In short. A motor circuit needs protection against short circuit (fuse or breaker), overload (thermal or electronic relay), and increasingly against locked rotor, phase loss, unbalance, earth fault and undervoltage. Feeders need overcurrent and earth-fault protection coordinated with the board. On generator supply, starting currents and voltage dips need extra care.
Motor protection functions
| Function | Purpose |
|---|---|
| Short-circuit | Fuse or instantaneous trip set above starting current |
| Thermal overload | Models heating from running current, with trip class matched to the starting time |
| Locked rotor / stall | Trips when starting current persists beyond the safe time |
| Phase loss / unbalance | Prevents single-phasing damage. See unbalance protection |
| Earth fault | Detects insulation failure. See earth-fault protection |
| Undervoltage | Prevents restart or stalling at low voltage |
| Temperature (RTD, PTC) | Winding and bearing monitoring on larger motors |
Starter and relay choices
- Direct on-line: simple, high starting current.
- Star-delta, soft starter, drive: reduce starting current. See starting kVA.
- Electronic motor protection relay: thermal model, trip class and diagnostics.
Feeder protection
Feeder circuits need overcurrent for short circuit and overload, earth-fault protection and coordination with the upstream device. Cables must be protected within their thermal limit. See cable protection.
On generator supply
- Starting currents cause voltage dips, which can drop contactors and trip undervoltage relays. Settings and sequencing should allow for this.
- Staggered starting avoids several motors starting at once. See motor starting.
- Lower fault current may require a check that short-circuit elements pick up.
Setting steps
- Take motor nameplate data: full-load current, starting current and time, stall time, service factor.
- Set the overload to motor full-load current and choose the trip class.
- Set the short-circuit element above starting current peaks.
- Set the locked-rotor time below the motor’s stall withstand.
- Check coordination with upstream feeder devices.
Illustrative example
A 90 kW pump motor starts in 6 seconds with a starting current of about six times full load. The overload relay is set to full-load current with a trip class that permits the 6-second start, the locked-rotor element is set shortly beyond the start time, and the short-circuit element is above the peak starting current. On generator supply, the voltage dip prolongs the start, so the stall timer is checked against the longer start.
Motor protection functions
| Hazard | Protection |
|---|---|
| Overload | Thermal model or inverse-time overload |
| Locked rotor or stall | Stall protection with start-time supervision |
| Short circuit | Instantaneous overcurrent, with setting above starting current |
| Earth fault | Earth-fault element, often with core-balance CT |
| Phase loss or unbalance | Negative-sequence or phase-failure function |
| Frequent starts | Start-count and cooling-time limit |
| Undervoltage | Release and restart logic |
Starting current and settings
Direct-on-line starting draws roughly 6 to 8 times full-load current for a few seconds. Instantaneous settings must sit above this, typically with margin for asymmetry, while thermal protection must tolerate the start but trip on a stall. See motor starting.
Feeder protection on a generator supply
A generator offers lower fault current than the utility, so instantaneous elements may not pick up on a remote fault. Check feeder settings against generator-only fault level and confirm discrimination with upstream devices. See coordination.
Restart after supply transfer
When power returns after a transfer, many motors restart together and demand a large inrush. Stagger restarts through timers or sequencers.
Practical checklist
- Confirm motor nameplate data and starting method.
- Set thermal class to the motor’s thermal limits.
- Check CT ratio against full-load current for accuracy.
- Test overload, earth fault and stall functions.
Common mistakes
- Overload class that trips on every start.
- Undervoltage relay without delay on generator supply.
- No unbalance protection.
What to send us
Send the motor schedule and starter details. Send your application details.
Questions engineers ask
What is a trip class?
A measure of how long an overload relay allows the starting current before tripping.
Why does a motor trip on generator but not on grid?
The generator has higher voltage dip and different fault characteristics. Longer starts and undervoltage can trip it.
Is a fuse or a breaker better for motors?
Both work. Fuses limit current well, breakers reset and offer adjustability. Use what the design needs.
Do small motors need all these functions?
No. Scale protection to motor size and criticality.
This guide gives general principles. Check motor data for settings.