Electrical Advice

Hello All,

I'm working on a home automation project - this is my first project that attempts to incorporate automation.

My device uses a NEMA 23 Stepper Motor to drive a platform 2ft (total travel). At the end of the movement, I need to turn on and off an electromagnet w/ +300 lbs of holding force.

I see a couple posts that show how I could control one or the other device, but I'm struggling to find a control / power supply that can handle both items. Below are some of the details - Any advice or thought on how to control these two items would be greatly appreciated. Again, I'm a novice - please type slowly :wink:

NEMA 23 Stepper Motor; P/N: 25311-02, https://www.inventables.com/technologies/stepper-motor-nema-23)
2.3V, 2.8A

Electromagnet; P/N: EM300-3-222, http://catalog.apwcompany.com/item/electromagnets/3-0-diameter-round-electromagnets-em300/em300-3-222?
3V, 5A

Thanks

It will be easier for you if you use 5V or 12V devices.

They will take less current and you will be able to source a power supply much more easily.

Thank you for the reply - I was able to locate both components in 12V options. I imagine this will get me moving in the right direction. Thanks again

This may be overkill, but you could buy a reprap (open source 3D printer) RAMPS board, which is basically a sheild that plugs into a Arduino Mega 2560 and can drive multiple stepper motors (5 I think ) and also has FET's for switching heater loads, which take far more current than your requirements.

e.g.

http://www.ebay.com/itm/Brand-New-3D-Printer-Controller-for-RAMPS-1-4-REPRAP-MENDEL-PRUSA-for-Arduino-/231210615229?pt=LH_DefaultDomain_0&hash=item35d53a01bd

And just get the stepper drivers separately (as you don't need are 5 stepper motors drivers and they are a couple of dollars each)

With Arduino there are tons of tutorials for controlling stepper motors you can find them easily just googling them. Even in the Arduino IDE you can find some. You may require an aditional chip as a driver to interface with Arduino, but don't worry because they are very cheap (for example, for unipolar stepper there is the 1 dollar ULN2003 chip)

And controling the magnet lock is easy with Arduino too because you just have to turn on and off a 12 V device (I've a magnet lock at home and control it with Arduino and a transistor).

WAIT A MOMENT ...

The voltage of a stepper motor is largely irrelevant. It is the max coil current that matters, and of course the torque. First requirement is to ensure the motor has sufficient torque for your application. A bipolar stepper motor such as you mention in your first post will be fine, IF it has sufficient torque. If it has more torque than necessary you may be able to find a motor that requires less current.

Bipolar stepper motors should be driven with a proper stepper motor driver board. This Pololu A4988 is an example and the web page has very good information. However the A4988 can only drive a motor that takes 1 amp, or maybe 1.5 amps at a stretch. For a 2.8 amp motor you will need a bigger driver (they all work in much the same way) which will be more expensive as you are moving out of the hobby market. Hence you may want to consider a lower amp motor - but only if it has enough torque.

The key feature of proper stepper motor drivers is that they can use a high voltage to get better performance from the motor because they can limit the current so the high voltage can't damage the motor. The voltage of 2.3v in your first post is just the voltage that will cause 2.8 amps to flow in the coils based on Ohm's law. The motor would perform very poorly if only supplied with 2.3v. It may work reasonably at 12v if you don't need it to have high torque at high speed but 24v or 36v would be better.

I suspect you will wish to review your approach to selecting a power supply.

...R

@Robin2,

Assuming the OP's initial spec if correct that the torque he needs would be delivered by a motor taking 2.3V, 2.8A.

At 12V like these motors normally work at, it would be around 1/5 of the current i.e around 530mA which would be fine on the standard stepper driver module.

As the Reprap RAMPS 1.4 boards are only $10 and the Pololu A4988 can be had for around $5 each if you shop around, I don't think building this would be hard,.

But it does presume that the torque calculations are correct.

The OP may be using a reduction gearbox, they really specify.

I'm not sure of the Specifics but I know the reparp Ramps board will operate at 24V as well, but I know that 24V PSU's are a bit harder to source then 12V ones.

rogerClark:
Assuming the OP's initial spec if correct that the torque he needs would be delivered by a motor taking 2.3V, 2.8A.

At 12V like these motors normally work at, it would be around 1/5 of the current i.e around 530mA which would be fine on the standard stepper driver module.

My calculations (which are often wrong) suggest a coil resistance of 0.82 Ohms and that should give an Ohm's law current of 14 amps at 12v. So I don't understand where 0.53 amps comes from.

I wonder if you are thinking that the current will be lower to maintain the same number of watts with a higher voltage? Alas, it doesn't work like that. The magnetic force just depends on the current in the coils and doesn't care about the voltage. You could, of course dial the A4988 down to 1 amp or less, but then you would not get the specified torque from the motor.

...R

Hi Robin2

I was just taking the power i.e V*I (posted by the OP) and working out how much current would be required at 12V give the same power.

I've no idea about the coil resistance of these motors, but I suspect that as the stepper drivers produce quite high frequency square waves, the inductive reluctance would come into play.

I took a quick look and there are power drivers for NEMA 23 as seen in this video RAMPS Merlin NEMA23 via THB6064AH 50volt 4amp stepper driver - YouTube

But as we have no idea what weight the OP is trying to lift with the magnet or what gearbox he is using etc, its hard to give an accurate answer :wink:

rogerClark:
I was just taking the power i.e V*I (posted by the OP) and working out how much current would be required at 12V give the same power.

That may be valid for the average current in the motor but the peak current will need to be controlled by the stepper driver so it doesn't exceed the allowed maximum. The purpose of using a high voltage is to get to the max current as quickly as possible (and for as long as possible during each step) in the face of the inductance and the back emf. Also a stepper motor is just a resistance when the motor is not moving.

Generally when they give the volts ans amps for a stepper motor the resistance can be derived using Ohm's law. In fact I suspect the makers know the resistance and max current and calculate the voltage using Ohm's law - because the voltage as little relevance.

...R