The Overload the Plant Wants Turned Up
A 50 hp, 600 V, 3-phase TEFC motor drives a screw conveyor at a gravel pit. Nameplate: 52 A, service factor 1.15, 40 °C ambient rating. It is started across the line from a combination starter 60 m away, with class 20 thermal overloads dialled to 52 A. Operators report trips after 25-40 minutes of what they call normal running, and the frame is too hot to hold a hand on. You measure line voltage 600/612/588 V at the starter, running current 56/56/44 A, balanced winding resistance, insulation resistance 500 megohms at 500 V, and 38 °C ambient at the motor. While looking at the starter you notice the only bonding means for the motor is the EMT, one coupling turns by hand, and there is no bonding bushing at the motor. The plant supervisor wants the overload "turned up a bit" so production stops calling.
1. Is this a bad motor or a supply problem? Show the unbalance calculations and the ratio rule you use to decide.
Read the worked answer
The numbers point at the supply, not the windings.
Voltage unbalance (use line-to-line voltages): Average = (600 + 612 + 588) / 3 = 600 V. Maximum deviation from average = 12 V. % unbalance = 12 / 600 x 100 = 2.0 %.
Current unbalance: Average = (56 + 56 + 44) / 3 = 52 A. Maximum deviation = 4 A. % unbalance = 4 / 52 x 100 = 7.7 %.
The ratio rule: on a healthy motor, current unbalance runs roughly 6 to 10 times the voltage unbalance. Here 7.7 / 2.0 = about 3.8 times. That is inside the expected band, which means the current unbalance is being produced by the voltage unbalance and not by a shorted turn or a bad connection inside the motor. A winding fault typically gives a current unbalance far out of proportion to the voltage, and often an insulation-resistance or resistance-imbalance signature; this motor meggers at 500 megohms with balanced resistance.
Confirming tests:
- Re-measure voltage at the motor terminals with the motor running and again at the MCC - if the unbalance grows at the motor, the drop is in the 60 m run or the starter (loose lug, pitted contact, fuse clip).
- Measure voltage at the service with the conveyor off. If it is already unbalanced with no load, the problem is upstream - utility, transformer, or a single-phase load elsewhere on the pit's transformer.
- Clamp each conductor at both ends of the run and compare; a difference between MCC and motor indicates a high-resistance joint in the raceway.
The motor is the victim. Turning up the overload treats the thermometer by breaking it.
2. What maximum continuous current can this motor be expected to carry at this unbalance, and what do you do in the short term versus the long term?
Read the worked answer
Derate before you decide. NEMA MG1 guidance for a 2 % voltage unbalance is a derating of about 0.95 of nameplate. So the motor can be expected to carry only about:
0.95 x 52 A = 49.4 A continuously, and that is at 40 °C ambient with the motor at rated temperature rise.
It is running at an average of 52 A with one phase at 56 A. It is being asked for more than its derated capability, on a motor already too hot to touch, in 38 °C ambient. The overload is doing exactly its job; the trip is information, not a nuisance.
Short term (production running, do not raise the setting):
- Leave the overload at 52 A, or set it to the derated value only if the drive train can take the reduced output.
- Find and fix the unbalance source: torque every line lug from the MCC to the motor, check the starter contacts and fuse clips, and check whether a single-phase load or a capacitor bank on the pit's transformer is causing the imbalance. A 2 % unbalance is usually a loose connection or a utility issue, both fixable.
- Reduce the conveyor load if possible (feed rate, blocked chute, bearing drag). Confirm actual shaft load with a power meter - "normal running" from the operator's chair is not a measurement.
Long term: do not rely on service factor 1.15 to cover a supply defect; service factor is a margin for occasional overload and voltage variation, not for continuous operation above the derated capability. Document the voltage unbalance in writing and give the utility or the plant a target of 1 % or less.
3. What is wrong with the bonding at this motor and starter, what does the Code require, and why does it matter beyond the current overload problem?
Read the worked answer
What is wrong:
- The EMT is being used as the only bonding means, and one coupling is loose enough to turn by hand. A loose coupling is a poor, high-resistance connection that will arc under fault current - and it may not even be electrically continuous if the fitting is a setscrew type that has been disturbed.
- There is no bonding bushing at the motor. Where a raceway terminates at a concentric or eccentric knockout, the remaining metal is a poor fault path, and Section 10 requires bonding around that reduced area or the use of a listed bonding means.
- The starter enclosure and motor frame must be bonded to the system bonding conductor with a conductor sized from Table 16 based on the rating of the upstream overcurrent device - not by the raceway alone where continuity is questionable.
What the Code requires: Section 10 requires that all non-current-carrying metal parts that may become energized be bonded to the grounding system, and the bonding conductor is sized from Table 16 (for a 100 A device, 8 AWG copper, for example - confirm the actual overcurrent device rating). Raceways are recognized as bonding means only if they are continuous and properly made up with listed fittings.
Why it matters beyond the overload: the motor frame is on a conveyor in a gravel pit where people walk. With the bonding path compromised, a fault from a damaged winding to the frame will sit there, raising the frame to line potential until somebody touches it and the earth path provides the return. The overload trip and the bad bonding are two symptoms of a maintenance culture that needs a documented electrical maintenance program, not one parameter change.