28523855 ac motor ac motor protection part 1 of 2

7/31/2019 28523855 AC Motor AC Motor Protection Part 1 of 2

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AC Motors - Javed 1

AC MOTOR & AC MOTOR PROTECTION GUIDE

Part-1 of 2

(Javed Ahmed)

INTRODUCTION

Intent of this guide is to cover aspects of large capacity AC Motors with the intention of assisting Electrical

Engineers who are in the business of dealing with Control & Protection of AC Motors in Industry &

Utilities. However, most of AC Motors are covered to some extent.

Simplest and most widely Motor in the World is Squirrel cage Induction Motor. This is because it is most

robust and can be highly efficient and less complex in its design and simplest means required for its

control. Motors find their application in many areas like locomotion, fans, pumps, control systems, tiny

electronic circuitry of wrist watches just to mention a fraction of many.

Therefore, only two types of Motors are considered here. They are Induction Motors & Synchronous

Motors.

Induction Motors:

These Motors are supplied with AC power supply only to its Stator Winding. Stator current produces a flux

in the Rotor crossing over the air-gap between the Stator & Rotor. Necessary additional flux required in

the Rotor to produce a rotation is obtained by induction principle. Thus energy is transferred from Stator

alone to the Motor which results in rotation of the Rotor producing mechanical energy. These Motors are

called poly phase or single phase type based on supply voltage to the Stator.

Rotors, however, can be either poly phase type (wound rotor) or bar type bare un-insulated conductors

(squirrel cage rotor). In case of Wound rotor Motors, each phase is insulated from the Rotor body and its

end is brought out through slip rings for connection to starting control which is stationary system. In case

of Squirrel cage rotor, bare conductors are located in slots around the periphery almost parallel to the

Rotor linear axis and are shorted at both ends by means compatible metallic ring forming like what looks

like a traditional Squirrel cage.

Synchronous Motors:

These Motors are poly phase with Stator winding almost identical to that of an Induction Motor and is

connected to the AC supply source. However, its rotor is also connected to separate supply source, whichis a DC source. This DC supply is provided to the Rotor Poles (usually salient poles) which are matching

with stator phase winding poles. This DC supply to the Rotor is called field excitation. There are many ways

of supplying the Field excitation to the Rotor with or without brush & slip rings. For starting a Synchronous

Motor, a number of bar conductors are embedded in the Rotor pole faces. These bars get induced

voltages from Stator current and thus Motor rotates on same principle as an Induction Motor. DC


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AC Motors - Javed 2

excitation to rotor poles is not supplied until appropriate rotor speed is attained (this speed is called cut in

speed). Until such speed is reached, the rotor winding is shorted through starting & discharge resistor to

limit the induced current at lower speeds. At rated speed, there is no induced voltages as both fields

rotate at exactly same speed and hence no rate of change of flux.

MOTOR CONTROLS

Squirrel Induction Motors are supplied with AC source only to its Stator and rotor is inaccessible.

Therefore, its control like staring, reversing, stopping during normal operation as well as disconnection

during a Stator fault is also associated with AC supply only (associated with the Stator only).

Wound rotor IM & Synchronous Motors additionally require means of control in the rotor circuit.

However, all Motors are supplied with AC power to the Stator and hence require means of Stator supply

disconnection & connection means during normal operation as well as disconnection of supply during a

Stator fault.

There are two control functions required which are:

Normal switching

Fault disconnection

Normal switching action involves starting & stopping. In some cases additional requirement is reversing its

rotation.

Starting an Induction Motor:

When AC supply is applied to the Stator of a Motor when its rotor is at standstill (zero speed), the Motorwill behave like a Transformer (stator winding as primary & rotor bars as secondary). The secondary is

being short circuited by end rings. Rotor at this point of operation is referred to as at 100% slip from its

synchronous speed ns (ns =120f/P, where f= frequency, p= number of stator poles). Stator winding current

(limited by stator winding resistance & reactance as well as rotor bar resistance & reactance reflected to

the stator side) produces rotating AC flux (same as frequency of supply voltage) which cuts the rotor bars

and induce voltage in it. Since Rotor bars are not coils, they have dominant resistive component. Induced

voltage in the rotor bars results in current in the rotor bars. This current produces IR heat in rotor bars.

