Permanent Magnet Motor

2.2.26 The d.c. permanent magnet (PM) motor

The response times for PM motors are very fast and the torque increases directly with the input current, independently of the speed or the angular position. Multiple-pole machines maximize the output torque per watt of rotor power. Commercial PM motors are available in many sizes from 35 milliNewton-metres at about 25 mm diameter to 13.5 Newton-metres at about 3 m diameter.

Direct-drive rate and position systems using PM motors utilize d.c. tachogenerators and position sensors in various forms of closed-loop feedback paths for control purposes.

3.3.2.5 Ultrahigh Speed Permanent Magnet Motor Applications

The application of PMMs with extra high speeds (up to 12,000 RPM) in ESP systems has definite advantages but necessitates the use of special equipment. More details are given in the following, based on the data from a major manufacturer [43].

The operating speed range of ultrahigh speed (UHS) equipment is between 1,000 and 12,000 RPM, although their special pumps can operate at speeds up to 15,000 RPM. Because of this extremely high operating speed the whole ESP system is significantly shorter, less than half to one-third of the length of conventional units. The compact design of the ESP system makes it possible to assemble and test the unit at the manufacturer's facility; it is delivered to the well fully assembled and is ready to run in the well. Before installation, the MLE must be attached to the motor only; this feature considerably decreases installation time and labor costs as well as minimizes rig time and any human errors. Typical installations require one-third to one-fourth of the installation time of a traditional ESP system. The compact design of the UHS ESP system permits deeper pump setting depths and running through high dogleg severity sections.

The UHS submersible pump is a modular multistage centrifugal pump designed for a speed range of 1,000–12,000 RPM. The floater design pump is equipped with stages distributed in several pump modules, with seven to nine stages per module depending on pump model. Pump stages are manufactured on computer numerical controller machines from solid pieces of high-grade stainless steel. Radial stability of pump stages at UHSs is crucial, all bearings and bushings must be of abrasion-resistant quality and made from hard materials (bimetallic, tungsten carbide facing radial bearings and silicon carbide toward pump shaft). Axial loads in the floating impellers are taken up by downthrust washers made of a hard material and every module has its thrust bearing to carry the load from the discharge pressure. The use of hard alloys in manufacturing maximizes the ESP's abrasion and corrosion resistance. The shaft of low-rate pumps may be hexagonal and may have double keyways with separate keyways for each module to minimize shaft dimensions and increase pump efficiency.

Thanks to the much higher motor oil volume and the active cooling system the UHS motors' thermal behavior is better than that of the IMs. Heat transfer between motor housing and reservoir fluid depends on the same parameters (fluid rate, casing size, etc.) as in standard systems and similar fluid velocities around the motor are required for proper cooling. UHS motors are designed for low voltage and low current and have a flat, at least 91%, power efficiency over a wide range of motor loading.

Comparison of UHS PMM and standard IM installations has shown that considerable power savings can be realized with UHS equipment because of the higher motor efficiency and lower current requirement [44]. However, when UHS systems are compared to normal- or high-speed (up to 6,000 RPM) PMM installations, similar power efficiencies can be expected. In such cases the advantages of using UHS systems come from other features such as shorter pump and motor lengths and better corrosion and abrasion tolerance.

It is also important to note that UHS ESPs have significantly better performance in low-rate and slim-hole applications. For example, the efficiency of UHS pumps producing 100–200 bpd is greater than 50%, whereas standard-speed pumps have efficiencies typically below 40%. The higher efficiency of the UHS pumps leads to longer system run life and less problems associated with high pump temperatures, such as scale deposition, MLE overheating, and bearing problems.

Control

The flux demand has been set to zero because the flux is provided by the magnets. However, if the field current demand was not set to zero, the control system would provide a flux component of stator current that could either increase or decrease the effect of the magnet's flux, depending on the polarity of the reference signal. Clearly there will be an upper limit on the flux because of saturation of the magnetic circuit, and in practice, reducing the flux is a more attractive proposition because it allows us to operate in a field weakening mode and so extend the speed range into a constant power region, as discussed at the end of Section 9.5.

