Slip Power Recovery (Kramer)

33.3.3.4 Slip Power Recovery (Kramer)

In this type of converter, which is described in Table 33.8, the rotor current of a slip-ring wound-rotor induction motor is rectified and the power then reconverted to AC at fixed frequency and fed back into the supply network. For traditional designs the low frequency slip ring currents are rectified with a diode bridge and the DC power is then inverted into AC power at mains frequency.

The traditional designs had poor AC mains dip immunity, high torque pulsation and high levels of low frequency AC supply harmonics. The latest generation of this type of drive is called the Rotor Drive and uses PWM-VSI inverters for the rotor and AC supply bridges.

This keeps sine wave currents in the AC rotor circuits and the drive has many advantages over traditional circuits including:

• No torque pulsation • Low AC harmonics • Very high immunity to AC supply dips • Very cost-effective if a limited speed range is required,

but still requires a separate starter

892 Y. Shakweh • Inherent ability to run at rated speed without electronic

The PWM VSI drives offer the highest possible performance circuits

of all variable speed drives; refer to Table 33.9. Recent improve- • Converter cost reduced by 2:1 if uses the ± speed ability ments in switching technology and the use of micro-controllers to give a speed range

have greatly advanced this type of drive. The inverters are now able to operate with an infinite speed range. The supply power factor is always near unity. Additional hardware is easily added

if there is a requirement to regenerate power back into the The availability of power electronic switches with turn-off mains supply. Motor ripple current is related to the switching capability; e.g. FETs, BJTs, IGBTs, and GTOs have currently frequency and in large drives the motor may be derated by less favored drives with voltage-fed PWM converters on induction. than 3%.

33.3.3.5 PWM-VSI Converter

TABLE 33.9 Drives features Type

DC DRIVE

AC DRIVE

PWM-VSI Motor type

DC Cyclo

CSI (FCI)

CSI (LCI)

Kramer

• DC motor

• Induction and

• Induction or

wound rotor synchronous

motors

induction motor

Power • Up to 10 MW

0.5 to 50 MW • 0.5 to 2 MW • Speed range

• < 1200 rpm • 10,000 rpm speed

• Limited by

motor capability

Performance • High torque

• High torque • High torque over speed

• High torque

• Poor dynamic

• High torque

over speed over speed range

over speed

response

over speed

range • High dynamic

range

• Low starting

range

range

• High dynamic • High dynamic performance

• High dynamic

performance performance

performance

Advantages • Simple

• Regenerative • Good Power regenerative

• High stall

• Tolerant to

• Slip ring

supply dips

wound rotor • Standard

• Robust motors

• Minimal

motor

• High over-load robust

• High over-load

• Minimal

derating Disadvantages

capacity

• Stall torque

• Complex rating

• Motor custom

• Complex

• Motor custom

• Complex

• Motor custom • Expensive • Motor

design

• Poor dynamic

design

• Regeneration at maintenance

• Low AC supply

extra cost • Custom motor

Power factor

Applications • Mill drives

• Pumps, fans, • Process lines (ball and sag)

• Mill drives

• Pumps, fans,

• Pumps, fans,

• Paper machines • Marine

(ball and sag)

compressors • Traction propulsion

• Mine winders

generation • Process lines

• Mine winders

• Marine

• Mills (ball and • Conveyors

• Mill drives

33 Drives Types and Specifications 893

motor drives are used for duties other than continuous. As the Table 33.9 summarizes the main features of all types of con- output attainable under such deviating conditions may differ

33.3.3.6 Comparison

verter drives discussed above and assesses their merits and from the continuous rating, fairly accurate specification of the drawbacks. It also illustrates typical applications.

duty is an important prerequisite for proper planning. There is hardly a limit to the number of possible duty types.

In high performance applications, such as traction and

33.4 Load Profiles and Characteristics

robotics, the load and speed demands vary with time. During acceleration of traction equipment, a higher start-up torque

The way the drive performs is very much dependent on (typically twice the nominal torque) is required; this is usu- the load characteristics. Here are four load characteristics ally followed by cruising and deceleration intervals. As the described.

torque varies with time so does the motor current (and motor flux linkage level). The electric, magnetic, and thermal load- ing of the motor and the electric and thermal loading of the