Vd for high starting torque motor

[coulter]

Question(1) of the week (for me anyway):

I looking for opinions (and the science/regulation/industry practices behind them). Probably lots of ways to attack this - likely most will work - I haven't settled on a good one. And I am missing some understanding - which I am working on.

The installation:
MCC is 480V, 3ph, Z grounded. Motor is 31FLA, 225LRA. Feeder length ~700 ft. Load is likely considered high starting torque.

My thinking: (I'm open to critique on this)
Normally I would size the conductors to no more than 5% at 115% FLA (for an sf of 1.15). For those that are even wondering, yes, also at least the minimum required by art. 430.

However, since this is a high starting torque load, I'm thinking maybe I should be sizing conductors to a specified Vd at starting current (locked rotor). If so, what to specify? I'm thinking the NEC spec for firepumps is a good place to start - no more than 15% (art 695.7) - but at the motor, not the controller.

Oh yeah, I not looking at this as a code issue. I expect to be over the minimums.

Any thoughts?

carl

 

[kingpb]

The motor should be sized for the driven load, so the fact that it is going to be high starting torque, is not related to the load, assuming the motor selector picked the right motor for the application.

Something you will need to consider is the amount of time that it will take the motor to come up to full speed, given the expected Vd. If the Vd is to great, the reduction in torque will cause the motor to take longer to start and come up to speed then desired. This in turn causes an increase in heating of the motor, and if the time is too long, the motor protection may trip the motor due to limits based on thermal damage curve. Be sure to check cold and hot start damage curves.

What you need to look at is motor and load torque versus speed curves, and the motor capability at different voltages. Typically, 85% bus terminal voltage is considered minimum drop, but it really depends on the other loads on the bus, and what can the rest of the system handle and for how long, assuming the motor even starts with the connected load. Ultimately, a system model should be analyzed for dynamic motor starting, however, this may be unrealistic due to project budget constraints.

Don't forget you can slightly overvoltage the bus by using taps on transformers to help increase voltage for starting and running. If you do this, keep in mind it can make the voltage too high on a lightly loaded or no load bus.

The curves you need should be available from manufacturers, but it may be like pulling teeth. You will most likely only get typical curves.

 

[coulter]

king -
Thanks for taking time to look at this.

We're okay on the existing installation. It starts fine and runs fine. The existing bus is pretty stiff - 1500kVA - no large motor loads. I'm a bit puzzled by the size of the existing conductors - #2, 700ft. I'm not sure why the feeder was speced out so large. Doesn't hurt anything - just seems a waste of copper.

We have a similar installation running on a 50kW gen - and it does start and run. I haven't had a chance to check the sag on starting.

Why am I looking at this? Well, we are comtemplating some changes, and since I am puzzzled by the existing conductors, I want to make sure I understand the science on Vd on starting.

carl

 

[weressl]

Question(1) of the week (for me anyway):

I looking for opinions (and the science/regulation/industry practices behind them). Probably lots of ways to attack this - likely most will work - I haven't settled on a good one. And I am missing some understanding - which I am working on.

The installation:
MCC is 480V, 3ph, Z grounded. Motor is 31FLA, 225LRA. Feeder length ~700 ft. Load is likely considered high starting torque.

My thinking: (I'm open to critique on this)
Normally I would size the conductors to no more than 5% at 115% FLA (for an sf of 1.15). For those that are even wondering, yes, also at least the minimum required by art. 430.

However, since this is a high starting torque load, I'm thinking maybe I should be sizing conductors to a specified Vd at starting current (locked rotor). If so, what to specify? I'm thinking the NEC spec for firepumps is a good place to start - no more than 15% (art 695.7) - but at the motor, not the controller.

Oh yeah, I not looking at this as a code issue. I expect to be over the minimums.

Any thoughts?

carl

Our Corporate quideline calls for #4 up to 815" for a 25HP/34FLA application at 0.87 Power Factor. If your power factor is lousy, and that was true for old motors, you would need to oversize.

 

[coulter]

Our Corporate quideline calls for #4 up to 815" for a 25HP/34FLA application at 0.87 Power Factor. ...

That sounds about right. Start with a #8, bump two wire sizes for 800 feet. Although for a non-abnormal load, under 400 feet, I would have thought #8 is enough.

Just curious, how did Corporate come up with this?

carl

 

[weressl]

That sounds about right. Start with a #8, bump two wire sizes for 800 feet. Although for a non-abnormal load, under 400 feet, I would have thought #8 is enough.

Just curious, how did Corporate come up with this?

carl

Calculations for typical circuit configuration with three single conductors enclosed in a magnetizable conduit.