E&S - Tony Narusevicius - 20-09-2005 - English 1

Which MV motor starter to choose?

E&S - Tony Narusevicius - 20-09-2005 - English 2

The MV starter and motor are the links between the electrical network and the mechanical load

LoadMotor

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starter

E&S - Tony Narusevicius - 20-09-2005 - English 3

The motor starter must be designed to limit the constraints to the network, the motor and the load

The type of motor starter will influencethe perturbations on the networkthe mechanical strength to the mechanical loadthe starting torque on the shaft

The protection will protect the motor

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OvercurrentThermal (I²t) rotorThermal (Pt100) stator

Protection LoadMotor

E&S - Tony Narusevicius - 20-09-2005 - English 4

The network has constraints

Acceptable inrush current

Acceptable starting undervoltage

Acceptable power factor

short circuit withstand

Acceptable harmonics

E&S - Tony Narusevicius - 20-09-2005 - English 5

The load has constraints

Resistant torque curve

Starting inertia

Acceptable mechanical stress

operating conditions Starting time number of starts automatic starting operating cycle overload locked rotor current deceleration control

........

E&S - Tony Narusevicius - 20-09-2005 - English 6

The motor-starter must be designed to control, protect and supervise the motor for maintenance

Motor control: local or remote; automation....

Electrical motor protection short circuit under-voltage ........

Preventive protection temperature through thermal image or sensors bearing vibration sensors moisture derating for altitude .........

The design of the motor starter should decrease the number of spare parts....

.........

E&S - Tony Narusevicius - 20-09-2005 - English 7

The most common motor: asynchronous motor

the 3 windings of the stator are placed at 120° and are fed by the service voltage

these 3 coils create a rotating magnetic field around the axis of the stator

this magnetic field creates a current in the rotor the rotation of the rotor N is lightly slower as the

magnetic field one Ns

There are 2 types of asynchronous motors single or double cage motors (rotor is short

circuited) slip ring rotor motor (resistance of the rotor can

be modified by introducing external resistances)

phase 1

phase 2 phase 3

120°

E&S - Tony Narusevicius - 20-09-2005 - English 8

Influence of the service voltage on the motor torque

N (speed)Ns

starting torque

V"s > V's

Vs

V's >Vs

resistant torque

Torque

acceleration torque

The lower the voltage; the lower the motor torqueIf the motor torque is lower than the resistant torque the motor does not start

E&S - Tony Narusevicius - 20-09-2005 - English 9

Asynchronous motor

single cage rotordouble cage

deep slot rotor motor

starting torque lowfrom 0.6 to 1 Tr

slightly higherfrom 0.8 to 1.2 Tr

maximum torque around 2 to 2.2 Traround 2 to 2.2 Tr

(slightly higher for deep slots)

starting current from 4.5 to 5.5 Ir from 5 to 6.5 Ir

these motors are ideal for intensive use and hazardous environments

excellent mechanical and electrical robustness simplicity of design no brushes

relatively low power factor 0.8 to 0.9 on full load if asynchronous installed motor power is high reactive, power compensation may be required

E&S - Tony Narusevicius - 20-09-2005 - English 10

Asynchronous slip ring rotor motor

the impedance of the rotor winding can be modified by introducing external resistances.

by decreasing external resistance on starting (from Rr" to R), the characteristic of the torque is translated and the starting torque adapted to the torque of the machine.T

N

Ts

T max

T load

R= rotor resistanceRr ’ >Rr ’' >

E&S - Tony Narusevicius - 20-09-2005 - English 11

Synchronous motor

The main differences with asynchronous motors are: their constant speed (synchronous speed) the rotor circuit supplied with Dc the power factor which may be set by the exciting current

In a conventional synchronous motor controller the DC power is applied from an external source brushes and slip rings carry the current to the field winding

A brushless synchronous motor it generates its own DC power for its field the amount of DC power is controlled through induction low current DC power applied from the controller

E&S - Tony Narusevicius - 20-09-2005 - English 12

Load torque curves of machines

compressor

E&S - Tony Narusevicius - 20-09-2005 - English 13

Starting conditions:motor torque must always be greater than the load torque

the motor starts

the motor does not start

running point

E&S - Tony Narusevicius - 20-09-2005 - English 14

Some of the different types of MV motor starter

Full voltage on the stator or direct on line FVNR or DOL

Reduced Voltage through reactance

Reduced Voltage through auto-transformer RVAT

Reduced Voltage trough soft starter RVSS (SoftStart)

E&S - Tony Narusevicius - 20-09-2005 - English 15

Direct stator starting on full voltage (FVNR)

Locked rotor current on starting is around 4 to 7 In depending on motor characteristics

starting time of 1 to 10 seconds depending on the moment of total inertia (motor + machine), motor torque and load torque

the network must be able to withstand the starting current without disturbing other loads

the machine being driven must be able to withstand the mechanical impact due to the motor torque

This mode is very popular (simple, robust, not expensive)

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Protection LoadMotor

E&S - Tony Narusevicius - 20-09-2005 - English 16

Reduced voltage by reactance

This starting mode reduces current inrush on the network in the simplest manner. Since the motor starting torque is low, the machines being driven must have a relatively low torque during star up: compressors; centrifugal pumps; converter sets....

