Industrial drives 1 (motors and generators)

List 6 types of Wound-Rotor Motor controller types.

1) Faceplate Starters 2) Faceplate Speed Regulators 3) Multiswitch Starters 4) Drum Controllers 5) Motor-driven Drum Controllers 6) Magnetic Starters

List some possible conditions that can cause AC hum in a relay coil.

1-Broken shading coil 2-Misalignment between the armature and magnet assembly, the armature is then unable to seat properly 3-Operating voltage too low 4-Dirt, rust, filings, and so forth, on the magnet faces. The armature is unable to seal in completely 5-Wrong coil 6-Jamming or binding of moving parts (contacts, springs, guides, yoke bars) so that full travel of the armature is prevented 7-Incorrect mounting of the controller, as on a thin piece of plywood fastened to a wall, for example, so that a "sounding board" effect is produced.

Name the three types of electrical situations that can cause a large increase in motor temperature.

1-Low line voltage increases the current drawn from the line or power source. Since an increase in current causes an increase in temperature, it is important to make sure that the motor windings are properly protected from such an increase in temperature. 2-Jamming the motor and causing it to stop when the power is on will also cause temperature increases. This takes place almost immediately and a good sensing device is needed to protect the windings. 3-Large increases in voltage can cause excess current to flow through the windings. The increased current will cause increased heat to be generated.

What are the four types of starting methods that are discussed in this section of the text book?

1. Directly across the line 2. By Autotransformers (Compensators) 3. By resistance in series with the stator winding 4. By means of a step down transformer

What are four characteristics of a Class D squirrel cage motor?

1. High starting torque 2. Low starting current 3. High slip (15 to 20 percent) 4. Low efficiency

Compound Connected Motor (Cumulative and Differential)

A compound DC motor connects the armature and fields windings in a shunt and a series combination to give it characteristics of both a shunt and a series DC motor.[5] This motor is used when both a high starting torque and good speed regulation is needed. The motor can be connected in two arrangements: cumulatively or differentially. Cumulative compound motors connect the series field to aid the shunt field, which provides higher starting torque but less speed regulation. Differential compound DC motors have good speed regulation and are typically operated at constant speed.

Slip Rings

A conductor band, mounted on an armature and insulated from it. A conductor strip slides on the band as the armature rotates. The function of the slip ring system is essentially the same as a commutator and brushes. Slip rings are also used to transmit current from the armature in a generator application.

Commutator

A cylindrical device mounted on the armature shaft and consisting of a number of wedge-shaped copper segments that are arranged around the shaft and insulated from both the shaft and each other. The motor brushes ride on the periphery of the commutator and electrically connect and switch the armature coils to the power source.

Across the line starting (direct on line starting)

A direct on line (DOL) or across the line starter applies the full line voltage to the motor terminals, the starters or cubicle locations, can usually be found on an ELO drawing. This is the simplest type of motor starter. A DOL motor starter also contains protection devices, and in some cases, condition monitoring. Smaller sizes of direct on-line starters are manually operated; larger sizes use an electromechanical contactor (relay) to switch the motor circuit. Solid-state direct on line starters also exist. A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit. The maximum size of a motor allowed on a direct on line starter may be limited by the supply utility for this reason. For example, a utility may require rural customers to use reduced-voltage starters for motors larger than 10 kW. DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor belts. In the case of an asynchronous motor, such as the 3-phase squirrel-cage motor, the motor will draw a high starting current until it has run up to full speed. This starting current is typically 6-7 times greater than the full load current. To reduce the inrush current, larger motors will have reduced-voltage starters or variable speed drives in order to minimize voltage dips to the power supply.

Bearing

A mechanical engine part that resembles a short length of bronze tubing with grooves to direct oil flow. Sleeve type bearings are preferred where low noise level is important. Ball bearings are used where higher load capacity is required or periodic lubrication is impractical.

What type of insulation is commonly used today and under what name is it known?

An insulation that is commonly used today is a polyvinyl formal type of varnish called Formvar

What is the minimum number of windings required in a three phase motor so that it can be operated properly with a Wye/Delta starter?

