bradyw
Posts: 8
Joined: Wed Jul 16, 2014 8:34 pm
Location: Minneapolis

High Power Stepper Motor

Wed Jul 16, 2014 8:54 pm

I'm trying to operate an Automation Direct Stepper Motor Nema 23 Single Shaft 166 oz-in Bipolar w/ 1ft cable with the Raspberry Pi (model b). Here is a link:

http://www.automationdirect.com/adc/Sho ... -MTR-23055

I have tried using the L293D motor driver, but I fried it (along with a Pi). I've also tried using L298N Stepper Motor Driver board, but shortly after connecting something popped so I unplugged the power supply. The smell of burning plastic soon followed, but it has no apparent damage. I've included links to the exact products mentioned above.

https://www.adafruit.com/product/807
http://www.ebay.com/itm/like/141146299102?lpid=82

I really just want some advice on how to get this thing moving. I need it to open and close a window and to be able to hold the window in an open position. Any ideas at all are appreciated. Thanks.

rgrbic
Posts: 128
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Contact: Website

Re: High Power Stepper Motor

Thu Jul 17, 2014 12:32 am

These drivers cant handle such current. L298N can be used in parallel mode - google it
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BMS Doug
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Location: London, UK

Re: High Power Stepper Motor

Thu Jul 17, 2014 8:31 am

rgrbic wrote:These drivers cant handle such current. L298N can be used in parallel mode - google it
+1

the L298 can only handle 2A, above 2A the L298 can be connected in parallel.
Doug.
Building Management Systems Engineer.

Ravenous
Posts: 1956
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Location: UK

Re: High Power Stepper Motor

Thu Jul 17, 2014 8:59 am

What voltage are you running it at?

The datasheet claims 0.8 ohm resistance (per phase) - at 12V that is rather a lot of current! :o

Does the supplier recommend a particular driver for this motor, and if so what is it?

rgrbic
Posts: 128
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Re: High Power Stepper Motor

Thu Jul 17, 2014 9:25 am

The serial resistor can be added to motor winding (choose appropriate wattage!). Better solution is to control current.
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bradyw
Posts: 8
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Location: Minneapolis

Re: High Power Stepper Motor

Thu Jul 17, 2014 1:34 pm

I'm not finding anything on using the L298 in parallel mode. Can someone point me in the right direction?

Also, I'm using an 18V 2A power supply.

Ravenous
Posts: 1956
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Re: High Power Stepper Motor

Thu Jul 17, 2014 2:15 pm

18V - be very careful, I think that's completely the wrong sort of power supply for this setup...

If the stepper coil resistance is 0.8 ohms as stated, this will draw a current of 18/0.8=over 20 amps! According to Ohm's law (and give or take a bit for losses in the driver itself).

This stepper will draw 2.8 amps from a 2.25V supply, in theory. (Again from Ohm's law)

Really, this stepper isn't made for driving from a fixed voltage driver as we amateurs use. It's built for running from a proper current controlled driver ("Chopper" driver) as rgbric said above. These things are designed to push 2.8 Amps (say) into the driver at whatever voltage is needed - this also gets decent high speed performance from the stepper. (Normally a stepper will not be able to turn as quickly if driven from a simple constant voltage.)

(Look at the fancy drivers recommended for this, on the page you linked to.)

The stepper you have is decent, but to drive it from a simple driver like an L298 you should probably use a much lower voltage power supply (a bit over 2.25V - say 4 volts, to overcome losses in the L298 itself).

Alternatively if you have to use a higher voltage, you could use a resistor in series with each of the two windings, again as said above. It would need to be a power resistor though, because it will get hot! Also it will be very inefficient - all that energy is wasted as heat in the resistors. I think for 18V at 1A per phase you will need two 15-16 ohm (or higher for less current) Power Resistors rated at least 15 Watts... Calculating all that needs a bit of Ohms law work again.

Finally the advanced option is to use the raspi as a Chopper driver directly - don't just switch "on" the stepper, instead drive its coils with PWM signals so they are "on" only for about 10-20% of the time. This is an advanced topic though, one software glitch and you could turn it on fully and everything would overload immediately again.

