Motion System Design

Honey, they shrunk the drive!

Through evolving technology, drives continue to physically shrink. The latest advance, application specific intelligent power modules, enable engineers to design full-featured ½ to 2 hp, adjustable-speed ac drives that fit in the palm of your hand.

Physically smaller drives solved several problems for a manufacturer of metering pumps used in food, laundry, and process industries. Earlier pump models used a two-speed motor with contactors to handle speed switching. A NEMA 12 enclosure protected the contactors from dust, but took up a lot of space. In addition, the drives required constant maintenance because the contactors wore over time, causing arcing and eventual failure.

A GPD 205 drive with matched 3-phase motor, from MagneTek Drives & Systems, New Berlin, Wis., replaced the two-speed motor and contactors and solved these problems, Figure 1. This drive measures 2.7-in. wide, 5.0-in. high, and 3.4-in. deep — small enough to be installed in a NEMA 4 wash-down enclosure. Mounted on the metering pump motor in such an enclosure, it is unharmed by exposure to water and spray. A heat sink keeps the drive cool from the excess heat of the motor.

Using new technology, this drive was able to offer features not normally found in drives its size. These features helped improve the pump system’s performance. According to the OEM, the drive helps extend pump-system life four to five times over the previous two-speed motor arrangement. Part of this life extension comes from the drive’s use of soft-start and soft-stop profiles that help reduce the amount of wear and tear on motor windings and mechanical components. The OEM also reports that some of its customers gained a 40 to 50% savings in energy use.

How they shrank the drive

Over the last several years, manufacturers have been using intelligent power modules (IPMs) to reduce the size of drives. These modules use powerful microprocessors and surface mount technology to squeeze more features and functions onto smaller printed circuit boards. However, because the power converter of an ac drive consists of a gatedriver board and a 3- phase inverter section, the overall size of the drive remained basically the same. Early versions of gate-drive printed circuit boards, Figure 2, contained the power supply for both control and gate-driver power, and for the gate-driver circuits. These circuits provide isolation and translate logic-level signals used to control the transistor gate signals. The power section typically contains the input rectifier, bus filter capacitors and bi-polar or insulated- gate bi-polar transistors (IGBTs).

Engineers, though, were able to design a smaller drive by using the next generation of intelligent power modules — Application Specific IPMs (ASIPMs). These modules, Figure 3, are manufactured by Mitsubishi.

They integrate power and gate drive circuitry into one package, eliminating the gate-driver printed circuit board and the isolation components between control and power converter sections. This reduces package size and lowers parts count, which improves reliability. A 1 hp drive, for example, can fit in a shirt pocket, Figure 4.

In addition, engineers designed the gate driver and power converters to operate using the low signal levels (+5 and +15 V) available from a microprocessor, microcontroller, or custom IC circuit. This use of low-voltage supplies and low powered gate-firing circuitry helped reduce package power losses by 25%. Use of PWM technology also saved energy by reducing motor heating.

ASIPM circuit configuration

The main circuit configuration of an ASIPM contains:
• A 3-phase ac input section, consisting of a diode rectifier circuit.
• A 3-phase IGBT inverter output section consisting of three positive-side and three negative-side IGBTs.
• Gate drive circuitry.
• A motor phase-current detection circuit.
• Protective and fault logic.
• A dynamic braking IGBT.

Completing the power converter section are bus precharge circuitry, bus capacitors, and metal oxide varistors (MOVs), which protect against line surges. Three capacitors are also required for the module’s charge pump circuit.

To interface to the module requires signals from the main control card (5-V logic level) and Schmitt trigger circuitry to receive fault logic signals.

The module has interlocking circuitry to protect upper and lower IGBTs located in the same phase from being activated at the same time. The module requires only +5 and +15-V power sources. This eliminates the multiple power supplies from a switching regulator.

The +15-V power source is required for the gate drive circuits of both upper and lower IGBTs. However, the lower and upper gate circuits do not have a common return path. The power converter control section has a common return path, thus charge pump circuitry is added to provide the required isolation and sufficient power to the upper IGBT devices.

A 5-V PWM signal controls the switching of the IGBTs. Switching frequencies can be up to 20 kHz for low-noise operation. The IGBTs enable more than 100% starting torque at 1.5 Hz. Peak currents are reduced by as much as 42%.

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An application specific IPM independently incorporates protective functions, eliminating reliance on the speed and priority of the microprocessor for protection. The protective features in the module are current limit, short circuit, overcurrent, overheat, overvoltage and control-power fault. There is also analog current detection for all three output phases without the use of current transformers (CTs) or Hall-effect sensors.

Applications for drives with application specific IPMs include commercial laundry equipment, where limited space requires small drives; food processors and mixers, which benefit from a small package but require flexibility and soft start features; pumps, drill presses, and commercial routers; as replacements for motor-control starters; and in conveyor and material handling applications where the drive’s reduced size allows it to become an integral part of a motor.

Application specific IPM-based drives are available for 115 and 230 Vac, single or three-phase input power applications, with 460 VAC packages expected soon.

Power ranges are 1/8 to 2 hp. Performance is best in V/Hz and speed-controlled applications requiring a maximum output frequency of 400 Hz.

Information for this article was provided by Ron Koehler, manager, AC Products, MagneTek Drives and Systems.

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