An old folk told me that with his 18 years working experience, that increase in ambient temperature (above 40C) will cause an AC motor to lower its RPM......True?? Please advise.....
It is true that an increase in ambient temperature will cause an induction motor's speed to decrease in the absence of other influencing factors such as voltage change and load change. An induction motor's speed is the speed of the rotating magnetic field minus the motor's slip. Temperature is one of several factors that influences the motor's slip. Most of the speed change takes place when all of the parts of the motor are at room temperature before power is applied. As the motor comes up to its normal operating temperature, the temperature of the motor’s rotor can increase from 25C to 100C and the full-load speed could go down by 6 RPM or so. If the ambient temperature increases from 25C to 45C, the speed might go down another 2 RPM. In most situations, it would be difficult to detect such a small speed change, or to know that the speed change was not caused by a voltage change or load change. However, such changes are readily apparent when running dynamometer tests in a laboratory. Most of the speed change is due to increase in rotor resistance with temperature.
This is true. Most require to operate in a minimum amount of free air. The minimum with the pump I experienced was one cubic foot. If this volume is reduced and the motor gets too hot it will start lowering its RPM and could eventually stall. This can be a bit of a pain if the motor was driving a fuel pump or some other device.
There is absolutely no relation whatsoever, between the ambient temperature and the RPM of a motor. The RPM is decided only by the supply frequency and the number of poles in the motor. Just to quip, may be, when the ambient temperature goes very high and the load on the motor is also at rated levels, may be - just may be - the motor insulation may fail and it would come to a stop. That is the only way that I can think of any relation between the temperature and RPM. This is from my 26 years of experience in the field.
Never known this to be true. However, do you know what type of motor? The motors own internal termperatures may exceed 40C and ambient may have nothing to do with motor operation. 40C is only 104F...which is a lot of times a typical temp in the summer outside or in a cabinet. Motors should not change much in this type of ambient. Could be several things in my experience: 1. Things tend to grow as they heat up...perhaps binding of some sort? 2. Could be that the ambient from the weather affected the source frequency from the power plant changing the motor speed? Though not sure how. 3. Line voltages feeding the motors could be lower due to demand in the summer months? Making the motor weaker and thus at the same time slower when running at load? 4. Perhaps your folk has worked at only one place in his life and the loads would get higher as the temperature rose, causing the motors to slow down? 5. It could be that the motors get more resistive with higher temps, thus getting weaker and slowing down. But then again, 104F(40C) is not all that hot for an AC motor..this is a temperature in Arizona in the summer. There are a number of ways that a motor can perceptively slow down during high ambient...but they are not designed or behave that way normally. Would need to talk to your folk to get a more detailed explanation. I suspect the load is what changes. But no...they should not slow down.
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