.massmind.org/techref/io/stepper/linistep/lini_tun.htm

Hope that helps!

7. Originally Posted by naveen.a019
Thank you so much, now i get it if a motor is specified 0.8A/phase that means the total current comes up to 1.6A that is full stepping mode i can select a power supply of 2A for single motor for Half stepping mode. please correct if i am wrong.
Errr... hang on... in a unipolar driver only one coil at a time is turned on, or if both coils are on, they are not on all the way... a 1.5 amp per phase motor needs 1.5 amps of supply current. {Roman, please correct me if I'm wrong on this?}

Now, having said that, the Linistepper has a special high current half step mode with actually uses 1.4 time the per/phase rated current.
techref.massmind.org/techref/io/stepper/linistep/halfstep.htm

And the linistepper itself needs a bit of power, so for example, for a 1 amp per phase motor, the system should draw about 1.2 to 1.6 amps and Roman suggested a 2 amp supply.

Originally Posted by naveen.a019
Can you suggest any Interface board for linistepper.
Yes! If you are buying a kit, I strongly recommend my 4 Axis board:
techref.massmind.org/techref/io/stepper/linistep/4axis4build.htm which does everything... no really... it does more stuff that it should. If you are going open source and want to make your own, look at any of the many PMinMO BOB designs. (see the lower left corner for "interface boards")

8. Originally Posted by James Newton
Errr... hang on... in a unipolar driver only one coil at a time is turned on, or if both coils are on, they are not on all the way... a 1.5 amp per phase motor needs 1.5 amps of supply current. {Roman, please correct me if I'm wrong on this?}
...
Hi I just saw this thread. The Linistepper uses microstep phasing currents so typically there is about 1.5 times the per-phase current, maybe a bit less. So with a 1.5A motor in microstep mode it will draw around 1.5v * 1.5 = 2.25A. Each microstep can draw different currents so I would say somewhere from 2A to 2.3A.

However if you are using FULL STEP MODE both phases run full on so it would be 1.5A * 2 which is 3A and that is at all times.

Manufacturers can rate their motors under different systems. Most of the old unipolar motors you pull out of old equipment have label ratings for 2 phase on. So 1.5A means both phases always running at 1.5A.

But newer motors (like most square ones) are rated with the expectation of microstepping which will be around 1.41 times the per-phase current at all times. I hope that is not too confusing!

Originally Posted by James Newton
...
Thanks, but actually, you are wrong: Roman Black is the founder (designer, publisher) of the Linistepper. I just sell the kits, answer questions, and document his knowledge. But I do spend a lot of time answering questions and so I very much appreciate your recognition of that.
I'll throw that back in your court. For all my few weeks of the product design and some later updates you have done years of web page creation and support and customer tech and sales support and marketing... I might be the designer of the circuit and software but you are definitely the founder of "Linistepper the open source product".

9. I'm putting together a page at:
techref.massmind.org/techref/io/stepper/power.htm to document these issues. Any input on that page would be appreciated.

Standard Half step mode uses just the rated current?

Is "Always provide double the rated current" a good rule of thumb?

10. James I made an error above (bad memory!). I checked the Linistepper source code today and there are 3 main values for the currents used to generate all the microsteps;
A B
100% 0%
100% 25%
100% 55%

The average current is 126%. So the average current when running in microstep mode is 126% of the single phase current. For any particular microstep the minimum is 100% and the maximum is 155%.

In FULL step mode the current will be 200% at all times.

The half-stepping mode the LiniStepper uses is 140% average current.

I think they only need to allow for double the current if they are using full step mode, I would suggest that 140% is the best generalisation for half stepping and microstepping modes.

Regarding that web page I would add a section specifically for the Linistepper as it differs from chopper supplies. You can use the current data in this post, I have just double checked them with the latest Lini firmware.

11. can i make sure i am understanding this correctly. i have 4 stepper motors running 2a per phase. should i have a power supply about 10 amps to get the best out of them

12. What type of stepper motor drivers are you using?

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