Every AC induction motor runs slower than its synchronous speed. The difference is called slip, and it is not a defect — it is the fundamental operating principle of the induction motor. Without slip, the rotor bars would see no changing magnetic field, no current would be induced, and no torque would be produced. Understanding slip is essential for correctly sizing motors, predicting actual shaft speed, and troubleshooting motor performance issues.
Slip varies with load, motor design, and supply voltage. A motor running light might show 1 percent slip while the same motor at full load operates at 3 to 5 percent slip. The rated slip at full load is stamped on the nameplate as the full-load RPM value, and it tells you a great deal about the motor's torque characteristics and efficiency class.
This guide covers synchronous speed calculations, slip percentages, NEMA design classifications, and practical guidance for selecting the right motor design for different load types.
Synchronous Speed and Pole Count
Synchronous speed is the speed of the rotating magnetic field produced by the stator windings. It is determined entirely by the supply frequency and the number of magnetic poles: N_sync = 120 x f / P, where f is frequency in Hz and P is the number of poles.
For 60 Hz systems, the common synchronous speeds are: 2-pole = 3600 RPM, 4-pole = 1800 RPM, 6-pole = 1200 RPM, 8-pole = 900 RPM, 10-pole = 720 RPM, and 12-pole = 600 RPM. For 50 Hz systems, multiply each value by 50/60 (e.g., 4-pole at 50 Hz = 1500 RPM).
The number of poles is fixed by the stator winding configuration and cannot be changed in the field. A motor designed as a 4-pole machine will always have a synchronous speed of 1800 RPM on a 60 Hz supply. The actual shaft speed will be lower by the amount of slip.
When a VFD changes the supply frequency, synchronous speed changes proportionally. A 4-pole motor fed at 45 Hz has a synchronous speed of 1350 RPM. The slip percentage remains approximately the same as at 60 Hz, so the actual shaft speed would be roughly 1350 minus the slip RPM.
Calculating Slip and Its Significance
Slip is expressed either in RPM or as a percentage. Slip RPM = N_sync - N_actual. Slip percentage = (N_sync - N_actual) / N_sync x 100. A 4-pole motor with a nameplate speed of 1750 RPM has a slip of 50 RPM or 2.78 percent at full load.
Slip increases with load. At no load, slip is nearly zero and the motor runs very close to synchronous speed. As mechanical load increases, the rotor must slow down relative to the stator field to induce more rotor current and produce more torque. This is a stable equilibrium: if the load increases, the rotor slows, slip increases, more torque is produced to match the load.
High-efficiency motors (NEMA Premium) have lower slip than standard motors because their rotor bars have lower resistance. A premium efficiency 4-pole motor might have a nameplate speed of 1770 RPM (1.67 percent slip) compared to 1745 RPM (3.06 percent slip) for a standard efficiency motor. This matters when replacing a standard motor with a premium motor on a fan or pump — the higher actual speed can increase power consumption by 2 to 5 percent due to the affinity laws.
If a motor's actual speed under load is significantly lower than the nameplate speed, the motor is overloaded, the voltage is low, or one phase may be lost. A motor running at 1680 RPM when the nameplate says 1750 RPM is drawing excessive current and overheating.
NEMA Motor Design Classifications
NEMA classifies induction motors by their torque-speed characteristics using design letters A, B, C, and D. Each design has different starting torque, breakdown torque, starting current, and slip characteristics tailored to specific load types.
Design B is the general-purpose workhorse, used in roughly 90 percent of industrial applications. It provides normal starting torque (100-200 percent of full-load torque), moderate starting current (600-700 percent of full-load current), and full-load slip of 1 to 5 percent. Fans, centrifugal pumps, machine tools, and conveyors all use Design B motors.
Design A is similar to Design B but with higher breakdown torque and higher starting current. It is used where brief overloads are expected and the power supply can handle the inrush. Design A motors are less common and are gradually being replaced by Design B in most applications.
Design C provides high starting torque (200-250 percent of full-load torque) with moderate starting current and slip similar to Design B. It is specified for hard-to-start loads like loaded conveyors, reciprocating compressors, and crushers — applications where the motor must accelerate a high-inertia load from a dead stop.
Design D has very high starting torque (275+ percent of full-load torque) and high slip (5-13 percent at full load). The high slip acts as a cushion for impact loads like punch presses, shears, and hoists. The tradeoff is lower efficiency and more heat generation due to the higher rotor resistance. Design D motors are uncommon outside specialty applications.
Selecting the Right Motor for the Load
Motor selection starts with the load requirements: horsepower (or kW), speed, torque profile during starting and running, duty cycle, and environmental conditions. The required horsepower at the shaft determines the motor rating, with a service factor margin. Most industrial motors carry a 1.15 service factor, meaning they can operate continuously at 115 percent of nameplate HP under standard conditions.
Match the motor speed to the driven equipment. If the equipment needs 1200 RPM, select a 6-pole motor rather than a 4-pole motor with a speed reducer — the direct-drive approach is simpler and more efficient. If an intermediate speed is needed (say 900 to 1200 RPM range), a VFD on a 4-pole motor provides exact speed control with no mechanical losses from belts or gearboxes.
Consider the starting conditions. A Design B motor on a centrifugal pump starts under low torque because the pump is unloaded at zero flow. But the same motor connected to a loaded screw conveyor must produce high torque from the first revolution. If the starting torque requirement exceeds 150 percent of full-load torque, consider a Design C motor or a soft starter that ramps voltage to control acceleration.
Voltage and phase must match the available power supply. Most industrial motors above 1 HP are three-phase, available in 208V, 230V, 460V, or 575V. Higher voltage means lower current for the same power, which allows smaller wire and reduces I²R losses in long cable runs. A 460V motor draws half the current of a 230V motor at the same horsepower.
Slip Behavior on VFD-Driven Motors
When a motor runs on a VFD, the supply frequency varies and the slip behavior changes in subtle ways. At frequencies below the motor's base frequency (typically 60 Hz), the VFD maintains constant volts-per-hertz ratio, producing approximately constant flux and constant available torque. Slip in RPM stays roughly the same as at 60 Hz, but slip as a percentage of synchronous speed increases at lower frequencies.
Above base frequency, the VFD cannot increase voltage beyond its DC bus limit, so flux weakens as frequency rises. Available torque decreases with the square of the frequency increase. This is the constant-power region and is used primarily for spindle drives in machine tools, not for general industrial pump or fan applications.
Modern vector-controlled VFDs compensate for slip in real time. They measure or estimate the actual rotor speed and adjust the output frequency to maintain the commanded speed precisely. This is called slip compensation and is essential for applications requiring accurate speed control, such as winders, extruders, and CNC feed drives.
One practical consideration: running a motor at very low frequencies (below 10-15 Hz) reduces the self-cooling airflow from the shaft-mounted fan. Below about 20 percent speed, a separate blower motor may be needed to prevent overheating, especially under sustained full-torque loads.