Home› Level 1› Unit 4› Section 2› 2.6 Effects of Load & Voltage Changes
Unit 4 — Electrical Fundamentals
Section 2 — Introduction to Motors

2.6 — Effects of Load & Voltage Changes

Motor current, temperature, efficiency, and power factor all shift in predictable ways when mechanical load or supply voltage changes. Understanding these relationships allows technicians to diagnose overloads, voltage problems, and sizing issues from measured electrical quantities alone.

Jump to

2.6.1 — Effects of Load Changes

An induction motor is a self-regulating device: it draws exactly the current needed to produce the torque required by its mechanical load. As the shaft load increases, slip increases, the rotating magnetic field induces more rotor current, and the stator automatically draws more current from the supply. Every electrical and thermal parameter shifts as a result.

⇧ Increased Load
Shaft speed ↓ Decreases
Slip ↑ Increases
Stator current (amperes) ↑ Increases
Input power (watts) ↑ Increases
Winding temperature ↑ Increases
Efficiency ↑ Improves (to peak)
Power factor ↑ Improves (to peak)
⇩ Decreased Load
Shaft speed ↑ Increases (toward Ns)
Slip ↓ Decreases
Stator current (amperes) ↓ Decreases
Input power (watts) ↓ Decreases
Winding temperature ↓ Decreases
Efficiency ↓ Drops (fixed losses dominate)
Power factor ↓ Drops significantly
💡
No-load current is not zero

Even with no mechanical load on the shaft, a motor draws 30–60% of its full-load current. This magnetising current creates the rotating magnetic field and is largely reactive (lagging). It is why lightly loaded or no-load motors have very poor power factor. Always measure and record running current under actual operating conditions — not at startup or no-load.

Overload Relay Sizing

Overload relays protect the motor from sustained over-current caused by overloading. They must be sized to the motor’s actual full-load current (FLA) as measured in the field — not necessarily the nameplate FLA, which is measured at rated voltage. If the supply voltage is consistently above or below nameplate, actual FLA will differ. Canadian Electrical Code (CEC) and NEC allow 115–125% of nameplate FLA for standard overload relay sizing, with the exact percentage depending on service factor and motor type.

⚠️
Ambient temperature affects overload relay trip point

Bimetal overload relays are calibrated for a standard ambient temperature (typically 20–40 °C depending on manufacturer). In high-ambient locations such as rooftops in summer, the relay trips earlier than its marked setting. In cold locations it trips later. Ambient-compensated relays correct for this automatically. Always verify the relay type before adjusting trip settings to resolve nuisance trips.

2.6.2 — Effects of Voltage Variations

NEMA standards permit ±10% of rated voltage. Most motors operate acceptably within this range, but both high and low voltage cause problems — often different problems than technicians expect. High voltage is not always “safe”, and low voltage is frequently the cause of motor failures that appear as overloads.

💡
High Voltage — +10%
e.g. 264 V on a 240 V motor
Starting torque +21% ↑
Running torque Increases ↑
No-load current Increases ↑
Full-load current Slightly less →
Speed (full load) Slight increase →
Iron core temperature Rises (saturation) ↑
Insulation stress Higher ↑
Power factor Decreases ↓
⚡
Low Voltage — −10%
e.g. 216 V on a 240 V motor
Starting torque −19% ↓
Running torque Decreases ↓
No-load current Decreases ↓
Full-load current +10–15% ↑
Speed (full load) Decreases ↓
Winding temperature Rises substantially ↑
Starting ability May fail to start ↓
Power factor Increases slightly ↑
🚫
Low voltage causes more failures than high voltage

Low voltage forces the motor to draw higher current to produce the same shaft output. Higher current means higher I²R heating in the windings. The relationship is approximately: a 10% voltage reduction causes a 15–20% increase in full-load current and a 20–25% increase in winding temperature rise. This accelerates insulation degradation and is the leading electrical cause of premature motor failure. Always measure voltage at the motor terminals, not at the panel — conductor voltage drop can be substantial.

2.6.3 — Torque Is Proportional to Voltage Squared

The relationship between supply voltage and available motor torque is not linear — it is proportional to the square of the voltage. This means small voltage deviations have a disproportionately large effect on starting ability and breakdown torque.

T ∝ V²

Torque available at any given slip is proportional to the square of the applied voltage. A 10% voltage reduction reduces available torque by approximately 19%, not 10%.

100% Nominal Voltage
100% rated torque
Baseline
+10% High Voltage (110%)
121% rated torque
1.1² = 1.21  →  +21%
−10% Low Voltage (90%)
81% rated torque
0.9² = 0.81  →  −19%

The practical implication for HVAC/R technicians: a compressor motor with marginal torque at rated voltage will fail to start reliably when the supply voltage drops even slightly. A motor that starts fine in the morning (cooler, higher voltage) may fail to restart after shutdown at mid-afternoon peak load (hotter, lower voltage). This pattern — starts fine when cool, locks out on hot restart — is a classic symptom of low voltage combined with a borderline starting device.

📊
Measure voltage during the starting attempt — not before

Supply voltage often sags significantly during motor starting due to the high inrush current. A supply that reads 240 V at no load may drop to 210 V during starting, reducing available starting torque by 24% compared to nominal. Always use a true RMS voltmeter and observe the voltage during a timed start sequence to capture voltage sag, not just the static voltage.