I think, unless you can suddenly get a 4V (ish!) power supply at a few amps, the two power resistors trick is the quickest way to get you running. Be very careful with that 18V though, easy to fry things. (I fried a raspi once with an 18V unregulated supply I made!)

Disclaimer - I have never driven a stepper motor this powerful, just giving you the theory, and I've never tinkered with the proper controllers. (I've seen them running though - very nice industrial stuff and worth every penny.)

ps sorry about the length. I tried to shorten this, I really did! :ugeek:

bradyw
Posts: 8
Joined: Wed Jul 16, 2014 8:34 pm
Location: Minneapolis

Re: High Power Stepper Motor

Thu Jul 17, 2014 2:45 pm

Thank you for the in depth response. I have considered getting the driver from Automation Direct (the same company that makes the stepper), but I really don't want to drop the $155. If I can get this stepper to work I would be installing them in multiple locations, so adding that much to the cost would be a deal breaker.

rgrbic
Posts: 128
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Re: High Power Stepper Motor

Fri Jul 18, 2014 10:21 am

The cheapest solution is to use L297 + L298N. With such configuration you can control the current through windings to match the required torque (to pull the window and hold it in desired position). And in the same time to prevent heating of the motor.

You can buy such drivers or you can do it yourself. If you make it yourself, put a good heatsink on L298N. It is wise to use optocouplers between RPi and motor driver.
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boyoh
Posts: 1351
Joined: Fri Nov 23, 2012 3:30 pm
Location: Selby. North Yorkshire .UK

Re: High Power Stepper Motor

Fri Jul 18, 2014 3:31 pm

bradyw wrote:I'm trying to operate an Automation Direct Stepper Motor Nema 23 Single Shaft 166 oz-in Bipolar w/ 1ft cable with the Raspberry Pi (model b). Here is a link:

http://www.automationdirect.com/adc/Sho ... -MTR-23055

I have tried using the L293D motor driver, but I fried it (along with a Pi). I've also tried using L298N Stepper Motor Driver board, but shortly after connecting something popped so I unplugged the power supply. The smell of burning plastic soon followed, but it has no apparent damage. I've included links to the exact products mentioned above.

https://www.adafruit.com/product/807
http://www.ebay.com/itm/like/141146299102?lpid=82

I really just want some advice on how to get this thing moving. I need it to open and close a window and to be able to hold the window in an open position. Any ideas at all are appreciated. Thanks.

One question, how is the stepper motor mechanically linked
To the windows. Are the current test you are doing on-load
Or off-load. Is the window mechanical load balanced to the stepper
motor mechanical out/put load.

Suggest you use Opto Isolators between the Pi and buffer stages
Due to different voltage levels
BoyOh ( Selby, North Yorkshire.UK)
Some Times Right Some Times Wrong

richardamullens
Posts: 5
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Contact: Website

Re: High Power Stepper Motor

Tue Jul 22, 2014 12:22 am

According to the stepper specification, it has an inductance per phase of 2.4 mH

Thus using the relationship E= -L di/dt, if you ignore the resistance of the coil, an applied voltage of 2.4 V will give di/dt =1000 Amps per second, so in 2.5 mS the current will rise to 2.5A

For 24 volts, 250 micro seconds will suffice.

You can keep the current in bounds by repeatedly pulsing the enable lines on the L298.

This is a rough calculation, you need to factor in the resistance of the coil to arrive at a more accurate time period.

RaspISteve
Posts: 98
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Re: High Power Stepper Motor

Tue Jul 22, 2014 2:04 pm

Guys,
If you're needing to drive a 'High Power Stepper Motor' and do it reliablely you need a proper driver and power supply able to keep it all going.

In essence steppers and motors in general run on current going through the coils. The more current then the more power. The problem is to get the necessary current through the coils. If time isn't a constraint it's a simple ohms law thing where coil resistance and applied voltage determine current. However, if you are switching the coils quickly either as a stepper or via a commutator inductance raised its head and will slow the rise in current. Attempts to switch coils too fast will limit the peak current and limit power to the point where the motor stalls in the case of a stepper or just won't go round any faster for commutating motor.

