[Tristar500]

I've got a machine running a Vexta Oriental stepper rated at 2 amps.
The machine is stalling and missing steps.

this is the motor in question https://catalog.orientalmotor.com/it...E1mKn-pqU3MLQL

Question? The nameplate says it's rated at 2 amps which is what I have the Gecko 201 set at. The motor stays cold even when running for a long time.

I got to thinking about the amp rating. Amps by themselves really don't mean anything without knowing the voltage. I think what would be more helpful would be a rating in Watts or volts X amps.

If the voltage coming from the gecko is low I see no reason to increase the amperage to see if I can get more torque.

Any thoughts?

[joeavaerage]

Hi,
a stepper drive uses PWM to cut the supply voltage down to that necessary to cause the required current to flow.

Often as part of the specs there is a winding resistance. That is the correct way to calculate the heat generated.

P=I²*R

The power is proportional to the square of the current. Thus if you increase the current by 50% the heat generated in the coil increases by 125%. In short increasing the current above the
spec is likely to overheat it and wreck it.

I used Vexta stepper on my first mill, they were brilliant. They were second hand when I got them and then I used them for seven years. I've since given them to a friend who is rebuilding that same mill.
They were five phase stepper, 24size and 1.4A. 1.4A sounds pretty measly but at any one time there are four windings powered in a five phase stepper for a total of 5.6A. Either way I used these up to 2400rpm.
I used Vexta drivers and they were direct off-line, ie 230VAC input. The DC link voltage, the effective supply for the steppers is north of 150V. These little motors absolutely sang. I detuned them to 0.9A per phase
because I felt they were getting too hot.



If they are missing steps, especially at speed then you should increase the voltage NOT the current. Increasing the current, even a little bit will cause an overheat. Increasing the voltage increase the speed before it starts to miss steps.

Current=Torque
Voltage=Speed

The problem with your steppers is that they are high inductance....and that counts against going fast. At the moment of switching the inductance causes the current to build up slowly, and during that
time it has very little torque and cannot thus move the load ie it stalls or misses a step. A high voltage will 'drive' current through that winding.

Vexta steppers are very good, but I think you have the wrong model for CNC.

For a 23/24 size stepper look for 1mH -2mH, and reject anything over 2mH.
For a 34 size stepper look for 2mH-4mH and reject anything over 4mH.

Your steppers are 56mH (bipolar series) and 14mH (uinpolar) F****king hopeless!!!. Does not say what they are when connected bipolar parallel, but I would guess 5mH-7mH which is still too high
in my book. If you are to use these things you need to wire them bipolar parallel and should have as high a voltage driver as you can. There are quite a few Chinese drives rated to 110VDC,
they might be enough.

Craig

[Tristar500]

Hi,
a stepper drive uses PWM to cut the supply voltage down to that necessary to cause the required current to flow.

Often as part of the specs there is a winding resistance. That is the correct way to calculate the heat generated.

P=I²*R

The power is proportional to the square of the current. Thus if you increase the current by 50% the heat generated in the coil increases by 125%. In short increasing the current above the
spec is likely to overheat it and wreck it.

I used Vexta stepper on my first mill, they were brilliant. They were second hand when I got them and then I used them for seven years. I've since given them to a friend who is rebuilding that same mill.
They were five phase stepper, 24size and 1.4A. 1.4A sounds pretty measly but at any one time there are four windings powered in a five phase stepper for a total of 5.6A. Either way I used these up to 2400rpm.
I used Vexta drivers and they were direct off-line, ie 230VAC input. The DC link voltage, the effective supply for the steppers is north of 150V. These little motors absolutely sang. I detuned them to 0.9A per phase
because I felt they were getting too hot.

What model number are your motors?

If they are missing steps, especially at speed then you should increase the voltage NOT the current. Increasing the current, even a little bit will cause an overheat. Increasing the voltage increase the speed before it starts to miss steps.

Current=Torque
Voltage=Speed

Craig

The motors in question are Vexta PK299-01AA Like I mentioned, they run ice cold so I see no harm in increasing the amperage while monitoring their temperature.
They are 4 wire motors so bipolar.

I'll have to check the voltage going to them to see if it is sufficient to run them at their rated torque.

I built a machine some years ago with some 5-phase Vexta motors. They had incredible torque and speed compared to the 2 phase variety. I'm not sure why they aren't more popular except for the limited supply of 5 phase drivers.

[joeavaerage]

Hi,

They had incredible torque and speed compared to the 2 phase variety. I'm not sure why they aren't more popular except for the limited supply of 5 phase drivers.

Cost. There were only a few manufacturers of five phase steppers at the best of times Oriental Motors, Pacific Scientific, Sanyo Denki and Berger Lahr being the best known.
For the same price as one of these steppers you can buy a modern AC servo which will eat any stepper ever made, including five phase steppers.

My new machine, well about three years now, uses 750W Delta B2 series servos and they are just so vastly superior to steppers that I will not be going back.

Craig

[Algirdas]

steppers are 56mH (bipolar series) and 14mH (uinpolar)

are you sure it is not µH ?

[joeavaerage]

Hi,
can't think of any stepper as low as uH. I took that direct off the Vexta data sheet, and my experience is that Vexta data is accurate and well formed so I'm tempted to believe
what is written....but at 56mH its no wonder its missing steps.

Craig

[joeavaerage]

Hi,
my guess is that there are four coils. That would allow the coils to be hooked up as bipolar series, bipolar parallel and unipolar.

The specs say that the winding inductance of the unipolar setup is 14mH. In uniploar only one coil is hooked up at a time, so at a guess I'd say the inductance of one coil is 14mH.
I would also guess the four coils will all be identical.

In bipolar series two coils are hooked in series, or alternately might be considered as one coil but with double the number of turns. The inductance of a coil is proportinal to the square
of the number of turns so the combined or series coil is 14 x 2²=56mH.

If you were to wire the motor in bipolar parallel then the inductance of the two 14mH coils in parallel is the reciprocal of the sum of reciprocals, ie 1/( 1/14 + 1/14)= 14/2 =7mH.

There are times when my electrical engineering training comes in, this is one of them.

To OP, you need to wire this things in bipolar parallel. Note that the rated current is 1.4A per coil, but if you had two in parallel then that would be to set the driver at 2.8A per phase.

Craig