Soft Starter with a 200HP Motor

[stang]

In our facility we build hydraulic power units. We have recently built systems with 200HP motors on them but don't have the setup to run them. We have a 400A service coming into the building. If I were to put a soft start on the motor, do you still think the 400A service will be enough. I understand the FLA is ~250A for this motor. If I reduce the voltage to lets say 25% using the soft starter, will I be able to reduce the inrush below the 400A ampacity and actually get the motor to start/accelerate. Also note that the load (hydraulic pump) is essentially removed from the system during starting (using an unloader valve) so the motor will see minimal load when starting.

Thoughts?

 

[Bwas]

Not enough information provided. What other loads are on the service and are they running when the motor starts?

My guess would be that your service isn't big enough.

 

[stang]

I am sure there are other loads but not too much. I will have to look into this to be sure.

For this exercise, let’s assume we have the full ampacity so I can help understand what we might be capable of if we had the full bus.

This is where I get lost in the calculations and how the motor would behave during inrush if there is not enough current to get it to start accelerating. In theory the motor would have just over 1.5 it’s FLA during start but is this enough to get it going.

I am curious what size HP motor could we reliably start given the constraints if the above is not going to work. How is this calculated to determine that.

Any feedback is appreciated

 

[topgone]

Please check if your torque requirements are met when you decide to reduce the supply voltage. Reducing the voltage to 75% will result in a reduction of the motor torque to about 56% of the rated motor torque. Say for example, for a class B motor design, the available starting torque will be just about 175% of FLT. That means, reducing the voltage to 75% will reduce the motor starting torque of a class B motor to 0.56*1.75 = 0.98~ 98% of FLT --> a condition where the motor might stay "crawling" and not reach full speed if the load plus rotor inertia requires torque greater than 100% of motor FLT.

 

[hillbilly1]

Unloaded hydraulic pumps usually are easy to start, but if starting under load, that may give you issues, or if they have a flywheel.

 

[Jraef]

Even unloaded, the motor will draw around 250-300% current at some point in the starting process, so on a 250A motor, that's at least 625-750A. So no, you do not have enough to start even one of them on a 400A service using any simple starting method.

Your only chance is to use a VFD, even though varying the speed may be unimportant. A VFD can accelerate the pump at 100% of it's full load amps, because it can do so for as long as it might take; seconds, minutes, even hours. You still will not be able to run two of them, but you can start either one.