[rincewind23]

Hi all,

I'm trying to make a CNC conversion of my Axminster X1 micro mill (I believe it's the same as the Sieg X1). I've got three stepper motors, and three driver circuits. The Steppers are MotionKing 23HS8404, the driver circuits are from here:

http://www.quasarelectronics.com/315...tor-driver.htm

I'm running them from the 12v line of an old 300w ATX PSU.

The problem I'm having is that the stepper motors seem to be drawing too much current. They're rated at 40vdc/4.2amp, but they're getting too hot to touch - and on the driver circuit the MOSFETs got so hot that they melted the solder and now they're leaning against each other! This is after only being powered up for a couple of minutes.

I tried to measure the current flowing through the circuit, but the 10A fuse on my multimeter blew when I tried to measure it, so I guess it's drawing more than 10A.

I'm new to all this, so please let me know if there's any information I missed out that's needed, or if I'm posting in the wrong place.

Cheers,
KEv.

[bibster]

Do the steppers actually work? I mean: You get them to do steps?

The coils are 0.6 Ohm each, according to the DataSheet, the Mosfets are about 0.18 Ohm when On, to that adds up to 0.6 + 0.18 + 0.18 = 0.96 Ohm.
12 Volts over 0.96 Ohm = 12.5 Amps!!! Sure your fuse blows.

There seems to be NO current limiting whatsoever in the schematic, are you sure the board is supposed to work with the steppers? It seems as tough the board's for those tiny motors pictured in the page you linked to, and not for those (rather) though NEMA23 motors you're using!

Regards, Paul

[rincewind23]

Thanks Paul

The steppers do actually work - I have written some test software for my PC to drive them and they're working fine.

These boards are supposed to be able to work with steppers - the website says they will drive 5 to 50Vdc motors at currents up to 5 Amps max per phase - so this means I've definitely been running them outside of specs!

I'm a software guy, haven't done hardware work like this for over 15 years, so the fact you explained how you calculated the current really helped - thanks

I guess I need to put another couple of ohms of resistance in series, and that should drop the current to about 4Amps, right?

Cheers,
KEv.

[Mariss Freimanis]

I looked at your board schematic and it's screwed up. It doesn't show a current limit resistor in series with the winding which it needs because the drive isn't a switching type.

Here's what you need to do:

Your motor is about 2V rated. You have a 12V supply. You must lose 10V (12V - 2V) across this resistor at 4.2A. Ohms Law makes it 2.4 Ohms. 4.2A times 10V means the resistor must be at least 42W rated.

Get two 2.4 Ohm, 50W resistors. Put each in series with your motor coils. It will work properly then. Warning: The resistors will probably cost more than the drive. Good luck.

Mariss

[rincewind23]

Thanks Mariss

I managed to find the resistors here in the UK for the equivalent of about 3 bucks each, picking them up tomorrow. I guess I'll need to put one in series with the input to the driver circuitry, too?

Secondly, you say the motor is about 2V rated - how do you calculate that from the 40V max rating on the data sheet?

Apologies if these are simple questions - like I said, I'm a software guy

Cheers,
KEv.

[gandalf1]

Hi Kev,
I use the same boards and found them pretty much bomb proof as I found when I put a capacitor in the wrong way round with explosive results, a new cap got the board working again with no other problems. I use Nema 23 motors with 5 volt 1 amp coils and have been running on 13,8 volts parallel Bipolar connections with 8.2 Ohm series resistors to each coil which if I worked it out correctly gave more or less a 5 volt drop at around 1 amp for each coil. Although the boards stayed relatively cool I did add PC power supply fans to the enclosure just in case but never really reached much morethan warm. The motors were a different thing though and to me did seem excessively hot so each had a power supply fan to cool them down fitted after which they barely seemed to hreat up. Latest incarnation is to use a PC power supply and run the motors as previously set up on 12 volts but with 50 mm fans instead of the 80 mm used before. this is for testing the modified machine after which I am going to use the little motor drivers from ARC set to 0.9 amp and from a 24 volt transformer suitably rectified and output smoothed.
The resistors I am using at present came from Farnell, were ordered over the phone just before 5 pm one evening, card details quoted and they arrived first post the next morning. The person I spoke to on the phone was very helpfull and suggested alternative resistors (same value and wattage) to those I asked about as those in the online catalogue needed to be ordered from the US and were much more expensive so I considered them to be a good company to deal with.
John (Sawtry Cambs)

[Mariss Freimanis]

KEv,

Put a resistor in series with each winding. Don't put them anywhere else. The '2V' guess comes from the motor being a NEMA-23 size rated at 4.2A per phase. Rule of thumb is a double-stack 23 size motor is 8W per coil, (V = W / I).

Mariss

[rincewind23]

Thanks for all the replies. I got a couple of 2A, 50W resistors from RS components, and put them in series with my coils. It's definitely fixed the overheating of the motor / driver circuitry, now all the heat is in the resistors but once I've mounted them on a heat sink of some description I should be good to go.

Thanks again for all the help!

