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Bilge Rat
Join Date: Aug 2007
Location: Fernley, Nevada (near Reno)
Posts: 651
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I'm in the USA, things might be different overseas. An 18.5KW motor is about 25HP. Of all the 25HP motors I've connected (probably 50 or so), I can think of only one or two that have been started by any method other than across-the-lines. (Direct on line, or DOL).
Generally speaking, with any induction motor, the DOL starting current will be about 6 times the running current. There are a number of different starting systems available.
1) Wye-delta, sometimes known as star-delta. If the motor voltage is 230/460, it must be delta wound, and all 12 leads must be brought out. If it's single voltage, again, it must be delta wound, and all 6 leads must be brought out. If it's 240/415 volts, it may be wye-delta started only on 240 volts. These starters come in open-transition, and closed-transition. In the open-transition type, there's a short (1 or 2 second) time during the transition from start to run when there's no power delivered to the motor. The closed-transition type maintains power at all times. Usually, the open-transition type is OK for just about anything except close-coupled pumps. They're usually OK for fans. Starting current is usually about 2-3 times running current, and starting torque is about 1/3 of across-the-lines locked rotor torque. Long starting times (up to several minutes) are acceptable.
2) Part-winding start. The motor must usually be designed specifically for this type of starter. These motors are usually single voltage, and the lead numbers are usually 1-2-3, 7-8-9. This type of starter first energizes leads 1-2-3, then 7-8-9 after a short time delay. The time delay is usually limited to 1 or 2 seconds. This type is usually used where the inertial load is light, like a close-coupled pump. Some 230/460 volt motors can be started this way on 230 volt only. Starting current is about 3 times running current, and starting torque varies alot based on motor design. It's usually about 1/2 the across-the-lines locked rotor torque, but it can be alot lower.
3) Autotransformer, sometimes known as a compensator. This system will start any type of motor, it simply applies a lower voltage to the motor during starting via a transformer. Most of these have multiple taps, usually 50%, 65%, and 80%. Like the wye-delta type, they come in open-transition, and closed-transition. Starting torque is roughly whatever the tap is set at, and current is proportional. Long starting times are OK here, but not as long as a wye-delta.
4) Reactor type. This is somewhat similar to the autotransformer type, but it uses a current-limiting reactor in series with the motor during starting. These are usually found only on medium-voltage motors (more than 600, less than 35,000 volts), because only 2 contactors are needed. The wye-delta and the autotransformer type both need 3. Medium voltage contactors are expensive, and take up alot of room.
5) Electronic soft-start. These are gaining popularity quickly, mainly because they're getting to be much more reliable, cheaper, and more versatile than the other types. Like the autotransformer type, they work with any motor. They can serve as a substitute for just about any other type of starter. The current/torque curves can be set up many different ways, the most popular being either constant current or ramped current. Most have built-in bypass contactors that put the motor across the lines after starting. this cuts down on heat, and doesn't subject the semi-conductors to voltage and current spikes. Alot of them can start a centrifugal pump (with its unique torque vs. speed curve) fairly smoothly. Some can also decelerate the motor smoothly as well. The other types are all coast-to-stop.
6) VFD. Really not a starter, more of a controller, but it can accelerate or decelerate a motor using an exact curve, regardless of load.
Rob
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