Motor damage and starting curves

[joshtrevino]

I am doing a coordination study for an existing switch gear that formerly operated at 2300V, but will now be operating at 4160V. The one motor that remains on the switchgear will be rewound to operate at 4160V.

The motor is 300hp, 2-pole (3550rpm). My issue is that my ETAP library does not have 2-pole motor curves built into it, only 4-pole motor curves. ETAP technical support does not appear to have a library to download with 2-pole motor starting and damage curves. I have contacted Siemens, but I am unsure how much it will help as this is an existing motor that will be rewound.

Question - Does motor rpm / number of poles significantly change the starting and damage curves of a motor? Could I conservatively use the 300hp 4-pole curves built into the ETAP library to substitute for the 2-pole motor?

Please excuse me if this is an ignorant question as I am new to software coordination studies.

 

[Jraef]

The thermal damage curves are based on heat generated by current flow. So to that end, a 2 pole motor will have lower FLC than a 4 pole motor of the same HP, therefore a 4 pole thermal model ends up being slightly conservative in that regard. Where you can get into more trouble is going the other way; basing it on 4 pole when the motor is 6 or 8 or even 10 pole and the FLC is significantly higher.

Starting curves are typically based on PERCENTAGE of speed, not actual RPMs.

 

[templdl]

I am doing a coordination study for an existing switch gear that formerly operated at 2300V, but will now be operating at 4160V. The one motor that remains on the switchgear will be rewound to operate at 4160V.

The motor is 300hp, 2-pole (3550rpm). My issue is that my ETAP library does not have 2-pole motor curves built into it, only 4-pole motor curves. ETAP technical support does not appear to have a library to download with 2-pole motor starting and damage curves. I have contacted Siemens, but I am unsure how much it will help as this is an existing motor that will be rewound.

Question - Does motor rpm / number of poles significantly change the starting and damage curves of a motor? Could I conservatively use the 300hp 4-pole curves built into the ETAP library to substitute for the 2-pole motor?

Please excuse me if this is an ignorant question as I am new to software coordination studies.

I only sold one large motor, an 800HP wound rotor type but provided many proposals for others. As I can recall the only standard was that they were designed for one hot and two cold starts per hour unless the RFQ requested otherwise.

 

[Besoeker]

I only sold one large motor, an 800HP wound rotor type but provided many proposals for others. As I can recall the only standard was that they were designed for one hot and two cold starts per hour unless the RFQ requested otherwise.

Common in my experience.
Another is four starts per hour evenly spaced and twice from cold.

 

[Jraef]

On a project I did to re-power a 4kV 500HP Banbury Mixer (rubber shredder) for a tire retreading plant, the owner got a deal on his motor from a junk dealer and was concerned over the fact that the shaft was only 2" diameter, so he wanted me to use a soft starter, which I did. But the small shaft had me worried a little too, so when getting ready to program the protection relay, I got (bought) the motor specs from Westinghouse (this was before they sold out to Teco). Turned out this was a specially designed OEM fan motor for going over pot lines in an aluminum mill, designed to run continuously and never shut down unless the power failed or for scheduled maintenance. It was therefore rated for one start PER DAY maximum! Needless to say, he had wasted his money, soft starter or not... Google a picture of a Banbury Mixer, you'll see what I mean. It's basically two rotating lobed shafts and when the big block of solid rubber is fed into them, the pressure from being squeezed into a gap less than half the thickness of the block makes the rubber explode into shreds, then it is fed into the ovens to be melted. There is a big gear box, but the shaft torque on that motor is still horrendous, and they start and stop this machine a half-dozen times per shift.

Better yet, video...