## 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