This is initial starting current of Motor (which is also called as Locked rotor amperes, LRA). This produces

Locked rotor Torque, LRT. LRA initial value is directly proportional to the applied Voltage. LRT is

proportional to the square of the applied Voltage. Thus if 80% voltage is applied, the LRT will be 64% ofLRT at 100% applied Voltage.

This current in the Rotor bar produces flux which interacts with flux due to stator winding current

resulting in its rotation. Thus rotor slip starts continuously falling (as it continuously picking up speed). The

current keeps falling from initial LRA.


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AC Motors - Javed 3

Rotor resistance (Rr) is independent of slip & frequency and hence will remain constant from starting

point (zero speed) to rated speed. Rotor reactance (Xr) depends on the frequency and slip. At start point

s=1 and f=rated frequency of supply and hence Xrstart is equal to 2fL (where L is its inductance). At full

rated load Xr thus will be equal to s times at start = s [i.e. Xrrun =sXrstart]. This means if rated slip is 5% [s=

(ns-n/ns) x100 =5], then Xrrun=0.05 times Xrstart. Thus rotor current is function of slip (excluding effect of

load).

Below examples show the effect of Rotor slip, frequency & losses of Motor:


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Some important notes on SCIM Starting & load:

For Direct Online Start SCIM, typical Starting currents are in the range of 5 to 7 times the Full load current.

Starting torque can be up to 350% of full load torque. It is thus important to match the load torque with

Motor torque right from the starting till its rated load is reached (generally it is necessary to see that the

Motor torque Vs. speed curve must always be above load Vs. speed curve). When starting current

becomes unacceptable, other means of starting shall be considered like Star-delta starting, Auto-

Transformer starting etc for SCIM. Whenever starting is performed at reduced voltage, it is essential to

note that torque is proportional to Voltage squareif for example 80% voltage is used at start as per

standards, starting torque will be as low as 64% of that which is obtained when started with 100% voltage.


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AC Motors - Javed 8

At start, there is no rotation. Hence there is no air cooling in operation when the fan is inherent part of the

Motor (mounted on its shaft without external power). This instant is also involved with highest IR loss in

Rotor and stator. Thus it becomes essential to optimally select a Motor duty in accordance with the

requirement of numbers of starting sequences, cooling system etc. Below represents various duty cycles

for Motors as per IEC standard:

Typical Starting Means:


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Difference between an SCIM & SCIG (Induction Motor & Induction Generator) and some aspects of

Induction Generator:

Electrically, there is no difference between a SCIM and SCIG (Squirrel Cage Induction Generator). However

a SCIM always rotates at a speed lower than [slip speed = (ns-n) is positive] and a SCIG always at a speed

higher than its synchronous speed [slip speed = (ns-n) is negative]. Below is a figure to indicate SCIM &SCIG torque-speed curve (SCIM behaves according to curve above X-axis & SCIG below X-axis):


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Slip of a SCIM:

Typical slip of an SCIM at full load is usually less than 5% (meaning its rpm is higher than 95% of

synchronous speed). NEMA standard has defined various SCIM slips as below:


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Brief theory of an Induction Motor:


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MOTOR CONTROL CENTERS (MCC)

SIEMENS


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MOTOR ENCLOSURES


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ELECTRICAL FUNDAMENTALS FOR LT MOTORS


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DESIGNATIONS & DIMENSIONS (IEC-NEMA)


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COMMON MOTOR TYPES & APPLICATIONS


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ADJUSTABLE SPEED DRIVES


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ENGINEERING BASIC DATA


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ELECTRIC MOTOR CONTROLS


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MOTOR SELECTION GUIDE


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28523855 ac motor ac motor protection part 1 of 2

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