A speed-adjustable electric drive consists of three main systems:
- A synchronous motor with permanent magnets (PMSM)
- A power supply system
- A control system
The main element that contributes the advantages of the drive is PMSM.
PMSM is an electrical machine, in which the excitation field is created by strong permanent magnets in the rotor, and switching of stator phase windings is implemented by using semiconductor devices according to the position of the rotor relative to the stator. This type of machines provides constancy of the electromagnetic torque, good power performance and flexible regulation characteris-tics, while keeping high efficiency.
Typically, PMSM has a stator with a magnetic core and a multiphase winding, which is in-stalled in the slots that are uniformly distributed around the circumference of the magnetic core; and a rotor with magnetic multipolar system, consisting of ferromagnetic poles and alternate blocks of per-manent magnets, as shown in figure 1.
Figure 1 - PMSM cross-section (а) and phase current and voltage oscillograms (b), (here Т is a period: Т = 60/p/n)
The interaction of the rotor magnetic flux with the current layer, formed by the currents flow-ing trough the stator winding, create the electromagnetic tangential torque. Torque value depends on the relative position of rotor poles and the stator current layer. There is an optimum relative position for which the electromagnetic torque is maximum (for a given current value). As rotor moves, the rotor poles move along the machine boring circumference. To maintain the optimum position of the stator current layer relative to rotor poles, phases switching is performed, i.e. switching-off of the phase that runs off the pole and switching-on of the phase that runs on the pole. This switching occurs synchronously with rotor motion. This keeps a permanent position of the current layer relative to ro-tor pole (with accuracy to one slot pitch) and appropriate to operational conditions with the maximum torque.
The required switching positions are determined by using a rotor position sensor, built in mo-tor. It’s also possible to work without a rotor position sensor, but phase voltage analysis should be performed instead.
Many PMSM units, such as a stator, rotor cylinder, frame with the end bearing shields, cooling system, are practically identical in design and manufacturing technologies to similar units of com-mercially manufactured synchronous and asynchronous motors.
The basic principle design features of PMSM are: the presence of permanent magnets in the ro-tor and a specific geometrical configuration of the machine magnetic circuit.
PMSM power and control systems, regarding to their composition and operating principle, practically does not differ from the frequency converters that are serially manufactured for asynchro-nous and synchronous motors. The function of the PMSM power supply system (PSS) is to convert the supply DC or AC voltage into AC voltage, required for supplying of PMSM phase windings, or AC currents of stator windings.
The structure of a modern PSS for speed-adjustable motors is generally based on a double en-ergy conversion principle which allows meeting all PMSM demands.
The principle of double energy conversion consists in the following. At first, the industrial fre-quency supply voltage is converted to DC voltage (current) by using mains controlled or uncontrolled rectifiers. Then the DC output is smoothed with the help of an inductive, capacitive, or combined inductive-capacitive filter. Finally, the smoothed DC voltage (current) is converted to AC voltage (current) by using current or voltage inverter, at that the form of voltage or current, as well as its fre-quency, are determined by the requirements of PMSM phase windings.
It should be noted that such PSS structure enables us to obtain voltage or current with a fre-quency exceeding the mains frequency significantly. If the output converter voltage (current) fre-quency is less than supply voltage frequency, PSS can be built on the principle of direct conversion of the supply voltage.
In each case the type and design principle for the power supply system are determined by a combination of PMSM characteristics and technical requirements stated for the drive as a whole.
The main point in the design of the adjustable-speed drives based on PMSM, that are manufac-tured by RPA “ Russian Electric Drive ", is an integrated approach to the design of both the machine and the power supply and control systems that ensures optimal matching of their parameters and op-erating modes. This improves the efficiency of the drive as a whole.
Main advantages of electric drives based on PMSM, as compared to serially produced electric drives based on synchronous (SM) and asynchronous motors (AM), are as follows:
- improved efficiency due to a absence of losses in excitation system (by permanent magnets)
- smooth start without any disturbance in the mains (see figure 2).
- possibility of 100% speed adjustment;
- high efficiency in the whole range of power and speed adjustment (see figure 3);
- higher specific characteristics as compared to commercial SM and AM (see figure 4);
- absence of the power supply line to the rotating elements (slipping contacts are excluded);
- ow level of vibration and noise.
Comparative characteristics of synchronous motors with permanent magnets (PMSM), asynchronous motors (AM) and asynchronous motors with soft start (AM with SS)
Figure 2 – Starting performance of the electrical drives: current (а) and full consumed power (b).
Figure 3 – Efficiency of PMSM and PMSM-based drive (a) and efficiency of PMSM and AM (b)
Figure 4 – Comparative overall dimensions of 14 MW motors: PMSM (yellow) and commercial SM (blue)