The point of using a higher starting voltage it to get the required current flowing soonest. Next problem is to stop the current rising too far and stuffing the driver or cooking the motor. Various tricks can be used to manage this from turning off the drive current at a limit point but importantly allowing it to flow via flywheel diodes or just current limit the source.

Very simple ways of managing current flow can be done by supplementing the coil resistance with a big power resistor such to obscure the coil impedance. You still need a higher voltage source to start the current but resistor takes the hit. Adding a capacitor across the resistor will shunt it on the pulse edge to increase current though the coil but this needs care in design and really only works in unipolar motor systems.

If you do design your own driver you need to understand that coils/inductors will store current in the same way a capacitor will store voltage. This manifested as a high voltage output as the inductance tries to keep the current flowing after you took it away. Switch mode powers supplies use this effect and include a mystery diode to maintain current flow on each switch cycle. Your design must include similar diodes, usually in a bridge around each motor coil to manage the back emf generated. Your supply will also need appropriate decoupling capacitors to absorb this regenerated power then use it to supplement the PSU on the next pulse.
Share and Enjoy.

rgrbic
Posts: 128
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Re: High Power Stepper Motor

Tue Jul 22, 2014 9:16 pm

+1

The stepper motor performance can be improved by adding serial resistor to each coil. Basically, the motor coil can be modelled as serial RL circuit where R is resistance and L is coil inductance (lets forget the emf). When the DC voltage is applied to such circuit, the current gradually increases (defined by time constant L/R) till the steady state value is reached. Steady state current value is equal to U/R where U is value of DC voltage. So what happens when you add serial resistor? The total reistance of the circuit is R + Rd where Rd is added serial resistance. The time constant of the circuit is changed but also the steady state current is decreased. This means you can apply the higher voltage to get the same current as without serial resistor. Since the time constant is decreased, the current change is more rapid thus improving stepper motor performance.

The figure below illustrates what is happening with the current in coil when DC voltage is applied to motor coil. The used values for motor coil are R = 0.8 ohms, L = 2.4 mH. The value of serial resistor is 3.3 ohms. Blue line shows the current values when 1V is applied to the coil without serial resistor. The red line shows the current when serial resistor is added but the applied voltage is equal to 12V.

The main disadvantage is additional power dissipation on serial resistors.
Attachments
stepper_current.png
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RaspISteve
Posts: 98
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Location: Cheltenham, UK

Re: High Power Stepper Motor

Tue Jul 22, 2014 9:51 pm

Guys, there's been a couple of folk here playing with step motors on various applications and going DIY on the driver which is OK for the little noddy applications. However, this topic is for a "High Power Stepper Motor". It can save a lot of headaches by using a commercial module to do all the hard stuff and they usually include a load of useful features like a maximum current setting and micro stepping leaving one to chase down the usually not to be underestimated software end.

Consider these things: http://www.arceurotrade.co.uk/Catalogue ... er-Drivers. I have no connection with this supplier other than as a customer and offer this as an example only. These folk also do a couple of chunky step motors. There are other suppliers out there.

Also, nice description above on including a series resistor.
Share and Enjoy.

bradyw
Posts: 8
Joined: Wed Jul 16, 2014 8:34 pm
Location: Minneapolis

Re: High Power Stepper Motor

Fri Jul 25, 2014 2:04 pm

I appreciate the suggestions, but I think most of this electrical stuff is way over my head. I decided to spring for the driver recommended by the stepper provider. I got it in the mail yesterday and I works like a charm.

Ravenous
Posts: 1956
Joined: Fri Feb 24, 2012 1:01 pm
Location: UK

Re: High Power Stepper Motor

Fri Jul 25, 2014 2:16 pm

Very nice... proper industrial stuff and it should deliver the full torque and speed.

Out of interest, are you still going to be connecting the controller to a Raspi, or does that controller wire to some sort of forward/backward/stop switch?

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