Motor protection — what to install and why
An electric motor without protection is a matter of time before it burns out. Overload, phase loss, short circuit, overheating — any of these can destroy a winding in minutes. Rewinding a 15 kW motor costs from 8,000 UAH, and equipment downtime costs even more. Proper protection runs 5–10% of the motor price and prevents 90% of failures.
Main threats to a motor
Before choosing protection devices — know what you are protecting against:
- Overload — current above rated for an extended period. Cause: worn mechanism, jamming, wrong motor selection
- Short circuit — sudden current spike, tens of times above rated. Cause: insulation breakdown, cable damage
- Phase loss — one of three phases disappears. The motor keeps running on two phases, but current rises 1.5–2 times and the winding overheats
- Overheating — winding temperature exceeds the limit. Cause: contamination, overload, high ambient temperature
- Undervoltage — below 85% of rated voltage the motor draws higher current and overheats
- Phase imbalance — unequal voltage or current across phases. Even 3% voltage imbalance produces 18% current imbalance
Protection devices: simple to advanced
Circuit breaker
Minimum protection — against short circuits. A D-curve breaker (for motors) withstands inrush of 10–20 times rated and trips on a short circuit in milliseconds. But overload protection from a breaker alone is poor — the thermal trip is slow and imprecise.
Thermal overload relay
Classic overload protection. A bimetallic strip heats up with current and opens the contactor when the setpoint is exceeded. Trip time depends on overload magnitude: at 120% rated — 10–20 minutes, at 200% — 1–2 minutes.
Downside: a thermal relay does not protect against phase loss and does not account for ambient temperature. At +40 °C the relay may not trip in time.
Electronic overload relay
Replaces the thermal relay and adds features: phase-loss protection, phase imbalance, rotor stall, underload (belt break). Costs more, but for motors from 7.5 kW — a justified investment.
Phase monitoring relay
A standalone device that monitors presence of all three phases, phase sequence, and voltage level. Disconnects the motor on phase loss or undervoltage. Costs from 500 UAH — cheap for that level of protection.
Thermistors (PTC / PT100)
Temperature sensors embedded in the motor winding. PTC is a simple variant: resistance jumps sharply at the threshold temperature. PT100 gives precise temperature reading in degrees. A thermistor relay module is needed to disconnect the motor on overheat.
Variable frequency drive
A VFD is more than just protection — it is a full motor control system. It has built-in protection against overload, short circuit, phase loss, overheating, and stall. Plus soft starting, which reduces mechanical stress at startup.
Soft starter
A soft starter limits inrush current and provides basic overload and phase-loss protection. It does not replace a full protection set, but for motors with infrequent starts it is a good addition.
Typical protection schemes
| Motor power | Minimum protection | Recommended protection | Approximate cost |
|---|---|---|---|
| 0.37–2.2 kW | Breaker + thermal relay | Breaker + thermal relay + phase relay | 800–2,000 UAH |
| 3–7.5 kW | Breaker + thermal relay | Breaker + electronic relay + thermistors | 2,000–5,000 UAH |
| 11–30 kW | Breaker + electronic relay | VFD or breaker + electronic relay + thermistors + phase relay | 5,000–25,000 UAH |
| 37–75 kW | Breaker + electronic relay + thermistors | VFD with full protection suite | 25,000–80,000 UAH |
Common mistakes in motor protection
We see these mistakes constantly:
- Breaker only, no thermal relay. A breaker will not protect against slow overload at 120–150%. The motor will keep running and heating until the winding burns.
- Thermal relay with wrong setpoint. The setpoint must equal the motor rated current from the nameplate. Too low — nuisance tripping. Too high — no protection on overload.
- Ignoring phase loss. On older networks phase loss is common. Without a phase relay the motor continues on two phases and burns out in 5–15 minutes.
- No thermistors on high-power motors. From 11 kW, thermistors are probably the cheapest insurance policy. Most modern WEG and ABB motors come with PTC sensors factory-installed.
Frequently Asked Questions
Is a circuit breaker alone enough to protect a motor?
No. A breaker protects against short circuits but not against overload or phase loss. Minimum is a breaker plus thermal relay. For motors from 7.5 kW add a phase monitoring relay.
How to set a thermal overload relay correctly?
Set it to the motor rated current from the nameplate. For a 7.5 kW / 380 V motor the rated current is approximately 15.5 A — set the relay to 15.5 A.
Does a VFD replace all other protection equipment?
A VFD replaces the soft starter, thermal relay, and phase relay. But an input circuit breaker is always needed — to protect the cable and the VFD itself from short circuits.
Why are thermistors needed if there is a thermal relay?
A thermal relay measures current, not temperature. If motor cooling is compromised (blocked fins, broken fan) — current is normal but temperature rises. A thermistor catches overheating even at normal current.
Which phase monitoring relay to choose?
For most applications, a relay with phase-loss detection, phase-sequence monitoring, and undervoltage protection is sufficient. Cost starts at 500 UAH — compared to a rewind at 8,000+ UAH, that is a negligible investment.
Summary
Motor protection is not a cost — it is savings. A breaker + thermal relay + phase relay costs 1,500–3,000 UAH and prevents a rewind costing 8,000–25,000 UAH plus downtime. For motors above 11 kW consider a VFD — it combines protection and control in one device.
