[mdframe]

I have a question I hope someone can help me with. I am currently building my CNC machine and I have actually started getting to the point of building the sub assemblies. I have 3 PowerMax II nema 23 bipolor stepper motors.

These motors are rated for 60 volts. I have caught snippets here and there that you should use linear power suppliers for steppers with 5 to 10 times the rated voltage to get the best performance. First I don't understand why you would need more voltage than rated, what does this mean. Secondly, does anyone have a good supply for a these type of power supplies? I am finding that power supplies like this are very expensive.

Thanks,

Matt

[jeffs555]

Matt,
There are actually two different voltage ratings for a motor, a maximum voltage and an operating voltage. The 60 volt rating is the maximum voltage rating based on the insulation of the wires inside the motor. If you apply a voltage greater than this even momentarily, the insulation on the wires could breakdown and the motor would short out, so your power supply should be no more than 60 volts.

The operating voltage is sometimes specified, and sometimes not. Some motors specify an operating voltage and current, some specify an operating voltage and resistance, and some specify an operating current and resistance. Ohms law says that voltage is equal to current times resistance, so if you know two of these you can calculate the other. Your PowerMax motors specify an operating current and resistance. There are many different versions of the PowerMax II motors, but doing the Ohms law calculations, they all work out to an operating voltage of between 2 and 3 volts. Your power supply should be 5 to 10 times this value, so a 24 volt power supply would be the most likely choice, but a 12 volt supply would work.

The reason for using a higher voltage is that it gets the power to the motor faster, so it can step faster. Most modern stepper drivers pulse the voltage to the motor on and off, so even with a 24 volt supply, the average voltage is only 2 or 3 volts. Most drivers have some way to set the average current, but since the resistance of the motor remains constant, if you know the average current, you can calculate the average voltage using Ohms law. Since your motors specify an operating current, you don't have to calculate anything, you just set your drivers to the current your motor specifies.

Jeff

[Al_The_Man]

I have 3 PowerMax II nema 23 bipolor stepper motors.

These motors are rated for 60 volts. I have caught snippets here and there that you should use linear power suppliers for steppers with 5 to 10 times the rated voltage to get the best performance.
Matt

Are you may be confusing motor rated voltage with the Powermax amplifier rated to go with these motors, some which are rated at 60vdc?
Your motor voltage is probabally going to be much lower, Pacific Scientific usually only specify current in their specs.
The Torque of a stepper will decreases with speed due to the inductance (resistance in ohms) change as speed increases.
This would normally decrease the current through the windings and decrease torque, so a couple of methods are used to offset this, one is to apply a high DC voltage with a series resistor which progressively controls the current as impedance increases.
The other method is to use the same higher voltage but chop the DC so that a mean level of current is fed through the winding as impedance increases with speed. You will need to look up the current rating per winding of your particular motor in order to implement either method. I have most of the P.S. specs. if you need it looked up.
Al

[ger21]

Al, most of the Pacsci steppers have a MAX voltage rating on them. But it doesn't say MAX voltage, just something like 65V. And there documentation metions nothing about the voltage, just current and resistance.

Mdframe, what is the current rating of your motors? Or the model#. This might make a difference as to what drivers would work best.

[mdframe]

The model number for these PowerMax steppers is: P21NSXS-LSS-NS-04. I have built and tested a driver board with these motors that seems to work. I was only using a small power supply with them and the rotation was slow but had very high torque. The chip I am using on the driver board is ST Microelectronics L6208 and the mode is half step. I got the specs on the board from http://www.otocoup.com/CarteL6208_e.htm .

I want to thank everyone in this forum for helping out, this is truly a great place to share and learn and I really appreciate it!

Thanks,

Matt

[Al_The_Man]

According to the PacSci info I have it shows they may be a special. The first S in SXS in the part number indicates a special construction and the second S which indicates winding type does not list S, so it also may be a special. sorry.
The shaft designation shows it as non-standard also.
Al

[mdframe]

Al,

You are correct these motors were special built for someone but I talked with PacSci and they stated that the inner workings were the same as their standard nema 23 biplor stepper motors, it has a double ended shaft. They were used in a fluid metering system and seem to work fine with the board I built from the previous post. They are in very good and clean condition. The label on the motor states the following:

POWERMAX II 1.8 STEP MOTOR
MODEL: P21NSXS-LSS-NS-04
CUST. P/N: 30028
Is(dc): 1.16A BIPOLAR SERIES Vs(dc): 65V
Po: 30 W T:90C MAX
W: 1500 RMP AMBIENT: 40C DATE: 975014713

Here is the link they originally gave me about this motor:
http://www.pacsci.com/products/step_...xproducts.html

Thanks for looking into this for me.

Matt

[ger21]

I'd run those with a Bipolar Xylotex, 2.3a bipolar parallel, at 24V-28V. That's the cheapest way and will get you pretty good performance.

Both Xylotex and HobbyCNC have Unipolar kits that can be run at up to 40+ Volts. Those might give you a little higher top end speed, but you'll lose about 30% of your torque at lower speeds.

For the best performance (high speed) go with Geckos and run them at 40-60V bipolar parallel.

Higher voltages give you higher top speed. Usually by doubling your voltage, you'll double the highest useable rpm of your motors. If you can only run them at 200rpm on 12V, you should be able to get around 400rpm @ 24V. Other factors can come into play, but this is roughly what you can expext.

[flsteam]

I understand ohms law but I am trying to come to grips with the design of the power supply in this example. If the motor is rated for example 1.6Vdc @ 6 amps then what is the power supply requirement. Is it 1.6 x 6 = 9.6 VA or would it be 6 times the power supply voltage which in this example would be 60 Vdc or 360 VA.

[ger21]

Depends on the drive, but with a chopper type drive like a Gecko, or a Xylotex, you only need to supply about 2/3 of the rated current, or 4 amps per motor using your example. To get best performance (higher speeds) from steppers, you need to supply 10 to 20 times the rated voltage due to the inductance.

[ger21]

You might want to read this for more info. http://www.geckodrive.com/photos/Step_motor_basics.pdf

[flsteam]

Thanks for the link to Gecko's info page(s). Their example shows a power supply based on the motor current and the power supply voltage not the motor rated voltage.

John

[WilliamD]

That's a pretty good write up on the Gecko site! How do you set the current for the motor then. I'm building a PS with a microwave transformer that I rewound. I'm going to have it at 60v, and it can do 13 amps. How do I set the current? The write up said that you want the current set at the rated amount. I have the capacitor for my power supply, is there something else I need to add?

[flsteam]

The data sheet for the respective drives give formulas to calculate a resistor that is installed on the Gecko board which limits the current the drive can supply the stepper.

John