Newbie- Stepper motor question, and help finding a driver please!

[doublec4]

Hi Everyone,

I'm new here and inexperienced with electronics so I'm looking for a little help. I've read a few things and learned a bit, but I have a question regarding stepper motors that I couldn't find an answer to.

Basically, I need to use 2 size 17 double stacked linear stepper motors for a project, but am having trouble selecting a driver. There are a lot of options out there, but I'm not looking to spend a fortune, and I'm hoping to have the driver interface with my "Arduino" processor to receive commands.

The two stepper motors are rated at 2.33V, 2.6amp/phase each. My power supply is rated at 24V, 200W and 8amp. I've heard good things about chopper drives, but I can't seem to find one to deliver the needed 2.6amp continuous for the motors I have. I know that chopper drives deliver "constant" amperage, but my question is this. Because I'm sending 24V to the driver, does that decrease the amperage draw (continuous / static) for my motor? Or do I still have to buy a driver that is rated for at least 2.6 amp continous?

If that is the case, does anybody know of a driver that meets my requirements? I'm feeling a little overwhelmed. Thanks!

[RomanLini]

Try a pair of these stepper drivers;
SLAm Stepper Motor Controller, Allegro SLA7062M, Unipolar
They are compact, easy to build and very rugged. And well supported on that site.

I use 3 of them on my machine, also running 2.6A motors. Your 24v 8 amp PSU will be plenty! They will draw around 2 amps total from the PSU under most operations.

[James Newton]

It would be good to hear a little more detail on those motors... Linear motors you say?

[doublec4]

Thanks for the suggestion Roman,

the motors are linear hybrid stepper motors from haydon kerk. Size 17, captive

I think I have a solution to this issue. I actually have not bought the components yet, so I can change my "plan."

I'm thinking I should switch to 5V motors, which have a 1.33amp/phase rating. Then I could use a more common 2amp continuous driver, and buy a 12V to 48V voltage converter. Then I would need to use resistors (I'm guessing? or if theres something else I could buy) to drop the 48V supply down to the 40V max rating of many stepper drivers.

How does that plan sound?

[packrat]

Originally Posted by James Newton It would be good to hear a little more detail on those motors... Linear motors you say?

Very neat motors for a lot of things. Check here for the Haydon Kerk; Stepper Motor Linear Actuators


Another company; Stepper Linear Actuators

A lot of the surplus electronics places and Ebay have them too.

cary

[RomanLini]

James- they are just a size 17 stepper motor with a tiny leadscrew through the middle of the motor, usually used for low power low speed linear positioning.

Packrat- Convert 12v to 40v??

What is your application? If portability is important and you are running from a 12v battery you may just be able to run the motors (ie drivers) from 12v. That will be fine for slow speeds up to a few revs per second.

This sounds like an unusual application with specific needs, so to make good design choices you need to give more details on what it does and how it needs to do it.

[doublec4]

Hi Roman, thanks for your interest.

The application is to actuate an aerodynamic element on a vehicle, hence the 12V DC power from the vehicle electrical system. I'd like to get it up to 40V so that I can match the thrust/velocity curve that I sized the motor from on the haydon kerk website. It's critical that I get good thrust and speed from this motor.

The original driver you posted for me in this thread, I noticed it does 40V, chopper etc. however it was unipolar. Do you know of one that is rated for 40V, greater than 1.33 amp continuous, and is chopper for bipolar motors? I would prefer the ability to microstep or at least half step, for better speed response.

Thank you, you've been very helpful!

[Mariss Freimanis]

Avoid using more than a 24VDC power supply with NEMA-17 motors. NEMA-17 motors are relatively low current and more importantly, low inductance. This combination of specifications places this size motor in the "coffin corner" regards to overheating due to eddy current losses. A good rule of thumb is to keep ripple currents (switching and running AC ripple current) to 20% or less of the motor's rated current.

For you, this is about 260mA. If you use a switching type drive, the switching ripple current (peak-to-peak) is equal to (Vsupply / Inductance) times 1/2 the switching period. Solving for supply voltage Vs, use I * 2f * L / 5 as your maximum voltage.

Example: Say you use a 20kHz switching drive, your phase current is 2.6A and your motor inductance is 1 mH.

Vmax = 2.6A * 2 * 20,000Hz * 0.001H / 5 = 20.8 VDC

You can go over this limit for short periods of time (< 5 minutes) because the equation gives you the continuous duty cycle limit voltage. When all is said and done, do not allow the motor case temperature to exceed the rated maximum value (usually 85C). Use any means, fair or foul, (such as heatsinking the motor) to keep it under the listed maximum.

Mariss

[doublec4]

Originally Posted by Mariss Freimanis Avoid using more than a 24VDC power supply with NEMA-17 motors. NEMA-17 motors are relatively low current and more importantly, low inductance. This combination of specifications places this size motor in the "coffin corner" regards to overheating due to eddy current losses. A good rule of thumb is to keep ripple currents (switching and running AC ripple current) to 20% or less of the motor's rated current. For you, this is about 260mA. If you use a switching type drive, the switching ripple current (peak-to-peak) is equal to (Vsupply / Inductance) times 1/2 the switching period. Solving for supply voltage Vs, use I * 2f * L / 5 as your maximum voltage. Example: Say you use a 20kHz switching drive, your phase current is 2.6A and your motor inductance is 1 mH. Vmax = 2.6A * 2 * 20,000Hz * 0.001H / 5 = 20.8 VDC Mariss

Thank you Mariss,

Just a quick question about this. I think you did the calculations there for the original motor. The new 5V motor I have selected is 1.3A, inductance of 8.3mH, so with your calculation, it gives:

1.3A*2*20,000Hz * 0.00821H / 5 = 85VDC

Is this correct? I should be safe running 40V if this is true? Sorry for being such a newb, I just want to match up the right motor and driver and not waste money. Don't want to have any expensive mistakes! Thank you

[Mariss Freimanis]

My reality check alarm is blaring.:-) Honestly, I think you may have slipped a decimal point for the inductance because 0.8mH makes far more sense than 8mH. These are tiny motors relative to NEMA-23 size; 1.3A at 8mH is reasonable for that size, certainly not for NEMA-17.

I'd go so far as to say if the specs say 8mH, then the specs are in error. It violates the laws of physics

Mariss

[doublec4]

Originally Posted by Mariss Freimanis My reality check alarm is blaring.:-) Honestly, I think you may have slipped a decimal point for the inductance because 0.8mH makes far more sense than 8mH. These are tiny motors relative to NEMA-23 size; 1.3A at 8mH is reasonable for that size, certainly not for NEMA-17. I'd go so far as to say if the specs say 8mH, then the specs are in error. It violates the laws of physics Mariss

Not trying to argue with you, you obviously are much more knowledgable than me with this sort of thing. However, I'm just going on the specs presented on the haydon-kerk website:

Size 17 Double Stack Stepper Motor Linear Actuator

If you look under the 5V motor, it says 8.21mH / phase inductance. Does it make a difference that I selected a double stacked motor? Because thats the one I need...

Thanks again!

[RomanLini]

If I understand you right and this is to operate some air spoiler etc on a full sized vehicle, be aware that the aerodynamic forces can be enormous at higher speeds or even low speeds and cross winds etc.

A size 17 stepper motor with a tiny little leadscrew through it's shaft would be very unsuitable.