75 kva step up transoformer OCP/Breaker

[MortsMotor]

Just starting a small repair shop for electrical motors. Have a 3 phase 75 kva 120/208Y 480/277 step up transformer
Amp rating: 208/90
Have a 100A disconnect with 90A fuse in it.
Right now, I have a 200A Breaker on the primary side, I get a feeling its too small and I need to go with a 250A
Problem right now is, when I attempt to start a 75 HP motor, it trips the primary (200A) Breaker right away in the switch gear.
Does not blow the fuses on the secondary side (100A) Disconnect. I can start a 30HP motor up just fine. I'm getting the inrush current it too much for the 200A breaker?
Meg tested motor its good, no open, ground fault or short, has high resistance reading on all windings.
When energized, shaft tries to rotate then trips.
PM if you like

 

[petersonra]

Just starting a small repair shop for electrical motors. Have a 3 phase 75 kva 120/208Y 480/277 step up transformer
Amp rating: 208/90
Have a 100A disconnect with 90A fuse in it.
Right now, I have a 200A Breaker on the primary side, I get a feeling its too small and I need to go with a 250A
Problem right now is, when I attempt to start a 75 HP motor, it trips the primary (200A) Breaker right away in the switch gear.
Does not blow the fuses on the secondary side (100A) Disconnect. I can start a 30HP motor up just fine. I'm getting the inrush current it too much for the 200A breaker?
Meg tested motor its good, no open, ground fault or short, has high resistance reading on all windings.
When energized, shaft tries to rotate then trips.
PM if you like

a 75hp motor is 177 FLA at 208V. A 200A Cb is way undersized IMO.

 

[iwire]

You might get it running with a VFD to ramp it slowly but other than that I think you are out of luck with those breakers.

You can legally protect the 75 trans much higher than 200 but that may become very expensive if the switchgear does not have a place for it.

 

[petersonra]

You might get it running with a VFD to ramp it slowly but other than that I think you are out of luck with those breakers.

You can legally protect the 75 trans much higher than 200 but that may become very expensive if the switchgear does not have a place for it.

he might be able to get it running with a soft starter as well. but it seems iffy.

 

[kwired]

I agree you are pushing the limits on the 75 HP motor, as far as tripping the breaker and not the fuses, the fuses are seeing motor current only, the breaker is seeing motor load plus any transformer loses. Another consideration is if the breaker would happen to have a magnetic trip adjustment. 200 amp breakers may or may not have adjustment, but anything over 200 often will have adjustment.

 

[MortsMotor]

So for bench test power of motors 75 HP and up I should have a bigger transformer setup?
Only have 120/208Y 3 phase coming into building. and the 208V side of transformer is what
should be paying attention to and not the 480V side?
I also should clarify, they are being started with no load.

 

[ActionDave]

So for bench test power of motors 75 HP and up I should have a bigger transformer setup?
Only have 120/208Y 3 phase coming into building. and the 208V side of transformer is what
should be paying attention to and not the 480V side?
I also should clarify, they are being started with no load.

Our shop has a step up transformer wired to a two position switch. We start motors on low voltage and then switch over to higher. Don't have as much inrush this way.

 

[meternerd]

Problem right now is, when I attempt to start a 75 HP motor, it trips the primary (200A) Breaker right away in the switch gear.
Does not blow the fuses on the secondary side (100A) Disconnect. I can start a 30HP motor up just fine. I'm getting the inrush current it too much for the 200A breaker?
Meg tested motor its good, no open, ground fault or short, has high resistance reading on all windings.
When energized, shaft tries to rotate then trips.
PM if you like[/QUOTE]

Inrush can be 5 to 7 times full load, even if there is no load on the motor. It doesn't last very long if there's no load, but a breaker that is not designed for motors will trip too soon to get the motor up to speed. Breakers rated MCP (Motor Circuit Protector) are designed to handle the inrush, so you can size them closer to the full load rating of the motor and still not trip when starting. Most have adjustable trip settings. Adjustment is typically 5 to 10 times breaker rating. The fuses you have must be time delay or they would probably be blowing too. Seem a bit too small for a motor that size. According to Table 430.250, a 75 HP at 208V draws 211 Amps and at 460V it draws 96 Amps. We use 250 Amp breakers at 480 Volts for our 75HP pump motors, and those are either part winding start or soft-starts. Not saying you couldn't get by with a smaller one, but reliability is an important consideration for a water utility like ours.

 

[kwired]

a 75hp motor is 177 FLA at 208V. A 200A Cb is way undersized IMO.

T430.52 says 177 amps for a 60 Hp @ 200 volts, 75 Hp @ 208 volts is 211, so even higher than what you mentioned.

I don't doubt the OP couldn't start and run this motor with no load, but will need either higher overcurrent protection settings, different trip curve devices, or reduced voltage or soft starting methods.

 

[gk351]

Our shop has a step up transformer wired to a two position switch. We start motors on low voltage and then switch over to higher. Don't have as much inrush this way.

Just curious on how this setup works....

 

[kwired]

Just curious on how this setup works....

I bet it involves a manual transfer switch, or two contactors and a two way control switch.

 

[ActionDave]

Just curious on how this setup works....

If you wire an unloaded motor up high voltage but hit it with lower voltage it will start and run and the inrush and running amps are lower.

I bet it involves a manual transfer switch, or two contactors and a two way control switch.

Bingo!
Two contactors. One contactor and a single pole switch turns the power on. One double pole switch switches the power to either a step down transformer or regular 240V three phase (our shop is on a delta system). We don't use 480V at all in our shop.

 

[kwired]

If you wire an unloaded motor up high voltage but hit it with lower voltage it will start and run and the inrush and running amps are lower.
Bingo!
Two contactors. One contactor and a single pole switch turns the power on. One double pole switch switches the power to either a step down transformer or regular 240V three phase (our shop is on a delta system). We don't use 480V at all in our shop.

That should say double throw switch​. Which I'm sure you intended.