Motor Stall Times 3

[Kelley8]

Is it a fair assumption that on average, hot and cold stall times for smaller motors (< 30 Hp) are longer than for large motors (> 150Hp)?

I have data sheets for 2 newer GE motors (one is stock, other is efficient) and there hot and cold stall times are 65, 77 (stock motor) and 48, 54 (efficient motor) seconds.

Those numbers are much larger than the stall times I've seen for bigger motors.

Thanks.

 

[aolalde]

Stall withstand in seconds depends on the temperature rise due to thermal energy injected to the windings against the winding mass and the insulation class. If no heat transfer is considered:

H (Thermal energy) = 3 x I^2 x R x t
I= stator current per line
R= Phase resistance
t = time stalled

R= p * L/A
p= wire resistivity
L= equivalent wire length per phase
A =equivalent wire cross section area.

TR =k1 x H/Cumass
Cumass= 3 x A x L x K
TR= Temperature rise
K= conductor specific mass
k1 = constant for units used.


then; TR =k1 x 3 x I^2 x p x L / A / (3 x A x L x K)

TR= K2*( I/A)^2
K2=k1 x p / K

The temperature rise at stall condition is a function of the current density squared.
The allowable temperature rise depends on the insulation class.
These two depend on the manufacturer winding design rather than the motor size.

 

[electricpete]

I think it is true on average.

Here is the rough way I see it. Please don't take it as gospel. I'm sure I will be corrected.

Large motors are more likely to be rotor limited due to skin effect heating. Economic design will force large motors toward lower start time.

In smaller motors rotor heating during starting is not such a big factor and a motor designed to meet steady state running conditions will likely meet long stall times without significant extra expense.

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[GGOSS]

Hello Kelley8,

The max stall time of a motor is moreso dependant on its design that its size/rating.

The stall times you quote in your post appear exceptionally long, so much so that I would question whether or not they were correct.

From experience I can advise that I have come accross many small motors with stall times (from cold condition) in the order of 8 to 15 seconds. In fact that would appear to be the norm.

Regards,
GGOSS

 

[Kelley8]

Yes, my colleagues are familiar with ~ 12 to 18 seconds.

But those high numbers I posted earlier come straight from the GE motor design performance data. Stock motor, model #5KS254SAB204 hot-65.7, cold-77.3. Efficient motor, model #5KE254SAC204 hot-48, cold-54.

 

[GordS]

Most motors I deal with have stall times in the range of 10-20 sec. However, I do have GE motors on a high inertia application where the stall time is about 45 sec. These motors are physically larger than shorter stall time designs of the same horsepower - I think because more thermal capacitance is required to be built into the rotor to absorb the generated heat during a start.

 

[Kelley8]

GordS,

unfortunately the motor is 37 years old and I do not have any documentation that states it was built for the large inertia start. It is built on a 324 frame and my thinking is that in the old days, they built them a little better with more back-iron. However, could the efficiencies of today make new motors withstand more lock rotor current?

Those are the questions I must answer.

GE will do it.... for $10,000.

 

[aolalde]

30 HP could be considered a small motor.

For small motors, the stator winding temperature rise is the limiting parameter for stall time endurance.

For large motors ( >100 HP) most probably the rotor is the limiting factor as electripete referred above, due to high reactance of the rotor bars bottom during the starting condition.

If your motor is 37 years old, send it to a good shop for re-winding, ask for premium quality maximum amount of copper cross section area in that new stator winding. Record the winding parameters and calculate the inrush current density and stator copper mass. The stalled time allowed for the stator will be based on previous discussion; TR= k*j^2*t. Most probably this old motor with a modern winding will exceed any actual design built into a smaller frame size.
The basic physics laws are; what is the heat injected against the motor mass.

 

[electricpete]

I vote you a star aolalde for your first post.
Welcome to the forum and I hope you stick around awhile.

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[jbartos]

Suggestion. The efficiency motors have the smaller thermal accummulation capability; therefore, the stall time (or locked rotor time) are shorter. The efficiency motors may have larger locked rotor currents. Therefore, the stall time is shorter.

Definition: Locked Rotor Time or Stall Time - Time in seconds that a motor can withstand locked rotor (stalled) current without damage.

