Upgrading control system, help picking what I need

[TTalma]

I am chucking the old control system on a CNC. It uses an integrated circuit board for the power supply and the stepper drivers.

I plan to replace this with a smooth stepper.

The stepper motors are

PrimoPal,
Model: PHB57S112-40595-K3A4,
1.8 deg,
6A .
5 Ohm

I am planning on using Gecko G201X stepper drivers (https://www.geckodrive.com/products/...tal-step-drive)

Are these the correct drivers to use with this motor?

How big a power supply do I need I was thinking 60V, is that overkill? Most of the power supplies I se are around 6A, but is that enough if each motor is 6A? The current PS, driver board is tiny, and I would have a hard time believing it can supply 18A. Doing the math (V=IR) the 6A 60V PS should be plenty big enough.

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[joeavaerage]

Hi,
Gecko 201X's have a good reputation and will do fine.

The higher the voltage the faster the steppers will go before they start missing steps, so 60V is not an overkill, in fact I would say 72V or even 80V if the G201's are rated for it.
You are correct that all three steppers should draw rated 6A all at the same time is rare....but its common to use a supply that can accommodate 2/3 of the total full rated current , ie 12A.

A 1000W Antek linear supply is a good choice. Linear supplies are preferred despite the extra cost over switch mode supplies, they are more forgiving and have good overload capacity. A 1000W
supply for instance will provide 1500W or even 2000W for brief periods without problem whereas a switch mode supply will fault out instantly.

https://www.antekinc.com/1000w/

Doing the math (V=IR) the 6A 60V PS should be plenty big enough.

By the way this calculation is woefully incorrect for stepper motors. Despite the supply being say 60V the continuous or steady state voltage applied to the motor by the drive will be way WAY
WAY less, something like 4V or 5V, and that is the value that you would have to use if you want to do a power calculation. In reality steppers are so quirky in this regard power
calculations are likely to throw you off, the manufacturers torque/speed curve is the only definitive source of that sort of information.

Craig

[joeavaerage]

Hi,
imagine a stepper that is rated for 6A and it has a phase resistance of 1 ohm, which is quite typical. Imagine also a 60V driver.

If the stepper is energized but stationary what would happen. If the full voltage of the driver was applied to the winding then the current would be:
I=V/R
=60/1
=60A !!! This is clearly not sensible the power supply and/or the stepper would blow up instantly.

The driver uses PWM to cut down the voltage to the stepper such as to not exceed the rated current, 6A The applied steady state voltage would be:
V=I.R
=6 x 1
=6V

Thus the duty cycle of the stepper drive would be approx 1/10
V_out=V_in x 1/10
=60 /10
=6V

You might imagine if the output current from the driver into the stepper is 6A then the current from the power supply to the driver must also be 6A.
If that were the case the power delivered to the driver is:
P=V.I
=60x6
=360W

But this is when the motor is stationary....so its not producing any rotational work so the 360W is being disapated by the drive and the stepper and they are going to get
hot and blow up in short order. Clearly this does not happen .....so what gives?

The driver uses the inductance (of the stepper) to make a 'sort of transformer'. As you know a transformer works with AC only, but could
transform 60V at 0.6A down to 6V and 6A....but again that only works for AC right? Well in fact no....by clever arrangement and very rapid switching
inside the driver it too can behave like a transformer ....but for DC. Thus while the steady state voltage and current of the stepper is 6V and 6A, the voltage
and current from the power supply is 60V and 0.6A. I've glossed over the switching losses etc just for clarity...but this is a pretty good approximation of how it works.

A couple of interesting points:
1) Even while stationary the steady state voltage of the stepper is 6V and 6A or 36W. At 36W of dissipation you might expect the stepper to get hot....and they do,
but hopefully not enough to damage themselves.
2) The only time that the full voltage (60V) gets applied to the stepper is right at the moment of switching from one winding to the next. The high voltage 'pushes' current
through the combined inductance and resistance of the winding in very VERY short time, which in turn allows the stepper to spin fast. The higher the voltage the faster
the current builds up in the new winding and the faster you can switch yet again. Its for this reason that you want the highest voltage you can get for your driver, it allows
best possible speed from a given stepper.
3) The transformer action of the driver means using voltages and currents to work out the power of the stepper is by no means straight forward like it would be with a DC
motor. Steppers are, a least to a first approximation, 'a constant power device'. A 23size (57mm) stepper is commonly about 100W output. At low speeds it has high torque
such that the product of torque and speed is 100W. At medium speeds the torque reduces to say half but still such that the product of torque and speed is still 100W.
At the steppers top speed, just before it stalls, it has very low torque, maybe only 5% of its low speed torque but still the product of torque times speed is 100W. So the
power is constant but the torque reduces linearly with speed. This is only a broad overview and of limited accuracy...so don't try to use for calculation purposes but
is a fair 'seat of the pants' understanding of how steppers perform in practice.

Craig