The relationship between the motor’s full-load speed and its actual speed
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Discuss related issues based on a case study. There is an induction motor, and the parameters from its data sheet are as follows: rated capacity of 500 HP, 2 poles, 3-phase voltage of 4000 Volts, frequency of 60 Hz ； Full load: 3578rpm, 995Nm, 66.7A, efficiency 95.2%, power factor 0.848 ； 75% load: 3583 rpm, 51.2 A, efficiency 94.8%, power factor 0.832. This motor was used to drive a water pump for testing, and the following test parameters (speed/power) were obtained: 3431 rpm/328 kW, 3434 rpm/301 kW, 3434 rpm/292 kW, 3434 rpm/256 kW, 3438 rpm/191.6 kW, 3440 rpm/149 kW. The following questions pertain to situations where frequency conversion is not used; the frequency is assumed to be 60 Hz: 1. Are the values listed on the motor’s data sheet theoretical calculations, or are they test results from prototypes with the same power? 2. When the load is below full load, the rotational speed increases gradually. Why? So no-load should be the maximum speed of this motor. 3. During actual testing, as the load decreases, the power drops while the speed increases. However, it is always below the full-load speed of 3578 rpm, indicating that the actual full-load speed of the motor cannot reach 3578 rpm. Does that mean the actual full-load torque is greater than 995 Nm? (Provided that the rated power is the same) 4. Are the actual speed, torque, and power of a motor caused by factors such as manufacturing errors? If prototype testing reveals a full-load speed that differs from the value stated in the data sheet, why not adjust the full-load speed value accordingly? After all, 3578 rpm seems to have little practical significance, and it deviates significantly from the actual value. 5. For a motor that has been manufactured, does the speed of the motor under load depend on the load or on the power supply parameters (actual voltage, frequency, current)? Is it torque balance first, followed by matching of the rotational speed, or is the rotational speed kept relatively constant while the power supply automatically adapts to the load characteristics?
75% load: 500*0.7457X0.75=279.6kw; the actual speed at this power level is 3434 rpm, which is lower than the theoretical speed of 3583 rpm. Is the output torque constant at a constant rotational speed? If it remains constant, then the power also remains constant? The actual load decreases, and so does the power. Based on the motor’s starting torque-speed curve, shouldn’t there be a corresponding torque (power) at each speed level below the full-load torque? That is, as the power output decreases, both the output speed and torque decrease as well. During the process of decreasing output power, does a situation of constant speed or constant torque output occur?
The values in the data table are calculated values (they also correspond to the measured values stated in the type test report, but this does not mean that the actual parameters of each motor necessarily match those listed on the nameplate)
2. Regardless of the load, the rotational speed increases gradually; it is independent of whether the machine is running under no load or at full load, and depends only on the start-up time
The startup process involves a gradual increase in rotational speed. At steady-state operation, are the rotational speeds the same for 50% and 75% load rates? That is, the actual speed will vary depending on the load level (although the difference is not significant). So, is this difference in rotational speed the result of torque balance? Or is the rotation speed unrelated to the load and depends only on the power supply?
The rotational speed is certainly related to the load; a higher load increases the slip rate, which in turn causes the actual speed to deviate from the value indicated on the nameplate, though the difference is not significant. Since torque is proportional to the square of the terminal voltage, it follows that the rotational speed is also related to the terminal voltage.