Single and Three Phase Motor Testing

What motor testing actually involves

A motor that hums, trips, runs hot or simply will not start is telling you something, but the symptom on its own rarely says what. The same "it trips on start" can be a failing winding, a weak supply phase, a sticking contactor, an overload relay set too low, or a pump that has seized. Motor testing is the discipline of measuring each of those separately so the repair fixes the cause, not the guess.

Richie Rich Electrical tests single phase and three phase motors and the controls that drive them across Cape Town homes, complexes, workshops, farms and commercial sites. The tests are done with the motor isolated and locked off, the readings are compared with the nameplate, and you get a plain explanation of what was found before any part is replaced.

Single phase motors

Single phase motors run pool pumps, borehole pumps, pressure pumps, gate motors, compressors, extractor fans, workshop machines and most household equipment. They depend on a start winding and, usually, a capacitor to get moving, which is why a motor that hums loudly but does not turn is such a common call. What gets checked:

• Supply voltage at the motor terminals under load, not just at the board.
• Start and run capacitors: value, leakage and swelling or venting.
• Centrifugal switch or start relay operation where fitted.
• Insulation resistance of the windings to earth, and earth continuity to the frame.
• Running current against the nameplate rating, and whether it settles after starting.
• The isolator, cable, plug and any float switch, pressure switch or timer in the circuit.

Three phase motors

Three phase motors drive larger pumps, compressors, lifts, conveyors, HVAC plant, woodworking and engineering machines. They have no capacitor to fail, so faults tend to sit in the windings, the supply balance or the control gear. What gets checked:

• Voltage on all three phases, and the imbalance between them, measured at the starter and at the motor.
• Current draw per phase under load. A phase pulling noticeably more than the others points to a winding or supply problem.
• Winding resistance balance across the three windings, and insulation resistance to earth.
• Phase rotation, so a replaced motor or repaired supply does not run the machine backwards.
• Terminal box condition: star or delta links, tightness, heat marks and moisture.
• Earth continuity to the frame and back to the board.

Controls, starters and drives

Many "motor faults" are control faults. The motor is fine; the equipment around it is not. Testing includes the whole control chain:

• Direct-on-line and star-delta starters: contactor contacts, coil, timer and the changeover sequence.
• Thermal overload relays: setting against the motor full load current, trip class and whether the reset is being abused.
• Control circuits: stop and start buttons, float and pressure switches, interlocks, emergency stops and the control transformer or supply that feeds them.
• Variable speed drives and soft starters: fault codes, parameter settings, input and output checks, and cooling.
• Motor circuit breakers and fuses, sized correctly for starting current.

Where the equipment is under a manufacturer's warranty or a specialist maintenance contract, the electrical boundary is confirmed first so the right party handles the right part.

Electrical fault or mechanical fault?

A motor draws more current when the load on it increases. A seized bearing, a blocked impeller, a slipping or over-tight belt, or a pump running against a closed valve will all trip a healthy motor. Part of a proper test is separating the two: the motor is run uncoupled or unloaded where practical, and the current compared with the loaded reading. If the motor is sound and the load is the problem, you are told that, rather than being sold a rewind.

What you will get

• Readings compared with the nameplate and explained in plain language.
• A clear finding: repair the motor, replace it, repair the control gear, or fix the driven load.
• A price for the recommended work before it starts.
• Correct overload and protection settings after any repair, and a test run under load.

Helpful to have ready when you call

• A photo of the motor nameplate (kW, voltage, full load current, connection).
• What the motor drives, and whether the fault is on starting, after warming up, or at random.
• What trips: the overload, the breaker, the earth leakage, or nothing at all.
• Any recent work, water ingress, storms or supply problems in the area.

Related pages

For the supply side see three phase installations, 3 phase circuits and 1 phase circuits. For starters and control gear see all types of electrical starters and motors. Business premises are covered under commercial electrical services, and any intermittent trip that is not motor related belongs with fault finding.