T/Tn T L1shunted

T with L1

Tr

N

Nn

resistant torque

N1

1

2

Tmax

IId/In

N

NnN1

2

4

InIf(Tr)

I L1shunted

I with L1

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L1Protection LoadMotor

E&S - Tony Narusevicius - 20-09-2005 - English 17

Reduced voltage by auto-transformer

This system is recommended for high power motors. It has the advantage of not dissipating energy.

It can be used to obtain, for the same decrease in current, less reduction in torque than with the reactor starter.

N1

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Ct3

Ct2

Ct1

Protection

T/Tn

Tr

N

Nn

resistant torque

N1

1

2

Tmax

Ct1 and Ct2 closedCt2 closedCt2 and Ct3 closed

IId/In

N

NnN1

2

4

InIf(Tr)

Ct2 and Ct3 closed

Ct2 closed

Ct1 and Ct2 closed

LoadMotor

E&S - Tony Narusevicius - 20-09-2005 - English 18

Auto Transformer starting Sequence

Second stage: the motor is running in series with part of the autotransformer winding.

The motor is always supplied during the second stage, when CT1 is open.

First stage:

CT1 must be closed before CT2.

CT1 and CT2 are closed

CT3 is open

CT1

CT2

CT3

M

Second stage:

CT1 is opened

CT2 remains closed

CT3 remains open

CT1

CT2

CT3

M

CT1 and CT2 are closed

CT1 is openedCT2 are closedCT2 and CT3 are closed

Third stage: the motor runs at full voltage.

First stage :the motor is starting with reduced voltage.

Third stage:

CT3 is closed

CT2 remains closed

CT1 remains open

CT1

CT2

CT3

M

E&S - Tony Narusevicius - 20-09-2005 - English 19

Reduced voltage by Soft Starter

The soft starter is an solid state device which controls the starting period of the motor

depending on the application several settings are possible. When the motor is running a contactor shunts the electronic starting

module. Advantages

Reduction of starting current Reduction of starting torque and mechanical shock ( the torque may

be adjusted to the resistant torque) smooth acceleration and deceleration of the motor

It is a solution when auto-transformer motor starter is not possible.

But it is not a variable speed starter Only the price limits the use of the Soft Starter

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Soft starterProtection LoadMotor

E&S - Tony Narusevicius - 20-09-2005 - English 20

Soft starter setting: different possibilities

current limitation setting torque control setting mix of both ........

deceleration possibilities

E&S - Tony Narusevicius - 20-09-2005 - English 21

Soft starter setting: current limitation

T/Tn

0,6 Un

0,85 Un

Un

Tr

N

Nn

resistant torque

N1

1

2

Tmax

fix frequency

T=function UUn( )

2I and Torque

Is/In

Il

Tsn

Tlimited

limited T= Tn * (In / Il)2

N

NnN1

2

4

InIf(Tr)

Limit of the starting current

The voltage is increased until the starting current reaches the limit, then voltage is maintained at this current, and then the voltage is increased again up to the service voltage

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Soft starterProtection LoadMotor

E&S - Tony Narusevicius - 20-09-2005 - English 22

Soft starter setting: torque control

The voltage is set to have a motor torque just above the resistant torque.If the resistant torque is constant the starting voltage may be close the full voltage; the inrush current maybe close the direct on line inrush current

T1 = k.I1

N and TC

t

t1

Tn

Nn

resistant torque

I and T

tt1

I1

In

x In

Tn

T1U1

I starter

Un

resistant torque

torque control

U starter=>

speed ramp fonction oftorque ramp

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Soft starterProtection

E&S - Tony Narusevicius - 20-09-2005 - English 23

Soft starter: normal setting

The initial torque setting applies sufficient voltage to the motor to cause the motor shaft to begin to turn.This voltage is gradually increased overtime (as per the ramp time setting) until either:

the motor accelerates to full speedthe ramp time expiresa current limit is reached.

If the motor has not reached the full speed at the end of the ramp setting, the current limit setting will proportionally control the maximum output torqueThe current remains limited for as long it takes the motor to accelerate to full speed.

When the motor reaches full speed and the current drops to running level, the by pass contactor shunts the soft starter module.The motor is now running at the full voltage.

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Soft starterProtection LoadMotor

E&S - Tony Narusevicius - 20-09-2005 - English 24

Soft starter setting: deceleration

The starter may be used to control the deceleration by slowly reducing the voltage to the motor upon initiating a stop command

the most common application for the decel feature is pumping application, in which a controlled stop prevents water hammer and mechanical damage to the system.

