Motor Stall Times
Motor Stall Times
(OP)
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.
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.





RE: Motor Stall Times
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.
RE: Motor Stall Times
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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RE: Motor Stall Times
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
RE: Motor Stall Times
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.
RE: Motor Stall Times
RE: Motor Stall Times
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.
RE: Motor Stall Times
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.
RE: Motor Stall Times
Welcome to the forum and I hope you stick around awhile.
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RE: Motor Stall Times
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.
RE: Motor Stall Times
Thoughts?
Quando Omni Flunkus Moritati
RE: Motor Stall Times
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 www.lmphotonics.com/forum
Regards,
GGOSS
RE: Motor Stall Times
RE: Motor Stall Times
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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RE: Motor Stall Times
RE: Motor Stall Times
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.
RE: Motor Stall Times
RE: Motor Stall Times
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.