At least three.

What can happen if the fit of the brush in the brush holder is too tight or too loose?

If the brush is too loose in the brush holder or the brush spring pressure is too light, pre mature electrical wear can result. However, when brush pressure is too strong heavy mechanical wear can be expected

For a double cage rotor which cage has the lower resistance and which cage has the higher resistance values? Also what class of motor is this?

In a double cage rotor the outer winding has high resistance and the inner winding has low resistance. These are class "C" motors and have high starting torque with low starting current.

Describe the current to voltage relationship of a resistance starting control.

In a resistance type starter, a heavy duty resistance is put in series with the line conductors. The voltage drop over the resistor will produce a reduced voltage at the motor terminals and therefore the starting current of the motor is reduced in direct proportion to the reduction of the voltage applied to the motor terminals

Why is it required that the centrifugal switch contacts open when a split-phase AC motor is accelerating?

It is required that the centrifugal switch contacts open when a split-phase motor has accelerated to 75 or 80 percent of its synchronous speed to prevent the motor from drawing excessive current from the line and to protect the starting winding from damage due to heating. When the contacts are opened the starting windings are disconnected.

Jogging

Jogging describes the repeated starting and stopping of a motor at frequent intervals for short periods of time. A motor would be jogged when a piece of driven equipment had to be positioned fairly closely- for example, when positioning the table of a horizontal boring mill during setup. If jogging is to occur more frequently than five times per minute, NEMA standards require that the starter be derated.

Eddy Current Losses

Localized currents induced in an iron core by alternating magnetic flux. These currents translate into losses (heat), and their minimization is an important factor in lamination design.

Formula for efficiency

Output/Input x 100

Relays are usually operated by electro-magnetic means but what are the other three means that can be used to operate a relay?

Pneumatic, Hydraulic, Mechanical

Horsepower

Power rating of the motor. It takes 746W of electrical energy to produce 1 horsepower. Horsepower=Torque x Speed/5252 To convert horsepower to kilowatts (kW), multiply by 0.746

Shunt Connected Motor/Generator

Shunt DC Motors operate on direct current. As such, the field windings and armature are connected in a parallel combination, and in electrical terminology a parallel combination is known as a shunt. This type of motor is a "shunt-wound" DC Motor and the type of winding is called a shunt winding. This type of motor has good speed regulation even as the load varies, but does not have the starting torque of a series DC motor (starting torque is about 125%). It is typically used for industrial, adjustable speed applications, such as machine tools, winding/unwinding machines and tensioners.

What two major components make up a squirrel cage motor?

Stator and rotor

Name four things to check for when performing general maintenance on a motor or generator

- Brush Wear - Commutator Wear - Damage to windings - Lubrication

Name the five types of brush material used and list the appropriate ampere rating per square inch for each.

- Carbon Graphite (35-45 amps per square inch) - Electro Graphite (75 amps per square inch) - Natural Graphite and Artificial Graphite (50-65 amps per square inch) - Metal Graphite (150 amps per square inch)

Since Wound-Rotor Motors are suitable for extremely heavy loads what types of applications are they well suited? List as many as you think of.

- Cranes - Hoists - Elevators - Conveyors - Fans and Blowers

What are some of the conditions of wear that can be found on a commutator that are not favourable?

- Pitch Bar Marking - Streaking - Threading - Copper Drag - Heavy slot bar-marking - Grooving

What are the four methods that can be used to resurface a commutator?

- Sandpapering - Hand Stoning - Grinding - Turning

If you are troubleshooting a squirrel cage motor that is noisy while running list four items to check and the possible remedy for each.

1. Motor running single phase - Stop motor, then try to start. (It will not start on single phase) Check for "open" in one of the lines or circuits. 2. Shaft Bumping (sleeve-bearing motors) - Check alignment and condition of belt. On pedestal-mounted bearing, check end play and axial centering of rotor. 3. Air gap not uniform - Centre the rotor and, if necessary, replace bearings. 4. Vibration - Driven machine may be unbalanced. Remove motor from load. If motor is still noisy, rebalance motor.

