A sensorless efficiency test system for a high-speed permanent magnet synchronous motor
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We present a sensorless efficiency test system with energy recovery for a high-speed permanent magnet synchronous motor (PMSM). In the system, two identical high-speed PMSMs are used as the motor under test (MUT) and the load machine (LM), respectively. A new sensorless vector control (VC) method based on a hypothetical reference frame is presented to control both the MUT and the LM. Also, a regenerating unit is used to implement energy circulation to save energy. Experiments were carried out on a prototype, with a digital controller based on the TMS320F28335, to verify the adequacy of the sensorless VC method. As a result, the efficiency test system achieves the load test at the speed of 21000 r/min without any reduction equipment. During the test, the energy regenerated by the LM could be fed back to the MUT by the regenerating unit, and 81.31% electrical power was saved. In addition, with the proposed sensorless VC method, both the MUT and the LM can work at id = 0 without a position sensor.
KeywordsDistributed generation High-speed permanent magnet synchronous motor (PMSM) Sensorless control Energy saving
With the advantages of small size, high efficiency, and high reliability, permanent magnet synchronous motors (PMSMs), especially high-speed PMSMs, have been used extensively for distributed generation and industrial drives in recent years [1, 2, 3, 4]. In order to determine the performance and characteristics of PMSMs, a loading facility is required for the load test. However, a conventional loading facility, such as an eddy dynamometer, mechanical dynamometer and hysteretic dynamometer, requires a reduction gearbox for the high-speed PMSM. Some types of facility also require a water cooling system for high power operation. Not only will it be extremely noisy but it also wastes energy and water during the test [5, 6, 7].
In order to save energy, there have been attempts to produce electric dynamometers that can implement energy circulation. In , Wang et al designed an electric dynamometer with a DC generator as the load motor. The recovered energy is fed back to the power grid through the energy regenerating unit. With the energy recovery system, this DC dynamometer is more efficient than traditional ones. However, according to the commutator, the DC dynamometer cannot run at a high speed. Therefore, recent research into electric dynamometers focuses on squirrel cage induction motors which can be operated at a higher speed.
Yang et al presented a novel AC electric dynamometer based on a squirrel cage induction motor with an improved direct torque control (DTC). The experiments showed that the proposed dynamometer realized energy recovery and had good dynamic performance during the test [9, 10]. Liu et al presented an asynchronous dynamometer using an induction machine by DTC . In , the asynchronous dynamometer was in the generating mode by DTC, while the experimental motor supplied the driving torque by employing field-oriented control, and the two electric machines were connected with a common-DC-bus to implement energy circulation. As the energy was circulating between motor and dynamometer, this method could save test energy without any harmonic pollution to the power grid. However, DTC brought torque ripple which could be a problem for the performance test. Therefore, some scholars like Zhang paid attention to the vector control (VC) . In , the proposed dynamometer was connected to the power grid by a back-to-back pulse width modulation (PWM) converter to feed back energy. An improved VC based on flux-oriented rotor was adopted to make the induction motor operate in generating mode with controllable torque. The effectiveness of the proposed VC was verified by MATLAB-based simulation. In , Chu presented a dynamic testing system with energy recovery for an electric vehicle. By the VC, the dynamic testing system can be controlled from light load to full load to accomplish the dynamic test. His test showed that the testing system saved 65%~70% of the energy by simulating the full-range speed and torque output.
However, existing asynchronous dynamometers are still restricted for high-speed PMSM. First, due to the extreme high speed, the PMSM and dynamometer need to have accurate alignment. However, it is difficult to achieve precise coaxial operation with two facilities of different size. In addition, the VC requires the rotor position which is detected by position sensor such as encoders and resolvers to commutate and control current. However, these features increase the cost and size of the dynamometer and reduce reliability.
This paper presents a sensorless efficiency test system with energy recovery for the high-speed PMSM. A novel sensorless VC method based on a hypothetical reference frame is presented to control both the motor under test (MUT) and the load machine (LM). Then experiments are carried out with a 75 kW high-speed PMSM to verify the adequacy of the method. As a result, the proposed system can be used at different speeds and power levels with energy recovery. During the whole test, considering the d-q frame, both MUT and LM can be operated with zero phase current in the d axis (id = 0) without any speed or position sensor.
2 Structure of efficiency system
3 Sensorless VC method for MUT
For the MUT, the purpose is to make it work as a motor to supply the driving torque. In addition, we hope the whole system has good dynamic performance and high efficiency. VC is considered to be an ideal method for PMSMs. However, traditional VC methods require rotor flux position, which cannot be measured in high-speed PMSMs. Therefore, the sensorless method is essential for the system. The typical sensorless VC method for PMSMs is to estimate the rotor flux position using the instantaneous voltage equation of the motor and the detected current, and observers are usually used to enhance the robustness of the control system [14, 15, 16, 17, 18, 19]. However, similarly to the use of observers, these sensorless methods require excessive computing time in the case of high-speed PMSMs, because of the higher carrier frequency, which is unacceptable. Therefore, a convenient sensorless VC method striving for id = 0 is presented in this paper.
As is apparent in the figure, the d axis is the actual rotor flux axis, whereas the δ-γ frame is the hypothetical coordinate system assumed in the controller.
Thus, self-synchronization can be achieved according to iγ as decreasing ωr for iγ < 0 (∆θ > 0) and increasing ωr for iγ > 0 (∆θ < 0). As there is no unknown quantity needing to be estimated, this sensorless VC method is simpler and more convenient than the traditional sensorless methods.
