Servo Controller Boost Current and Output Wire Size

[bchasta]

I have a customer with a 60Hp, AC servo motor that has the following ratings:

Vmax = 400 V
Io = 66 A
Imax = 264 A
fn = 225 Hz

Under normal operating conditions, 60Hp is adequate to move the load, however, they want to get maximum acceleration out of the load which requires considerably more current than a 60Hp servo controller can source. As a result, they want to put a 150Hp servo controller on the motor with a peak output boost current of 234A and continuous output current of 156A. In theory, the boost current would only be applied for a fraction of a second during acceleration, but my concern is that a failure in the drive control or an unintentional change to a drive parameter could result in the controller sourcing that current for a longer period.

An additional concern is that the system is intended to be portable with the servo controller and the motor separated by 10' to 100' of individual conductors, not in conduit. The distance is within the manufacturers recommendation, but their recommended conductor size is 4/0 and we cannot fit 4/0 terminals on the motor binding posts.

The customer contends that because the boost current is of such short duration, the output wire need only be sized for the 66 A Io of the motor. My contention is that the wire must be sized based on the drive output capability for safety.

Am I correct, or paranoid?

 

[petersonra]

I would not be too worried about a mistaken parameter change causing a problem. That is no different than if someone mistakenly puts the wrong overload heaters in a starter. No motor protection system is adequate if it is improperly applied.

It is common especially for small motors not to have a drive small enough so that the drive is at max current when the motor is. The ONLY way these drives work and the motor is protected is by properly setting the drive parameters.

It is a common practice with DC motor drives to pump some additional current into a motor for short periods of time when it is needed. The DC drive typically has parameter settings that will limit the overload to a short enough period of time that the wiring will not be damaged. Keep in mind that the current ratings of wiring are based on continuous current at maximum rated ambient temperature. You are talking about very brief periods of overloads. The insulation will not have a chance to overheat. No reason you cannot apply this kind of logic to a servo drive, within limits of course. The manufacturer can tell you what is appropriate.

The other thing is this. The OCPD for the branch circuit is sized for the drive and provides short circuit protection. The drive typically provides over current protection for the motor and wiring, just like overload heaters do on a conventional motor starter.

Think about this too - when a motor starts across the line, it can have 6X normal current for a short period of time. No harm. No need to upsize the wires.