Chapter 7 Experimental Validation of the Designed Topology 7.1 Introduction The position of permanent magnet Stepper motor is measured without using sensors, [58-60] but till now no references are available to measure steady-state torque of PMH stepper motor without using any sensors. This Chapter discusses about experimental arrangement of prototype PMH stepper motor, which is designed as Topology 6, i.e. uniform airgap with airgap length of 0.137 mm without extra teeth between stator poles to measure steady-state torque without using any sensors by conducting no-load and load tests. 7.2 A Novel Method to Measure the Steady-State Torque using DSP Processor A novel experimental set-up is arranged to measure steady-state torque of Topology 6 without using any sensors. A prototype motor which is considered for experiment is having the design specifications as shown in Table 3.1. Two such identical PMH stepper motors of same rating of 1.5 Nm are coupled using special aluminum coupling forming PMH stepper motorgenerator set as shown in Fig. 7.1. This arrangement forms PMH stepper motor-generator set. One of the machines in this set is excited programmatically through DSP processor working as PMH stepper motor and drives the other machine which works as generator and induces voltage in the phase windings. The speed of the motor is increased by increasing drive speed in steps/second by varying time constant in the DSP processor programme which controls PMH stepper motor excitation sequence. This generated voltage in the generator mode machine phase windings is measured at no-load and speed is measured at different load currents by applying resistive load. The block diagram of experimental setup is shown in Fig. 7.2. 191
Fig. 7.1 Prototype PMH stepper motor-generator set Fig. 7.2 Block diagram of PMH stepper motor-generator experimental setup 7.2.1 No-Load Test on Motor - Generator Set The PMH stepper motor of motor - generator set is excited by MOSFET converter connected to a 12 V DC voltage source. The motor phase currents are independently controlled by two hysteresis-based controllers which generate the MOSFETs drive signals. The output switching commands of the DSP control board are obtained via its digital ports and interfaced with the converter through opto-isolated gate drive circuits. Output of this bridge is controlled by DSP IC through Matlab programme using DSP code-composer as shown in Fig. 7.3. 192
Fig. 7.3 Block diagram of the experimental system using DSP controlled MOSFET bridge for PMH stepper motor excitation Firing pulses of one of the MOSFETs in the bridge are shown in Fig. 7.4 and Fig. 7.5 for drive speeds of 25 steps/second and 5 steps/second respectively. The total experimental set-up for steady-state torque measurement is shown in Fig. 7.6.The generator mode machine phase winding terminals are kept open without connecting any load and connected to the storage oscilloscope and digital multimeter to obtain generated phase voltage waveform and magnitude 193
respectively. The wave shape of generated voltage per phase is shown in Fig. 7.7 which is stored in storage oscilloscope. The speed of the motor is varied from 1 rad. / sec to 30 rad. / sec by varying time constant in DSP IC and corresponding generated voltages are measured and tabled in Table 7.1. Fig. 7.8 shows relation between PMH stepper generator generated voltage in Volts to speed in radians/second. Fig. 7.4 Firing pulses of one of the MOSFETs in the bridge for drive speed of 25 steps/second Fig. 7.5 Firing pulses of one of the MOSFETs in the bridge for drive speed of 5 steps/second 194
Fig. 7.6 Proto type PMH stepper motor - generator experimental setup Fig. 7.7 Generator mode machine phase winding generated voltage wave shape in storage oscilloscope Table 7.1 No-load induced voltages in PMH stepper generator at different speeds Speed (radians/second) 0 05 10 20 30 No-load induced voltage (V) 0 5.8 11.6 23 34 195
Fig.7.8. No-load Induced voltages in PMH generator at different Speeds 7.2.2 Load Test on Motor-Generator Set A resistive load (0 25 Ω) is connected to generator mode machine terminals. Speed of the motor is noted down at different load currents of generator and shown in Table 7.2. The relation between speed in rad. / sec and load current in A is shown in Fig. 7.9. Table 7.2 PMH motor Speeds and no-load induced voltages at different load currents Load current (A) 0.2 0.5 1 1.5 Speed (rad. /sec) 30 29 28 27 No-load induced voltage (V) 34 32.5 30.8 27.2 Fig. 7.9 PMH motor speed at different load currents 196
7.2.3 Steady-State Torque Calculation using No-Load and Load Tests Results From the results of no-load and load tests on PMH stepper motor-generator set, steady-state torque is calculated and is shown in Table 7.3. Table 7.3 Steady-state torque of PMH stepper motor at different load currents with the corresponding speeds Current (A) 0.2 0.5 1 1.5 Speed (radians/second) 30 29 28 27 steady-state torque (Nm) 0.23 0.56 1.1 1.51 Speed corresponding to rated current 1 A is obtained as 28 radians/second from load characteristic. No-load induced voltage corresponding to this speed of the motor is obtained as 28.5 V from no-load characteristic. The product of no-load voltage and load current of PMH generator is considered as input of generator mode machine which is equal to mechanical output of PMH stepper motor. Torque of PMH stepper motor is obtained dividing this power with corresponding angular speed. The corresponding toque calculated at rated current of the motor (1 A) is equal to 1.1 Nm. The same procedure is repeated at 1.5 A load current and torque calculated is equal to 1.51 Nm. 6.3 Summary This novel method is used to measure steady-state torque of prototype PMH stepper motor which is similar to Topology 6, without any sensors. It is proved that the prototype PMH stepper motor steady-state torque which is obtained by experimental method (1.1 Nm) is almost equal to that which is obtained by FEM and equivalent circuit analysis in unit 4 (1.1422 Nm). It is also observed that the drive consumed 1.5 A to get rated steady state torque of 1.51 Nm. 197