Synchronous Motor

1.2 Synchronous Motors

Synchronous motors, as the name implies, rotate at a constant (synchronous) speed. The rotor of this type of motor is a wound rotor, which receives the excitation (magnetizing) current from its excitation system (a separate direct current source with controller). Synchronous motors, although they are more costly and require more maintenance compared to induction motors, are used in applications requiring constant speed (such as in the textile fiber and paper industries), a high degree of operating efficiency, and a controllable power factor. The last two factors are particularly important above 1000 kW. Synchronous motors are often used in large plants to drive the central air compressor. A large synchronous motor can be used to control the power factor of the whole plant, compensating the lagging power factor of a large number of medium and small induction motors.

3.2.3 Starting of Synchronous Motors

Therefore we need some means for starting synchronous motors. Once the rotor reaches a speed close to the synchronous speed (>95%) through some starting means, the field winding will be excited, so the motor locks into synchronization.

There are some techniques employed to start a synchronous motor. As a simple method, a separate motor is used to drive the rotor to run close to the synchronous speed. A second method is to start the synchronous motor as an induction motor. In this case, the rotor has a special damping winding for the purpose of starting, which is similar to the squirrel-cage arrangements of an induction motor. When the synchronous motor is starting, the torque will be produced by the induced current in the damping winding, so it can start as an induction motor. In addition, the damper winding has another function to help keeping the synchronous motor in synchronism. If the speed of the motor is increased or decreased from the synchronous speed, then a current will be induced in the damper winding and will develop a torque to oppose the change in speed.

As for the starting method of PMSMs used in variable speed drives, we can start the motor slowly at a reduced frequency by using a PWM inverter. In this case, a high starting torque can be developed by using the information of the rotor initial position.

2.2.20 Synchronous motors

where T_max is the maximum rated torque and δ is the load angle.

It is evident from equation (2.81) that synchronous motors have no starting torque and the rotor must be run up to synchronous speed by some alternative means. One method utilizes a series of short-circuited copper bars inserted through the outer extremities of the salient poles. The rotating magnetic flux induces currents in these ‘grids’ and the machine accelerates as if it were a cage-type induction motor (see Section 2.2.21). A second method uses a wound rotor similar to a slip-ring induction motor. The machine is run up to speed as an induction motor and is then pulled into synchronism to operate as a synchronous motor.

The advantages of the synchronous motor are the ease with which the power factor can be controlled and the constant rotational speed of the machine, irrespective of the applied load. Synchronous motors, however, are generally more expensive and a d.c. supply is a necessary feature of the rotor excitation. These disadvantages, coupled with the requirement for an independent starting mode, make synchronous motors much less common than induction ones.

Synchronous Motors

Synchronous motors have definite advantages in some applications. They are the obvious choice to drive large, low-speed reciprocating compressors and similar equipment requiring motor speeds below 600 rpm. They are also useful on many large, high-speed drives. Typical applications of this type are geared, high-speed (above 3600 rpm) centrifugal compressor drives of several thousand horsepower.

A rule of thumb that was used in the past for constant speed applications was to consider the selection of a synchronous motor where the application horsepower was larger than the speed. This, of course, was only an approximation and tended to favor the selection of a synchronous motor; it would be considered too severe by current standards. However, the rule can aid in the selection of the motor type by giving some insight as to when the synchronous motor might be chosen. For example, applications of several hp per rpm often offer a distinct advantage of the synchronous over the induction motor. In fact, at the lowest speeds, larger sizes and highest hp/rpm ratios may be the only choice.

One interesting characteristic of synchronous motors is their ability to provide power factor correction for the electrical system. Standard synchronous motors are available rated either 100% or 80% leading power factor. At 80% power factor, 60% of the motor-rated kVA is available to be delivered to the system as reactive kVA for improving the system power factor. This leading reactive kVA increases as load decreases. At zero load, with rated field current, the available leading reactive kVA is approximately 80% of the motor-rated kVA. The unity power factor machine does not provide any leading current at rated load. However, at reduced loads with constant field current, the motor will operate at leading power factor. At zero load, the available leading kVA will be about 30% of the motor-rated kVA.

Because of their larger size, 80% power factor motors cost 15 to 20% more than unity power factor motors, but the difference may be less costly than an equivalent bank of capacitors. An advantage of using synchronous motors for power factor correction is that the reactive kVA can be varied at will by field current adjustment. Synchronous motors, furthermore, generate more reactive kVA as voltage decreases (for moderate dips), and therefore tend to stabilize system voltage better than capacitors, since they supply maximum leading kVA when the voltage is at a minimum.