The new NEMA Design E motor specification has its own NEMA minimum efficiency tables in excess of the EPAct minimum and it allows higher locked rotor current than the Design B specification.
//See NFPA 70-1996 Table 430-151B for minimum locked-rotor amperes for different motor design letters, including NEMA design letter E\People often confuse the NEMA Design E specification with the EPAct standard. They are not related.

 

[jraef]

Interesting discussion. I never thought about it enough before, but it brings up another issue regarding overload settings. If &quot;most&quot; motors have a stall time of 12-18 seconds, and you install an overload relay with a class 20 trip element, are you not in fact risking rotor and/or stator damage? By definition, Class 20 neans that it will trip in no more than 20 seconds at 600% of setting, commonly acccepted as Locked Rotor Current. If so, why are so many Class 20 overload relays sold? This may deserve a new thread but I wanted the context of this thread to tie into.

Thoughts?

Quando Omni Flunkus Moritati

 

[GGOSS]

Hello jraef,

You certainly would compromise the motor in that case. Your only options would be to install a class 10 relay or class 15 relay, (subject to actual max locked rotor time) however that would in fact over-protect the motor and compromise productivity/profitability particularly in those applications which experience transient overloads under normal operating conditions.

The best solution (also the most expensive) is to install an electronic motor protection relay that offers motor thermal modelling. This would allow the protection curves within the device to be matched to the motors thermal capabilities.

There's an FAQ on this subject that's worth reading. Also there is good discussion thread in the motor protection section at

http://www.lmphotonics.com/forum

Regards,
GGOSS

 

[vanstoja]

For squirrel cage induction motors we have found by much locked rotor testing that axial expansion of the rotor cage due to thermal expansion of the rotor bars is the governing criteria for locked rotor endurance (LRE) time. For our motors, cage axial expansion was constrained by nonmagnetic end rings used to compress and retain the rotor punching stack. Sometimes rotor bars can &quot;lock-up&quot; at one end so all the locked rotor power goes in expanding the rotor bars in one direction from the locked-up end rather than expanding them in both axial directions from the middle. Locked rotor endurance time for new motor designs was specified based on long-time-delay trip settings of the circuit breakers used to protect the motors. When testing showed that the specified LRE time (eg, 14 seconds for a 700 HP water-cooled motor)could not be met, the only alternative was to reset the breaker LTD trip times to accomodate the the motor design limitations. Presuming the cited GE motors are squirrel cage design, then the designers have apparently found a way to offset the bar axial expansion constraint limitations to be able to offer such long LRE times. There appears to be much more locked rotor testing being conducted by manufacturers in recent years compared to the meager LRE data available from decades-old motor designs.

 

[electricpete]

Just thinking about some characteristics that might make small motor have higher stall time.

We already mentioned large motor rotor heating due to skin effect does not apply to small motors.

Small motors likely have die cast aluminum rotors. From the looks of them I think they are solid (not hollow) while bigger motor fabricated rotors are hollow with spider. If I am correct that aluminum rotor is solid (does anyone know?) it would have more heat capacity (relatively speaking).

TEFC and other small motor construction will likely be more efficient at dissipating heat heat to the frame without any fan action (locked rotor) than open drip proof (ODP more common in larger motors).

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[edison123]

The safe heat capacity (watts/area that is dissipated) is typicaly more for small round wires (used in small motor stators) than for the rectangular bars/strips (used in large motor stators). This would account for larger stall times for small motors.

 

[Kelley8]

Some interesting and elightening points have been made.

Unfortunately, I do not know the kinds of information that would be used in aolalde's equations. I know little about the rotor and stator.

I don't think that the GE motors w/ longer stall times are special, they only cost $650.

 

[jbartos]

Suggestion: Double cage squirrel-cage induction motors will have a longer stall time since the heat dissipation / distribution in the rotor is more directed to the rotor inner volume instead of the rotor surface where it heats the stator windings.

 

[tmahan]

Electricpete
With regards to small motor rotor construction. I once worked for a motor manufacturer in Houston... At that plant all small motors were made by pouring molten Al into the stacked laminations. Therefore all rotor bars were solid Al.and the rotor was a solid unit.

In my opinion there is also an economic issue that contributes to the tendency of smaller motors to have larger LRT's. Smaller motors are tested in a destructive manner so that those parameters are well known. The guys in whatever company's marketing group are more comfortable citing longer accepable times for smaller,less expensive, well tested designs.