12-1 Induction motor characteristics The starting current for an induction motor is several times the running current and the starting pf is much lower than the pf at rated speed. Both of these features tend to cause the supply voltage to dip during sta rt-up and can cause problems for adjacent equipment. The torque -speed/slip characteristic of a general purpose induction motor is shown below along with a typical mechanical load torque. The rated torque is usually slightly smaller than the starting to rque so that loads can be started when rated load is applied. The curve has a definite maximum value which can only be supplied for a very brief period since the motor will overheat.
The motor/load combination will operate at the intersection of their torque -speed curves. For general purpose induction motors this will typically correspond to a value of slip in the range of 0.02 to 0.08. When slip is relatively small, torque will be proportional to slip, and this feature is commonly used when determining the motor load and operating speed.
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12-2 Example A three-phase induction motor is rated: 2.4 kV, 200 hp, 3456 rpm. It is used to drive a compressor rated: 250 hp, 3600 rpm. The power input of the compressor is directly proportional to speed3 (speed cubed).
The Essay on Motor Speed Control
Introduction and Objectives In this lab you will control the speed of a motor. Figure 1 shows the hardware setup, which is the same as for Week 1 of Lab 4. You will use the potentiometer on your evaluation board to set the desired speed of the motor, and you will control the speed through the PWM output of the HCS12. You will measure the speed of the motor using an input capture pin, and display ...
Neglect rotational losses and determine the hp output of the motor and its speed in rpm when connected to the compressor.
Motor torque Tout & TL operating point
Load torque T L = K L n2
Motor torque Tout = Kout s 0 1.0 Speed Slip ns 0
Solution Ignoring rotational losses means P out = P D and Tout = TD. We have to determine the x-y co-ordinates of the operating point as shown in the figure. This means that we have to build equations for torque vs speed for the motor and the compressor. The power input to the compressor is proportional to speed cubed.. This means that the torque is proportional to speed squared. Rated conditions
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12-3
Non-rated conditions
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12-4 Important characteristics of induction motors
Starting current
Magnitude of Input Current
Rated current
No-load current Rated speed 0 1.0 Speed Slip ns 0
Starting current is several times larger than the rated current since the back emf induced by Faraday’s law grows smaller as the rotor speed increases.
Rated pf
The pf may also be plotted against speed. Note the drop in power factor past rated conditions. If motors are underloaded excessive pf correction will be required.
Power Factor Rated speed
0 1.0
Speed Slip
ns 0
Rated efficiency
The efficiency-speed curve starts at zero and rises nearly directly proportional to speed until max. η is reached. Efficiency then drops sharply as the motor approaches no-load. Installing an oversized motor wastes power.
Efficiency Rated speed
0 1.0
Speed Slip
ns 0
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