Motor overload feasibility.

[junkhound]

5HP rated 184 frame 240V single phase motor.

Running with 10 HP load, 50A breaker.

a. How long can it run before overheat?

b. how much extra cooling air would be needed to run (albeit at lower efficiency) continuously at 10 HP.

 

[gar]

230420-2352 EDT

junkhound:

If you can get the motor up to speed, and then mechanically load it to this level without exceeding max torque of the motor the power dissipation in the motor will go to about 4 times the rated full load value. The duration that you can go to this level will be a function of the thermal time constant of the motor. Obviously motor lifetime is dependent on the amount and duration of overload.

.

 

[ptonsparky]

How often do you want to press the thermal reset on the motor?

 

[gar]

230421-1026 EDT

Further comments.

There will be a maximum hot spot location in the motor, and this location may change somewhat as the motor heats up.

So suppose our criteria is to not exceed this maximum hot spot criteria. Then the loading during startup will determine how long we can run at a certain overload. When that time is reached we stop and let the motor cool.

Now consider a DC brush type motor with a fixed DC magnetic field. At zero RPM maximum torque is produced, and might be 10 X full rated load capability. When this motor reaches 1/2 final speed, then its maximum load current is 1/2 that of starting.

I believe it is quite possible to start a DC motor at 10X rated load, but tht load must appropriately drop as speed increases. Note that 10X current means 100 times power.

Use these basic DC motor characteristics to drive your thinking on your particular application.

..

 

[junkhound]

Note that 10X current means 100 times power.

resistive load OK, but fpr a motor?? Need that one explained if true. IIRC motor torque proportional to current.

Of course, should have qualified the original question statement and specified fixed frequency, VFD at 120 Hz would almost double capability efficiency ostly depending on lamination thickness.

 

[retirede]

Note that 10X current means 100 times power.

resistive load OK, but fpr a motor?? Need that one explained if true. IIRC motor torque proportional to current.

Of course, should have qualified the original question statement and specified fixed frequency, VFD at 120 Hz would almost double capability efficiency ostly depending on lamination thickness.

Gar was talking about a DC motor, so yes, 10 times current means 100 times (input) power.

 

[Jraef]

Here is a typical “Thermal Damage Curve” for a motor on a pump. As you can see, at 100% of rated load you can run forever. At 200% of rated load, the motor damage will start at about 180 seconds. There is no reasonable amount of ambient cooling air volume that will significantly change that more than a few percentage points. I have done cryogenically cooled pumps that are in liquified gasses and the thermal damage curve is only slightly different, ie you might get 300 seconds at 200% current.

 

[junkhound]

Say a 5HP 184T motor has an 85% or so efficiency, air flow needed to keep to 40C rise is approximately 50 CFM.
If we duct a furnance fan blower on it and drive airflow to 500 CFM around and thru the motor, even if efficiency drops should not the motor stay well below 40C rise and run continuously at say 10 HP?
Or, would 500 CFM be considered and unreasonable amount of ambient cooling air volume?

 

[retirede]

Say a 5HP 184T motor has an 85% or so efficiency, air flow needed to keep to 40C rise is approximately 50 CFM.
If we duct a furnance fan blower on it and drive airflow to 500 CFM around and thru the motor, even if efficiency drops should not the motor stay well below 40C rise and run continuously at say 10 HP?
Or, would 500 CFM be considered and unreasonable amount of ambient cooling air volume?

The heat deep in the windings and rotor bars has to get to the surface that’s exposed to the airflow. At some point, that becomes the limiting factor, not the airflow itself.