When higher than standard pull-out torque is required, 80% leading power factor motors should be considered. The easiest way to design for high pull-out is to provide additional flux, which effectively results in a larger machine and allows a leading power factor. The leading power factor motor may, therefore, be less expensive overall. However, leading power factor motors are generally “stiffer” electrically, and may need to be evaluated against the higher current pulsations that will result from reciprocating compressors or other pulsating loads. Larger flywheels can normally be applied to compensate for this effect.

Direct-connected excitors were once common for general purpose and large, high-speed synchronous motors. At low speeds (514 rpm and below), the direct-connected exciter is large and expensive. Motor generator sets and static (rectifier) excitors have been widely used for low-speed synchronous motors and when a number of motors are supplied from a single excitation bus.

Speed

Synchronous motors are classified according to their speed. They are either high-speed or low-speed machines. Those operating over 500 RPM are designated high-speed motors.

The speed of a synchronous motor is dependent on the frequency of the power source and the number of poles the stator has. RPM increases directly with frequency and inversely as the number of poles. The same formula used to calculate the speed of the induction motor is used to determine the speed of this type of motor. The motor will run at synchronous speed and will not have the slip required by the other three-phase induction motors. Varying loads within the rating of the motor will not cause the RPM to change.

Power Factor for Alternating Current

The power factor is the factor by which the apparent kVa power is multiplied to obtain the actual power, kW, in an alternating current system. It is the ratio of the in-phase component of the line current to the total current [39].

In induction motors, the magnetizing component of the current always lags by 90°. Therefore, the line current lags at all loads; the magnitude depends upon the magnetizing current load.

Review the types of motors proposed for a process plant with a qualified electrical engineer; such an evaluation of the mix of synchronous and induction motors will help the new power factor for the plant, because a net lagging factor for plants means that all power to that plant will cost more than if the factor were unity or leading. From Brown and Cadick [40]:

Apparent power = EI, or VA or kVA

Active power = EICosθ, or W, or kW

Note: θ = vector diagram angle of current between apparent power and active power

Reactive power = EISinθ, or VAR, or kVAR

Compute power factor:

F_p = active power/apparent power

F_p = EICosθ/(EI) = cosθ

F_p = W/(VAR) = (kW)/(kVAR)

Power charges are based on kVAR demand; thus, the lower the power factor, the higher the demand charge. See Plankenhorn [41], Valoda [42], and Lazar [43] for helpful discussions of this subject. Power charges are based on VAR demands; thus, the lower the power factor the higher the demand charge.

Most process plants must be careful to maintain a favorable power factor for their system, otherwise a penalty may be placed on power costs. If the power factor falls below some set value – say 0.8 – power costs increase because the actual power (as current) going into work (horsepower) is considerably less than the total supplied to the plant system. The difference is that which goes into the magnetizing field (reactive current), which does not represent actual work. By adding synchronous motors or capacitors to an otherwise all-induction load system, you may raise the power factor from a lagging condition to unity (or nearly so). The synchronous motors may be designed to furnish varying amounts of leading power factor. This is a study or balance that must be recognized at the time of plant design, and recommendations should be prepared by competent electrical power engineers.

The usual synchronous motor power factors are unity (1.0) or 0.8 leading. Values of 0.7 or 0.6 leading will give more leading correction to an otherwise lagging system.

This is always lagging for an induction motor. For a synchronous motor of power factor (PF) = 1.0, the kVA and kW are equal and for any PF less than 1.0 – that is 0.9, 0.8, 0.7, etc., the PF is leading. Also see References [44–46].

30.4.1 Introduction

Variable-speed synchronous motors have been widely used in very large-capacity ($>\mathrm{M}\mathrm{W}$) pumping and centrifuge-type applications using naturally commutated current-source thyristor converters. At the low-power end, the current-source SPWM inverter-driven synchronous motors have become very popular in recent years in the form of permanent-magnet brushless dc and ac synchronous motor drives in servo-type applications. There are certain features of three-phase synchronous motors that have allowed them, especially the lower capacity motors, to be controlled with high dynamic performance using cheaper control hardware than is required for the induction motor of similar capacity. Since the average speed of the synchronous motor is precisely related to the supply frequency, which can be precisely controlled, multimotor drives with a fixed speed ratio among them are also good candidates for synchronous motor drives. This section begins with the performance of the variable-speed nonsalient-pole and salient-pole synchronous motor drive using the steady-state equivalent circuit followed by the dynamics of the vector-controlled synchronous motor drive.

AC synchronous motor

When current U is at 0 degree, current V has a delayed phase difference of 120 degrees (2π/3 rad), while current W leads with a phase difference of 120 degrees (2π/3 rad). In Fig. 11.10, it can be assumed that positive current produces a magnetic flux away from the rotor, and negative current produces flux in the opposite direction. The thin arrow vectors on the figure represent the magnetic fields produced by each coil, while the wide arrow vectors represent the resultant magnetic field that is a sum of the total magnetic flux.

