synchronous speed

In an AC induction motor, synchronous speed is the rotational speed at which the stator's magnetic field turns inside the machine. This speed is determined entirely by the AC supply frequency and the number of magnetic poles wound into the stator. For a motor running on 60 Hz single-phase or three-phase power with two poles, the synchronous speed is 3600 rpm; with four poles it drops to 1800 rpm. The relationship is fixed: synchronous speed in rpm equals 120 times the frequency in hertz, divided by the number of pole pairs.

The reason synchronous speed matters is that an induction motor's rotor never actually rotates at this theoretical speed. The rotor lags behind the rotating magnetic field by a small amount called slip, typically 3 to 5 percent under full load. This slip is what allows the motor to develop torque: the relative motion between the field and the rotor induces current in the rotor bars, creating the force that turns the shaft. Remove the slip and the motor produces no torque at all.

The term synchronous comes from synchronous motors, which do rotate at exactly the same speed as the magnetic field, requiring special construction with permanent magnets or a DC winding to lock the rotor to the field. Induction motors, by contrast, are asynchronous: they always run slightly slower than synchronous speed. Despite this, engineers still reference synchronous speed when describing an induction motor's operating point, since it sets the baseline from which slip is measured and it determines the motor's available speed range.

In industrial settings, knowing the synchronous speed is essential for motor selection and load matching. A 60 Hz four-pole motor has a synchronous speed of 1800 rpm and full-load speed around 1750 rpm. A two-pole version of the same motor hits 3600 rpm synchronously, with full-load closer to 3500 rpm. Many industrial processes require specific output speeds; choosing the wrong pole count means either running a geared reducer or accepting mismatched speed at the driven load.

Frequency conversion adds a layer of complexity. In regions or facilities using 50 Hz power, a four-pole motor's synchronous speed is 1500 rpm, not 1800. Variable frequency drives (VFDs) change both the frequency and voltage sent to the motor, allowing its synchronous speed to be adjusted from near zero to well above the nameplate speed. The motor's actual speed still lags by slip, but now that slip occurs at whatever frequency the drive is outputting.