Current on rotor connected to rheostat

[philly]

At full load torque, the rotor will take full rated rotor current. This is often given on the motor nameplate or data sheet.
To a good approximation, you can get it from P = sqrt(3) x Vr x Ir/1000
where
P is the motor power in kW
Vr is the rotor open circuit voltage
Ir is the rotor current at full load.

For example, we have a drive on a 3,065 kW machine with a stator voltage of 11kV and a rotor voltage of 2,150V. The above calculation gives a rotor current of 823A. This is pretty close to the nameplate value of 814A

So with this I guess your saying that the power in the stator is equal to the power in the rotor minus any losses between the two. The power in the stator is a function of stator voltage and current, while the power in the rotor is a function of the rotor voltage and current, with the voltage in the rotor being determined by the open circuit rotor voltage (commonly referred to as "a") and the slip.

So when the rotor takes on full load rotor current as mentioned above, then all of this current is traveling through the rotor, brushes, and cable to the rheostat in an application with a rheostat?

Do motor datasheets other then wound rotor motors typically give all this secondary information such as rotor voltage, rotor resistance, and rotor inductance? I know the datasheets usually have the primary stator resistance but I've never seen the primary stator reactance or inductance since its frequency dependent. I'm curious how you build the motor model circuit and plug in all the values from information given on a datahseet. Or do you need more?

 

[LarryFine]

So with this I guess your saying that the power in the stator is equal to the power in the rotor minus any losses between the two.

Sounds a lot like a transformer, doesn't it?

Added: However, with the controller on full speed, there's very little voltage across the secondary, so even high current will calculate as not a lot of power; most of the "power loss" is actually expended by the motor rotation.

 

[Besoeker]

So with this I guess your saying that the power in the stator is equal to the power in the rotor minus any losses between the two.

Only at standstill. When the motor is spinning, you have to consider the mechanical output power and at rated speed the mechanical output accounts for the majority of it.

So when the rotor takes on full load rotor current as mentioned above, then all of this current is traveling through the rotor, brushes, and cable to the rheostat in an application with a rheostat?

Yes, it is. But if the shorting contactor is closed, there is little power in the circuit external to the rotor. Other than conductor losses.

Do motor datasheets other then wound rotor motors typically give all this secondary information such as rotor voltage, rotor resistance, and rotor inductance? I know the datasheets usually have the primary stator resistance but I've never seen the primary stator reactance or inductance since its frequency dependent. I'm curious how you build the motor model circuit and plug in all the values from information given on a datahseet. Or do you need more?

For the motor model, you need the equivalent circuit values:
X1
R1
Xm
Rm
X2'
R2'
Plus Windage & Friction losses.

This information is not generally given on the regular datasheets. If you know efficiency and power factor at a number pof points (typically 2/4, 3/4, and 4/4 of rated power) it is possible to estimate equivalent circuit values.