Resoluciones Varias

In generator no-load operation mode, the machine is driven externally by the load ma- chine at constant speed with an open stator winding terminal. In order to verify the cal- culated air-gap flux density and the proper design of the stator winding, the induced voltage 𝑈 was measured for different speeds up to 10000 min in the no-load genera- tor mode with the NORMA 5000 power analyser, which has an accuracy of 0.1 %. The rotor was warmed up previously to 100 °C to measure the induced voltage with a mag- net operating temperature of 𝜗 100 °C. The measured induced voltage fundamentals are depicted in Fig. 5.1 over rotation speed, and they are compared to numerically cal- culated induced voltages. The measured induced no-load voltages are 4.86 % lower than the calculated values of the 2D FEM analysis: e.g. 𝑈 64.6 V 67.9 V⁄ , measured / calculated voltage at 𝑛 4167 min . This result comes from the flux leakage at the axial ends of the rotor magnets and the additional leakage of magnetic flux between adjacent rotor sections due to the rotor staggering, as well as the magnet insulation and rounded magnet edges, which reduce slightly the magnets’ active mass. These effects were not considered in the 2D FEM calculation.

Measured induced phase voltage and induced line-to-line voltage over time using the oscilloscope (Agilent Technologies, DSO7034B) at rated speed 𝑛 4167 min are shown in Fig. 5.2. The filtering effect of the short-pitched distributed stator 3-phase winding with 𝑊 𝜏⁄ 5 6⁄ (for reduction of 5th _{and 7}th _{field wave harmonics) with 𝑞}

2 number of slots per pole and phase and the step-skewed rotor (reducing the 11th _and

13th _{field wave harmonics) attenuate mainly induced voltage harmonics. Hence, the}

voltage waveforms over time are nearly sinusoidal. The total measured harmonic distor- tion of the induced phase voltage is 1.36 %.

The measured no-load torque values, due to air friction, bearing friction and the no-load iron losses, and their linear interpolations at two different magnet temperatures of 𝜗 35 °C and 𝜗 100 °C are shown in Fig. 5.3 as a function of rotation speed 𝑛. Because of the low no-load cogging torque from the stepped skewing of the rotor and the low

114

mechanical friction of the ball bearing, the break-away torque is only about 0.2 Nm or 0.4 % of rated torque. The dependence of this value on magnet temperature is discussed in the following paragraph along with no-load losses 𝑃 .

Fig. 5.1: Measured and numerically calculated no-load induced phase voltage (RMS) of the prototype electric machine up to 10000 min with a magnet temperature of

𝜗 100 °C

Fig. 5.2: Measured waveforms of the induced phase voltage and the induced line-to- line voltage of the prototype electric machine at 4167 min with a magnet tempera-

ture of 𝜗 35 °C: the amplitude of the induced phase voltage fundamental is 𝑈 , 98.7 V, and the amplitude of the induced line-to-line voltage fundamental is

𝑈 _, 169.3 V √3 ∙ 98.7 V -200 -100 0 100 200 0 2 4 6 8 10

Vol

ta

g

e

U

/ V

t

The no-load losses 𝑃 consist of friction losses 𝑃 _, and the no-load iron losses 𝑃 _, as the sum of no-load hysteresis and eddy current losses in the stator and in the rotor lam- inations, and eddy current losses in the segmented rotor magnets 𝑃 , . There are two

possible methods to measure the no-load losses. The first method is measuring the gen- erator no-load shaft torque 𝑀 and speed 𝑛 at thermal steady state (Fig. 5.3). For this measurement, the prototype machine is coupled with a speed-controlled load machine through a torque transducer. The measured mechanical power represents the no-load losses at the given speed. In order to consider the influence of the magnet temperature on the losses, the no-load losses were measured at two different magnet temperatures of 𝜗 35 °C and 𝜗 100 °C, which correspond to the calculated steady-state tempera- tures of the magnets at no-load operation and rated operation (OP 2), respectively (Fig. 5.4). Since the magnet remanence flux density decreases with increasing temperature, given the negative magnet remanence temperature coefficient of 0.11 %/K, and since the iron losses are proportional to the square of the magnetic flux density (𝑃 _, ~𝐵 ), the no-load losses decrease with the rise in magnet temperature. The second measure- ment method is the motor spin-down test. For this measurement, the prototype machine is operated alone as a motor, uncoupled from the load machine. The rotor is first warmed up to the steady-state temperature of the magnets at no-load operation 𝜗 35 °C. Then, the prototype machine is driven up by the inverter to the maximum speed of 𝑛 10000 min . As the machine rotates at maximum speed, the driving power

Fig. 5.3: Measured no-load torque values 𝑀 and their linear interpolations at no- load generator operation of the prototype machine with two different magnet tem-

116

from the inverter is switched off, the rotor decelerated, and the changing rotation speed over time is measured as the rotor decelerates (Fig. 5.5). The no-load losses are calcu- lated using the reduction of kinetic energy 𝑊 of the rotor with increasing time, using the calculated rotor moment of inertia of 𝐽 8.27 ∙ 10 kg ∙ m in (5.1). The measured no-load losses 𝑃 _, with the spin-down test are shown in Fig. 5.4. Comparison with the first method at a magnet temperature of 𝜗 35 °C shows good agreement, espe- cially below 5000 min-1_.

𝑃 , 𝑑𝑊 𝑑𝑡 4𝜋 ∙ 𝐽 ∙ 𝑛 ∙ 𝑑𝑛 𝑑𝑡 (5.1) 𝑊 1 2∙ 𝐽 ∙ 2𝜋 ∙ 𝑛 (5.2)

Using PT100 resistive temperature sensors, the temperature increase of the stator wind- ing in the stator slots and in the winding overhang, and the temperature of the magnets (signal access via rotor slip rings for the sensor leads) were measured during generator no-load operation at rated speed of 𝑛 4167 min , using a liquid cooling flow with a flow rate of 7 l min⁄ (Fig. 5.6). The measured steady state temperature (and, in brackets, the temperature rise over the coolant average temperature) was 31.1 °C (1.8 K) in the stator slot, 30.9 °C (1.6 K) in the winding overhang, as the stator ohmic losses are zero,

Fig. 5.4: Prototype machine: Measured no-load losses 𝑃 from a) measured no-load torque 𝑀 at no-load generator operation with two different magnet temperatures of 𝜗 35 °C and 100 °C and b) measured no-load losses from deceleration test 𝑃 ,

and only 34.6 °C (5.3 K) in the magnets, mainly due to air friction losses. The eddy cur- rent losses 𝑃 _, in the magnets are according to calculation negligibly small.

Fig. 5.5: Prototype machine: Measurement of the rotation speed over time during the deceleration of the rotor from maximum speed 𝑛 10000 min with magnet

temperature of 𝜗 35 °C 0 2000 4000 6000 8000 10000 12000 0 4 8 12 16 20 24

Rota

tion spe

e

d

n

/ mi

n

t

Fig. 5.6: Measured temperature rise in the prototype machine at no-load, rated speed 𝑛 4167 min 20 24 28 32 36 40 0 30 60 90 120 150 180 210 240 270

Temperature

ϑ

/

°C

t

Ambient Coolant outlet Coolant inlet

118