inverter drives

An inverter is a driving unit for three phase motors.

It controls motor speed by outputting a three phase voltage of variable frequency.

Supply for smaller inverters is normally 230 V single phase, with a three phase 230 V output.

Large ( 2.5 + kW ) inverters normally require a 400 V three phase supply and have a 400 V output.

Commercial units normally have features such as overload protection, slow start, controlled slow stop......

Hope this helps.

Dear Irshaad:

Just to add a little more to Johnathan Barnes' response:

An inverter is so named because it takes DC bus voltage and inverts this voltage, as expressed at the output. For half the cycle, anyway. ;>)

They can also go by the name Variable Frequency Drive (VFD), when used with motors. Your usage "inverter drive" indicates it is used with a motor, but it could be any sort of load. However, inverters are not "required" to provide adjustable frequency output, and VFDs typically are expected to provide this capacity.

Commonly there is a section that converts AC to DC, then a section that fabricates either single or three phase power from the DC supply. There will typically be some sort of power factor correction in the AC->DC side, to minimize current draw. The output (DC->AC) side can have a whole host of features.

The devices that you plug in a cigarette lighter that provide some few amps of single phase 115vac power are called "inverters".

There a great number of caveats to using a motor at a frequency other than what is on its nameplate.

David A. Smith

Thx for your replies, it really helped. I would like to post this section below, i found it on the archieves of groups.google.com

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Exactly. There are two types of electronic frequency converters for motor drive applications, cycloconverters and rectifier-inverters. A rectifier-inverter drive converts the input (fixed-frequency) AC to DC, then puts it through another converter (called an inverter) that produces the variable-frequency AC to control the motor speed. OTOH, a cycloconverter synthesizes the variable-frequency AC by chopping up little slices of the three input AC phases and combining them to produce lower-frequency AC. It uses fewer parts and is cheaper than a rectifier-inverter, but is less flexible. A rectifier-inverter can produce output at any frequency, independent of the input frequency; you could think of a radio transmitter as a rectifier-inverter drive with output up in the megahertz range. A cycloconverter can only produce AC output, IIRC, at frequencies below one-third of the input frequency. If the motors are designed to work with the specific drive, you can make it all work together just fine.

Other posters have talked about variable-pitch hubs. These are used in a different kind of drive, in which the engine runs at a constant speed (because the ship service generator is driven from the same shaft) and drives the propellor shaft through a reduction gear. Thus, shaft RPM is constant, and the only way to control power and reverse thrust is by shifting prop pitch.

Both systems have been used for decades. The Normandie used AC electric drive, though they varied the speed by varying turbine-generator RPM and had separate ship service turbine generators. I've been on cruise ships that used both kinds; Sagafjord had the gear-driven fixed-RPM variable-pitch system and Nieuw Amsterdam has diesel-electric drive with variable-frequency motors, and, I presume, fixed-pitch propellors. When QE2's steam system was changed out, the new system was a similar diesel-electric drive. I belive electric drive is becoming more popular for several reasons:

• With modern solid-state electronics, fairly fancy motor drives have become much cheaper and just as reliable as huge high-powered precision reduction gear sets.

• Electric drive gives designers a lot more flexibility in arranging the machinery, because they don't have those big shafts passing through the machinery spaces getting in the way, and shaft alleys getting in the way of everything else between the engine rooms and the stern. The thrust blocks and motors can be located right next to the shaft penetrations in a compact assembly; electrical cables are flexible and don't take up much room.

• Engines can be run at the optimum speed, and not all engines need to be running if you don't need 100% power at any given time. This improves fuel efficiency a lot. On Sagafjord, when we weren't in a hurry, they ran with one shaft shut down (so the running engine was operating efficiently) and carried maybe 15 degrees rudder to compensate. This is a lot of drag they don't need if the ship uses electric drive.

I worked on several Navy ships that had yet another style of drive; DC diesel-electric (a lot like pre-1985-or-so American railroad locomotives). With this sytem, main propulsion diesel generators are wired directly to the motors. The diesels run at fixed rpm, and you control power to the motors by controlling generator excitation, which varies the voltage in the circuit, and fine-tuning things by controlling motor excitation. This system was used on craft like tugs and submarine rescue ships (like the USS Grasp, one of my customers, much seen on TV working plane wrecks lately). Its advantage is that it can put full horsepower in the water at any speed, so the ship can steam at 20 knots or more by itself and can also supply full power at maybe 3 knots towing a much larger wreck. Interesting problems, but not much relevant to cruise ships.

-- Hasta Luego

Irshaad~ Faster than Bruce Lee

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