2.6.4 — Voltage Unbalance in Three-Phase Systems

In a three-phase system, if the three line voltages are not equal, voltage unbalance exists. Even a small voltage unbalance causes a disproportionately large current unbalance, because the negative-sequence component of voltage drives a much larger current response than an equal magnitude positive-sequence deviation.

% Voltage Unbalance = (Max deviation from average ÷ Average voltage) × 100

Example: Phase voltages of 240, 236, 248 V → Average = 241.3 V → Max deviation = 6.7 V → Unbalance = 2.8%

Voltage Unbalance vs. Current Imbalance — Why Small Is Dangerous
Balanced System — 0% Voltage Unbalance
L1
100%
L2
100%
L3
100%
3% Voltage Unbalance → ~27% Current Imbalance
L1
88%
L2
100%
L3
127%

The winding carrying the highest current overheats, even if the average current appears normal. The NEMA de-rating rule for voltage unbalance is approximately: 1% voltage unbalance causes 6–10% current unbalance, and motor efficiency and rated load capacity must be de-rated according to NEMA MG-1 tables. At 5% voltage unbalance, a motor must be de-rated to about 75% of its nameplate horsepower or it will operate at elevated temperatures.

⚠️

Sources of Voltage Unbalance in HVAC/R Systems

  • Single-phase loads (lighting, receptacles) unevenly distributed across the three phases of the panel — the most common cause in commercial buildings.
  • Faulty or open utility transformer winding — produces large unbalance and usually requires utility company involvement to resolve.
  • Loose or corroded connection at the motor terminals, starter contacts, or fused disconnect — creates resistance heating and unequal voltage drop.
  • Open delta transformer bank with unequal load on each leg — common on older commercial sites.
  • Single-phase fault to ground on one feeder conductor in a multi-wire circuit — presents as sudden, severe unbalance.
⚠️
Check all three voltages — not just one

Always measure L1–L2, L2–L3, and L1–L3 (or phase-to-neutral on all three phases) when diagnosing a suspected overheating or overcurrent complaint on a three-phase motor. A single voltage reading between two phases will not reveal unbalance. Measure at the motor terminals, not at the panel, to capture any conductor-level resistance differences.

2.6.5 — Practical Diagnosis — Load & Voltage Problems

Most motor problems in the field involve some combination of mechanical overload, voltage supply issues, or both acting together. The following principles allow a systematic approach to diagnosis using a clamp-on ammeter and a true RMS voltmeter.

Step-by-Step Diagnostic Procedure

  1. Measure supply voltage at the motor terminals under load — not at the panel, not at idle. Record all three phases (or L–N on single-phase). Compare to motor nameplate rating. Voltages more than ±10% outside nameplate indicate a supply problem that must be resolved before any other diagnosis.
  2. Measure running current on all phases with a clamp-on ammeter during steady-state operation. Compare to nameplate FLA. Current within 10% of FLA is normal. Current significantly above FLA indicates overloading, low voltage, or a motor fault.
  3. Calculate voltage unbalance using the three measured phase voltages. Maximum deviation from average divided by average, times 100. Values above 2% require investigation and de-rating; values above 5% require shutdown and correction.
  4. Check for conductor voltage drop by measuring voltage both at the panel and at the motor terminals simultaneously. A large difference (>3 V on a 240 V circuit) indicates undersized conductors, poor connections, or excessive run length. Long wire runs in equipment rooms and rooftop installations are a frequent cause of low voltage at the motor.
  5. Verify the mechanical load is within the motor’s rated HP. Check for fouled evaporator or condenser coils, restricted airflow, bearing drag, seized components, or refrigerant overcharge causing high discharge pressure on compressor motors.
  6. Check winding temperature after sustained operation by placing a temperature probe on the motor frame. Compare to the motor’s ambient plus rated temperature rise. Insulation class limits (from 2.01): Class B = 130 °C, Class F = 155 °C, Class H = 180 °C total (winding temp = ambient + temperature rise).
  7. Verify overload relay setpoint is set to actual measured FLA, not just nameplate value. Account for ambient temperature correction if using bimetal relays in high-ambient locations. Replace with ambient-compensated relays where appropriate.
✅

Quick Reference — Current vs. Probable Cause

Measured Current Voltage Normal? Most Likely Cause
Well below FLA (<70%) Yes Motor lightly loaded; normal if at design condition (e.g., economiser mode)
At or near FLA Yes Normal operation at designed full load
Above FLA (110–125%) Yes Mechanical overload — check driven equipment
Above FLA (110–125%) Low −10% Low voltage forcing higher current — check supply and conductors
Significantly above FLA (>125%) Yes Severe overload, jammed load, or winding fault — trip and investigate
Unequal between phases Unbalanced Voltage unbalance — measure all three phases and trace source
Two phases high, one very low One phase low/open Single-phasing condition — motor running on two legs; shut down immediately
🚫
Single-phasing is a motor emergency — act immediately

If one phase of a three-phase supply is lost (open fuse, loose lug, or utility fault), the motor attempts to continue running on two phases. Current in the remaining two windings rises to 150–200% of normal. A motor that trips on overload and resets repeatedly should be treated as a single-phasing suspect until proven otherwise. Measure all three phases before assuming the overload relay is simply undersized.

Test Your Knowledge
↑ Top