Contacts and Motor Starters

CHAPTER 8

 

>> Motors

AC-Motors

Parts of an Electric Motor

A. Stator : Stationary Frame

B. Rotor : Revolving Part

The rotary motion in an ac-motor is caused by the fundamental law of magnetism.

This law states that like poles repel and unlike poles attract.

>> Diagram of an ac-motor

This shows a three phase, two pole stator.

Where A, B, and C are the three phases

1

>> Diagram of the Three Phases

Fig. 13-2 Pg. 244

Poles 1 and 4 are at their greatest magnetic field at time equal to one, because phase A (red line) is connected to those poles, and the same for the other poles when their corresponding phases are at maximum current magnitude.

2>> Synchronous Speed

Speed at which it takes the motor to go one cycle and one revolution.

S=[120*frequency}]

(# poles)

Example:

For a three-phase, 60 Hertz, 2 pole motor:

S=[120*60]/2=3600 revolutions per minute

>> Polyphase Squirrel-Cage Induction Motors

  • The most common three-phase motor
  • Does not have solid poles
  • Instead, it has laminations: numerous flat sheets held together in a package. They are insulated from each other (this reduces Eddy currents) making up the stator
  • The difference between induction and synchronous motors is that the rotor for an induction motor can travel at a different speed than the stator. This is called Slip.
  • slip= Syn. rpm – Motor rpm *100                          Syn. rpm

3>> Single-Phase Motors

lSupplied by single source of ac voltage

lRotor must be spun by hand in either direction, does not have a starting mechanism

lHas no starting torque

lThree different types of single-phase motors: split-phase, capacitor start, permanent split-capacitor, and shaded-pole motors

>> Resistance Split-Phase Motors

lHas a start winding and a main winding

lWinding currents are out of phase by 30 degrees, this produces a flux field that starts the motor

  • Main winding current (IM) and start winding current (IS) lags supply voltage (VL)

lStart (inrush) current is high

lNeeds centrifugal starting switch or relay to disconnect the start winding (protects it from over heating)

lEfficiency is between 50-60%

4>> Capacitor-Start Motors

Has the same winding and switch mechanism arrangement as split-phase but adds a short time-rated capacitor in series with the start winding

The time shift phase between the main and start winding is close to 90 degrees

IS leads VL

Efficiency is between 50-65%

Capacitor controls the inrush current

5>> Permanent Split-Capacitor Motors

Winding arrangement is the same as the capacitor and split-phase motors

Capacitor can run continuously, rated in microfarads for high-voltage ratings

No centrifugal switch is needed

IM lags VL, while IS leads VL

Efficiency is between 50-70%

6>> Shaded Pole Motors

Simple construction, least expensive

Has a run winding only, shading coils are used instead of the start winding

Stator is made up of a salient pole, one large coil per pole, wound directly in a single large slot

A small shift in the rotor causes torque and starts the motor

Efficiency is between 20-40%

7

>> DC Motors

  • Consists of an armature winding and a stator winding
  • Armature windings act as the rotor
  • Has three different classifications: constant torque, constant horsepower, or a combination of the two
  • Standard industrial dc motors are shunt wounded
  • Modifications of the dc motor are: shunt wound, stabilized shunt exciting fields, compound wound motors, and series wound motors

>> Armature Voltage Control

Is used for motor speeds below base speed

Output torque= T=k*ø*IA

k is machine constant

ø is the main pole flux

IA is the armature current

>> Brushless DC Motors

Three phase ac power is converted into dc by the input side of the motor to charge up a bank of storage capacitors

These capacitors are called the Buss

The purpose of the buss is to store energy and supply dc power to transistors in the output side as the motor requires the power to start up

Figure 13-21, page 264 shows the input power section

It consists of three fuses, six diodes, a choke, and two capacitors

The fuses protect the diodes

The choke protects against line transients

The motor control may run at very low speeds at very high torques while drawing little current from the ac line

This picture is a representation of the encoders (rotor part of the motor) telling the corresponding transistors (stator) to turn on in order to get maximum torque from the motor

8

>> Motor Control Starters

Motor will draw high inrush current while the starter will slow current down

Starter reduces the amount of torque needed to start the motor

9

>> Magnetic Motor Starter

Normally open contacts

Not always possible to control amount of work applied to the motor

Has overloads

–Motor may be overloaded resulting in damage to the motor

–Open due to excessive motor current, high temperature, or a combination of both

>> Full-Voltage Starter

Contains one set of contacts

Motor is directly connected to the line voltage

9

>> Reversing Motor Starter

Contains two starters of equal size

Two starters connect to the motor

Interlocks are used to prevent both starters from closing their line contacts at the same time

Figure 14-4A

>> Reduced-voltage Motor Starter

Applies a percentage of the total voltage to start (50% – 80%)

After motor rotates, switching is provided to apply full voltage

Torque will be reduced when starting

Four types:

1) Autotransformer

2) Primary Resistance

3) Wye – Delta

4) Part Winding

>> Autotransformer Starter

Two contactors are used:

1) Start contactor

– Closes first and connects motor to the line

through an autotransformer

– Deenergizes

2) Run contactor

– Motor switches to this contacter which has

full voltage

>> Primary Resistor Starter

Two contactor

1) Line contactor

– First to energize connecting motor to the

line voltage through a resistor

– After preset time, contactor opens

2) Accelerating contactor

– Energizes

– Causes smooth acceleration to full voltage

>> Wye – Delta Starter

Three contactors are used

1) Line contactor and start contactor

– Energizes first and connects motor in wye

putting about 58% of line voltage across

each motor phase

– Contacts open after preset time

2) Run contactor

– Energizes connecting motor in delta and

putting full voltage on the motor

>> Part Winding Starter

Starter supplies about 48% of normal starting torque

Not truly a reduced-voltage means

Two Types

1) Two-Step – one winding connected to

full voltage line and, after a preset time,

the other connects

2) Three-Step – one winding is connected in series

with a resistor to the voltage line; after interval, resistor

is shorted out and then second line is connected to

full voltage line

>> Solid-State Motor Starter

For lower starting torque and smooth acceleration

Used on conveyors, pumps, compressors, etc.

10

>> Standard Modes of Operation

Motor voltage gradually increases during acceleration

Creates a kick start pulse of 500% of full load amperage for high friction

Used when necessary to limit current

Used when motor requires a full voltage start

Source : Erik Redd & Jeremy Roberts, Motors & Motor Starters.

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