Electrical Motor Slip

Overview

Engineering reference data for Electrical Motor Slip in electrical engineering.

Key Formulas

Ohm's Law

$V = IR$

Voltage = Current × Resistance.

Power

$P = VI = I^2R = V^2/R$

Electrical power.

Energy

$E = Pt$

Energy = Power × Time.

Variables

Slip Variation with Motor Size

Electrical induction motors exhibit slip characteristics that vary significantly with motor size. Smaller motors typically experience higher slip percentages under full-load conditions.

Slip and Motor Characteristics

Slip Under Load

Slip increases proportionally with the mechanical load on the motor shaft. This increased slip allows the rotor bars to cut more magnetic flux lines, which in turn generates the greater torque required to drive the heavier load.

Starting Conditions

When an induction motor is initially energized and the rotor is stationary, the slip is at its maximum value of 100%. Under this condition, the motor draws the highest current from the supply (locked-rotor current).

Slip and Motor Current

As the rotor begins to accelerate and the slip decreases, the motor current also decreases toward its full-load running value. The relationship is inverse: lower slip corresponds to lower current draw.

Slip Effects on Electrical Parameters

Slip Frequency

The frequency of the currents induced in the rotor is directly proportional to the slip. This is called the slip frequency. $f_{\text{slip}} = s \cdot f_{\text{supply}}$ where $f_{\text{supply}}$ is the stator supply frequency (e.g., 50 Hz or 60 Hz).

Slip and Inductive Reactance

The inductive reactance ($X_L$) of the rotor windings depends on the slip frequency. $X_L = 2\pi f_{\text{slip}} L$ where $L$ is the rotor inductance.

Slip and Rotor Impedance

The total impedance of the rotor circuit is the vector sum of its constant resistance ($R$) and its slip-dependent inductive reactance ($X_L$). $Z_{\text{rotor}} = \sqrt{R^2 + (s \cdot X_{L,\text{max}})^2}$ where $X_{L,\text{max}}$ is the reactance at standstill (slip=1).

Starting (High Slip): The impedance is predominantly inductive, resulting in a low lagging power factor and high current. Running (Low Slip): The inductive reactance decreases, and the power factor improves, approaching unity as the resistance component becomes dominant in the impedance.

Interactive Charts

References

• Original Source