Motors & Motor Starters

Motors & Motor Starters Prepared By: Erik Redd & Jeremy Roberts

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

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.

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

Example. A 2 pole, 60 Hz motor runs at a full-load speed of 1760 rpm. What is the slip?

Ans. %slip= 3600-1760*100 3600 =51.1%

Single-Phase Motors • Supplied by single source of ac voltage • Rotor must be spun by hand in either direction, does not have a starting mechanism • Has no starting torque • Three different types of single-phase motors: split-phase, capacitor start, permanent split-capacitor, and shaded-pole motors

Resistance Split-Phase Motors • Has a start winding and a main winding • Winding 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) • Start (inrush) current is high • Needs centrifugal starting switch or relay to disconnect the start winding (protects it from over heating) • Efficiency is between 50-60%

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

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%

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%

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

Shunt Field Control • Is used for motor speeds above base speed • Horsepower, (HP)= Torque*rpm 5252 Where torque is in lb-ft

Speed Regulation • Speed Regulation (IR)= no load rpm- full load rpm full load rpm

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

Brushless DC Motors • 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

Brushless DC Motors • 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

Picture of a Brushless Motor

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

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

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.

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

Motors & Motor Starters