Question

1. A rotor spins with 71kJ of energy. If plugging is used to stop the motor, how much heat will the rotor have to dissipate (numeric answer only in k-Joules)?

2. A rotor spins with 55kJ of energy. If DC Braking is used to stop the motor, how much heat will the rotor have to dissipate (numeric answer only in k-Joules)?

3. A technician has a 600:5 CT and an ammeter that is scaled 300A for 5A input. If the technician runs two primary turns through the CT and wires its secondary to the ammeter, what will the meter read when the load current is 252A (numeric answer only in amps)?

4. A 10 pole SCIM is rated 500HP, 600V, 60Hz, 590A. At what approximate voltage should its stator operate at when driven at 28Hz off a variable frequency drive (numeric answer only volts)?

5, An 8 pole SCIM is rated 1000HP, 1,127V, 60Hz, 260A. At what approximate voltage should its stator operate at when driven at 80Hz off a variable frequency drive (numeric answer only volts)?

Answer 1

I will answer the first two questions.

1) The heat Dissipated in the rotor during plugging period (initial rated speed to zero speed) is three times the original kinetic energy of all the revolving parts.

Therefore in the given problem the kinetic energy of rotor is 71 KJ.

Heat Dissipated in the rotor during plugging= 3 × 71 =213 KJ.

2) In DC braking, the heat dissipation in the rotor is equal to the original kinetic energy stored in the rotor.

In given problem rotor spins with 55KJ of energy.

Therefore heat dissipation in the rotor = 55KJ.

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