norminal voltage
400(3 phase)
power consumption at nornimal voltage
Arms 16.8
norminal power 6
continuos output current at 4 Hz
Arms 25

My question is why the output current is higher than the input current because for the servo all the output current is Iq which is active current

 

The short answer is that they are at different voltages.

Regards
cww

 

>The short answer is that they are at different voltages.<

Thank you, Can you explain a little for me , or give me some reference to see.

 

To explain a little:

It's very difficult with this email setup as the previous discussion is not visible, but you were wondering why the output current could be different than the input current.

This is a complete system where power is converted from AC to DC and then, if an AC servo, back again. Energy is being stored and then used or returned, so there is only one loose relationship between input and output. That is that power in must equal power out plus losses. Since the input voltage is always the same and the output voltage likely changes, the current cannot be the same between the two
and keep the power relationship constant. It's much more complex than this, but that is about the simplest way I can think of to explain it.

Normally this sort of thing is just a given and it is good exercise to have to explain it. I could say it's like a transformer where the current ratio is the inverse of the voltage ratio, but it's not really like a transformer, much more dynamic.

Regards
cww

 

16.8 Amps at 400 Volts equals 25 Amps at approximately 600 Volts. Why 600V? That's just (nominally) the standard DC voltage created by drives, servo or otherwise. There's nothing particularly magical about 600V, it's just what the industry uses most often.

As Curt has pointed out, it's a bad idea to simply use 600V as the output voltage and try to calculate current from the other numbers. You really have to look at what the drive ratings are. And the motors, for that matter, since certain motors can't pull full power from the drive. And it gets even weirder when you consider PEAK current, because the capacitors in the drive mean you can double or even quintuple the max continuous current for brief periods.

The continuous POWER output must of course equal the continuous power input, minus a bit for losses. This is often a better way to think about servos. For one thing, you almost NEVER care about the actual value of the output current. You worry about input current because of fusing and incoming supply. But for OUTPUT all you're really worried about is "can I move this load the way I want to?" That's a power question, and the amps vs. volts breakdown is largely irrelevant. Okay, okay, you have to size your cables. But that's usually just a question of what cable the servo manufacturer tells you to use with a given drive / motor combo.

-James Ingraham
Sage Automation, Inc.

 

Thank you, sorry for the late reply, and I wish for you everything goes well in 2009.

I agree with you the most part, there is only one issue: that obviously you think that the output voltage is lower than the input voltage in a servo system, is that right?

To explain more, how I start this topic is that in a drive system (with asychronous motor), always the input voltage and output voltage are the same, and the output current is more than the input current (with input chock) because at the motor side there is inactive current to flux the motor, and the active current is the same as the input current (with chock). So from this point of view for a servo motor (PM motor), there is no need of inactive current. That is why I think the input and output current should be more or less the same when there is input chock.

 

Thank you for your reply, wish you have a good 2009.

I don't understand why "16.8 Amps at 400 Volts equals 25 Amps at approximately 600 Volts", and also thank you for your advice when using servo, I'm just wondering how come these values.

As for why I am asking this question you can refer to the reply to Curt, so I don't need to copy and paste.

 

When a PWM output drive is used, current to the motor will differ from current to the drive largely because of the varying PWM duty cycle. In this case, and considering the power equation in a very simple fashion, power in equals power out. For a given motor current, the input voltage is - pretty much - constant and the input current changes, but the PWM output voltage varies in order to hold the motor current constant. In a very simplistic fashion, as the motor speed increases, the PWM duty cycle increases and the drive's input current starts approaching the drive's output current.

With regards to the statement that 16.8 Amps at 400 Volts equals 25 Amps at approximately 600 Volts, I don't see any sense in this at all. These are (2) different power levels. James, shouldn't this be ~16.8A @ 600V=25A @ 400V??

 

>I don't understand why "16.8 Amps at
>400 Volts equals 25 Amps at
>approximately 600 Volts",

You don't understand because I screwed up the numbers. It would be worrisome if you understood.

>I'm just wondering how come these values.

So am I, really.

>obviously you think that the output voltage is
>lower than the input voltage in a servo system

Actually, no. In most servo systems, that magic 600V I mentioned before is common. Most servo systems have a HIGHER output voltage. And yet they also have higher amps output, even though they obviously can't output more power. Servos are weird.

-James Ingraham
Sage Automation, Inc.