Quote:
Originally Posted by cts_casemod
Thats just what the literature tells you. But from that you cant calculate the torque.
let me clarify:
Theory says that torque falls by the square of the voltage, so technically at twice the nameplate speed one would have 25% torque as the V/Hz ratio has now fallen to 50%.
Some sources disagrees and state that you'll have half of the torque, since the horsepower is constant. Clearly something is not right here. How can you have the same horsepower if the torque has fallen to 25 and the speed has doubled!? Perhaps one could double the slip, which makes a lot of sense, but this is never mentioned in technical papers.
The idea is to keep the rotor power constant. For that, if the motor is operating at half the V/Hz ratio, one doubles the rotor frequency, by increasing the slip.
The net result would in fact be constant torque, since the primary V/Hz ratio and hence the flux has fallen to 1/2, but the rotor flux is kept constant, but I haven't found any sources mentioning anything about the slip, so for the time being, this is just me putting 2+2 together.
Adding to the problem is how reluctance will play here. Not only the V/Hz ratio is increasing, but the impedance of the motor (XL) is also increasing. More impedance = less current.
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I have a similar question relating the output power of a motor, related to the bus voltage. The reason I'm wondering about this is because I would like to know how the motor's performance is compromised - and not just top speed - if the bus voltage is reduced. This will be a significant help in my battery calculations.
Theory says P = IV. Great, so if we hold current constant, we should get a simple linear reduction in power if we linearly reduce voltage - right? Where this seems to fall apart is exactly where it should be textbook easy. If you're lucky, the motor supplier or controller supplier will provide a torque - speed curve based on a voltage. They may even provide a power - speed curve as well.
For my motor, I'm relying on the data produced by ORNL's testing of the 2010 Prius. For the Prius motor, the torque output is pretty much linearly ( in fact about 1:1 ) related to current. In the region before the base speed, torque output is constant. BEMF has little to do with this because it's almost negligable at low speed - about 10V or 20V at 500rpm. Additionally, they did all these tests at the same switching frequency (5kHz) and used their own inverter so they knew inverter output wouldn't affect the data.
Motor stator resistance is very low ( about 0.7 Ohms L-N ) and the inductance is fairly low at about 1mH. ( Ld and Lq are different - I can't find exact numbers) For all the tests, efficiency was in the 90% range.
Now for the strange part:
At 650Vbus, the torque output is 200Nm from 0 to about 2000 rpm.
At 500Vbus, the torque output is 150Nm from 0 to about 2000 rpm.
At 225Vbus, the torque output is 120Nm from 0 to about 1000 rpm.
This is not linear. Any suggestions about what is going on?
-E*clipse