Variable speed drives; Ratings, standards and testing--Motors, motor control and drives (part 3)

Variable speed drives

The next section will cover details of different types of power electronic circuits which can be used in conjunction with the motors described to provide speed control and position control of the rotor shaft. A more detailed treatment can be found in the reference. The concept of quadrants in the torque-speed plane is crucial to the specification for a variable-speed drive. A drive operating with positive torque and positive speed (or speed in the direction which the user defines as forwards) is said to be operating in the first quadrant of the torque-speed plane. If the torque is reversed, braking or generating action takes place and power is extracted from the load. This is second quadrant operation. Many applications require the drive to operate in both motoring and braking modes; these are divided into drives that simply dissipate the regenerated power in resistors and drives that return power to the ac supply by a dedicated inverter. If the speed falls to zero under the braking action and the torque is maintained, the third quadrant will be entered in which both speed and torque are negative and the drive is motoring in the reverse direction. By reversing the torque once more, braking is achieved as the drive moves into the fourth quadrant.

Basic drive systems only operate in the first and third quadrants; the more sophisticated systems will operate in all four. The speed and smoothness of the transition between quadrant is a significant test of the quality of the drive; dc motor drives have excelled in this respect and much of the development of ac motor drives has been targeted at improving this aspect.

DC motor drives:

These are probably the simplest motors to control, since separate control over field excitation and armature current is normally available. Typically the field is supplied through a single-phase controlled rectifier and the armature through a three-phase controlled rectifier. The reference gives a full description of the operation and also discusses the control aspects of these drives.

AC motor drives:

Earlier, we explained the dependence of the speed of the induction motor on supply frequency. It follows that by supplying the motor from a suitable source of variable frequency, the synchronous speed of the motor can be varied. For correct operation, the ratio of supply voltage to frequency also has to be kept sensibly constant so the voltage also has to vary with the flux. This gives rise to the use of a frequency inverter for controlling an ac induction motor. PWM inverters are now by far the most common in small and medium sizes, using the high switching speeds of IGBTs or MOSFETs; older designs of 6-step inverters are still found in very large sizes. A full discussion of different circuit topologies is given in Section 36 of reference 10C. In recent years, much effort has gone into improving the transient performance of inverter drives in an attempt to take market share from dc drives where high control bandwidth is required. This has led to the development of vector control, field- oriented control and direct torque control by different manufacturers, all of which control the position of the rotor current in relation to the position of the rotating flux wave in the air gap. Such systems require rotor position feedback, which can be derived by hardware such as a shaft encoder, or software such as an observer.

In larger sizes, and commonly above 1 MW, synchronous machines are preferred to induction motors. These are normally driven by a different style of thyristor inverter, which uses the back emf of the machine itself to commutate the thyristors.

Switched reluctance drives

The switched reluctance motor cannot be operated without its power converter. While it’s possible to operate it from a variable frequency inverter, with appropriate changes to the control, such complexity is not required. The most common configuration for the power converter, although many other variants exist, many of them offering a reduced number of switches at the expense of reduced control flexibility. In all cases, the power switches are in series with a phase winding, offering a number of advantages to the converter designer, not least the possibility of reduced switch size. Switching speeds are normally lower than in ac inverter drives, since there is no requirement for PWM at high speeds to synthesize a sinusoidal voltage.

Commissioning of drives

Most commercially available drives have control systems which allow the drive to be tailored to suit a variety of applications. This is especially true of general purpose drives sold from a catalogue; these generally have to be tuned to suit the parameters of the load, particularly if transient performance is an issue. Some systems have a degree of self-tuition and simply require the execution of a set-up program to allow the system to test the load so that correct responses can be stored for future operation.

With the improvements accruing from cheap on-board signal processing, most systems now have comprehensive and intelligible fault signaling ability, and setting up new drives is now much simpler than with first-generation drives.

Ratings, standards and testing

National and international standards exist for specifying the rating and performance of several types of motor. Many of these also specify the test methods to be used, although these tests are often complex and can be done only by the manufacturer or a specialized independent test site. Contract testing can be undertaken by universities or consulting organizations. In the Electrical Drives academic center for testing machines and drives of all types on a contract basis, having testing capacity up to 750 kW and up to 15 000 rev/min at low power. Where the motor is a component in a drive system, the situation is much less clear, since there are few standards, other than EMC and Harmonic Limits which apply to complete systems. For free circulation within the European market, CE marking under the Low Voltage and EMC Directives is required.

Table 1 indicates the most commonly encountered standards, with approximate national and international equivalents where appropriate. In spite of ongoing attempts at harmonization, there remains in some cases no simple correspondence between these standards and further advice (for instance from catalogues of national standards) should be taken for details of the equivalence of individual parts.

For motors other than standard induction motors, and particularly for drives, it’s usually difficult to find a standard which is entirely relevant. In these cases, a detailed specification drawn up between the supplier and the customer is generally preferred, referring where appropriate to particular parts of other standards to cover specific aspects of construction and performance.

Table 1 National and international standards for electric motors and drives; Performance and rating:

Noise and vibration:

Safety:

EMC and harmonics:

Construction and dimensions: 6s: 4999, especially parts 105, 141 and 147; 5000 part 10 EN 60035-6 and EN 60035-7 EN 60529 60072 601 36 IEC: 60034-5 NEMA MG-13 BS: 4999 part 102 5000 part 10 EN 60034-1 and EN 60034-12 60072 EN 60034-9 and EN 60034-13 EN 60704 60072 3456 (many parts now superseded); EN 60335 5345 5501 (potentially explosive atmospheres); IEC: 60335 60079 UL 1004 BS: EN 50081 EN 50082 EN 55014 EN 55104 EN 61800 IEC: 1000 50081 50082 55014 55104 61800 IEC: 60034-2 NEMA: MG-10 6s: IEC: 60034-2 NEMA: MG-3 BS: NEMA: MG-2