Stepper motor current test

S3S4S5S6

Stepper motor current test [Copy link]

In the above figure, the red dotted line is the current on the motor phase line, and the blue line is the voltage on the phase line. The current probes from Tektronix and Keysight are too expensive and I cannot afford them!

Can the pulse in the figure be converted into a sinusoidal voltage waveform through an integrating circuit? In this way, the current waveform can be derived by measuring the voltage waveform of the integrating circuit without the need for a current probe. Is this feasible in principle?

maychang

What you need is to measure the current waveform. To measure the current waveform, you only need to connect a small resistor in series with the motor winding and measure the voltage waveform across the resistor. This resistor is often called a sampling resistor. The sampling resistor should be as small as possible to reduce power loss. The resistor should be connected in series with the end of the winding close to the ground to facilitate measurement with an oscilloscope.

maychang

The lowest (most sensitive) range of an oscilloscope is usually 2mV per grid, so 5 grids is 10mV. If the sampling resistor is 0.1 ohm, then a peak current of 0.1A is the full scale of the oscilloscope. If the peak current you need to measure is 1A, then the sampling resistor should be 0.01 ohm.

maychang

"Can the pulses in the figure be converted into a sinusoidal voltage waveform through an integrating circuit? In this way, the current waveform can be derived by measuring the voltage waveform of the integrating circuit without the need for a current probe?"

The pulse voltage must be converted into a sinusoidal voltage before measurement. There is no need to use an integration circuit, and low-pass filtering is enough. Passive filtering is best, because active filtering, like integration circuits, requires the use of chips such as op amps, and the ability of these chips to withstand overload is far worse than that of resistors, and power needs to be provided for the chips.

S3S4S5S6

maychang posted on 2022-1-15 14:57 What you need is to measure the current waveform. To measure the current waveform, you only need to connect a small resistor in series with the motor winding and measure the voltage waveform on the resistor. The resistor...

Do you mean inserting the resistor in the way shown in the picture?

maychang

S3S4S5S6 posted on 2022-1-17 08:27 Do you mean inserting the resistor in the way shown in the figure?

"Do you mean inserting resistors in the way shown in the diagram?"

Yes. However, if neither end of the winding is grounded, you will need to use a differential probe with your oscilloscope. If the end of the winding connected to the resistor is grounded, then you can just use a normal probe.