Starting Method for a Variable-Frequency Synchronous Reluctance Motor
Abstract Generated abstract
This paper proposes a starting method for frequency-controlled synchronous reluctance motors intended to reduce starting current, shorten acceleration, and prevent sticking near half synchronous speed. The authors analyze the stator current components during direct starting and relate the torque dip in the mechanical characteristic to rotor asymmetry and the reverse-field component. They argue that starting at operating voltage with twice the operating supply frequency, then switching to the operating frequency at subsynchronous speed, avoids the torque dip while reducing converter current demand. Experimental characteristics and transient oscillograms for a DRS-150M motor indicate reliable starting under rated load and a reduction in starting time of about 1.2 seconds.
Full Text
A Method for Starting a Frequency-Controlled Synchronous Reluctance Motor
Yu. M. Achkasov, B. A. Zakharov
(Presented by the scientific and technical seminar of the Research Institute of AEM)
Low-power synchronous reluctance motors (SRM) are most often started by the method of direct (asynchronous) starting. Direct starting of a simple synchronous motor under the control law
\[ \frac{U}{f}=\mathrm{const}, \]
where
\[ \begin{aligned} U&\text{— mains voltage;}\\ f&\text{— mains current frequency,} \end{aligned} \]
is accompanied by a large starting current and by the presence of a dip in the static mechanical characteristic because of the asymmetry of the starting winding due to the salient-pole construction of the rotor, which considerably increases the starting time and may cause the motor to “stick” during acceleration under load.
Elimination of the motor “sticking” during acceleration under load, reduction of the starting time and of the starting current of the frequency converter can be obtained if starting is carried out at an operating supply voltage and frequency (corresponding to the law \(\frac{U}{f}=\mathrm{const}\)) that are twice the operating values, and then, when the motor reaches subsynchronous operating speed, the frequency of the supply current is switched to the operating value. The method will be clear from the following description (Fig. 1):
a) the static mechanical characteristic of the synchronous reluctance motor with the known starting method;
b) the static mechanical characteristic with the proposed starting method;
c) the part of the static mechanical characteristic along which the motor accelerates to subsynchronous speed;
d) the static torque on the motor shaft.
Let us consider the expression for the stator current during direct starting of a synchronous reluctance motor:
\[ \dot I_c=\dot U_e\left[ \frac{Y_d+Y_q}{2}\cdot e^{j\omega t} + \frac{Y_d+Y_q}{2}\cdot e^{-j[(2s-1)\omega t-2\gamma_0]} \right], \tag{1} \]
where
\[ \begin{aligned} U &\text{— phase voltage;}\\ Y_d, Y_q &\text{— conductances along the longitudinal and transverse axes (from the motor equivalent circuit);}\\ \omega_1 &\text{— angular speed of rotation of the stator magnetic field;}\\ t &\text{— time;}\\ s &\text{— slip;}\\ \gamma_0 &\text{— value of the angle between the material axes of the stationary and rotating coordinate systems at } t=0. \end{aligned} \]
From expression (1) it is evident that the stator current consists of two components:
\[ \dot I_c=\dot I_{c1}+\dot I_{c2}. \tag{2} \]
The first component \(I_{c1}\) corresponds to the forward-traveling field wave, inducing in the stator winding the frequency \(f_1\). The second component \(I_{c2}\) arises from the electromotive force induced in the stator winding at the frequency \(f_1(2s-1)\) by the reverse field, which appears owing to rotor asymmetry. At slip \(s=0.5\), the current \(I_{c2}\) is equal to zero. For small deviations from one half of the synchronous speed, the current \(I_{c2}\) has a low frequency.
Fig. 1. \(a\) — mechanical characteristics of a simple SRM; \(b\) — connection diagram of a capacitive DRS—150M
Entering into interaction with the counter-rotating field of the rotor, the current creates an additional rotating torque, which is added to the principal rotating torque when \(s<0.5\) and subtracted when \(s>0.5\). As a result, a dip appears in the static mechanical characteristic, because of which the starting time increases substantially and the motor may “stick” near a speed equal to one half of synchronous speed if the minimum value of the starting torque developed by the motor becomes smaller in magnitude than the resisting torque.
Starting a synchronous reluctance motor at an increased frequency makes it possible to eliminate the dip in the static mechanical characteristic and, consequently, to shorten the acceleration time. With this method, at the moment when the motor reaches the subsynchronous speed corresponding to the operating frequency, by switching the supply-current frequency to the operating value, the motor is pulled into synchronism and reaches the synchronous operating speed; i.e., acceleration is carried out along that part of the static mechanical characteristic corresponding to twice the operating frequency, where addit-
additional torque from the current \(I_c\) is added to the principal rotating torque. Since starting of the motor is carried out at an unchanged supply voltage (the operating voltage) and at a frequency twice the operating frequency, the starting current decreases in accordance with the increase in the reactive resistance of the SRM. This makes it possible to reduce the power of the frequency converter while preserving a starting torque sufficient for starting the motor with rated load.
The operating voltage may be equal to the rated voltage or differ from it, depending on the required starting torque and the required rotational speed of the motor.
Fig. 2. Experimental mechanical characteristics of the DRS—150 M.
Fig. 3. Graphs of motor-starting transients.
Oscillograms of the static mechanical characteristics \(n=f(M)\) for the improved SRM of type DRS—150 M with the known starting method are presented in Fig. 2,a, and with the proposed method—in Fig. 2,b. The motor was started up to a speed of 750 rpm at an operating frequency of 25 Hz according to the circuit shown in Fig. 1,b. In this case, because of the zero-sequence currents, a dip in torque is observed with the usual starting method. With the proposed method the dip is absent, which ensures reliable starting.
It follows from the oscillograms of the motor-starting transient with rated torque on the shaft \(n=f(t)\) (Fig. 3,a, b) that the starting time with the new method is less by 1.2 sec.
The method can be used for starting frequency-controlled synchronous reactive motors with a short-circuited winding on the rotor, designed for supply by three-phase, two-phase, or single-phase current; various circuits of capacitor SRMs and capacitor induction motors; and other types of synchronous motors with electrical or magnetic asymmetry on the rotor.
REFERENCES
- D. A. Zavalishin et al. Low-Power Electrical Machines. Gosenergoizdat, Moscow—Leningrad, 1963.