Test and Verify Efficiency of Electrical Motor as per New Zealand Guidelines | Carelabs

Electric motors consume the majority of industrial electricity, and their efficiency is usually taken on trust from a nameplate that may be decades old and may no longer describe the machine. Verifying efficiency turns an assumption into a measurement, which is what allows replacement and rewind decisions to be made on evidence.

Why nameplate efficiency stops being true

Rewinding. The single largest factor. A motor that has been rewound, particularly where the old winding was removed using heat that was not properly controlled, can lose efficiency permanently through degradation of the core steel's interlaminar insulation. Losses of one to two percentage points per rewind are commonly reported, and a motor rewound three times may be materially worse than its nameplate claims.

Bearing and mechanical condition. Friction losses rise with wear and with lubrication that is incorrect, degraded or simply over-applied.

Operating point. Efficiency is quoted at rated load. A motor running at thirty percent load is operating well away from its efficiency peak, and oversizing is extremely common, particularly where a motor was replaced with the next size up as a precaution.

Supply quality. Voltage unbalance produces negative-sequence currents that generate heat without useful torque. A small unbalance produces a disproportionate increase in rotor loss. Harmonic distortion adds further loss.

Cooling. Blocked cooling passages and fouled fins raise operating temperature, which raises winding resistance and therefore losses.

The standards that apply

IEC 60034-2-1 defines how efficiency is determined, and the method matters as much as the result. The preferred approach for larger machines separates individual losses rather than simply comparing input and output power, because the input-output method's uncertainty can approach the efficiency difference being investigated.

IEC 60034-30-1 defines the IE efficiency classes — IE1 through IE5 — that allow comparison on a common basis.

AS/NZS 1359.5 covers three-phase induction motor efficiency in the Australian and New Zealand context, and minimum energy performance standards apply to motors supplied into this market within defined power ranges. This is why a failed motor frequently cannot be replaced with an identical unit: the original may no longer be compliant for sale.

How testing is carried out

In-situ measurement. Power analysers record input power, current, voltage, power factor and unbalance at the motor terminals under actual operating conditions, with load determined from process data or from slip. This does not produce laboratory-grade efficiency figures, but it answers the practical questions: what the motor draws, how loaded it is, and whether the supply is contributing to losses.

Laboratory testing. Removing the motor and testing on a dynamometer to IEC 60034-2-1 produces a defensible efficiency figure. Justified where a rewind-versus-replace decision involves significant capital, or where a supplier's claim is being verified.

Loss separation. No-load and locked-rotor tests separate iron, friction, windage, stator copper and rotor losses, which identifies where the loss is occurring rather than only how much there is. This is what distinguishes a core problem from a bearing problem.

Condition measurements alongside. Insulation resistance, polarisation index, winding resistance balance and vibration analysis. Efficiency and reliability are not the same question, and a motor can be efficient and about to fail.

Interpreting the result

Efficiency alone rarely drives a decision. What drives it is annual energy cost at the measured operating point, compared against a replacement at current IE class, adjusted for running hours.

That calculation produces conclusions that are frequently counter-intuitive. A large motor at continuous duty with a modest efficiency deficit may justify immediate replacement. A small motor running two hours a week will not, regardless of how poor its efficiency is. Running hours dominate, and they are the input most often estimated rather than measured.

The rewind-versus-replace decision benefits most from measurement. A rewind is cheaper immediately and more expensive over the life of the machine if it costs efficiency. Knowing the current efficiency, the rewind history and the duty cycle is what makes that comparison real rather than assumed.

Related reading: switchgear assessment covers the equivalent condition question for the supply side, and harmonic analysis addresses the supply distortion that contributes to motor losses.