AC stall?

[ptonsparky]

Going back to post 3,



is this happening anytime pressure is over 1000 or only when starting if pressure is already above 1000? If it is only when starting with high existing pressure, then there likely is either poor design or selection of motor, no unloading device, or a malfunction of an unloading device.

If it can not develop 1000 PSI when it is supposed to be able to develop 1700, there could be low voltage, improperly wired motor, or other installation issues.

Puts us back to you Post 12.

To many things to guess at without having some detail of the installation. Dartboard or Shotgun Diagnosis at the moment.

 

[kwired]

Puts us back to you Post 12.

To many things to guess at without having some detail of the installation. Dartboard or Shotgun Diagnosis at the moment.

True, I was just trying to narrow it down to if it is a starting problem only or if it does this starting and/or running, to determine which way to go from there.

 

[drbond24]

We seem to have fixed it. The motors were wired in wye, and we changed them to delta. Now they work just fine. I'll have to get out my circuits book to understand the implications of that, but it definitely appears they have a great deal more torque in delta.

 

[iceworm]

We seem to have fixed it. The motors were wired in wye, and we changed them to delta. Now they work just fine. I'll have to get out my circuits book to understand the implications of that, but it definitely appears they have a great deal more torque in delta.

Single voltage motors and all 6 leads were brought out. That that's different than most. So the motors were dual voltage, 575V/1000V. (joke) Not really common. And not one I would have guessed. Jaerf was pretty close though.

ice

 

[Jraef]

Lost me here. Looks to me like one would be connecting 332V coils across 575V Severe overvoltage. I don't see how the LRC would be reduced - should go up - saturated

ice

Yeah, sorry. Brain fart. I was going from the symptoms to try to explain what is happening but left out the details. Forget the dual voltage part I had mentioned earlier, that was a mistake. I was equating the effects to what you see when you use a Wye-Delta starter but fail to transition to Delta. If the motor is designed for Delta connection at your voltage but you have the leads available to connect it to Wye and mistakenly do so, you get 33% torque at starting and 33% of starting current. The lack of torque is sometimes not noticeable with light loads, but if you never transition to full voltage and the load increases, you never see Locked Rotor Current even when it stalls.

I need to remember to never try to rescue a mistaken concept, the hole just gets deeper...

 

[drbond24]

Yep, it was the consensus around here that it was strange to have all 6 leads brought out as well.

The motor says wye start, delta run on the nameplate. It must be made for a fancy starter of some kind that we don't have that would transition from one to the other. We have it hard wired and have to pick one, so apparently delta is the right choice for us.

 

[drbond24]

If the motor is designed for Delta connection at your voltage but you have the leads available to connect it to Wye and mistakenly do so, you get 33% torque at starting and 33% of starting current. The lack of torque is sometimes not noticeable with light loads, but if you never transition to full voltage and the load increases, you never see Locked Rotor Current even when it stalls.

This pretty much describes exactly what we were seeing.

 

[kwired]

We seem to have fixed it. The motors were wired in wye, and we changed them to delta. Now they work just fine. I'll have to get out my circuits book to understand the implications of that, but it definitely appears they have a great deal more torque in delta.

Think about the motor coils. They are fixed and designed for a specific voltage. If you apply 575 volts and configure them in a delta fashion each coil recieves full 575 volts. If you configure them in wye fashion you have a midpoint and two other coils also connected to that midpoint. voltage across each coil (assuming all three are same impedance and load) is going to be 575/square root of 3 which is about 332 volts. So each coil designed for full voltage of 575 is going to only see 332.

The wye-delta options are either for dual supply voltage reasons, or more likely for reduced voltage starting (connect in wye for start, and transition to delta for run) How big is this motor? You usually do not run into wye-delta starting for motors under 15 or 20 HP, unless the source has limitations making it necessary, but even then, these days VFD or soft starters are more common way to do this.

 

[drbond24]

The wye-delta options are either for dual supply voltage reasons, or more likely for reduced voltage starting (connect in wye for start, and transition to delta for run) How big is this motor? You usually do not run into wye-delta starting for motors under 15 or 20 HP, unless the source has limitations making it necessary, but even then, these days VFD or soft starters are more common way to do this.

It is a 100 hp motor. By our standards around here that is pretty small.