# inertia of the motor.....

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could any1 xplain the principle behind the synchronous motor....i need to know wat s meant by inertia of the motor.....

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We installed a synchronous inertia motor at a NexRad site by Nome, Alaska in about 1997. The motor drove a generator so all the incoming utility power had to go through the motor generator set. The motor had a large fly wheel that stored enough energy that when the power went off the wheel would keep turning long enough to maintain power through the generator for the time needed for the diesel generator to come online, a period of about 6 seconds. This was a good design since the NexRad site was unmanned and a battery UPS could be subject to extreme cold temperatures.

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It means that once you start the rotor turning it will tend to keep going. This is more important in split phase and capacitor motors than true synchronous motors like you see on 3 phase but all AC motors will try to lock to the line frequency with the speed based on the number of poles in the winding. Some designs do a better job than others

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| could any1 xplain the principle behind the synchronous motor....i need | to know wat s meant by inertia of the motor.....

Inertia has two effects on the motor. The rotor has mass, so it has inertia. When at rest, it wants to stay at rest. When a field tries to make the rotor turn, it "fights back". When the rotor is turning and the field is removed, the rotation generates electricty back. If you simply disconnect the motor, it can continue rotating for a while. If you connect the motor leads to a load (for example some light bulbs), that will brake the motor and slow it down at a faster rate. Basically, the rotating inertia generates "back EMF".

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Inertia in physics is a "reluctance to change of velocity". When at rest an object is reluctant to move. To get that object to move in a given direction at a given speed requires a certain amount of energy.

In this case the object is the motor rotor. If the rotor is heavy it will require more energy to get it spinning to the speed that's required, therefore the rotor has high inertia. If the rotor is light in relative terms it will require less energy to get it spinning. (less inertia)

Therefore high inertia motors can't have a load connected during starting for fear of physical damage, that's ignoring the poor starting torque characteristics of induction motors. The principle also applies when the rotor is turning at 'normal' speeds during operation.

Newsey

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----------------- While others have discussed inertia, it may help if you detail where your problem lies- starting?, interaction with the system, etc. The specific "synchronous motor" factor is that a synchronous motor must run at synchronous speed . At any other speed the torque will pulsate and have an average of 0. Such torque pulsations can cause severe damage as the machine tries to break free from its mounting bolts and go walkaround. This seriously limits starting because, due to inertia (due to its mass), it can't accelerate from 0 to synchronous speed before the torque reverses -not a good thing. Hence the need to find some way to bring it to near synchronous speed-such as induction motor action - and then energise its field.

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| We installed a synchronous inertia motor at a NexRad site by Nome, | Alaska in about 1997. The motor drove a generator so all the incoming | utility power had to go through the motor generator set. The motor | had a large fly wheel that stored enough energy that when the power | went off the wheel would keep turning long enough to maintain power | through the generator for the time needed for the diesel generator to | come online, a period of about 6 seconds. This was a good design | since the NexRad site was unmanned and a battery UPS could be subject | to extreme cold temperatures.

What is the spin-up time and power for these? I was looking at some of them from, I believe it was, Liebert a while back, but could not find any specs on spin-up. I would think if they can keep reasonable power flow and frequency at capacity for 6 seconds, they would either need some huge starting power, or a very long spin-up time.

I didn't think about it at the time I looked at these, but when I later realized the low fault currents from inverters and UPSes, I now wonder what kind of fault currents these can deliver, as unlikely as they would ever be called upon to deliver fault current to a short that just happens in that narrow 6 second time frame.

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