Year 3 - AC Magnetic Motor Control

What two common 3 phase connections are used for starting motors with autotransformers?

3 transformers connected in Y or two transformers connected in open delta

What standard taps are available on motor starting autotransformers ?

50, 65, & 80% of full line voltage rating

How many leads run between the Wye-Delta starter and the motor?

6

What type of device do you commonly use to manually control the resistance in the secondary circuit?

A drum controller

What is meant by the term automatic, speed sensing acceleration?

A means of automatically sensing speed of the rotor in order to determine the proper time for removal of resistance from the secondary winding

What additional components are necessary to convert an open transition, wye delta starter to a closed transition type?

An additional 3 pole starting contactor and 3 power resistors

What device in the controls is used to protect the amortisseur winding if its getting too hot?

An out of step relay

What happens to the voltage induced in the amortisseur winding once the motor is operating at synchronous speed?

At synchronous speed, the relative speed between the rotating stator field and the amortisseur winding is 0. Therefore no voltage is induced in the amortisseur winding.

Where are disk brakes normally mounted?

At the end bell of the motor

What is meant by the term automatic, definite-time acceleration?

Automatically removes the resistance in steps at the same time intervals each time the motor is started.

What is the purpose of the DC coil on the PFR?

It times the application of the rotor field excitation, so the field reaches fulls strength when rotor and stator poles are adjacent

What is the purpose of the field rheostat?

It controls the DC through the rotor field.

In a synchronous motor what percentage of sync speed is the dc excitation normally applied to the field windings?

It is normally applied somewhere between 92 and 97 of synchronous speed. This will vary depending on the load

Describe how several steps can be added to starting stage of the primary resistance starter.

By adding more resistors in series, each with an accelerating contactor.

How do you adjust the speed at which the field contactor of a synchronous motor starter pulls in?

By adding or removing non magnetic shims for the face of the PFR stator core

How do you reverse the direction of rotation of a wound rotor motor?

By interchanging any two of the line leads to the stator

How is motor speed sensing normally achieved?

By using frequency- sensing relays

What is the main disadvantage of using reactors to start induction motors?

It worsens the already poor ( lagging ) power factor of the induction motor circuits.

What will happen if the resistance of the field rheostat is decreased while the motor is running?

Causes the power factor to become more leading. Current through rotor field will be increased.

What should you check periodically in regards to resistors of a primary Impedance starter?

Check for loose connections and signs of overheating

List 4 actions suggested when troubleshooting the control operating of a synchronous motor.

Check the sequence of operation Check relay contacts for sparking Check contractors for flash when closing Check all switches

What type of starter is the primary resistance starter? Open or closed transition?

Closed Transition

Whats another name for a autotransformer starter?

Compensator starter

Which is generally more accurate: a regulator that uses regeneration or one that uses secondary resistors?

Controller using regeneration This controller compares set speed and actual speed and adjusts itself to bring the two together

What are the 4 key rules for maintaining electrical controls?

Keep it Dry Keep it Tight Keep it clean Keep it friction free

What is the major disadvantage of automatic, automatic, definite-time acceleration?

It is not sensitive to the requirements of the load. Under heavy load, resistance may be removed too quickly.

Explain the purpose of the field reactor coil.

It produces a volt drop; this volt drop is necessary so the AC coil of the PFR can sense the AC in the circuit

What type of time delay relay is commonly used for the out of step relay?

Dashpot timers

What is the purpose of the AC coil on a PFR?

It senses the speed of the rotor

What is the next step in a lockout procedure, after all sources of energy have been suitably locked out?

Do a bump test. ( try the controls too make sure nothing is operational)

List 3 types of friction brakes.

Drum Disk and band brakes

Explain what is meant by the term dynamic braking.

Dynamic braking is a method of retarding the motor speed or completely stopping the motor by making it act as a generator to supply a resistive load.

State two situations where it may be necessary to use starting reactors rather than resistors.

For high voltage/high current motors Where heating or physical construction of the resistors create a problem

Why is it sometimes necessary to provide larger than normal contactors for some motors?

Frequent repetitive operation under large starting currents greatly reduce life of the contacts

What type of motor duty is required for a service that must operate for definite, specified, alternate intervals of load and rest?

Intermittent duty

What should you check for when inspecting the secondary resistors of a wound rotor motor?

Loose connections and overheating

What is the major advantage of a speed regulator that uses regenerative over a speed regulator that simply uses secondary resistors ?

Much higher efficiency compared to speed regulator with resistors

Can a wound rotor motor be reversed by making changes in the rotor circuit?

No

If an AC motor will not start under load with rated voltage, will you be able to start it with reduced voltage?

No

Can all three phase motors be used for Wye-Delta starting ?

No, the motor connection box used must allow access to both ends of each phase winding external to the enclosure

Are friction brakes normally electrically applied and spring released?

No. Friction brakes are normally spring applied and electrically released

List four categories into which most electrical faults fall.

Opens Shorts Grounds High resistance

What is the most important consideration in determining whether or not a motor may be started directly across the line?

Permissible voltage fluctuations (disturbance) to the power distribution system

Explain what plugging means.

Plugging is a control function that provides braking by reversing the motor line voltage polarity or phase sequence so that the motor develops a counter torque that exerts a retarding force

What does the abbreviation PFR stand for ?

Polarized field frequency relay

It disconnects the DC field excitation if an Overload or voltage dip causes the motor speed to drop, and reapplies it if the speed returns to within synchronizing speed before the out of step relay opens its contacts

Polarized field frequency relay (PFR)

This Device times the application of DC excitation so that it will create a magnetic field in the rotor circuit that will reach full strength when two opposite poles are aligned with each other.