It would be possible to apply field weakening control by applying a speed-dependent term (− i_d), but this would require a good understanding of the machine characteristics, which is far from easy. A simpler approach can be applied if we think back to how we control a d.c. motor in field weakening (see Chapter 2). Steady-state operation up to base speed requires the stator voltage to increase with speed, but once base speed has been reached, the voltage, by definition, cannot increase any further. For the PM motor drive the situation is no different and so negative i_d must be imposed via the stator current, and a simple way to do this is shown by the additional “Voltage Control (Field Weakening)” loop at the top of Fig. 9.29.

Field weakening operation of PM motors presents an interesting practical problem that may occur if the control system should fail when running at high speed. For example, suppose the motor is running at three times base speed and for some reason the control system loses control. The component of stator current that is opposing the magnet flux disappears and so the rotor is spinning at three times base speed with full airgap flux, and the terminal voltage will rise to three times its rated value. The motor insulation systems and the power converter components are not normally rated to withstand such voltages and so catastrophic failure will result. Rating all components for such a situation would usually be cost prohibitive, and so other means of protection need to be sought. A common solution is to put a simple crowbar circuit near to the motor terminals, such that if such a fault does occur, and the terminal voltage rises, it is limited to an acceptable level.

6.1 Permanent magnet motor drives

Permanent magnet motor drives are a very important and rapidly growing sector of the drives market, which is why we have devoted a large section of this chapter to a theoretical study aimed at understanding what determines their steady-state behavior. In terms of their application in drives we have little new to introduce because the converter circuits used are exactly as we have already discussed in Chapters 7 and 8Chapter 7Chapter 8 for the induction motor, and the control is also very similar.

Our discussion of field-oriented control in Chapter 7 focused heavily on the ability to control the torque-producing and field-producing components of the stator current (i_T and i_F) independently and rapidly, in order to provide virtually instantaneous control of torque. We do exactly the same for the permanent magnet synchronous machine except we normally set the demand for i_F to zero, as there is no need to provide the working flux from the stator side because it is provided by the rotor magnets.

As with the induction motor drive the motor is supplied from a voltage source inverter, and again we are therefore controlling the stator currents via the voltage. The control strategy and the inverter are essentially the same as for an induction motor – indeed some manufacturers make life easier for users by offering a single inverter product suitable for both types of motor.

As with the induction motor control strategy, the critical issue is determining the flux position (θ_Ref), but in this case the task is very much easier as the flux is aligned to the rotor position. In high-performance systems a high-resolution absolute encoder would be fitted to the motor shaft to provide an accurate rotor position signal so that the flux position is known precisely at all times. However, if a position sensor is not used then the reference frame angle, θ_Ref, can be derived from computed motor voltages and currents in the same way as for the induction motor, but in this case we no longer have the complication of the temperature-dependent rotor time-constant.

As we have discussed before, model-based schemes break down at very low frequencies because the voltage components become very small. For low-speed operation it is therefore necessary to supplement the approach with an alternative such as a position-sensing scheme using injected high-frequency currents. Or, in systems where the load is reasonably predictable, such as in a domestic washing machine, the motor can be brought up to a particular speed using open-loop switching in much the same fashion as a stepping motor (see Chapter 10) until the motional e.m.f. is large enough to be used as the control signal.

We have said with some satisfaction that because we have magnets on the rotor, we do not need to provide current to develop the field, and consequently the field current demand in Figure 9.10 is set to zero. However, if the field current demand were not set to zero, the control system would provide a flux component of stator current that could either increase or decrease the magnet’s flux, depending on the polarity of the reference signal. Clearly there will be an upper limit on the flux because of saturation of the magnetic circuit, and in practice, reducing the flux is a more attractive proposition because it allows us to operate in a field weakening mode and so extend the speed range into a constant power region, as discussed at the end of section 5.

