Permanent Magnet Motor

As with the PM motor, the SR motor has no electric or magnetic losses in the rotor.

From: Encyclopedia of Energy, 2004

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Electrical and electronics principles

Charles J. Fraser, in Mechanical Engineer's Reference Book (Twelfth Edition), 1994

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

The d.c. permanent magnet (PM) motor is a continuous-rotation electromagnetic actuator which can be directly coupled to its load. Figure 2.56 shows the schematic representation of a d.c. PM motor. The PM motor consists of an annular brush ring assembly, a permanent magnet stator ring and a laminated wound rotor. It is particularly suitable for servo systems where size, weight, power and response times must be minimized and where high position and rate accuracies are required.

Figure 2.56. D.C. permanent magnet 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.

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Electrical Submersible Pump Components and Their Operational Features

Gabor Takacs Ph.D, in Electrical Submersible Pumps Manual (Second Edition), 2018 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.

Typical performance curves of an UHS submersible pump are shown in Fig. 3.35 at a nameplate speed of 10,000 RPM [43]. As seen, extremely high heads are developed by the stage because of the high speed; this usually results in a low number of required stages and a short pump.

Figure 3.35. Performance curves of a ultrahigh speed pump.

PMMs used in UHS service of a major manufacturer [43] have six poles and a rated speed of 10,000 RPM at a frequency of 500 Hz. Their rotors are considerably shorter than those of standard IMs and thus have a much higher power density. The heat generated by the motor's operation at such high speeds required the development of an enhanced cooling system. Cooling is provided by an active system (in contrast to the passive cooling system used in IMs) that consists of

an oil-circulating pump and heat exchanger built in the motor base

a protector with an increased oil capacity.

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.

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Synchronous, permanent magnet and reluctance motors and drives

Austin Hughes, Bill Drury, in Electric Motors and Drives (Fifth Edition), 2019


PM motors are almost always fed from a voltage source inverter, and so the control strategy is different from that for the current source inverter shown in Fig. 9.27. The arrangement of a typical field oriented control system for a PM motor is shown in Fig 9.29, in which the asterisk (⁎) is used to denote demanded quantities. As mentioned previously, the scheme is very similar to that for the induction motor (see Fig. 8.16).

Fig. 9.29. Field oriented control scheme for permanent magnet synchronous motor.

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 (− id), 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 id 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.

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Synchronous and Brushless Permanent Magnet Machines and Drives

Austin Hughes, Bill Drury, in Electric Motors and Drives (Fourth Edition), 2013

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 (iT and iF) 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 iF to zero, as there is no need to provide the working flux from the stator side because it is provided by the rotor magnets.

The arrangement of a typical field-oriented control system is shown in Figure 9.10.

Figure 9.10. Field-oriented control scheme for permanent motor synchronous motor.

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 (θRef), 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, θRef, 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.

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Basics of the Electric Servomotor and Drive

George Ellis, in Control System Design Guide (Fourth Edition), 2012

15.6.1 Windings of Brushless PM Motors

Windings of brushless PM motors are distributed about the stator in multiple phases. Usually there are three phases, each separated from the others by 120° (electrical). Brush motors can have many more phases, but having a large number of phases in brushless PM motors is impractical because each phase must be individually controlled from the drive, implying a separate motor lead and set of power transistors for each phase. A simplified winding diagram of a three-phase motor is shown in Figure 15.20.

Figure 15.20. Simple winding set for a three-phase four-pole motor.

Brushless motors rely on electronic commutation. The drive monitors the rotor position and excites the appropriate winding to maintain the commutation angle at approximately 90°. Consider Figure 15.21, which shows a brushless rotor in a sequence of three positions as it rotates clockwise. The large arrows show the flux created by the windings. To simplify the drawing, the field flux is not shown, but recall that it points out of the north poles and into the south poles. In brush motors, the commutation angle is maintained by mechanically switching phases in and out. Because the brush motor has many phases, each phase represents only a few electrical degrees of rotation and the torque from a brush motor is smooth. An equivalent technique is used on brushless motors in a commutation method called six-step, but it produces large torque perturbations at each transition because brushless motors usually have just three phases.

Figure 15.21. Clockwise commutation sequence of 60°-steps maintains the winding flux between the magnet poles.

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PIC Motor Applications

Martin Bates, in PIC Microcontrollers (Third Edition), 2011

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.

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Inset types of permanent magnet bearingless motor

Masahide Oshima, in Magnetic Bearings and Bearingless Drives, 2005

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).

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Correction Elements

William Bolton, in Instrumentation and Control Systems (Third Edition), 2021

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, xy plotters, robots, machine tools and a wide variety of instruments for accurate positioning.

There are two basic forms of stepper motor, the permanent magnet type with a permanent magnet rotor and the variable reluctance type with a soft steel rotor. Figure 6.35 shows the basic elements of the permanent magnet type with two pairs of stator poles.

Figure 6.35. The basic principles of the permanent magnet stepper motor (2-phase) with 90° steps.

