# Stepper motor phases and torque output from shaft

I have a 5 terminal 12 v unipolar stepper motor. Can I consider it as a 2 phase motor?

Secondly I am driving the motor with a uln driver from a micro controller. I know the input voltage and input current. With this how can I calculate the motor output power from the shaft? I know the speed in rpm too. With the following data , i should be able too calculate torque output from shaft inturn. Please help me

• (1) Why would you want to consider it a 2-phase motor? (2) What does the data sheet for the motor say? Jun 29, 2016 at 17:48
• It's a unipolar motor. So I considered as one coil but energising at different times Jul 4, 2016 at 18:43

5 terminals indicates that your motor has 4 coils with a common terminal. This is called a '4 phase' motor because the 4 coils are separately powered, one at a time.

The characteristics of a stepper motor are determined by its materials and construction (rotor size, tooth and lamination shape, VR/PM/hybrid type etc.). Just knowing the voltage and current alone is not enough, because it only tells you the input power, not how much of that power is converted into torque and output power.

To calculate torque and power for a given voltage, current and speed you need the specifications of your motor. If they are not available then you will have to test the motor to determine them.

The graph below shows test results for a particular 57mm bipolar (2 phase) permanent magnet stepper motor driven with constant current PWM:-

Notice how torque drops rapidly as speed increases. Since mechanical power = torque x rotational speed, even at its best this motor is less than 40% efficient. Over 60% of the input power is wasted.

4 phase unipolar motors are less efficient because only half of each winding is powered, so it must be wound with thinner wire which has higher resistance. Efficiency and power output is also affected by the driver. A unipolar motor driven with constant voltage through series resistors may peak at less than 30% efficiency. Without resistors it might be a bit more efficient, but have lower output power and maximum stepping rate.

• +1 I learned that its feasible to have a stepper motor that is less efficient than deisel .Is a stepper motor better than say an induction motor for torque?Can stepper motors be made to be as efficient as say an induction motor? Nov 27, 2016 at 4:00