15.6.1 Windings of Brushless PM Motors

Motor Drive

A 6 V permanent magnet motor is used since it will run from the same 5 V supply as the PIC. However, a large decoupling capacitor (C4) should be fitted because the motor will generate a lot of noise on the supply, particularly when it switches off. The motor current direction, forward or reverse, is controlled by switching on two of the four MOSFETs from RA4 or RA5. Q1 and Q3 are switched on if RA4 output goes high (motor forward), and Q2 and Q4 if RA5 goes high (motor reverse). Q1 and Q2 switch on when the gate is high (N-FET), but Q3 and Q4 switch on when the gate is low (P-FET), so an inverting bipolar stage is needed on each gate. The current flows diagonally through the bridge and motor to drive it in either direction. The bridge is rated at 30 A, so a range of small to medium dc motors could be driven successfully. In the simulation, the motor characteristics can be adjusted to represent different motors: nominal voltage, coil resistance, coil inductance, zero load rpm, effective mass of the motor and load, and the number of encoder pulses per revolution.

10.3 Rotor position control strategy

The inset type of permanent magnet motor has q-axis salient poles so that the operation strategy is based upon the relationship between the suspension force and the suspension winding current in a salient-pole permanent magnet bearingless motor, which ensures that the rotor radial position control can be performed [2]. The relationship between the suspension force and the current was shown in (7.21) and the system control diagram was illustrated in Figure 7.17. In addition, the voltages at the motor winding terminals can be expressed using (7.3) in synchronously rotating coordinates, and the rotational torque is given by (7.8).

6.5.3 Stepper Motor

The stepper or stepping motor produces rotation through equal angles, the so-called steps, for each digital pulse supplied to its input. For example, if with such a motor 1 input pulse produces a rotation of 1.8° then 20 input pulses will produce a rotation through 36.0°, 200 input pulses a rotation through one complete revolution of 360°. It can thus be used for accurate angular positioning. By using the motor to drive a continuous belt, the angular rotation of the motor is transformed into linear motion of the belt and so accurate linear positioning can be achieved. Such a motor is used with computer printers, x–y plotters, robots, machine tools and a wide variety of instruments for accurate positioning.

There are thus, in this case, four possible rotor positions: 45°, 135°, 225° and 315°.

There is another version of the stepper motor and that is a hybrid stepper. This combines features of both the permanent magnet and variable reluctance motors. They have a permanent magnet rotor encased in iron caps which are cut to have teeth. The rotor sets itself in the minimum reluctance position in response to a pair of stator coils being energised.

The variable reluctance stepper motor does not contain a magnet and this helps to make it cheaper and lighter and so able to accelerate more quickly. However, this lack of a magnet means that when it is not powered there is nothing to hold the rotor in a fixed position. The permanent magnet motor tends to have larger step angles, 7.5° or 15°, than the variable reluctance motor. The hybrid motor typically has 200 rotor teeth and rotates at 1.8° step angles. They have high static and dynamic torque and can run at very high step rates. As a consequence they are very widely used.

Some applications require very small step angles. Though the step angle can be made small by increasing the number of rotor teeth and/or the number of phases, generally more than four phases and 50–100 teeth are not used. Instead a technique known as mini-stepping is used with each step being divided into a number of equal size substeps by using different currents to the coils so that the rotor moves to intermediate positions between normal step positions. For example, this method might be used so that a step of 1.8° is subdivided into 10 equal steps.

VI.B Principles of Operation

The magnetic poles of conventional PM motors are fixed in one position and the rotational speed is tied in with the frequency. In a WPM the magnetic poles are more flexible; the machine incorporates a special coil that “imprints” poles onto a ring of magnetic material attached to the spinning rotor. This coil operates in much the same way as a tape recorder, but, unlike a tape recorder, which is designed to erase and record sound only occasionally, the Written-Pole coil writes and erases the magnetic poles with the line frequency during startup. The alternating magnetic poles serve to pull the rotor forward, which increases the rotor's speed during each rotation until the motor reaches full design speed. When the full speed is achieved, the pole writing procedure ceases and the poles remain as last written.

The Written-Pole technology does not work on its own to start the motor and it is not activated until the motor is brought up to 80% of its design speed by a squirrel-cage rotor. Whereas in ordinary motors the squirrel cage is made of low-resistance material in order to reduce the losses during running, the WPM uses high-resistance material in order to reduce the starting current. The lower starting current and, hence, the lower generated heat allow the WPM to start loads more than 10 times as heavy as those that can be started with conventional motors of the same size.

11.2.2.2 Output Torque of Permanent Magnet Synchronous Motor at d-q Axis Reference Frame