List eight items that you could find on the nameplate of a motor.

1. Volts 2. Phase 3. Amps 4. RPM 5. Duty 6. AMB 7. NO (manufacturers order or catalog number) 8. Form Factor

Synchronous motor

A motor which operates at a constant speed up to full load. The rotor speed is equal to the speed of the rotating magnetic field of the stator: There is no slip. These motors can operate at leading or unity power factor and thereby provide power-factor correction. They also often either have permanent magnets on their rotor or are DC excited. A synchronous motor is often used where the exact speed of a motor must be maintained.

Separately Excited Generators

A separately excited generator is one for which the field current is supplied by another generator, by batteries, or by some other outside source. When operated at constant speed with constant field excitation but not supplying current, the terminal voltage of this type of generator equals the generated voltage. When the unit is delivering current, the terminal voltage is less than the generated voltage. The total amount of voltage drop equals the drop due to armature reaction plus the voltage drop due to the resistance of the armature and the brushes. Separately excited generators, however, are seldom used.

Series Connected Motor/Generator

A series DC motor connects the armature and field windings in series with a common D.C. power source. The motor speed varies as a non-linear function of load torque and armature current; current is common to both the stator and rotor yielding current squared (I^2) behavior. A series motor has very high starting torque (175% or more) and is commonly used for starting high inertia loads, such as trains, elevators or hoists. This speed/torque characteristic is useful in applications such as dragline excavators, where the digging tool moves rapidly when unloaded but slowly when carrying a heavy load.

Runaway Condition

A series motor should never be started at no load. With no mechanical load on the series motor, the current is low, the counter-EMF produced by the field winding is weak, and so the armature must turn faster to produce sufficient counter-EMF to balance the supply voltage. The motor can be damaged by overspeed. This is called a runaway condition.

Capacitor start/Induction run single phase motor

A single phase general purpose design, with an electrolytic capacitor in series with the start winding, offering maximum starting torque per ampere. A centrifugal switch removes the auxiliary winding and capacitor when the motor approaches full load speed. The design is a heavy duty unit which has approximately 300% (of full load) starting torque. Common applications include compressors, pumps conveyors and other "hard-to-start" applications.

Capacitor start/Capacitor run single phase motor

A single phase general purpose design, with an electrolytic capacitor in series with the start winding, offering maximum starting torque per ampere. A second run capacitor remains in series with the auxiliary winding during full load operation. This type of design has lower full-load amps as a result of the run capacitor and is consequently used on most higher horsepower single phase motors.

What is the purpose of a speed-sensing switch?

A speed sensing switch can be used to sequence conveyors when it is necessary for one conveyor to be running at nearly full speed before a second conveyor is started. The switch can also be used to indicate which direction materials on a conveyor are moving from the rotation of a suitable driven shaft. The electronic speed switch is a rugged, self contained, rotary shaft speed detector.

Why is a synchronous motor sometimes run in some industrial environments with no load attached? Explain your answer.

A synchronous motor sometimes run in industrial environments with no load attached because doing so can bring the overall power factor of the building closer to 1. This is achieved because the rotor windings running alongside the stator windings bring the current and voltage of the motor in phase with each other. In our lab test when maximum resistance was added to the rotor windings it made the current lead the voltage on our meters, which would make it a capacitive load and good for power factor correction.

DC generator/motor

Converts mechanical energy into electrical energy or vice-versa. It is made up of a yoke/frame, armature, commutator, field/stationary windings, brushes/brushholders and endbells.

Copper Losses

Copper loss is the power lost in the heat in the windings due to the flow of current through the copper coils. This loss varies directly with the armature resistance and the square of the armature current. The armature resistance varies inversely with the cross-sectional area. Armature copper loss varies greatly mainly because of the variation of the electrical load on the generator and not because of any loss occurring in the machine. This is because most generators are constant-potential machines supplying a current output that varies with the electrical load across the brushes. The limiting factor in load on a generator is the allowable current rating of the generator armature. The armature circuit resistance includes the resistance of the windings between brushes of opposite polarity, the brush contact resistance, and the brush resistance.

