Why did the speed of the motor vary so much when measured at different times?

[boreilly]

I was measuring the speed of a motor with various voltages applied. The next day i was doing the same thing however on the first day the motor had been running a while and would have course been warm. onthe second day i started measuring when i first turned ont he motor and found that the speeds varied alot. (i used a tachometer and instead of staying within +/- 5 RPM of a value, it would vary hugely.eg, i would put the tachometer against it which would quickly show a result, let's say maybe 500RPM, and then slowly increase another 100rpm then drop 50. never making it easy for me to read.All the while the voltage staying at whatever i set it at. Then a while later it was just like the day before and the results were similar and stayed within an easy range for me to read a somewhat accurate result of the speed.
was this simply because the motor was warming up? Its a 1 KW motor(permanent magnet I'm sure)

 

[Navin A S]

Yes, it is likely that the motor was warming up, causing the RPMs to vary until it reached a steady state. Permanent magnet motors tend to require more power when they are first started up, as they are not yet spinning fast enough to create their own magnetic field. As the motor spins faster, it becomes more efficient and requires less power to maintain its speed. This can cause the RPMs to fluctuate until the motor reaches its operating temperature.