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Soft starterProtection LoadMotor

E&S - Tony Narusevicius - 20-09-2005 - English 25

Starting mode selection table

application Startingmode

Controlledby

Advantagesdisadvantages

Permanent orquasipermanentprocess<1/day

Machines requiringhigh start torque

1 Circuitbreaker or1 contactor

Frequentstarts>1/day

Motor with lowcurrent inrush orlow power

Direct On Line /Full voltage 1 contactor

Simplicity,less investment

On starting:High torqueHigh current inrushHigh mechanical stress

Pumps fans,compressors,frequent starts

Machines startingwith low torque Stator by

reactance

Reduction of current in rushon startingPoor power factor

Optimisationof startingcharacteristics

When startingcurrent must bereduced, but thenecessary startingtorque maintained

Stator byautotransformer

or soft starter

3 contactors

2 contactors

Optimisation of torque andcurrent inrush on starting

expensive

Optimisationof high torquestartingcharacteristics

The most difficultstarts

rotor generally3 contactors

Small current inrush andhigh starting torque

E&S - Tony Narusevicius - 20-09-2005 - English 26

Network requirement

Starting mode selection table

- accepts the current oscillation during an open transition?

-accepts the starting reactive consumption power?

Investment Increase

- deceleration control ?

- constant torque with high starting torque as crushers, grinders, mixers ......

- the drive between motor and load is able to withstand the mechanical stress of a direct on line start

- accepts full starting current?- accepts starting voltage drop?

yes

yes

yes

yes

no

no

no

nono

no

yes

yes

no

no

- accepts the harmonics during the starting period

VSDspecific

no

FVNR RVAT RVSS

yes

yes

yes

E&S - Tony Narusevicius - 20-09-2005 - English 27

main protection

overload overcurrent, thermal image, and thermal sensors

short circuits To protect the motor and the network fuses and instantaneous over-current relay

thermal protection bearing overload temperature simulated by relay thermocople RTD

Incomplete phase, phasereversal and unbalance

faults give rise to: increased current in the stator

insulation fault in thewinding

earth leakage protection(differential protection)

stator/ frame vital to comply with the worker protectionprotection windings and magnetic circuit

zero sequence current relay under and over voltage under-voltage: common protection, prevents the motor on

working on overload over-voltage: fluctuations on the network

incomplete starting orfrequent starts

prevents long motor starting and over temperature speed or current

locked rotor protection of the motor against thermal damage under power or

undercurrent mainly for pumps

other protection specified by motor manufacturers

E&S - Tony Narusevicius - 20-09-2005 - English 28

Rating Current of the contactor

The power of a motor is the mechanical power.To determine the rating of the contactor we have to calculate the full load current of the motor.

The electrical power of the motor will be the mechanical one divided by the efficiency.

Is= Pm/(Us x V3 cos where

Pm: power of the motor Us service voltage cospower factor efficiency of the motor

Ir contactor > Is * k (k derating factor) if the altitude <1000m k=1

Altitude (ft) Voltage Current

3300 (1000 m) and below 1.00 1.005000 (1500 m) 0.95 0.9910,000 (3000 m) 0.80 0.96

E&S - Tony Narusevicius - 20-09-2005 - English 29

What do you think?

For an asynchronous motor,true false

the highest starting torque is supplied by the Full Voltage Motor starterthe higher the voltage, the higher the starting torquewith a full voltage motor starter the network capability to accept the inrush current needs to be verifiedthe inrush current may generate a voltage drop at the terminals of the motorthe distance between the motor starter and the motor has no influence on motor startingmotor starting through an auto-transformer is always a cycle of three stepsit is possible to continuously operate an auto transformer motor starter on an intermediate stepthe auto transformer motor starter may be used for every applicationthe auto transformer is generally designed in accordance with the motor use for each projectdifferent starting configurations are available in the soft startersSoft starters use an electronic device to reduce the voltageSoft starters maybe used to adapt the running speedSoft starter maybe used to control the decelerationsoft starter needs 2 contactorswith a soft starter it is possible to adjust the starting torque to the resistant torque

E&S - Tony Narusevicius - 20-09-2005 - English 30

Do you agree?

For an asynchronous motor,true false

the highest starting torque is supplied by the Full Voltage Motor starterthe higher the voltage, the higher the starting torquewith a full voltage motor starter the network capability to accept the inrush current needs to be verifiedthe inrush current may generate a voltage drop at the terminals of the motorthe distance between the motor starter and the motor has no influence on motor startingmotor starting through an auto-transformer is always a cycle of three stepsit is possible to continuously operate an auto transformer motor starter on an intermediate stepthe auto transformer motor starter may be used for every applicationthe auto transformer is generally designed in accordance with the motor use for each projectdifferent starting configurations are available in the soft startersSoft starters use an electronic device to reduce the voltageSoft starters maybe used to adapt the running speedSoft starter maybe used to control the decelerationsoft starter needs 2 contactorswith a soft starter it is possible to adjust the starting torque to the resistant torquefor some application the soft starter may create inrush current similar to the full voltage motor starter