True or False: A Capacitor Start motor can be reversed at full speed.

False

True or False: Eddy Current losses can be reduced by using a larger diameter of copper wire.

False

True or False: Generators convert electrical energy into mechanical energy.

False

True or False: Single-Phase AC Capacitor run motors have the highest starting torque compared to other single-phase AC capacitor motors.

False

Left hand rule to determine the polarity of an electromagnet (AKA a coil)

Grasp the coil with the left hand so that the fingers pint int he direction of the current in the coil, and the thumb will point to the north pole of the coil.

Left hand rule to determine the direction of flux in a conductor

Grasp the conductor with the left hand so that the thumb points in the direction of current, and the fingers will point in the direction of magnetic flux.

How do you reverse the direction of rotation of a three-phase ac motor?

Swap any two motor leads.

Wound-Rotor Motor

This type of induction motor differs from the squirrel-cage type in that it has wire-coil windings in it instead of a series of conducting bars in the rotor. These insulated coils are grouped to form definite polar areas having the same number of poles as the stator. The rotor windings are brought out to slip rings whose brushes are connected to variable external resistances.

What are the three types of multi-speed squirrel cage motors called?

Three types of multi-speed squirrel cage motors are constant torque, constant horsepower and variable torque/horsepower.

What types of fuses are normally used in a motor circuit and what are they designed to protect?

Time delay fuses are normally used in a motor circuit. They are designed to protect the windings.

How do you reverse the direction of a single phase ac motor?

To change direction of rotation in a single phase motor the polarity of the starting (or auxiliary) windings must be reversed

True or False: A Split-Phase motor has an auxiliary (start) and a main (run) winding.

True

True or False: A permanent magnet motor has high torque at low speed.

True

True or False: A skeleton motor has only one multiple coil winding for creating a magnetic field.

True

True or False: Compound DC motors are rated for continuous duty.

True

True or False: The biggest wear items in a DC Motor are the brushes.

True

True or False: The difference in an AC three-phase motor running at full speed and synchronous speed is referred to as slip.

True

Brushes/Brush Holders

The brushes carry the current from the commutator to an external circuit. Usually they are a mixture of graphite and metallic powder. Brushes are designed to slide freely in their holders because the commutator surface is usually uneven, and the brushes and commutator wear. The freedom thus allows the brushes to have good contact with the commutator despite wear or uneven surfaces.

Slip

The difference between the speed of a rotating magnetic field (which is always synchronous) and the rotor in a non synchronous induction motor. Slip is expressed as a percentage of synchronous speed and generally increases with an increase in load.

How do you determine the direction of rotation for a Shaded-Pole motor?

The direction of a shaded pole motor is determined by the direction that the shading ring points

How is the direction of a compound motor usually reversed and why?

The direction of rotation on a compound motor is usually reversed by changing the polarity on the leads on the armature. This is because we usually use cumulative motors which need to have their shunt and series fields wound in the same direction. The direction could be changed by switching the polarity of the shunt and series field windings but changing the windings of one and not the other could turn it into a differential compound motor and cause serious problems.

Field Windings

The field windings are connected so that they produce alternate north and south poles. Connection is done that way to obtain the correct direction of EMF in the armature conductors. The field windings form an electromagnet that establishes the generator field flux. These field windings may receive current from an external DC source, or they may be connected directly across the armature, which then becomes the source of voltage. When the windings are energized, they establish magnetic flux in the field yoke, pole pieces, air gap, and armature core.

Field Windings

The field windings are connected so that they produce alternate north and south poles. Connection is done that way to obtain the correct direction of emf in the armature conductors. The field windings form an electromagnet that establishes the generator field flux. These field windings may receive current from an external DC source, or they may be connected directly across the armature, which then becomes the source of voltage. When the windings are energized, they establish magnetic flux in the field yoke, pole pieces, air gap, and armature core.