The mathematical model of the control system is now presented as follows.
4 Sensorless VC method for LM
In the proposed system, the LM works as a generator to load the MUT. Thus the purpose of the control method is to regulate torque current to run the LM in the fourth quadrant. In addition, the LM is expected to have good dynamic performance and high efficiency.
Firstly, assume θE = 0, the initial rotor position of the LM is the same as the MUT, which means the angular error is the same for the MUT and the LM side, as △θ = △θ′. As stated in Section 3, using sensorless VC, the δ axis is coincident with the q axis (△θ = 0) in the steady state on the MUT side.
Therefore, in the LM side, the δ′ axis will also be coincident with the q′ axis (△θ′ = 0) with the effect of the MUT. In that case, we use two current regulators to keep iδ′<0 and iγ′ = 0. Then according to the torque equation (6), the LM will supply negative braking torque to the MUT. The more negative iδ′ is, the more load is on the MUT, and more energy is fed back. In addition, because iγ′ = 0, the LM will work in the condition of id′ = 0 with minimum copper loss.
As iγ′ = 0, the torque current iq′ will have the same sign as iδ′ under the restriction of θE\(\in\)(− π/2, π/2), that is to say, the LM will also provide braking torque to the MUT if iδ′ < 0. However, according to (11), the current vector iδ′ contributes not only to torque current iq′, but also to the excitation current id′. Also, as the angular error trends to zero, the great mass of armature current will act on the q′ axis. In conclusion, with the restriction of θE\(\in\)(− π/2, π/2), the control method above is also available to the case of θE≠0. In addition, an applied method to reduce θE will be introduced in the experiments section.
5 Experimental results
5.1 Comparative tests for sensorless VC method
Specification of servo motor
Rated output PN (kW)
Rated speed VN (r/min)
Rated torque TN (Nm)
5.2 Efficiency tests
Specification of high-speed PMSM
Rated output PN (kW)
Rated voltage UN (V)
Rated speed VN (r/min)
Rotor inertia J (kgm2)
Armature resistor RS (mΩ)
Armature induct LS (μH)
A static current vector is created in the MUT side to coincide the d axis with the A phase axis to initialize the rotor position of the MUT. This is common in the conventional VC system. As the coupling of two rotors, the rotor of the LM will rotate to a constant position. Then it will record the rotor position of the LM.
The same process is performed in the LM side to coincide the d′ axis with the A′ phase axis. The rotor position of LM is again recorded.
An angular error can be obtained from the above two positions and a compensation with the angular error should be performed to the initial position of the δ′-γ′ frame. As a result, the initial position difference θE between the MUT and the LM can be approximately eliminated.
The experiment is performed with in three steps:
Figure 15a shows the responses of armature current at both the MUT and LM sides for the various load torques. It can be seen that the RMS of both iA and iA′ increase with the increasing load torque with iA from 11.62 A to 64.26 A, as well as iA′ from 13.3 A to 68.89 A. During the test, the current quality of the LM is inferior to the MUT side with more harmonic components that influence the measured results. Therefore, the RMS of the armature current on the LM side is slightly larger than on the MUT side. The current components on both sides are shown in Fig. 15b, and it is clear that with the (negative) increasing of iδ′ (from − 0.98 A to − 101.19 A), more load acts on the MUT. While on the MUT side, because of the sensorless VC, the torque current iδ automatically climbs from 9.43 A to 111.85 A to overcome the load torque. In addition, the magnetizing current iγ and iγ′ are around zero all the time. This indicates that for the various torque currents, both the MUT and LM work in the condition of id = 0 without any position sensors. The power curves for each part of the system are shown in Fig. 15c. Pe and Pe′ are the electrical power for the MUT and LM, respectively; Pin is the input power of the whole system from the power grid; and Pd is the output mechanical power of the MUT as measured by ET1103. It can be seen that with the increase of load, both Pe and Pe′ increase obviously as Pe from 2.24 kW to 22.47 kW and Pe′ from 0 kW to −19.44 kW. The negative power means the energy circulates from motor to the inverter. However, the total input power of the efficiency test system grows modestly from 3.11 kW to 4.2 kW as the losses of two PMSMs and inverters. That is to say, the proposed system achieves the load test with only 4.2 kW whereas 22.47 kW is needed for traditional dynamometers. And 81.31% electrical power is saved by the energy feedback method. The efficiency curves for both MUT and LM are shown in Fig. 15d. With the use of two identical PMSMs, the two efficiency curves are approximately coincident. The final efficiency of the MUT is about 93.95% with an output power of 21.11 kW.
6 Conclusion and discussion
Using an identical PMSM as the LM, the efficiency test system can achieve the load test without any reduction equipment with a speed of 21000 r/min.
During the test, the energy regenerated by the LM can be fed back to the MUT by the regenerating unit, and thereby 81.31% electrical power is saved.
With this sensorless VC method, both the MUT and LM work in the condition of id = 0 without a position sensor.
The sensorless VC method is based on estimation of back EMF, thus it is inapplicable at a very low speed.
The sensorless VC is independent of the armature resistance RS, but it needs accurate knowledge of the inductance LS.
As a single closed-loop control method, the proposed sensorless VC is a little weaker than the existing dual closed-loop system in dynamic performance.
The control system of the MUT and LM are interdependent.
This work was supported by the Science and Technology Project of State Grid Corporation, “Research on Key Technologies of Flexible Control Strategy for Variable Speed Pumped Storage Units” and the Fundamental Research Funds for the Central Universities (No. B18020574).
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