Each pole of the permanent magnet motor is activated by a current being passed through the appropriate field winding, the coils being such that opposite poles are produced on opposite coils. The current is supplied from a d.c. source to the windings through switches. With the currents switched through the coils such that the poles are as shown in Figure 6.35, the rotor will move to line up with the next pair of poles and stop there. This would be, for Figure 6.35, an angle of 45°. If the current is then switched so that the polarities are reversed, the rotor will move a step to line up with the next pair of poles, at angle 135° and stop there. The polarities associated with each step are:

Step Pole 1 Pole 2 Pole 3 Pole 4
1 North South South North
2 South North South North
3 South North North South
4 North South North South
5 Repeat of steps 1–4

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

Figure 6.36 shows the basic form of the variable reluctance type of stepper motor. With this form the rotor is made of soft steel and is not a permanent magnet. The rotor has a number of teeth, the number being less than the number of poles on the stator. When an opposite pair of windings on stator poles has current switched to them, a magnetic field is produced with lines of force which pass from the stator poles through the nearest set of teeth on the rotor. Since lines of force can be considered to be rather like elastic thread and always trying to shorten themselves, the rotor will move until the rotor teeth and stator poles line up. This is termed the position of minimum reluctance. Thus by switching the current to successive pairs of stator poles, the rotor can be made to rotate in steps. With the number of poles and rotor teeth shown in Figure 6.36, the angle between each successive step will be 30°. The angle can be made smaller by increasing the number of teeth on the rotor.

Figure 6.36. Basic principles of a three-phase variable reluctance stepper motor.

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 following are some of the terms commonly used in specifying stepper motors:



This is the number of independent windings on the stator, e.g. a four-phase motor. The current required per phase and its resistance and inductance will be specified so that the controller switching output is specified. Figure 6.35 is an example of a two-phase motor, such motors tending to be used in light-duty applications. Figure 6.36 is an example of a three-phase motor. Four-phase motors tend to be used for higher power applications.


Step angle

This is the angle through which the rotor rotates for one switching change for the stator coils.


Holding torque

This is the maximum torque that can be applied to a powered motor without moving it from its rest position and causing spindle rotation.


Pull-in torque

This is the maximum torque against which a motor will start, for a given pulse rate, and reach synchronism without losing a step.


Pull-out torque

This is the maximum torque that can be applied to a motor, running at a given stepping rate, without losing synchronism.


Pull-in rate

This is the maximum switching rate or speed at which a loaded motor can start without losing a step.


Pull-out rate

This is the switching rate or speed at which a loaded motor will remain in synchronism as the switching rate is reduced.


Slew range

This is the range of switching rates between pull-in and pull-out within which the motor runs in synchronism but cannot start up or reverse.

Figure 6.37 shows the general characteristics of a stepper motor.

Figure 6.37. Stepper motor characteristics.

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.

To drive a stepper motor, so that it proceeds step by step to provide rotation, requires each pair of stator coils to be switched on and off in the required sequence when the input is a sequence of pulses (Figure 6.38). Driver circuits are available to give the correct sequencing and Figure 6.39 shows an example, the SAA1027 for a four-phase unipolar stepper. Motors are termed unipolar if they are wired so that the current can only flow in one direction through any particular motor terminal, bipolar if the current can flow in either direction through any particular motor terminal. The stepper motor will rotate through one step each time the trigger input goes from low to high. The motor runs clockwise when the rotation input is low and anticlockwise when high. When the set pin is made low the output resets. In a control system, these input pulses might be supplied by a microprocessor.

Figure 6.38. Input and output for a drive system for a stepper motor.

Figure 6.39. Driver circuit SAA1027 for a 12 V 4-phase stepper motor.

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.

Section 4.4.2 shows the application of a stepper motor to the control of the position of a tool. A manufacturer’s data for a stepper motor includes: 12 V 4-phase, unipolar, Step angle 7.5°, Suitable driver SAA1027.


A stepper motor is to be used to drive, through a belt and pulley system (Figure 6.40), the carriage of a printer. The belt has to move a mass of 500 g which has to be brought up to a velocity of 0.2 m/s in a time of 0.1 s. Friction in the system means that movement of the carriage requires a constant force of 2 N. The pulleys have an effective diameter of 40 mm. Determine the required pull-in torque.

Figure 6.40. Example.

The force F required to accelerate the mass is


The total force that has to be overcome is the sum of the above force and that due to friction. Thus the total force that has to be overcome is 1.0+2=3 N.

This force acts at a radius of 0.020 m and so the torque that has to be overcome to start, i.e. the pull-in torque, is

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Electric Machines, Rotating, Construction and Theory

Leo Birenbaum, Enrico Levi, in Encyclopedia of Physical Science and Technology (Third Edition), 2003

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.

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Basic Control of AC Motor Drives

Kyo-Beum Lee, ... Yeongsu Bak, in Control of Power Electronic Converters and Systems, 2018 Output Torque of Permanent Magnet Synchronous Motor at d-q Axis Reference Frame

The torque of the synchronous permanent magnet motor can be obtained from the mechanical output of the motor. In terms of the d-q axis coordinates, the input power of the motor can be expressed as:


From the d- and q-axis coordinates derived early, substituting the stator voltage and linkage flux equation into the power equation, it can be expressed as follows:


In the above equation, the electrical input is expressed in terms of the stator copper loss, time variation of the integrated magnetic energy, and the mechanical output. The torque is obtained by dividing the mechanical output by motor speed ωr, and it can be expressed as a d-q axis variable as:


where P is the number of poles. The generated torque is same to the current as seen from Eq. (11.45). Moreover, there is a reluctance torque coming from the difference of the electromagnetic torque and the inductance generated from flux ϕf and current iqsr by the permanent magnet at the torque of the IPMSM.

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