Motor Won't Run at Rated RPM

I assume you are with a 3 phase motor.

An 1800RPM at 450 RPM will stall. Only possibility that I can see is (you are at 60Hz right?) the windings leads are all brought out and by mistake these are connected as an 8 Pole motor (450 = 3600/8) and then the synchronous speed is 450 and not 1800.

There may be a bit of winding reversal too but then it would have hunted / drawn excessive current so that is not a likely possibility.

Ask your electrician to check the winding connections.

Actually it should be winded 16 poles to run 450(not 8 SB at 8 poles it would run 900 at 60Hz) which is very difficult by the way or internal connections messed. (one phase reverced.) Make 3 tests reversing the leads of each phase that go to the 6 screw connector and if not OK reverse again and go to next.

Still the gratest !

Thanks for the tip. We uncoupled the motor and it did reach the rated rpm (no Load) It is an air compressor that we have comfirmed not to be the issue through inspection at the vendors facility. If one of the windings were reversed does it make sense that it could reach rated rpm with no load?

No It doesn.t. I assumed the mesurement was unloaded. But there is a chance of half pole reversal in winding that can give low torque such as your case. Still do the test but if zill make another 3 tests (unloaded) and only briefely in star:Completely disconnect one by one the phases (one at a time) to find which if disconnected allows to start. If this applies to all then have to open up and check if internal windig to winding connections are accessible and verifiable..In a good quality winding they should.

It is not a wound rotor induction motor by chance? though it does not seem likely by other data.

Even then stable speed at 25% synch is a surprising phenomenon.

it could still be a connection issue however. If the motor is a dual voltage AND set up for Y-Delta starting (12 lead), and if you connected to the high voltage windings but are supplying low voltage, there will be insufficient torque from the motor. It will run at the normal speed when uncoupled, but the extreme torque loss will cause it to stall with any appreciable load: torque varies by the square of the voltage reduction.

The reason why I bring this up is that some motors, especially Weg, are sold as "480V" for use in North America, but are actually dual voltage motors for use in Brazil where they use 760V. So the motors are actually 440/760V wound, and if you are using them in North America, you MUST connect them to the LOW VOLTAGE pattern, which is counter-intuitive to what most electricians do every other day of their lives. Here is a direct quote from the Weg Crusher Duty motor manual.

STAR-DELTA STARTING
It is fundamental for star-delta starting that three phase motor have
the required number of leads to allow connection on both voltages,

that is, 220/380V, 380/660V or 440/760V...

So if your motor is one that is wound for 760V and you are supplying 480V, your motor is only producing roughly 40% of it's rated torque and when you couple the compressor, it will stall. Double check your connections again.