[daman]

I read many stepper guides and some of them say you only need 1/2 or 2/3 of the rated current from the power supply. Why?

If I buy a 5A stepper (like KL23H2100-50-4B stepper from Keling) do I then need only a 3.3A power supply per stepper?
What current limit do I use from the controller? 5A or 3.3A?

[James Newton]

Could you point out an example of that? A guide that says you don't need more current from the supply than the motor will be driven to?

[jschmitt]

I think that is a rule of thumb for 3 axis setups. You add up the total motor amps buy a power supply 2/3 or larger.

[RomanLini]

You can use a much lower current PSU than many people realise BUT this only holds for chopper type stepper motor drivers, not for linear drivers.

With a microstepping driver the total motor current is about 1.4 * the phase current. So a 2 amp motor uses 1.4 * 2 = 2.8 amps total, on average.

As the chopper driver works as a SMPS (switchmode power supply) you get current leverage depending on the ratio of input voltage vs motor voltage (Vpsu / Vm).

The math is basically;
1.4 * phase current / ((Vpsu / Vm) * 0.70)

The 0.70 allows for around 70% efficiency of the SMPS chopper stepper motor driver.

So for my machine, it has 3 motors of 2.6A;
3* 1.4 * 2.6 / ((35 / 6) * 0.70)
= 10.9 / (5.83 * 0.70)
= 10.9 / 4.08
= 2.67 amps needed total from the PSU to run all 3 motors.

My psu has a ammeter on the box, and in use the needle stays at around 2.1 amps so the calculation holds fairly true. This current will be constant during full-power holding and all low and medium speed movement.

Now some people will chime in with the fact that during VERY fast accelerations the psu will need to supply more current to provide the power for acceleration and this is definitely true, although will be a lot less current than most people realise. My psu barely peaks at 2.3 amps (up from 2.1 amps) during max accelerations of over 1G. As a conservative estimation you might want to allow double the current value for your psu.

[daman]

Thanks for the info guys.

I ordered one of these:
http://cgi.ebay.com/ws/eBayISAPI.dll...#ht_500wt_1154

So I won't be out of current with three pieces of 5A steppers

[stirling]

RomanLini

Whilst I agree with the fact that a typical chopper drive system requires a PS current of around 2/3 rated motor current, I'm not sure your explanation of why is correct. I'd also question your comment about the PS delivering more current for hard acceleration. As far as I'm aware a stepper is a constant current device (as opposed to a servo for example) and the whole point of a chopper drive is that it too is a constant current device.

[Drools]

Daman, I have a question on your power supply purchase. How do you plan on modifying them to turn on?
These power supplies are usually plugged into a backplane and controlled by the server (equipment).

[daman]

Daman, I have a question on your power supply purchase. How do you plan on modifying them to turn on?
These power supplies are usually plugged into a backplane and controlled by the server (equipment).

That would probably not been a problem. But someone already figured that out.
http://www.rcgroups.com/forums/showp...6&postcount=37

[RomanLini]

...
Whilst I agree with the fact that a typical chopper drive system requires a PS current of around 2/3 rated motor current
...

It probably requires significantly less than 2/3, depending on the ratio of Vpsu to Vmotor. As the math shows. The 2/3 figure is not mine but is a common figure discussed on this forum, and is a very conservative value allowing a very large safe margin.

...
I'm not sure your explanation of why is correct.
...

I am. I've been a designer of SMPS and stepper motor drivers for quite a few years now. The relationship of Vin/Vout to Iin/Iout in SMPS and chopper stepper drives is well understood.

...
I'd also question your comment about the PS delivering more current for hard acceleration. As far as I'm aware a stepper is a constant current device (as opposed to a servo for example) and the whole point of a chopper drive is that it too is a constant current device.

You are right, but only when the stepper motor is at rest or operating slowly.

Once motor speed rises above a certain value the motor inductance means that the current cannot be maintained without raising the psu voltage. Hence the need to use a 50 volt supply to run a 6 volt motor if you want good high speed performance. At high speeds and/or high accelerations the motor requires more voltage to maintain the same current so the efficiency of the system goes down and motor power consumption goes up. Note that under high DEceleration the motor can use little or no power or even return power to the psu (regenerative).

[ger21]

Looks like a nice servo PS for $25

[stirling]

Once motor speed rises above a certain value the motor inductance means that the current cannot be maintained without raising the psu voltage. Hence the need to use a 50 volt supply to run a 6 volt motor if you want good high speed performance. At high speeds and/or high accelerations the motor requires more voltage to maintain the same current so the efficiency of the system goes down and motor power consumption goes up. Note that under high DEceleration the motor can use little or no power or even return power to the psu (regenerative).

