Electronic Soft Starters

1
Electronic Soft Starters
Fairford North America
How and Why they Work, and What they can do for
You.
2
The Starting Point

• The performance of a soft starter is crucially
  dependent on the performance of the motor it
  controls.
• Therefore, it is essential to understand motor
  characteristics in order to understand soft
  starter applications.

3
Statements of Fact - 1

• Energy cannot be created or destroyed - but it
  can be converted from one form to another.
• In a conversion system, an event such as a surge
  in demand, experienced in one form of energy is
  immediately reflected in the other form of energy.

4
Statements of Fact - 2

• An induction motor is an energy conversion
  device, converting electrical energy into
  mechanical energy and some heat energy.
• For an induction motor, torque is proportional to
  the motor terminal voltage squared. (V2). So,
  for example, half the voltage produces a quarter
  of the torque.

5
Induction Motors - 1

• Were invented about 100 years ago.
• Because of their simplicity and efficiency, they
  are the workhorses of industry throughout the
  world.

6
Induction Motors - 2

• The induction motor has two basic functions to
  fulfil
• 1 To accelerate itself and its load to full
  speed.
• 2 To maintain the load at full speed
  efficiently and effectively over the full
  range of loading.

7
The motor speed-torque curve
Pull Out (Peak) Torque (MK)
Torque
Locked Rotor Torque (LRT, MA)
Pull Up Torque
Full Load Torque (FLT, MN)
0
S
Speed
8
The load speed-torque curve
Torque
Torque
Motor
Full Load Torque (FLT)
Load
0
S
Speed
0
S
Speed
9
Putting the two curves together we get
Peak Torque
Torque
Torque
Locked Rotor Torque
Motor
T2
Full Load Torque (FLT)
The accelerating torque at any speed S is (T2
- T1)
Load
T1
S
0
S
Speed
10
What does this mean in practice?

• Because the load starting torque is usually near
  zero, an induction motor will develop far too
  much torque when connected directly to the
  supply. So,
• At the instant of start-up, there is an
  un-necessary heavy power surge on both the
  electrical supply and the mechanical drive
  components.

11
The mechanical effects

• The sudden impact at start up on the load,
  followed by the rapid acceleration to full speed
  causes excessive wear on -
• Belts and pulleys
• Gears and chains
• Couplings and bearings and -
• Cavitation in pumps etc........

12
The electrical effects

• A heavy current surge on the electrical supply
  which can be severe enough to cause voltage dips
  and flickering lights.
• Pitting and burning of contacts due to high
  currents which are many times the motor full-load
  current.

13
The user experiences-

• Continuous maintenance.
• Greater chance of unscheduled shutdowns
• Oversized mechanical and electrical components to
  cater for the power surge on start up.
• Short component life.

14
Direct-On-Line Starters

• Primitive, but familiar - Just a simple contactor
  in circuit between the motor and supply giving
  only unsynchronised on-off control
• No control over inrush currents or torque surge
  at start up
• Very rapid acceleration of load with large
  transient forces placed on drive train and
  electrical system

15
Direct-On-Line Starting
600
CURRENT
TIME
16
Star-Delta Starters

• Uses three contactors and a timing mechanism to
  switch between them.
• Installation costs are greater because of the
  need for 6 connections between the motor and
  starter.
• If the transfer occurs at less than 80 of normal
  speed, large current and torque surges can arise.

17
The Star-Delta Trauma
18
Star/Delta Start
19
Primary Resistance Starter

• Inserts a resistance in one or more of the stator
  phase connections. The resistance is
  progressively reduced as acceleration occurs.
• Starts are limited
• Installation and running costs are high.
• Smoothest and least stressful method of electro
  - mechanical starting

20
Other Means Of Starting

• Auto transformer
• Primary Reactance

21
Semiconductor Motor Controller

• Is based upon the silicon controlled rectifier or
  thyristor.
• By pulsing a thyristor, it switches from off to
  on until the current stops flowing though it -
  which occurs every half cycle in an AC. supply.
• By controlling electronically the thyristor turn
  on point, it is possible to regulate the energy
  passing through it.

22
Thyristor Control
23
Semiconductor Motor Controller (cont.....)

• By starting with a large delay angle and
  gradually reducing it , the motor terminal
  voltage is increased from a low value to full
  voltage, giving a smooth, stepless, start.

24
A Soft Start compared
DIRECT-ON-LINE
6-700
STAR-DELTA
CURRENT
SOFT START
100
TIME
25
The Result is -

• In a simple, reliable and cost effective piece of
  equipment, all the problems of electro-mechanical
  starting have been banished to the past!

26
Running Induction Motors
100
EFFICIENCY
0
1/2
1/1
LOAD
27
The Action of the Optimising Mode - 1

• Optimising continually monitors the motor to
  improve the part-load efficiency by reducing the
  degree of over-fluxing of the stator.
• The part load pF of the motor is improved.

28
The Action of the Optimising Mode - 2

• This produces a significant reduction in kVAr and
  kVA, as well as a useful reduction in kW.
• Electrical losses in the stator and feed cables
  are reduced.
• Motor performance is unimpaired.

29
Applications

• There is virtually no fixed-speed application of
  an induction motor which cannot benefit from the
  fitting of a QFE soft-starter.
• In particular, pumps and pumping systems benefit
  from the soft-stop feature and the reduction in
  cavitation effects.

30
Applications

• Fans, particularly high inertia ones, enjoy
  smooth, stepless acceleration at controlled
  current levels without the damaging effects of
  the star-to-delta transfer.
• The life of frequently-started drives and of
  reversing drives is considerably extended by the
  soft start action.

31
Applications

• Compressors,
• Chillers, Escalators,
• Conveyors,
• Mixers,
• Rollers,
• Screw feeders.
• MG sets (elevators)

32
Applications

• Ball mills, Crushers,
• Blowers, Pelletisers
• Grinders, Presses,
• Saws, Washers,
• Moulders, Sifters,
• Screens, Tumblers, etc..,etc...,

33
Applications

• All these and many other applications have been
  successfully softstarted by the QFE.

34
Reliability

• IEC 60947-4-2, EN 60947-4-2AC Semi-Conductor
  Motor Controllers and Starters defines the soft
  starter in every important aspect. Purchasing a
  controller conforming to this standard will
  safeguard the user from poor equipment.
• UL listed for the United States and Canada

35
The End