Continuous and Peak Current (hobby servos)

[alphaai]

Hello,

Can someone please clarify with me what continuous and peak current and how it relates to hobby servos.

I found this somewhere online:

"When a servo makes small movements it draws a higher peak current than when moving a larger distance. Why? When the servo motor is stationary, only its (low) armature resistance limits current flow. Once the motor starts turning it generates a voltage which opposes the power supply, so the effective voltage is less and current drops."

2) What do they mean by "When the servo motor is stationary, only its (low) armature resistance limits current flow"

Q: Are they referring to the opposing torque on the motor and how that can affect the current draw? Im sorry I dont understand it that well. Current draw is apparently different when there is a load and when there isnt a load. Why is that?

3) What do they mean by this "Once the motor starts turning it generates a voltage which opposes the power supply, so the effective voltage is less and current drops.""

Q: This sounds like back emf. Voltage Source - Back EMF = Voltage.
How does that relate to current drop and peak current?

4) I found this online somewhere:
"As revs rise so does the back EMF, opposing the Applied EMF and reducing the Current."

Q: if the current is reduced then this is obviously not peak current?

Sorry Im not too sure if right on track. hopefully someone can clarify this for me.

Thanks,

Ben

 

[KeepItSimpleStupid]

OK, lets take a stab at this:

First two equations

V = Vm - ImRm

Basically states that there is an internal motor resistance Rm and Im is the current through the motor.

Im is proportional to torque

Vm, not V is proportional to speed

Which means a motor can be used as a tachometer when used as a generator into no load.

Another law: the current through an inductor cannot change instantaneously

Basically what thy are saying is:
if you move 10 deg, rest 1 sec, move 10 deg rest 1 sec, move 10 deg rest 1 sec and move 10 deg will take more power than move 40 degrees.

Each time the motor starts, only the winding resistance is limiting the current until the motor starts to move. You always get a high current peak when starting a DC motor.

EDIT: more used to be less which is wrong. Thanks Pommie!

 

[Pommie]

Basically what thy are saying is:
if you move 10 deg, rest 1 sec, move 10 deg rest 1 sec, move 10 deg rest 1 sec and move 10 deg will take less power than move 40 degrees.

I can't see how this can be true. To move in 4 steps requires you to accelerate and decelerate the load 4 times and it is this that takes the most power.

My understanding is that the current required is proportional to the work done plus the static load. A stationary servo holding a weight (torque) equal to it's max output torque will draw the max current.

Try this, connect up a servo and measure the current taken. Now try and turn the servo, if it's a decent servo you won't be able to, however, the current will rise to near peak even though the motor isn't turning. Just holding a position against a load will draw a large current.

So, basically, the harder it works to maintain or try to move to a position the more current it will draw.

Mike.

 

[KeepItSimpleStupid]

You are 100% correct. Brain and fingers didn't agree. Moving incrementally will take more power.