Electric motor calculator to IEC 60034-1

What this calculator does

It brings together, in six tabs, the IEC 60034-1:2004 calculations most often needed when testing or accepting an electric motor: winding temperature rise and its limit, the correction when the first reading comes late, supply voltage quality (harmonics and unbalance), the operating zone by voltage and frequency, and the tolerances on test-report values. It is for motors and rotating machines. For power transformers there is the transformer test calculator.

1. Temperature rise by the resistance method

The average temperature of a winding follows from how much its resistance rises (8.6.2.2):

(θ₂ + k)/(θ₁ + k) = R₂/R₁ and Δθ = (R₂ − R₁)/R₁·(k + θ₁) + θ₁ − θc

with k = 235 for copper and 225 for aluminium, θ₁ and R₁ cold, R₂ at the end of the thermal test and θc the coolant temperature at the end of the test.

Limit (Table 7). For indirectly air-cooled a.c. windings measured by resistance:

With the superposition method, class B or F machines of 200 kW or less are allowed 5 K more. For S2 duty below 5000 kW, 10 K more (Table 9).

Site adjustment (Tables 9 and 10). The limits assume coolant at 40 °C and altitude up to 1000 m. If the site coolant is colder, the limit rises by the difference. If the margin between the thermal class and 40 °C plus the limit exceeds 5 K, that rise is reduced by the factor 1 − (class − (40 + limit))/80 K. If the site coolant is warmer (up to 60 °C), the limit falls by the difference. Above 1000 m, the 40 °C reference is replaced by the Table 10 value. If no ambient temperature is specified, the cooler air at altitude is assumed to offset the poorer cooling, and nothing is adjusted.

Test-site adjustment (Table 11). If the test is carried out at a different altitude, the limit is multiplied by 1 + (Hr − H)/10 000 m, counting altitudes below 1000 m as 1000 m.

Worked example

A 75 kW class F motor with copper windings. Cold: 1.200 Ω at 20 °C. At the end of the heat run: 1.560 Ω, with cooling air at 25 °C.

• θ₂ = 1.560/1.200·(235 + 20) − 235 = 96.5 °C
• Δθ = 96.5 − 25 = 71.5 K
• Table 7 limit for class F: 105 K. A 33.5 K margin: it passes, and would even meet the class B limit (80 K), which is how class F motors are usually designed.
• If the site air were at most 30 °C: margin = 155 − (40 + 105) = 10 K, above 5 K → the limit rises by 10·(1 − 10/80) = 8.75 K, to 113.75 K.

2. Stopping-time correction

With direct measurement, resistance is read after switching off. The reading needs no correction if it is taken within the Table 5 interval:

If the first reading comes later, but within twice that time, readings continue every minute until they clearly fall. The temperature curve is then extrapolated back to the Table 5 interval on a semi-log plot (8.6.2.3.3). The calculator fits ln θ against time using the readings from the maximum onwards. If a later reading is higher, the highest is taken. Beyond twice the interval, the method may only be used by agreement.

In the example, a 160 kW motor (90 s interval) whose first reading comes at 150 s, followed by 1.548, 1.541, 1.535 and 1.529 Ω at one-minute steps, gives 95.3 °C extrapolated to 90 s.

3. Harmonic voltage factor (HVF)

An a.c. motor on a fixed-frequency supply must withstand a voltage with (7.2.1.1):

HVF = √(Σ uₙ²/n), n from 2 to 13, with uₙ the harmonic of order n in p.u. of rated voltage.

On three-phase motors, harmonics divisible by 3 are not counted. The limit is 0.02, or 0.03 for design N motors (IEC 60034-12), unless the manufacturer declares otherwise. With 3 % 5th, 2 % 7th and 1 % each of 11th and 13th, the HVF is 0.016.

4. Supply unbalance

A three-phase motor must run with a negative-sequence component of up to 1 % of the positive sequence over a long period, or 1.5 % for a few minutes, and a zero-sequence component of up to 1 % (7.2.1.1). The three line-voltage magnitudes give the negative sequence:

β = (U₁₂⁴ + U₂₃⁴ + U₃₁⁴)/(U₁₂² + U₂₃² + U₃₁²)², u₂ = √[(1 − √(3 − 6β))/(1 + √(3 − 6β))]

The zero sequence needs the phase-to-neutral phasors. With 400, 396 and 404 V line-to-line, the negative sequence is 1.15 %: acceptable only for a few minutes.

5. Voltage and frequency zones A and B

Figure 12 of the standard defines two zones for motors on a fixed-frequency supply (7.3):

• Zone A: voltage 0.95 to 1.05 p.u., frequency 0.98 to 1.02 p.u., with two corners cut. The motor must perform its primary function continuously, though it may deviate somewhat from its rated performance. At the edge of the zone, temperature rise typically exceeds the limit by about 10 K.
• Zone B: voltage 0.90 to 1.10 p.u., frequency 0.95 to 1.03 p.u., also with cut corners. The motor must perform its primary function, with larger deviations. Extended operation at its edge is not recommended.

A 400 V motor supplied at 376 V and 49.5 Hz is in zone B.

6. Tolerances (Table 20)

An example of the efficiency rule: a 75 kW motor declared at η = 95.0 % passes if it measures at least 95.0 − 0.15·5.0 = 94.25 %.

What this calculator does not do

It does not apply to transformers. It only covers indirectly air-cooled a.c. windings: no hydrogen or direct water cooling (Tables 8, 12 to 14), and no field or commutator windings. It does not calculate efficiency from separate losses (IEC 60034-2-1) or IE class (IEC 60034-30-1). It follows the 2004 edition of IEC 60034-1. Later editions may change some values, so check which one your contract refers to. The result is indicative and must be validated by a competent engineer.

What it is measured with

Winding resistance is measured with micro-ohmmeters, and insulation with insulation resistance meters. Supply harmonics and unbalance are recorded with a power quality analyser.