# Stepper Formulas

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## Stepper Formulas

### What Voltage should the power supply be?

To calculate the voltage needed empirically take a 24 VDC power supply
or any power supply that you have on hand that is above the minimum for
the drive and hook it to the heaviest loaded axis.

Run that axis and increase the speed until you find the fastest speed
that it will run without missing steps with the test voltage.

Using the following formula you can determine your voltage needed.
Speed you want ÷ (Speed you got * 0.9) * Test voltage used = Voltage needed
Example (300IPM ÷ (150IPM * 0.9) * 24VDC = 53.3VDC
Just make sure that the voltage is within the range of your stepper driver.

### What Voltage is my Stepper?

Some steppers just have the resistance and current on the tag.
When I asked this question I had to do a slap on the forehead!
Using Ohm's Law R x I = E (Resistance x Current = Voltage)
Example 1.1 Ohms x 2.8 Amps = 3.08 Volts

### Compute Maximum Voltage vs Inductance

To compute the maximum voltage that you should use
depending on the inductance of the motor use this formula.
Maximum Voltage = 1000 * SQRT(inductance)

Example a motor that is rated at 1.5mH inductance per phase.
1000 * SQRT(0.0015) = 38.73 Volts MAXIMUM.
Example a motor that is rated at 6mH per phase
1000 * SQRT(0.006) = 77 Volts MAXIMUM.
Example a motor that is rated 2.5mH
1000 * SQRT(0.0025) = 50VDC MAXIMUM.

NOTE: Not all motors are created equal.
If you use this formula and your motor seems to be excessively hot, turn down the voltage
until the temperature is acceptable. Stepper motors are designed to run hot, but there's no
need to stink up the place with fried insulation.
Many motors are rated to withstand an 80 C temperature rise.
For my own purposes, I limit the temperature rise to 60 C.

### Compute Value and Wattage of Current-Limiting Resistors

Note: For L/R systems only.

This is a basic application of Ohms Law for a series circuit.
Your resistor must drop the difference in voltage between
the voltage at which your stepper is rated and your supply voltage:

Resistor voltage drop = Supply voltage - stepper rated voltage

Applying Ohms Law, divide by the rated current to get the resistance:

Resistor value = Resistor voltage drop / stepper current

Finally, and very importantly, you need to know how much wattage
you will be dissipating as heat, which your resistors must be rated to handle:

Resistor wattage rating = Resistor voltage drop * stepper current

For example: Steppers rated at 2.5V @ 5A, with a 26V power supply

Resistor voltage drop = 26V - 2.5V = 23.5V
Resistor value = 23.5V / 5A = 4.7 Ohms
Resistor wattage rating = 23.5V * 5A = 117.5 Watts

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Last edited May 25, 2008 6:50 pm by Fritz (diff)
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