6.4 Single-Phasing, Rotation Checks, and Screw Compressors

Single-phasing means one of the three supply conductors is open while the other two are still live. The rotating field collapses into a pulsing single-phase field. A motor already running may keep turning from its own inertia and from the remaining windings, which then carry far more than their share of current. A motor at rest will not produce starting torque: it hums, may buzz or growl, and will not accelerate. That difference — runs (badly) if it was already up, will not start from rest — is a field trademark of a lost phase.

What opens, and what the operator sees

Typical opens in an ammonia plant:

• One fuse blown on a three-pole disconnect or starter.
• One starter pole burned open or a contact that no longer makes.
• A loose or burned lug on the MCC stab, overload block, or motor terminal box.
• A damaged feeder conductor or a pole of a disconnect that did not close fully.
• A failed pole inside a soft starter or a VFD output that has gone open on one phase (the motor still “has a drive,” but it is single-phasing).

Symptoms:

• Magnetic hum, more vibration than usual, sometimes a smell of hot enamel.
• Two phases showing high current, one showing zero or near-zero at the motor (if the open is upstream of the clamp). If you clamp above the open, you can fool yourself.
• Will not start from rest; attempt draws high current on the remaining legs and the overload may eventually open — or the winding may fail first.
• A lightly loaded fan or pump that was already running may not trip the overloads for a long time. Overload heaters see line current. At light load, even the inflated two-phase currents can remain below the heater curve long enough to blister insulation. That is why “it didn’t trip, so it must be fine” is the wrong conclusion.
• On a loaded screw, single-phasing often stalls or trips quickly. Do not count on that. Treat any suspected lost phase as a stop condition.

Single-phasing is the extreme of the voltage-unbalance problem in the previous section. One line-to-line voltage may collapse; the other two readings look wild. Do not keep jogging the starter to “catch” a start. Each attempt dumps heat into two windings.

Supervisor action: stop, lockout, find the open

CIRO is a supervisor credential. The expected action is not to pad heaters or to run to the end of the shift.

1. Stop the motor and the driven machine with the normal stop if it is still running and it is safe to do so. If the compressor is making abnormal noise, use the established emergency stop practice for that room.
2. Lockout/tagout the disconnect. Verify zero energy. Three-phase motors store nothing like a charged capacitor bank, but VFDs and power-factor capacitors do: follow the package LOTO, including wait times on drives.
3. Find the open. With the circuit isolated and tested dead, look at fuses (all three — a second fuse may be cracked), starter contacts, overload block, lugs, and the motor box. A fuse pulled for a ground fault may be the symptom, not the root; inspect the motor and feeder before you replace fuses and spin it.
4. Only after the open is repaired, insulation is acceptable, and rotation is proven, restart unloaded and watch all three amperes.

Never replace one fuse and re-energize “to see if it runs.” If a fuse opened on a fault, the other two may open next, or the winding is already wounded. Never defeat an overload or an oil-pressure switch to get a single-phasing motor to stay online.

Rotation: two leads, opposite direction

On a three-phase motor, swapping any two of the three line leads reverses rotation. That is the entire rule. You do not reverse rotation by swapping all three, and you do not reverse it by changing only a control wire. After any work on T-leads, a starter, a disconnect, a VFD output, or a replacement motor, bump-check: momentary start, watch the coupling or the shaft against the rotation arrow, stop. Bump on an unloaded compressor with oil-pump permission as the manufacturer requires.

Mark leads before you lift them. If two people land T1-T2-T3 by memory after a weekend MCC swap, the screw may start backwards on Monday.

Why reverse rotation destroys screw compressors

A reciprocating compressor is unhappy running backwards; a rotary screw can be destroyed. The package assumes one shaft direction for:

• The oil pump (shaft-driven pumps may not feed, or may attempt to pump the wrong way). Loss of oil at the bearings and rotors is a wipe in seconds.
• The slide valve and capacity mechanism, which can be driven into a bind or a position the controls cannot recover.
• Discharge-check and gas-flow direction. Reverse rotation can fight the check, overheat, or unload oil from the rotor cavity the wrong way.
• Axial thrust and rotor timing. Screws are not designed for reverse torque at load.

Always bump-check after electrical work. Do not “start it loaded and see if suction falls.” By the time suction tells you the machine is wrong, the rotors may already have contacted.

Oil-pressure interlock is supposed to stop a start that never builds oil pressure. Treat it as a backup, not as the rotation check:

• Some packages have a delay that lets a reverse machine turn long enough to do harm.
• A jumped, drifted, or incorrectly valved oil-pressure switch will not save you.
• A separately driven oil pump can build pressure even if the compressor shaft is wrong, depending on the oil circuit.
• Interlocks do not replace a pair of eyes on the coupling and the arrow.

If rotation is wrong, stop, lockout, swap two leads, bump again, then allow an automatic start. Document the lead swap in the electrical log so the next outage does not “correct” it back.

Condenser fans and pumps also have required rotation (air throw, impeller direction). Reverse fans move some air and look “fine” until head pressure climbs. Reverse pumps may still move some liquid at poor efficiency and cavitate. The screw is the machine that fails catastrophically; fans and pumps fail quietly. Check all of them.

Insulation resistance (megger) when overloads keep returning

After single-phasing, flooding, a washdown, or a motor that trips overloads with a clean mechanical load, test insulation resistance with a megohmmeter.

• For 480 V motors, field tests are commonly at 500 V or 1000 V DC, phase-to-ground and (with care and disconnected leads) phase-to-phase as the shop procedure allows.
• A good dry motor often reads tens to hundreds of megohms. A crude low-voltage floor many plants still use is on the order of 1 MΩ, sometimes stated as 1 MΩ plus 1 MΩ per kV of rating. Treat manufacturer and IEEE 43 procedures as the authority; do not pass a motor that smells burned just because it cleared 1 MΩ on a humid morning.
• Temperature and moisture swing the reading. A cold, wet motor that sat after a leak will megger low until it is dried; that is information, not a license to keep starting it.
• Polarization index (10-minute reading / 1-minute reading) is used on larger machines to separate moisture from ruined insulation.

A low megger explains repeated “overload” trips that are actually ground current and heating. Combined with single-phasing, a winding that already has weak insulation is the one that goes to ground on the next attempt. Megger before you keep slamming a starter.

Putting the electricity chapter together on a screw

On a 300 HP, 480 V screw, the supervisor’s electrical picture is: nameplate FLA as the thermal baseline, efficiency and PF to sanity-check screen kW and amperes, reduced-voltage starting so inrush does not wreck the bus, voltage unbalance computed from three line-to-line readings, no single-phasing, correct rotation proved by a bump, and insulation still intact. Skip any one of those and the plant can lose a compressor that was mechanically fine. The next chapter takes the same motors into ladder interlocks, VFDs, and classified-room wiring.