Measuring Temperature Rise-Single Phase Motor

Status
Not open for further replies.
fifty60

fifty60

Senior Member
Location
USA
I have a single phase motor that is rated for 115V 50/60Hz. We have had some trouble with the thermal overload tripping at 50Hz. We supply an enclosure for the motor, and I think that the ambient temperature Is too high inside of this enclosure.

I can measure the ambient temperature inside of the enclosure easy enough, but I would also like to measure the winding temperature rise. I know for 3 phase motors you can measure the lead to lead winding resistince. This method will not work on single phase PSC motors will it? If not, what method can I use?
 
You should be able to measure the temperature of a winding in a PSC motor by the resistance method; you are 'simply' measuring the DC resistance across a coil. In essence you are treating the coil itself as a copper RTD.

You would need to disconnect the motor from the supply and connect a microohm meter, all fast enough to avoid temperature changes. As a PSC motor, it will be pretty small, and will cool off quickly. Making the connections would not be trivial, because you would want to make sure that the connection resistance didn't change from test to test.

You would get a bit of benefit from the fact that small motors have higher resistance windings.

If this is a lab/production research project, then I am sure you can build an instrument to get the job done using the resistance method. But it might be more practical to just get a motor or two wound with embedded RTDs.

-Jon
 
fifty60 said:
I have a single phase motor that is rated for 115V 50/60Hz. We have had some trouble with the thermal overload tripping at 50Hz. We supply an enclosure for the motor, and I think that the ambient temperature Is too high inside of this enclosure.

I can measure the ambient temperature inside of the enclosure easy enough, but I would also like to measure the winding temperature rise. I know for 3 phase motors you can measure the lead to lead winding resistince. This method will not work on single phase PSC motors will it? If not, what method can I use?
Click to expand...
Knowing from your previous postings that you work for an equipment OEM sending your products overseas, what are you doing with that 50/60Hz motor? Sometimes mfrs put ratings like that on there because they ASSume you would use that motor on something like a centrifugal fan. So if you apply 50Hz the fan spins slower, so the load is reduced very significantly, i.e. the fan now REQUIRES only 70% of the HP at 50Hz as it did at 60Hz, because it is a centrifugal machine. So even though the motor might be over fluxed at 50Hz and therefore wasting more energy as heat, it is at the same time running under LOADED, and by so much less that it more than makes up for that.

But if you apply that same motor to something OTHER than a centrifugal machine, the loss of speed as a result of the lower frequency means the V/Hz ratio of that motor is now significantly higher, meaning more of the motor power is now going to be lost in waste heat.
 
The OL manufacture should show what the ambient correction should be for the overload. Temp rise should be on motor nameplate don't know why you want windings.
 
It is an open enclosure motor. I have to add my own enclosure for the motor. It is indeed an air circulator motor, so the load is a centrifugal fan. I was wondering why they could rate the 50/60Hz motor at the same voltage. Jraef's explanation definitely makes sense. The torque is actually increasing though right? The impedance is lower at 50Hz, so more current is drawn resulting in more torque? So the RPMs go down, but the torque increases, so the HP may not necessarily decrease?

I want to run the motor at 115VAC at 50 or 60Hz. It is rated to run at this voltage at 50 or 60Hz. I think the our enclosure (added to the motor) design may be allowing the motor to get too hot, therefore causing us thermal overloads as well as motor failures in the field at 115V 50Hz. I am doing tests now.

The insulation is type F, and the service factor is 1.25. I am trying to get a simple measurement of the air 1 inch around the motor frame, and also another measurement on the motor frame. I am going under the assumption that the motor coils have a 30 C delta with the frame after 4 hours of operation.

Where on the frame of the motor would be the best place for measurement? The motor is mounted shaft down.
 
Would it be helpful to take motor frame temperature measurements? I can then compare the difference between the frame temperature at 50 Hz and 60Hz. Would this info be of any use to me? Where would be the best spot on the frame to take a temperature?

I am really comparing the temperature between the fan motor in Enclosure A, and the fan motor in Enclosure B. I have no doubt that the motor is rated for 50Hz and the same voltage as 60Hz...
 
This is tangent to your original question, but:

fifty60 said:
It is an open enclosure motor. I have to add my own enclosure for the motor. It is indeed an air circulator motor, so the load is a centrifugal fan. I was wondering why they could rate the 50/60Hz motor at the same voltage. Jraef's explanation definitely makes sense. The torque is actually increasing though right? The impedance is lower at 50Hz, so more current is drawn resulting in more torque? So the RPMs go down, but the torque increases, so the HP may not necessarily decrease?
Click to expand...

At 50Hz, the motor is being operated at a greater V/Hz ratio. This increases the magnetic flux, and increases the _available_ torque. The torque actually being produced will be whatever is being consumed by the load. For a fan, operating at lower speed will significantly reduce the torque consumed by the load.

-Jon
 
Status
Not open for further replies.
Top