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Ross77
23-03-2017, 01:07 AM
I have a small engraving machine that I'm hoping to get up and running to cut some pcbs. the current set up uses 6 wire steppers running of a unipolar drive. I can't easily tap into the divers so looking to replace them with new drivers.

I have looked around and I can run them as 4wire on a bipolar drive but will either have to lose speed or torque depending on which coil I use. existing PSU is 50v and my drives are 50v so not much option to up the voltage

Is it worth driving the old motors in bipolar or will I lose to much speed or power?

Ultimately I will upgrade the motors etc. but I could do with a quick fix to get it working to finish some other projects moving.

Gary
23-03-2017, 07:15 AM
If the motors worked ok on the machine as unipolar, then using a bipolar driver will give you better performance.
going full coil will give you more torque and going half coil will give you faster speeds at the same time sacrificing torque at the lower speeds.
its an engraving machine, so its likely whatever way you connect it torque will not be an issue?

Ross77
23-03-2017, 03:17 PM
Thanks Gary
I was going to use the M542-G's that I got off you a while ago...... Is it possible to have relays switching between full and half coil for rapids or will it damage the drives?

I'm going to use LinuxCNC to control it so should be possible to set up an output. (still learning)
Thanks

Gary
23-03-2017, 03:21 PM
That will kill the drivers and if it didn't would also cause positional errors.
I would not try and over think this, going full coil will most likely give you better performance than what the machine had originally.
Try it out in both configurations and see what performs better for you.

Ross77
23-03-2017, 03:55 PM
Thanks Gary

Always over thinking things :0)

I was looking to use this as bit of a test bed to learn Linux as well and it seemed like a good idea. As long as it will mill pcbs then I'll be happy.

Ross77
25-03-2017, 10:28 PM
Had another look at the machine today and noticed that the stepper is 1.65v and 4.7A, will it matter that the drivers are only 4.2A peak?

I bought the Gerber engraver and 3 vinyl plotters as a job lot and they all the same make of stepper. I have another motor that is higher voltage and lower current, would that work better?

john swift
26-03-2017, 12:39 PM
the other factor you need to take into account is the inductance of the motor coils

the higher the inductance , the longer it takes for the motor current to reach the current the stepper driver is set to

this delay will reduce the speed you can step the motor

by using higher stepper driver supply voltages you can speed up the increase in current
the heat due to the copper & iron losses as you increase the voltage is the limiting factor

as a guide to select the supply voltage

multiplying the square root of the inductance (in mH) by 32 = stepper driver supply

for 2.2mH coils

32 x 1.48 = 47V


for 5mH coils

32 x 2.236 = 71V


using a stepper driver at its 4.2A limit
to drive a 4.7A motor will reduce the motor torque to 89% of its maximum

John

Ross77
27-03-2017, 02:00 AM
I cant find the specs for the exact model but looks like my best option is to go for full coil as the current will drop to 3A, which is within the driver spec. ideally I should double the voltage to 60V but the drives are 50V max so will try with the original 30V (tested today and its 30V not 50V as first thought) and see how it goes.

m_c
27-03-2017, 11:04 AM
Try it and see.

You'll most likely find the original motors were not driven at maximum voltage or current, and even if they were, you'd just need to a tune everything a bit slower. However, you'll most likely find fitting modern drivers will give a performance boost anyway, so even at less than maximum voltage/current, you'll most likely be able to get better performance.