astronomy

Motor soft starter

Motor soft starter is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Motor soft starter rather than just read about it. In short: A motor soft starter is a device used with AC electrical motors to temporarily reduce the load and torque in the powertrain and electric current surge of the motor during start-up. This reduces the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network, extending the lifespan of the system.

Key takeaways

Motor soft starter belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Motor soft starter to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Motor soft starter from memory before moving on to harder problems.

Reference excerpt

A motor soft starter is a device used with AC electrical motors to temporarily reduce the load and torque in the powertrain and electric current surge of the motor during start-up. This reduces the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network, extending the lifespan of the system. It can consist of mechanical or electrical devices, or a combination of both. Mechanical soft starters include clutches and several types of couplings using a fluid, magnetic forces, or steel shot to transmit torque, similar to other forms of torque limiter. Electrical soft starters can be any control system that reduces the torque by temporarily reducing the voltage or current input, or a device that temporarily alters how the motor is connected in the electric circuit.

Operating principles

Whenever the armature of an electric motor is moving, both the motor action and generator action are occurring simultaneously; the electromagnetic force produced by generator action opposes the desired motor action and effectively creates a variable motor resistance which increases with motor speed. When a voltage is applied to the motor, this resistance dictates the current drawn by the motor. At rest, the resistance is relatively low, so the starting or inrush current can be high if the full line voltage is applied to the motor. Compared to DC motors, AC motors tend to have significantly higher stator resistance and correspondingly lower inrush current. Nevertheless, across-the-line starting of induction motors is accompanied by inrush currents up to 7-10 times higher than the running current, and higher-efficiency motors can experience inrush currents 10-15 times the running current. In addition, starting torque can be up to 3 times higher than running torque. The starting torque transient can create a sudden mechanical stress on the machine, which leads to a reduced service life. Moreover, the high inrush current stresses the power supply, which may lead to voltage dips. As a result, lifespan of sensitive equipment may be reduced. Another common side effect, especially in residential installations, is voltage sag in the site's power supply created by the high inrush current, visible as flickering lights. A soft starter continuously controls the motor's voltage supply during the start-up phase. This way, the motor is adjusted to the machine's load behavior. Mechanical operating equipment is accelerated smoothly. This lengthens service life, improves operating behavior, and smooths work flows. Electrical soft starters can use solid state devices to control the current flow and therefore the voltage applied to the motor. They can be connected in series with the line voltage applied to the motor, or can be connected inside the delta (Δ) loop of a delta-connected motor, controlling the voltage applied to each winding. Solid state soft starters can control one or more phases of the voltage applied to the induction motor with the best results achieved by three-phase control. Soft starters controlled via two phases have the disadvantage that the uncontrolled phase will always shows some current unbalance with respect to the controlled phases. Typically, the voltage is controlled by reverse-parallel-connected silicon-controlled rectifiers (thyristors), but in some circumstances with three-phase control, the control elements can be a reverse-parallel-connected SCR and diode. Another way to limit motor starting current is a series reactor. If an air core is used for the series reactor then a very efficient and reliable soft starter can be designed which is suitable for all types of 3 phase induction motor [ synchronous / asynchronous ] ranging from 25 kW 415 V to 30 MW 11 kV. Using an air core series reactor soft starter is very common practice for applications like pump, compressor, fan etc. Usually high starting torque applications do not use this method.

Applications Soft starters can be set up to the requirements of the individual application. Compared to variable-frequency drives, soft starters require very few user adjustments. Some soft starters also include a "learning" process to automatically adapt the drive settings to the characteristics of a motor load, to reduce the power inrush requirement at the start. In pump applications, a soft starter can avoid pressure surges that could lead to water hammer. Conveyor belt systems can be smoothly started, avoiding jerk and stress on drive components. Fans or other systems with belt drives can be started slowly to avoid belt slipping as well as air pressure surges. Soft starters are seen in electrical R/C helicopters, and allow the rotor blades to spool-up in a smooth, controlled manner rather than a sudden surge. In all systems, a soft start limits the inrush current and so improves stability of the power supply and reduces transient voltage drops that may affect other loads.

See also Adjustable-speed drive Braking chopper DC motor starter section of Electric motor Electronic speed control Korndorfer starter Motor controller Space Vector Modulation Thyristor drive Variable-frequency drive Variable-speed air compressor Vector control (motor)

References

Illustrations

Worked examples

Example 1 — a first encounter with Motor soft starter

Start with the simplest possible case. Write down what Motor soft starter claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Motor soft starter before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Motor soft starter ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Motor soft starter

In research: Motor soft starter appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Motor soft starter in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Motor soft starter is common in secondary-school and first-year university syllabi. It links to neighbouring topics Control engineering, Electric motors, Electric power systems components, so understanding it makes those chapters shorter.
In everyday life: Look for Motor soft starter outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Motor soft starter in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Motor soft starter means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Motor soft starter out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Motor soft starter in simple terms?

Why does Motor soft starter matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Motor soft starter?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Motor soft starter.