OK I will admit that I know little about small series wound AC motors, do they behave Like small DC motors?
Then it is impossible that a shop vac can produce more than 1500- 1800 wattts of power?
~ ... Sorta yes. From what I have been able to pick up and make sense of over a lifetime of experience, listening and reading, the series wound motor characteristic cant be lumped with other electric motors without a hard look.
,,,My 1st exposure came from Dad telling me that a starter motor was a series wound motor and should not be run without load because there was nothing to limit its speed and the rotor could explode. We watched the motor kick
hard when connected - and then rev up to high speed. You could hear tho that the speed was still increasing slowly. Dad said that in real life there actually was a load - the bearings and air resistance inside the motor. To a little kid this made enuf sense. Iv always marveled at those concise points he made. The deeper issues I learned later never sullied their accuracy.
The key to its characteristic is that the series wound motor has rotor windings/coils and field coils in series and thus the same amount of current flows in each. When the motor is stationary the low resistance of the copper coils allows a huge inrush current to flow. The rotor and field are
strong and maximum torque is produced.
Great for starting a reluctant load.
,,,As the motor accelerates a back EMF develops causing current to drop. Magnetics of the rotor and field are weaker and torque drops. This effect continues as rpm increases. Motor and load torque come into balance at some point and thats the operating RPM. The setup will slow down to a new balance point if more torque is needed by the load.
As an aside: it happens that while a strong field enables torque, a weak field enables speed. -- A DC permanent magnet motor speed limits due to its always very strong field and is more speed stable under load for the same reason. ... The universal motor demonstrates this as well, but must slow to get its stronger field. On the other side, the series wound U motor , speeding up, gets weaker while at the same time loses the field strength that would naturally suppress its speed. Even with this "weakening " the condition of NO load will result in steadily increasing speed. Both DC and Universal motors require adaptive control to stabilize speed well.
An AC induction motor speed is governed by the frequency of the power; for 60HZ - 60 rps -- 3600 rpm, or even numbered sub multiples. Torque rises steeply as the motor drops below frequency lock. Nominal HP may be produced around 3% slip - ~ 3500rpm ... Maybe double that HP at 3400rpm / 6%slip. Torque continues to rise even more as it slows til it finally breaks down and speed plummets. Im not sure where that occurs. Maybe about 10% slip. :confused3:
The characteristic efficiency of U motors is low -- ~30% vs 70% for an induction motor. When you add that into the mix it tends to underscore the handicap a universal motor has in producing more power as it slows. ... The U motor will
consume the watts but wont give them back as mechanical HP.