[bibster]

Well, I'd say, use'em to 'eat the shoppe!!!
:withstupi

On the other hand, a sort of waste of energy, not? Why not build a simple powersupply, with the correct voltage? Or at least a voltage more appropiate, like 5V or so?
A simple torroid, bridge and some caps, and you're in..

Paul

[rincewind23]

That's not a bad idea, and one that's on the todo list somewhere after finishing the conversion!

[huongthanh]

Well, I'd say, use'em to 'eat the shoppe!!!
:withstupi

On the other hand, a sort of waste of energy, not? Why not build a simple powersupply, with the correct voltage? Or at least a voltage more appropiate, like 5V or so?
A simple torroid, bridge and some caps, and you're in..

Paul

To speed up and more torque on stepper motor, normally we use a higher voltage (2 or 3 times of motor rated voltage) and add in series with motor coil a resistor to increase total resistance of circuit to reduce the rising time of the current through the motor coils (because the rising time T= L coil/R coil, L coil:Inductance of motor coil, R coil: Resistance of motor coil).
When you add in series with motor coil a resistor then rising time of the current through the motor coil will be T=L/(R coil + R added resistor).
It is importance to calculate the Resitance and Watt of the Risistor so that Amp and Volt through the motor coil as rated to protect your motor.
Example:
Your motor spec. 5V, 1A/Phase
You using a 12V power supply
--> need 7V drop on the Current Limiting Resistor.
You desired the current through motor coil is 1A.
--> Resistance of Current Limiting Risistor sure be: R = U/I = 7/1 = 7 ohm
--> Watt of Resistor: W=U.I = 7*1=7 watt

[bibster]

thankyou thankyou thankyou huongthanh!!!!

I completely forgot about that (I use PWM stepper controllers... thay sort of take care of that! )

It's all a matter of rise time, thus... (Or time constant ? 'Constante de temps' ) T=L/R
Rcoil=0.6, Rmosfet=2*0.18, L=1.8mH --> 1.8/(0.6+0.18+0.18) = 1,8ms
1/(1.8ms) = 533Hz, so you'd be able to make 533 pulses per second, still reaching 63% of the full current (Sorry, it's defined like that, 63% after T, 99% after 5T)

Adding the 2.4Ohm resistor in series, changes this to:
T=L/R=1.8/(0.96+2.4)=0.54ms --> 1866 Hz

Paul

[James Newton]

First, I just want to make it clear that I agree with what others have said here; especially regarding how to calculate your current draw, and how to use resistors, etc... to reduce it to livable levels.

I also have to point out that I'm biased: I sell the Linistepper driver kits,
http://www.linistepper.com :wave:
although the design of the Linistepper is open source and there are several board layouts available for do it yourselfers.

But if motor heat is your problem, other than reducing the drive current (which will reduce torque), you really only have two choices:

1. Go buy a chopper (e.g. PWM) driver that filters it's output waveform. Gecko makes some. But they do cost a penny...

2. Go to an actively regulated LINEAR driver, like the Linistepper, and deal with the heat at the driver, not at the motor. This is what those big power resistors do, as others have advised, but keep in mind that without active power regulation, those resistors will reduce your maximum step rate and torque.

Here's why:
Standard chopper drives stay cool because they pass the job of filtering the chopped waveform on to the motor. Using the motor to filter the drive signal introduces eddy currents and other AC losses that result in heating in the motor.

Heat in the motor reduces the strength of the magnetic field, lowers torque, and increases the change that they will miss a step. Not to mention that they can melt leads, coils, and generally destroy things.
(flame2)

A top quality chopper will filter or otherwise reduce the harmonics in the drive signal before it reaches the motor, so the motor will run cooler. That takes expensive and proprietary components. Geckos cost.


Linear systems have never had problems with motor heat, because they produce a smooth signal with no harmonics, noise, etc... On the other hand, linear drivers get hot. So you need a big heat sink or fan on the driver. But think about it: Wouldn't you rather have the heat at the driver than at the motor? It's a lot easier to cool a little transistor than to cool a motor, in it's case and all. And you don't loose power.
:idea:

Most linear systems are passive: They do not actively regulate the power supplied to the motor. So when a coil is first turned on, while it takes some time to build up it's field, it's not getting the maximum possible voltage; it just gets the standard voltage that it will continue to get while it holds that field before it gets turned off. An active system watches the current flow and increases the voltage to drive the coil up to the required current as fast as possible. Then it backs off on the voltage, dropping the excess power locally, to keep the motor from frying.

For medium to small, low voltage motors like yours, which are running hot, there isn't any better driver than a linear driver, and the best linear driver is the linistepper. I know that sounds like a sales pitch, but I truly believe it, and I'll put my money where my mouth is: If you buy a Linistepper and try it, record the top motor speed and temp with your chopper driver, and then again with the Linistepper, and if it isn't better, I'll refund your purchase.

This is a perfect test case for active linear drive.

--
James Newton

[rincewind23]

Thanks for the comprehensive explanation, James.

Unfortunately I can't take you up on your 'quicker and cooler or your money back!' offer, as I have 4-wire, bipolar stepper motors.

Shame, as your website makes your boards look pretty good!

Cheers,
KEv.