Field pole and Frame/Yoke

The frame or yoke of a generator serves two purposes. It provides mechanical support for the machine. It is also a path for the completion of the magnetic circuit. The lines of force that pass from the north to the south pole through the armature are returned to the north pole through the frame. Frames are made of electrical-grade steel. The method of construction of field poles and frames varies with the manufacturer.

What type of relationship exists between the amount of overload and the time of failure for a protection device? Explain how this relationship works.

The larger the overload, the more quickly the temperature will increase to a point that is damaging to the insulation and lubrication of the motor. An inverse relationship, therefore, exists between current and time. The higher the current, the shorter the time before motor damage, or burnout, can occur.

For a split phase AC motor at what percentage of speed does the centrifugal switch contacts open?

The open at 75%

Stator

The part of an induction motor's magnetic structure that does not rotate. It usually contains the primary winding.

Armature

The portion of the magnetic structure of a DC or universal motor that rotates

Hysteresis Losses

The resistance to becoming magnetized offered by materials. Reduced by using silicon steel laminations.

Rotor

The rotating member of an induction motor in a single phase. Current that is normally induced in the rotor reacts with the magnetic field produced by the stator. This produces torque and rotation.

Starting Torque

The turning effort or torque the motor exerts when full voltage is applied to its terminals at the instant of starting. It is usually expressed as a percent of full load torque.

Permanent split capacitor (PSC) motor

This design has an auxiliary winding with a "run" capacitor, but unlike the capacitor start / induction run motor, the capacitor and auxiliary winding remain in the circuit under running conditions. (There is no centrifugal switch on this type of motor). A permanent Split Capacitor motor has low starting torque and low starting current. PSC motors are generally used on direct-drive fans and blowers. They can also be designed for higher starting torque and intermittent applications where rapid reversing is desired.

Autotransformer (Compensator) Starting

Two or more contactors may be used to provide reduced voltage starting of a motor. By using an autotransformer or a series inductance, a lower voltage is present at the motor terminals, reducing starting torque and inrush current. Once the motor has come up to some fraction of its full-load speed, the starter switches to full voltage at the motor terminals. Since the autotransformer or series reactor only carries the heavy motor starting current for a few seconds, the devices can be much smaller compared to continuously rated equipment. The transition between reduced and full voltage may be based on elapsed time, or triggered when a current sensor shows the motor current has begun to reduce. An autotransformer starter was patented in 1908.

After resurfacing a commutator what are the two most important things to do?

Undercut the mica and bevel the commutator slots

Why does an external DC voltage applied to the excitation field bring an AC Synchronous Motor to synchronous speed?

When DC voltage is applied to the rotor it gives the rotor a fixed magnetic polarity. This makes the rotor act like a permanent magnet and causes it to spin at the same speed of the rotating magnetic field created in the stator.

Plugging

When a motor running in one direction is momentarily reconnected to reverse the direction, it will be brought to rest very rapidly. This is referred to as plugging. If a motor is plugged more than five times per minute, derating of the controller is necessary because of the heating of the contacts. Plugging can be used only if the driven machine and its load will not be damaged by the reversal of the motor torque. P|ugging is easier on your motor than dynamic braking.

Explain what happens to a three phase motor while running if the motor suddenly loses power to one of the phases?

When a three phase motor loses one of its phases the motor will still continue to run but with lower power and overheating. This situation is called "single phasing" and can severely damage or burn out your motor.

What will cause a brush to wear 2 to 5 times faster then normal and what is this condition called?

When the protective film on the commutator wears off it can cause the brushes to wear 2 to 5 times faster than normal. This is called dusting and it occurs due to the loss of the water vapor component of the film with lack of humidity.

Describe the operation on a Wye/Delta starter while in Wye mode.

While in Wye mode the motor runs more slowly. This is because in Wye mode the voltage is dropped over two sets of windings and therefore has a weaker field of flux. This is done to allow for a lower inrush of current on start up.