Sequence of Operation for 200 HP motor 1

[gpape]

I have a 200 HP Reliance motor, Model No. P44G5261, on a vertical sump pump that maintenance wants to operate as an on/off pump in case of heavy rain. Based on operation sequence, can anyone offer information on the pros and cons of going on/off versus VFD? Sump liquid is waste water with around 1.2 cP viscosity.

 

[electricpete]

There is an intuitive proof. It requires analysis of a parallel situation of braking.

Let's say the motor is spinning at no-load speed. Now apply a dc current to the stator to create a stationary magnetic field (again no stator resistance).

There can be no work put into or out of the power system by the dc voltage since the stator field is stationary and rotor is rotatying.

The rotor will come to a stop. We have put no energy into or out of the motor from the power system as discussed above, so where did the rotor ke go? It will have to be dissipated in the rotor resistance. Therefore for this dc braking situation we can prove by conservation of energy that the energy dissipated in the rotor heating is equal to the initial rotor kinetic energy.

Now compare that situation to a rotor start and you will seee they are the same except for the frame of reference (rotating or stationary) and we expect the heating dissipated in the rotor the same either way.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

I need to summarize my last few posts.

You cannot apply conservation of energy directly to a motor start to determine rotor heating = rotor KE.

But you can apply conservation of energy to the parallel situation of rotor braking using dc stator voltage to conclude rotor heating = rotor KE.

Analysis of these two scenario's shows that rotor heating should be the same so we can apply the braking result to the starting scenario.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.