[tulsah]

Hi All,

I am considering incorporating current meters / ammeters into the wiring for my stepper motors. The motors are rated at 7A per phase and are to be driven from a G203V.

My question is whether it is ok to wire from say Phase A of the G203V, through a shunt resistor and on to the stepper motor? The shunt is required for the ammeter to function correctly.

I would like to do this for each of the three steppers - more interesting than functional at this point.

Please advise.

Thanks.

[Den-Den]

The motors should not be affected by the shunts, so they will work fine. However, the ampmeters will not read properly while the steppers are moving due to the pulsed current.

[CJL5585]

Hi All,

I am considering incorporating current meters / ammeters into the wiring for my stepper motors. The motors are rated at 7A per phase and are to be driven from a G203V.

My question is whether it is ok to wire from say Phase A of the G203V, through a shunt resistor and on to the stepper motor? The shunt is required for the ammeter to function correctly.

I would like to do this for each of the three steppers - more interesting than functional at this point.

Please advise.

Thanks.

Well, for starters, I would not recommend putting any shunt in series with any motor winding. The inductance of the motor winding will be changed maybe by only a small amount, but it could affect torrque, acceleration and de-acceleration.

Also, when the steppers are going from a higher speed to a lower speed (de-accelerating) the stepper acts as a generator and produces currents and voltages out of phase with the applied voltage. This reverse EMF will most likely produce some magic smoke in the Ampmeter if it is not designed to read both positive and negative going currents.

On a third point, I do not believe that you will get a correct current reading regardless of where the ampmeter is inserted into the circuit. This would be due to the fact that the current in the stepper motor will be changing by some amount with each step pulse to the drive. Since the current changes constantly, you would only have a relative reading on the meter due to the fact that the meter movement cannot respond to the instaneous changes of the current pulses in the drive. (One would need an oscilliscope to see the actual current at any point in time.)

If you are determined to add an ampmeter, install it in the input voltage line going to the Gecko drive and protect it from the reverse emf voltages and currents that will definately occur.

[tulsah]

It doesn't sound like incorporating the meters would yield much useful information. Probably best if I save the money on the ammeters as they would likely be damaged anyway. Thanks for the info.

[rippersoft]

You can put the ammeter on the DC supply and at least see your total amps draw by the system. May want to also hookup a volt meter.

RipperSoftware

[tulsah]

You can put the ammeter on the DC supply and at least see your total amps draw by the system. May want to also hookup a volt meter.

RipperSoftware

This sounds like a good idea to me. Thanks for the suggestion.

[jonandyev]

Tulsah, I have read with interest your post, and the replies to using current meters to monitor the current drawn by your steppers. I am bemused by some of the replies. It is possible to monitor the current according to the oriental motor connection instructions. I have wondered myself wether a clampmeter could do the job? The only possible drawback would be in the frequencies generated by the drivers, a clampmeter is normaly used to measure 50 or 60 hertz. Another problem that I foresee is that the meter is used to measure the current through inductive coupling and can only be used around one of your connection wires. Can the wires be seperated far enough so that the phase of each wire does not cancel out the reading?
If im wrong,then my apologies, but as I said, Oriental motor gives instructions on how to wire up ammeters to adjust the current.

Regards Jonandyev.

[Mariss Freimanis]

There is no problem with putting a current shunt (very low Ohm resistor) in series with a motor winding. At 7A a 3W resistor can be no bigger than 0.06 Ohms. Make sure you use an isolated or better yet, battery powered voltmeter.

The problem would be the meaning of the reading. The winding current is a DC value while the motor is stopped, but what value? If the motor is stopped on microstep '0' (sin 4.5 degrees), the reading would be 0.55A on a 7A motor. If it was stopped on microstep '9' (sin 85.5 degrees), it would read 6.98A. Probably not too useful.

The winding current is an AC sinusoid while the motor is moving. A DC voltmeter would show zero amps then while an AC voltmeter would show zero amps when the motor is stopped. The AC current would read RMS current accurately only at 72RPM (60Hz). An op-amp rectifier circuit would be required to make sense of the readings while stopped and while running.

A DC ammeter in series with the power supply to a drive would be slightly more useful. You would note the current at the moment of motor stall due to overload at moderate and high speeds (doesn't work at low speeds). You could use this limit as a warning your motor is about to stall during normal operation. Problem is voltmeters update the display only 3 or 4 times a second. An overload can occur in a much shorter period of time, the meter would give no warning then either.

Mariss

[tulsah]

Thanks for all of the replies and advice. I think I have decided to simply use a single ammeter and voltmeter at the output of the DC power supply. This will give me the sum total of the output from the power supply.

Mariss, is there any formula here for the size of the shunt on the ammeter, or should the shunt be as appropriate for the rating of the ammeter?

Thanks,

Tulsah

[pastera]

Take a look at the ACS712 from Allegro - it's a hall effect bidirectional current sense IC. It only has 50kHz bandwidth but that should be enough if you are only interested in the average coil current.


Aaron

[mugabe]

There is no problem with putting a current shunt (very low Ohm resistor) in series with a motor winding. At 7A a 3W resistor can be no bigger than 0.06 Ohms. Make sure you use an isolated or better yet, battery powered voltmeter.

The problem would be the meaning of the reading. The winding current is a DC value while the motor is stopped, but what value? If the motor is stopped on microstep '0' (sin 4.5 degrees), the reading would be 0.55A on a 7A motor. If it was stopped on microstep '9' (sin 85.5 degrees), it would read 6.98A. Probably not too useful.

The winding current is an AC sinusoid while the motor is moving. A DC voltmeter would show zero amps then while an AC voltmeter would show zero amps when the motor is stopped. The AC current would read RMS current accurately only at 72RPM (60Hz). An op-amp rectifier circuit would be required to make sense of the readings while stopped and while running.

A DC ammeter in series with the power supply to a drive would be slightly more useful. You would note the current at the moment of motor stall due to overload at moderate and high speeds (doesn't work at low speeds). You could use this limit as a warning your motor is about to stall during normal operation. Problem is voltmeters update the display only 3 or 4 times a second. An overload can occur in a much shorter period of time, the meter would give no warning then either.

Mariss

Current can be measured from both coils and square root from sum of power of 2 current values should give true result . Of course circuit will be more complicated than expected . If it is about to detect threshold - then just comparator per coil should suffice.
Did i miss something ?

[Mariss Freimanis]

Only one winding needs to be looked at because the currents are identical. The only difference is the phase; one winding leads or lags the other winding in phase by 90 degrees.

The problem is the readings have no directly useable meaning.

Mariss

[mugabe]

I meant for the case when motor stopped in microstepping mode (the case you have described) that unique coil monitoring wont make sense.

[steve323]

Why not use the cable wiring directly as the sense resistor. 16ga wire has 0.00402 ohms per foot. 22ga wire has 0.01614 ohms per foot.

Route a sense wire to both ends of the cable wiring. A voltage can be detected across the wire depending on the current flowing through it. No additional resistors need to be added in series with the motors with this approach.

[Mariss Freimanis]

True enough but the results would be very boring after all that effort. The current reading would always match the set value.

Mariss

[Mariss Freimanis]

Why not use the cable wiring directly as the sense resistor. 16ga wire has 0.00402 ohms per foot. 22ga wire has 0.01614 ohms per foot.

Copper has a very high positive temperature coefficient. The error would approach 20% over a 0C to 100C teperature range (if I remember correctly).

Mariss

[mugabe]

there are also special current sensing ic fro motor applications . IR design note :
DN501: Accurate Current Sensing in High Voltage Motor Drives
http://www.irf.com/technical-info/designnote/dn501.pdf