Does an ac servo motor experience high "inrush current" every time it moves after initial power up

Question

we are currently building a motion platform for an airplane simulator and the linear motion actuators will be driven by 6 servo motors of type: Syntron 3KW AC 380V 130HMB-30100.

The datasheet says that the inrush current is 22.1A compared to 7A nominal current, and since the motors will be starting and stopping all the time we wondered if this could cause problems with our supply (63A 3 phase 400V).

Hence I wonder if ac servo motors will draw a significant high current each time they start after stopping, or if the motor only draws high current during the initial power up.

Eventually if anyone knows whether the motor driver (HS0300A-P22S) will help eliminate this in some way.

Answer 1

This will come down to how quickly you try and move the motors in your servo loop, the inrush current is caused by the motor seeing full mains applied, and a phase error appearing across the phases as the rotor is stationary at that moment, while mains may be up to 90 degrees out of phase (maximum torque)

when you move a servo slower than its maximum tourque limit, the phase error is smaller, and thus the amount of current is smaller, your motors are in a control loop, meaning they never really stop while powered on, instead you will generally feel a vibration as the loop keeps correcting back to the desired point,

Because of this control loop, the maximum current is only dependent on how much torque you try and have it apply when moving to a new set point, for a constant load scenario this would mostly feed back to acceleration and top speed,

As for your controller, I would rather not go digging through the controllers datasheet, but if it has jerk correction, I would keep it set on the low side.

Answer 2

The inrush current is due to the capacitor banks being charged when you apply power supply. Once the bank capacitors are charged then it will sink just the nominal current. If you start and stop the servo continuously is just a normal operation for the servo, but you could face lot of regenerative power during braking, in this case a braking resistor is needed.

You didn't provided any datasheet regarding your motor and driver.

If you have 6-axis servo system, then it's better to have a central power supply module with capacitor bank and separate power modules for each axis. You would need only one braking resistor (if needed) and further you could have also regenerative power supply, so the excess of energy is fed back into mains grid.

Answer 3

The datasheet says that the inrush current is 22.1A compared to 7A nominal current

If that's on the motor datasheet, and if the motor does not have a built-in servo drive, then this is not really an inrush current. It's peak current the motor can safely handle for a few seconds at most. The word "inrush" means something else and doesn't apply here.

The motor has no control over the current - it's the servo drive that maintains current control, and it's the servo drive that can be programmed to allow the motor to temporarily exceed the nominal current and operate up to the peak current.

The BLDC motor current ratings give what the motor can accept from an external current source - i.e. the drive - without failing.

The peak current is what the motor can handle on a short term, without saturating the magnetic path, and without mechanically overloading it. The motor cannot shed the resistive heating at this peak current in a sustained manner: it will overheat. On the other hand, the motor should be in a safe thermal equilibrium when the nominal current is applied continuously. But also see thermal derating curves that may be provided in the datasheet.

Expect servo motors running under heavy loads to get hot. Scorching hot. At the higher end of the ambient temperature range, it's pretty normal for the motor's external surface to be a serious burn hazard. High temperatures are needed for there to be enough heat outflow to balance the internal heating.

As for how much current the servo drive consumes on start-up and during operation: that's up to the servo drive design, its supply voltage, and the load/motion profiles of the motor(s) attached to it.