CookTek MCD3000 Specifications download pdf (Page 37)

MCD3000 Series

36 MG.15.A4.22 - VLT is a registered Danfoss trademark

Desi

n Guide

generated in the resistors during a start. A long

cool down period is required between starts.

3. Poor performance for heavy duty or extended

starts because heat build up in the resistors

changes the resistance value.

4. Cannot provide an effective reduced voltage start

for loads with varying start requirements.

Soft starters are the most advanced of the reduced

voltage starters. They offer superior control over

current and torque as well as incorporating advanced

motor protection and interface features.

The main starting advantages soft starters offer are;

1. Simple and flexible control over starting current

and torque.

2. Smooth control of voltage and current free from

steps or transitions.

3. Capable of frequent starting.

4. Capable of handling changing start conditions.

5. Soft stop control to extend motor deceleration

times.

6. Braking control to reduce motor deceleration

times.

■

Types of soft start control

The term ’soft start’ is applied to a range of

technologies. These technologies all relate to motor

starting but there are significant differences in the

methods used and the benefits available. Soft starters

can be broken down into the following categories;

• Torque controllers

• Open loop voltage controllers

• Closed loop voltage controllers

• Closed loop current controllers

Torque controllers provide only a reduction in starting

torque. Depending on their design they control one or

two phases only. As a consequence there is no

control of starting current as is provided by the more

advanced soft start formats.

Single phase torque controllers must be used with a

contactor and motor overload. They are suitable for

light applications with low to medium start frequency.

Three phase control should be used for repetitive start

or high inertia loads because single phase controllers

cause increased motor heating during start. This

occurs because almost full voltage current flows in the

motor winding not controlled by the single phase

controller. This current flows for a longer period than

during a DOL start resulting in increased motor

heating.

Two phase torque controllers must be used with a

motor overload but can start and stop the motor

without use of a contactor, however voltage is still

present at the motor even when it is not running. If

installed in this manner it is important to ensure

adequate safety measures are taken and that such

operation is permitted by local regulations.

Open loop voltage controllers control all three phases

and provide both the electrical and mechanical

benefits normally associated with soft start. These

systems control the voltage applied to the motor in a

preset manner and receive no feedback on starting

current. Control of start performance is provided to the

users through settings such as, Initial Voltage, Ramp-

up time and Dual Ramp-up time. Soft stop is also

commonly available and provides the ability to extend

motor stopping times.

Open loop voltage controllers must be used with a

motor overload and, if required, a line contactor. As

such they are a component part that must be

integrated with other items to form a complete motor

starter.

Closed loop voltage controllers are a variant of the

open loop system. They receive feedback on motor

start current and use this to stop the voltage ramp

when a user set start current limit is reached. User

settings and adjustments are the same as for the open

loop voltage controllers with the addition of a current

limit setting.

Motor current information is also often used to provide

a number of current based protection functions. These

functions include, motor overload, phase imbalance,

electronic shearpin, undercurrent etc. These systems

are complete motor starters providing both start/stop

control and motor protection.

Closed loop current control is the most advanced form

of soft start. Unlike voltage based systems closed loop

current technology uses current as the primary

reference. The advantages of this approach are

precise control of starting current and ease of

adjustment. Many of the user settings required by

closed loop voltage systems can be made

automatically by current based systems.

■

MCD3000 control principles

MCD3000 soft starters control all three phases

supplied to the motor. They are closed loop current

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CookTek MCD3000 Specifications Page 37

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