For example, when phase U is at 0 degree, the corresponding coil (U) produces maximum magnetic flux in the direction away from the center of the rotor due to the supplied maximum positive current. Simultaneously, coil V and W produce reversed magnetic flux due to the supplied negative current. The summation of these three fluxes produces a resultant magnet field in which the rotor rotates in the direction of alignment. Throughout the rotation of the rotor, the magnitude of the resultant magnet field remains constant with the direction changing at the rotation speed.

In the synchronous AC motor, the speed of rotation is synchronized to the frequency of the AC input current. Also, produced torque can be regulated with the phase difference between them. Therefore, the available torque for the variable load is adjusted automatically by changing the phase difference even if the rotational speed is constant in the synchronous motor.

The torque of a synchronous motor is produced by the phase difference between the rotating magnetic field and the rotor. The torque is low when there is a small phase difference between the rotating rotor and the rotating magnetic field. The torque increases as the phase difference increases, which is due to adding additional load on the motor. The torque will reach a maximum when this phase difference approaches 90 degrees. Therefore, a synchronous motor can produce suitable torque under variable load conditions. The magnitude of the maximum torque is regulated by controlling the peak magnitude of the input current. If the torque is required to be varied independently of load, vector control (also referred to as field-orientated control) can be applied.

Synchronous AC motors and brushless DC motors are used in many clinically available RBPs. This is primarily due to the sensorless speed control and torque control characteristics of the design. The use of synchronous motors also allows the integration of contactless electromagnetic bearing systems when vector control is employed. Some examples of clinically available pumps that employ these motors are the HeartMate II & 3 and the HeartWare HVAD.

9.2.3 The Synchronous Motors

Synchronous motors are built with PM excitation, with nonexcited high magnetic saliency or with electromagnetically excited rotors (for large powers). When used for variable-speed drives, in general, with voltage source converters (cycloconverters, matrix converters, or single- or double-level PWM inverters) no cage in the rotor is placed. Only for current source machine commutated inverter synchronous motor drives is a strong rotor cage required to reduce the machine commutation inductance L_c = (L′′d+L′′_q)/2 such that up to 150% rated current natural commutation is secured.

For the time being let us neglect the rotor damper cage presence, as is the case for most low and medium power drives.

The space vector equations in rotor coordinates are

Now for i_F =i_F0 = constant (L_dmi_F0 = Ψ_PMd) we have the case of PM rotor if L_q≥ L_d. On the other hand for i_F0 = 0 and L_d>>L_q we obtain the high saliency passive rotor.

For this latter case low remanent flux density (B_r< 0.4 T) low-cost PMs may be located in axis q, and thus

The torque T_eis

Thus (9.28)–(9.33) represent the space phasor model of practically all cageless rotor synchronous motors.

The PMs in axis qin Fig. 9.7c serves evidently to increase torque production (9.33) and to improve the power factor. This motor is called a PM assisted reluctance synchronous motor. Sometimes it may also be called the IPM reluctance synchronous motor.Based on the space vector diagram we alter the torque expressions. We may express the torque (9.31) in terms of stator flux Ψ_s and the torque angle β, instead of i_d, i_q:

Finally (9.31) yields:

Note again that for the d axis PM rotor L_d≤ L_q while for the d axis excited rotor L_d≥ L_q. However, the expression (9.36) of torque remains the same in both cases.

Finally for the q axis PM reluctance rotor (L_d≥ L_q) synchronous motor (IPM-RSM) the torque expression (9.33) becomes

In (9.37), the sign + is for positive δ and – for negative δ.

Let us notice that in both cases, to modify the torque we have to change either the stator flux amplitude Ψ_s or the torque angle δ, much as in the case of the IM, with the difference that now the coordinates are fixed to the rotor rather than to the rotor (stator) flux (for the IM). Also for the IM only the second terms in (9.36) and (9.37) are visible as no source of dc magnetization on the rotor was considered for the IM.The stator voltage equation of SMs in stator coordinates is identical to that of the IM:

It follows that all the rationale for the DTFC of IMs is valid also for the SMs—with a few pecularities:

• The state (flux, torque, speed) observers are to be adapted to SM model

• For the electromagnetically excited SM, where the power factor is unity or leading, the TOS is to be slightly changed [5]

• For the electromagnetically excited rotor SM, the excitation (field) current (voltage) control is introduced (additionally) to keep the stator flux under control and the power factor angle constant either at zero (φ₁ = 0) or negative (φ₁ = −(6–8)°); the case of DTFC for excited rotor SM will be dealt with in a separate paragraph

• The flux–torque coordination is to be treated in what follows

4.2.4.1 Advantages and Disadvantages of Brush and Brushless Excitation