Variable Frequency Drives

A relatively new innovation for use in electric motor compressor drives is the variable frequency power source. Fundamentally, the power source converts an existing three-phase source into DC then uses an inverter to convert back to a variable frequency supply. Thyristors or transistors are used to switch the output at the required frequency.

Figure 7-6. An integral geared compressor driven by a motor with an open drip-proof enclosure. The enclosure also includes 85dba sound attenuation. (Courtesy of Elliott Company


The electric motor is basically a standard, single-speed, single-voltage motor. When receiving a variable frequency, the motor will operate at a variable speed. The motor may be either an induction or synchronous motor. The decision as to which type to apply is essentially similar to that for a single-speed application.

While the motor is basically standard, there are several items that may be considered when the variable frequency system and motor are purchased:

1 The insulation may need to be upgraded to protect from surge volt ages.

2 The winding reactance may need to be decreased to improve thyristor commutation.

3. The base temperature rise may need to be decreased to improve motor performance.

The merits of a variable-speed motor would appear to be obvious, as many compressors in the past have benefited from the variable speed available in a steam turbine. A compressor may be adjusted as required to meet the process needs. The advent of the variable-frequency drive returns some of the benefits to the process operator that were lost when the more favor able electric energy caused motors to replace steam turbines.

The inverters are either voltage source or current source (see Figure 7-7a and b). There are other variations, but they apply to drivers smaller than the ones used with compressors. However, pulse-width-modulated (PWM) (see Figure 7-7c), transistorized units are less complicated and are relatively maintenance-free with reliable units available to at least 500 hp. For all but the smaller compressors, the current source inverter is the one typically used. With a six-step voltage source, a rule of thumb has been to size the motor at two-thirds of its rating so as not to exceed the insulation temperature rise. For current source motors, the output torque is not constant with decreased speed, which fortunately is compatible with most compressors, as torque tends to follow speed. For current source drives, one needs to upsize the motor captive transformer by approximately 15% to account for harmonic heating effects.

Inverters do not output a pure sine wave but synthesize the output wave with pulses. Because of the pulses, harmonics are presented to the motor and, hence, the somewhat higher losses. Common systems are either 6 pulse or 12 pulse. This definition comes from the number of

(C.) Pulse width-modulated source.

pulses used to simulate the output wave form. The more pulses, the less severe the harmonics; however, the cost also goes up. The same issue also applies to harmonic reduction at the drive's rectifier input with the external power supply receiving the benefit of the higher pulse count. Figures 7-8, 7-9, and 7-10 are schematics of the rectifier and inverter cir-



Figure 7-9. Schematic of the rectifier and inverter circuit for a current source inverter,

cuit for a basic six-pulse system. For a 12-pulse system, phase shift transformers are added together with an additional six thyristors. The phase shift transformers shift the output of the second set of thyristors by 30°. If more pulses are needed, additional transformers and thyristors can be added in groups of six.

While cost will probably decrease with increased usage, this is a factor to be evaluated before a decision to use the variable frequency is made. Because of the harmonics, the output torque contains some unsteady torsional components. These can be handled by evaluating the compressor train torsional response. This will be further covered in a later chapter.




Figure 7-10. Schematic of the rectifier and inverter circuit for a pulse width-modulated inverter.


Figure 7-10. Schematic of the rectifier and inverter circuit for a pulse width-modulated inverter.

Application, relative to size, is not too much of a problem as the variable frequency drive has been used to 40,000 hp. High horsepower drives have been in service in Europe for a significant period of time [4]. In the U.S., the applications have been somewhat more modest in size but, as the popularity grows, the size will doubtless follow.

Probably the last hurdle is the cabinet size, which houses the electronics. It is large enough to be a factor in location. If located in an air-conditioned space, the air conditioning must be sized to accommodate the additional heat load, which is significant on the larger drives.

Variable-frequency drive technology is constantly improving in step with the advances in power electronic device technology and with the associated microprocessor controls. The following list of desirable features is offered:

Minimum input harmonics

Maximum input power-factor throughout the speed range

Minimum output harmonics and torsional excitations

Minimal tuning and setup required

Minimum maintenance, with maximum reliability

Permits the use of a standard or, at least, a more-standard motor

Higher drive efficiency, lower cooling requirement

Lower component count with a smaller footprint

Lower cost

At present, PWM current-source drives are available in sizes ranging upward into the thousands of horsepower range, as are stepped-PWM

voltage source drives. Both of these newer type drives offer minimal extra voltage stress and do not require the derating of standard motors. Because of the rapid growth of the power electronic technology, improved drives will continue to join the marketplace. This should continue until the above list is thoroughly satisfied.

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