EASA Rules of Thumb for Three-Phase Motors — Full Professional Guide

This guide provides detailed explanations, practical examples, and worked calculations for three-phase induction motors, based on a 55 kW example motor. It is intended for engineers, technicians, and maintenance teams to ensure safe, reliable, and efficient motor operation according to EASA recommendations and industry best practices.

Example Motor Specification

• Rated Power: P_rated = 55 kW = 55,000 W
• Line Voltage: V_LL = 415 V (3-phase)
• Frequency: f = 50 Hz → 4 poles → N_sync = 1500 rpm
• Efficiency: η = 0.93
• Power Factor: PF = 0.90

1. Voltage Variation — Concept & Calculation

Explanation: Voltage variation measures how much the supply voltage deviates from the motor's rated voltage. Small deviations are normal, but large variations can cause overheating, reduced torque, or even insulation breakdown over time. For reliable operation, maintain voltage within ±10% of rated voltage; ±5% is ideal.

%Variation = (V_meas - V_rated) ÷ V_rated × 100

Example: V_meas = 385 V, V_rated = 415 V → %Variation ≈ -7.23%

2. Voltage Unbalance — Effects & Calculation

Explanation: Voltage unbalance occurs when the three-phase supply voltages differ in magnitude. This creates negative-sequence currents in the motor, which can lead to overheating, reduced torque, and premature bearing or insulation failure. Keep unbalance below 1% if possible; up to 2% is acceptable in practice.

V_avg = (V_a + V_b + V_c) ÷ 3
V_unb% = max(|V_a - V_avg|, |V_b - V_avg|, |V_c - V_avg|) ÷ V_avg × 100

Example: Va=418 V, Vb=410 V, Vc=420 V → Vavg=416 V, Vunb% ≈ 1.44%

3. Current Variation — Monitoring Load

Explanation: Current variation shows changes in motor load. Sudden increases may indicate mechanical blockage, short circuits, or overload. Small variations under normal load are expected; a change above ±15% should trigger inspection.

• ±10% variation is considered normal under varying load.
• Sudden >15% indicates potential mechanical or electrical fault.

4. Current Unbalance — Formula & Sample

Explanation: Unequal current in phases increases heating in one phase, which can damage insulation and reduce efficiency. It often results from unequal supply voltage, faulty connections, or asymmetrical loads.

I_avg = (I_a + I_b + I_c) ÷ 3
I_unb% = max(|I_a - I_avg|, |I_b - I_avg|, |I_c - I_avg|) ÷ I_avg × 100

Example: Ia=95 A, Ib=87 A, Ic=94 A → Iavg=92 A, Iunb% ≈ 5.43%

5. Full-Load Current (FLC)

Explanation: FLC is the steady-state current the motor draws at rated voltage and full load. It is used to set overload protection and evaluate energy consumption.

I_FL = P_rated ÷ (√3 × V_LL × η × PF)

Example: 55 kW, 415 V, η=0.93, PF=0.90 → I_FL ≈ 91.42 A

6. Locked-Rotor Current (LRC)

Explanation: LRC is the current drawn when the rotor is stationary at start. Protection devices must allow this short-duration high current without tripping. Typical values are 5–8 times FLC.

I_LRC ≈ 5 ~ 8 × I_FL

Example using 6×: I_LRC ≈ 548.5 A

7. Instantaneous Starting Spike

Explanation: At motor start, a short-duration spike occurs due to magnetizing current. It lasts only a few milliseconds but may reach 10–14× FLC. Protection devices and breakers must tolerate this brief surge.

I_spike ≈ 10 ~ 14 × I_FL

Example: I_spike ≈ 914 A (very short duration)

8. Locked-Rotor / Stall Energy (I²t)

Explanation: I²t measures thermal energy absorbed by the motor during start or stall. It is useful to compare with full-load energy to ensure thermal protection settings are adequate.

E_start = I_LRC² × t_start
E_FL1h = I_FL² × 3600
Ratio = E_start ÷ E_FL1h

Example: I_LRC=548.5 A, t=10 s → Ratio ≈ 0.10 → start energy ≈ 10% of 1-hour full-load energy

9. Number of Allowable Starts

Explanation: Each motor start causes heating. Frequent starts shorten insulation life. Use conservative limits or manufacturer duty class.

• Cold motor: 2–3 starts/hr
• Hot motor: 1–2 starts/hr
• Check manufacturer data for exact duty cycle

10. Slip — Definition & Formula

Explanation: Slip indicates how much the rotor lags behind synchronous speed. It is a measure of rotor speed relative to magnetic field speed, usually a few percent for induction motors under normal load.

s (rpm) = N_sync - N_rated
s (%) = (N_sync - N_rated) ÷ N_sync × 100

Example: N_sync=1500 rpm, N_rated=1460 rpm → s ≈ 40 rpm, s% ≈ 2.67%

Quick Reference — Rounded Values (55 kW Motor)

Protection Guidelines

• Set thermal overload relays to FLC × service factor to protect windings.
• Instantaneous protection must tolerate start currents without unnecessary tripping.
• Use reduced-voltage starters or VFDs for frequent starts or high-inertia loads to reduce mechanical and electrical stress.

On-Site Checklist

1. Read motor nameplate: P, V, FLC, rated RPM.
2. Measure Va, Vb, Vc and Ia, Ib, Ic; record ambient and bearing temperatures.
3. Calculate voltage variation, voltage unbalance, current unbalance; compare to recommended limits.
4. Measure start current and acceleration time if safe to verify against expected values.
5. Check protection settings and cooling times between consecutive starts.