Hydraulic-Electric Analogies: Torque-Speed Behavior, Part 4

The torque-speed curve also shows the difference between the synchronous and rated speed. This speed difference is necessary to keep relative speed between the rotating stator field (synchronous speed) and the actual rotor speed. The speed difference is called the slip of the motor, because it represents slippage between synchronous and actual speed. It’s necessary to create more induced rotor current to boost output torque with increased load.

Now consider what would happen if the rotor catches up to the stator field. There would be no relative speed between the rotor conductors and the rotating field, no induced voltage, and no rotor current/torque developed by the motor. In fact, this can happen only in an ideal motor; that is, a motor with zero friction. Because zero friction is impossible, an induction motor’s rotor can never reach synchronous speed in normal operation.

Popular jargon often states that “the (nominal) motor speed is 1,800 rpm” in a four-pole motor. However, that speed is really the synchronous speed of the stator field. The result is that a four-pole motor with a totally unloaded shaft will accelerate to near synchronous speed, about 1,795 rpm.

When the motor shaft is loaded with some external torque, there’s a decrease in speed, an increase in the relative speed (inducing voltage into the rotor), and an increase in rotor current. The effects of the rotor’s rotating magnetic field changes the phase of the voltage induced back into the stator, elevating the power delivered to the stator and in the end, delivering more power to the shaft load. The rotor’s increased magnetic field “tells the stator” that the shaft load has risen.

Alternator function in the induction motor

Bilateralism in the induction motor is the phenomenon that allows the electrical power source to “know” that the motor is experiencing a higher load, and that current should be increased to meet the rising output-power need. But bilateralism also means that the machine can operate as either a motor or an alternator to meet changing needs and operating conditions. Can an induction motor function as an alternator, and if so, what are the conditions? Figure 13 answers that question.

If there is a load, sometimes called an “active load,” it overpowers the motor, causing the motor speed to increase above synchronous speed. Under this condition, the motor simply switches to become an alternator and sends the shaft input power to electrical power, which is then sent back onto the electrical grid to be used by other machines on the bus. Such loads, called over-running loads, cause mechanical energy to be sent into what is normally the output.

The ability for induction motors to function bilaterally—that is, as either motor or alternator—is an important energy-saving feature. Some hydrostatic transmissions, for example, have the same bilateral capability. However, in that case, it regenerates energy only when used with electrical prime movers. Internal combustion engines are not capable of such bilateral operation. That would be like a vehicle consuming fuel to climb a grade, but putting fuel back into the tank when the vehicle goes back down the grade. This is absurd!

The nameplate on an induction motor always gives the rated frequency in Hz, the rated output power in hp or kW, rated voltage and current, and the full load speed. Full load speed of a four-pole induction motor is typically between 1,700 and 1,750 rpm.

The difference between the synchronous speed and the actual shaft speed is, as mentioned earlier, “the slip.” The “communication” that takes place between the stator windings and their fields and the rotor conductors and their fields lets the stator “know” that a load on the shaft must be overcome, which requires increasing the amount of input power. Both electrical and hydraulic machines share this feature, albeit by quite different physical means.