I more or less agree with what you've said here (apart from the high acceleration bit) - but that was not what I was questioning.

Now some people will chime in with the fact that during VERY fast accelerations the psu will need to supply more current to provide the power for acceleration and this is definitely true

A chopper drive has no means of knowing whether the motor is under load or accelerating or whatever - it simply senses when the current has reached the preset level and maintains that - period. Yes - we might like for more current as the load increases - but we arn't going to get it - not from a chopper drive. Now servo drives - that's a different story...

[RomanLini]

...
A chopper drive has no means of knowing whether the motor is under load or accelerating or whatever - it simply senses when the current has reached the preset level and maintains that - period. Yes - we might like for more current as the load increases - but we arn't going to get it - not from a chopper drive. Now servo drives - that's a different story...

You need to separate the 2 currents to understand the operation. The motor current and power supply current are totally different things.

The chopper driver gives closed loop regulation of MOTOR current. The psu current will be less, because it is a switchmode system that provides voltage and current transformation; from psu = high voltage low current to motor = low voltage high current.

Because motor current is the one regulated, the psu current will change according to the needs of the entire system, ie more psu current is drawn during acceleration.

[stirling]

You need to separate the 2 currents to understand the operation. The motor current and power supply current are totally different things.

The chopper driver gives closed loop regulation of MOTOR current. The psu current will be less, because it is a switchmode system that provides voltage and current transformation; from psu = high voltage low current to motor = low voltage high current.

Because motor current is the one regulated, the psu current will change according to the needs of the entire system, ie more psu current is drawn during acceleration.

I understand all this and again I don't generally have a problem with most of what you've said - EXCEPT this bit "more psu current is drawn during acceleration".

This implies that the chopper somehow knows that you're accelerating - how does it know that?

Also IF IF IF somehow the chopper does recognize acceleration and therefore increases the current to the motor then are you saying if the load on the stepper increases for other reasons - let's say I try to stop the stepper with my hand - then the chopper will also "recognise" that and increase the current to the motor?

[RomanLini]

I'm a little tired of spoonfeeding you the same information over and over, and I noticed you being argumentative and rude (in my opinion) in your other thread where myself and others posted many replies to help you.

If you wish to understand more and you can't grasp my explanations then please google for "Jones on steppers" which is still an excellent reference and read it thoroughly.

I'm out.

[James Newton]

I understand all this and again I don't generally have a problem with most of what you've said - EXCEPT this bit "more psu current is drawn during acceleration".

This implies that the chopper somehow knows that you're accelerating - how does it know that?

Also IF IF IF somehow the chopper does recognize acceleration and therefore increases the current to the motor then are you saying if the load on the stepper increases for other reasons - let's say I try to stop the stepper with my hand - then the chopper will also "recognise" that and increase the current to the motor?

What part of "The chopper driver gives closed loop regulation of MOTOR current." isn't clear? How could the driver NOT know the motor is accelerating? It would be bloody hard for the driver to cause the motor to accelerate if it didn't know, don't you think?

When the motor is moving, from one step (or microstep) to the next, the current built up in the coil, or which needs to be built up to set the magnetic fields from the coil in order to pull the shaft into the correct position, needs to be changed. If the coil is off, for example, and needs to be full on, then the driver must supply voltage to cause current to flow so that the field builds up. As the motor accelerates, it will need the coil currents changed more often, which will result in the driver forcing more current, so more current is pulled from the supply.

When you do anything to the motor shaft, the magnets move relative to the coils, and the current flow wants to change. The driver, if it regulates current, compensates for that change. If the current drops, the driver will increase the voltage to the coil, which then increases the current draw on the supply.

Of course, it isn't the current that holds the motor shaft in position, it's the field. Once the field is built up, it holds as strongly as it can. If you twist the shaft hard enough, it will not hold position and that is a skipped step. Since the drivers (typically) are not measuring shaft position, but only coil current flow, then the answer to your hypothetical question is no: The driver will not change coil current just because you moved the shaft past a step, except in as much as that movement introduces a current from the magnet into the coil; that change in current will be detected, and compensated for.

The driver doesn't know the shaft position, so it doesn't respond to that. It knows coil current, if it is a regulating driver, so it DOES respond to that. It also knows the step and direction signals, and what they translate into in terms of desired coil current, and so as the driver is stepped, it will consume current to make the coils come to the correct levels, and when it is being stepped faster, it will consume more current.

Seems clear to me...