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Motor Torque and High Load Inertia 1

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Kelley8

Electrical
Oct 27, 2003
15
I have an old (pre-1970) 15Hp GE motor (sf-1.15, FL Rpm-1770, 550V, FLA-15Amps, Type-K, Nema Design-B, Frame-324Z, Code-G, Insul.-B) that part of a 7.5KVA motor-generator configuration.

Attached to the motor end of the shaft is a 430lb. flywheel that provides 260lbs-ft2 of inertia. The flywheel is a design feature that allows the generator output to not be affected by brief interuptions on the motor power supply. Nema MG-1 code states that the rated load inertia for a 15Hp motor is 75lbs-ft2. I do not have any test data (speed-torque curves) or anything else that tells me that the GE motor is special, but the MG set was designed as a package with the flywheel.

This motor has CR123C16.3B size-2 thermal overloads that trip when the motor is started. To prevent this, the starting sequence has been altered to jog the starter, allowing the motor to spin-up to speed. I am trying to find out if I can use higher overloads without damaging the motor. Mfg. motor data would certainly help me out.

Problem: How can I find speed-torque curves for old GE motors. Does anyone know of any good contacts for this? The normal GE motor tech-desk can not find "archived data".

From what I have learned, new standard GE 15Hp motors have hot and cold stall times of > 40 seconds. Is this number high for older 15Hp motors?

Also, newer 15Hp motors are normally on a 254 size frame. Does 324 sound too large?

Thanks in advance.
 
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It's a tough problem to analyse without oem assistance. As you know, even speed torque curves alone won't answer your question unless you know safe stall time or something similar.

One thing working slightly to your benefit: the NEMA MG-1 rating assumes that the motor starts with 90% voltage and with a torque speed curve which varies with speed and hits rated torque at rated speed... does your mg set starts without load? If so that gives you just a little margin but I don't think anywhere near enough to analyse your way out of the problem based on nema alone (without oem help).

If you do end up replacing the motor, you might be able to find larger horsepower off-the shelf direct replacement which will satisfy starting requirements, since your frame size is so large.

A random comment:
I just finished specifying replacement motor for mg set with flywheel. We were concerned because one of the quotes specified it would accelerate the specified inertia "at rated load" when we had asked for 80%. Upon careful analysis of the problem, starting voltage doesn't really affect motor's ability to start an unloaded motor (although it may affect the relay settings).
 
That 324 frame motor, as you guessed is awfully beefy.... motor is overframed for some reason... suspect it either was for thermal dissipation or it has a lot of torque capability. 4-pole 324T frame normally houses a 40 HP motor.

However, you said it's an 'old' motor.... how old ?
Prior to the 1952 Rerate ... a 4-pole 10 hp motor was housed in a 324 frame.

At the 1952 Rerate, a 324U frame housed a 4-pole 25 HP motor ... and at the 1964 Rerate, the 324T frame housed the 40 HP 4-pole motor ..

Since the motor is not a standard frame size for the HP, your best bet is to take all of the nameplate information and contact GE for any data you require, including speed-torque curve...
 
typo alert: ...We were concerned because one of the quotes specified it would accelerate the specified inertia "at rated voltage" when we had asked for 80%..."
Sorry, it wasn't that related to the question anyway but I wanted to clear up the confusion.
 
"That 324 frame motor, as you guessed is awfully beefy.... motor is overframed for some reason... suspect it either was for thermal dissipation or it has a lot of torque capability."

Yes, I suspect that motor could be built to handle the torque or maybe size was picked to match shaft height of the generator? Your rerate history is interesting. The facility here was built 1965.

Unfortunately, the GE tech help doesn't seem to work for obsolete model numbers. I think I need someone with 25 plus years experience at GE.

Motor replacement is last option, I would like to figure this one out analytically if possible(?).

Is it possible to compare/reference newer stock motor curves to older ones? How much margin should be included for updated manufacturer efficiencies? Thanks.
 
When did this problem start if this was a old set up? Or the flywheel is added now?

 
The starter thermal overload tripping problem has existed for sometime (prior to 1980) which is why the operators use a jogged type of start. I believe it has existed from day one.

Its difficult to determine if the original design built the motor for this purpose, and the thermal overloads originally sized larger to prevent tripping.

The MG set is usually running and starting it is infrequent (< 3 starts per year).

 
Hello Kelly8

The maximum load inertia that can be started by a motor is proportional to the thermal inertia (capacity) of the rotor. I would assume that if the motor frame is well oversized, that this has been taken into acount at the design time and that there will be no problems. If you were doing frequent starts, then I would get concerned, but with a very low start frequency, it is posibly worth the risk.
Best regards,

Mark Empson
 
Kelly8

Just can't refer one motor design to another and draw any characteristic inferences that would be valid. Too many variables in the design...

Some years ago I had access to one of GE's Motor Engineers in Fort Wayne .. who was always very helpful in pulling up 'old motors' data for me.

Please write me at
jomega@email.com

and I'll be happy to put you in touch with him.
You will need complete motor nameplate data when contacting him.

jO
 
Thanks jOmega, please check your email.
 
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