Newbie- Why 1/2 or 2/3 of rated current?

[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]

Originally Posted by 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).

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

[RomanLini]

Originally Posted by stirling ... 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.

Originally Posted by stirling ... 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.

Originally Posted by stirling ... 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]

Originally Posted by RomanLini 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.

Originally Posted by RomanLini 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]

Originally Posted by stirling ... 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.