Polarized field frequency relay (PFR)

This device monitors the rotor speed so that I can time the application of the DC rotor field current to somewhere between 92 and 97 percent of synchronous speed

Polarized field frequency relay (PFR)

List 3 methods of reduced voltage starting

Primary impedance starters Autotransformer starters Wye-Delta startings Part winding Starters Solid State starters

What should you check for when inspecting dashpot timers or dashpot overload relays?

Proper size, freedom of movement

State two advantages that reduced voltage starting has over across the line starting for AC motors.

Reduced inrush current on start Reduced torque on starting Less disturbance ( line drop ) to power system

Explain what regenerative braking.

Regenerative braking is a method by which the motor is made to act like a generator and feed energy back into the system.

What is the reason for the positive overlap of the contacts in the field contactor?

The positive overlap ensures that the field will never be open circuited. An open circuit in the field circuit can cause a high voltage to be induced in the DC windings of the rotor field.

What is the difference between a schematic and a wiring diagram?

Schematic: Electrical relationships Wiring diagram: physical relationships

State two advantages that across the line starting has over reduced voltage starting for AC motors.

Simple control low component cost maximum starting torque Minimum acceleration time for motor

How are high resistance faults normally located?

Sometimes with an ohm meter but mostly when the circuit is loaded by checking the voltage drops.

Which type of brake is automatically applied if there is a system power failure?

Spring applied type of friction brake

Why is a wound rotor motor started with resistance inserted in its secondary circuit?

Starting torque increases and starting current decreases

What test equipment would you use to check the speed of a motor?

Tachometer

What action takes place first when a field contactor drops out?

The NC contact in series with the field discharge resistor closes

What action takes place first when a field contactor pulls in ?

The NO contacts that apply DC excitation to the field close.

If the armature of the PFR does not pull in right away when the motor is started or when the motor speed drops below synchronous speed what should be checked?

The PFR should be checked for a sticky mechanism or too large an air gap.

What will happen to the amortisseur winding in a synchronous motor if the motor is permitted to operate at less than synchronous speed for an extended period of time?

The amortisseur or damping winding will overheat

Why are synchronous motors often rated with a starting duty cycle?

The amortisseur winding is not designed to carry current for long periods of time.

Autotransformers are generally used for intermittent duty. What does this mean?

The amount of time a device can be in service before getting damaged

Why does speed regulation of a wound rotor motor become very unstable below about 50% of sync. speed?

The flat slope of the torque vs speed means that a small change in torque output of the motor requires a large change in speed

When adjusting the speed at which the contactor pulls in, what should you be trying for?

The highest practical speed of the application

Why is the autotransformer method of motor starting considered to provide more starting torque per line ampere of current than any other type of reduced voltage starter?

The line current (primary) Is a further reduction of the motor current ( secondary ) through transformer action In all other starters the line current and motor current are the same.

What is the difference between a reduced voltage starter with an open transition and a reduced voltage starter with a closed transition?

The open transition has a momentary loss of voltage which leads to a momentary torque dip and line disturbance

Explain how the out of step relay protects the amortisseur winding against overheating?

The out of step relay is a timer that starts and to time out whenever the rotor is turning slower than synchronous speed. If the relay times out, it opens a contact in the control circuit, which causes the line contactor to drop out and the motor to stop.

What might occur if the timing action of the out of step relay fails during startup, and its timed contacts remain closed?

The purpose of the PFR is to protect the amortisseur winding. If the motor is started under heavy load, the amortisseur winding may be damaged if the motor takes too long to come up to sync speed

How can the power factor of a synchronous motor be made more leading?

The resistance of the field rheostat must be reduced to allow more excitation current to flow through the rotor field winding.

Explain the difference between a wound rotor motor starter and a wound rotor motor speed regulator.

The resistors in a wound rotor motor speed regulator would have to be of a high wattage rating when in comparison to the starter.

Which type of electrical diagram is used to explain the operation of electrical circuits?

The schematic

Explain why a circuit should be loaded when testing for voltage drops across various parts of the circuit.

The voltage drops will only occur when current is flowing through the circuit

What type of protection do autotransformers starters generally use?

Thermal ( high limit ) switches equipped with heat sensors

As synchronous motor accelerates, what happens to the magnitude and frequency of the AC induced in the field coils?

They Decrease

What happens to the magnitude and frequency of the voltage induced in the rotor as the motor accelerates?

They Decrease

When the field is applied to the rotor, the ammeter in the rotor circuit dips slightly below zero and then swings up to the normal excitation level. What does this indicate about the polarity of the DC applied to the field winding?

This indicates the proper DC polarity on the PFR

Why should at least one shim always be left in the magnetic circuit of a PFR?

To prevent the armature from sealing against the stator core ( due to residual magnetism)

Why is the resistance in a wound rotor motor removed in steps as the motor accelerates?

To produce a smoother accelerating torque

What is the purpose of the field discharge resistor?

To protect against induced back voltages

How is automatic, definite-time acceleration normally achieved ?

Using time delay relays

What is different about a field contactor compared to a normal contactor?

When the field contactor pulls in, the normally open contacts close before the normally close contacts open. When the contactor drops out the normally closed contact close before the normally open contacts open

Explain why the NC contact from the PFR is in series with the field contactor.

When the motor is first started, or anytime the rotor turns at lower than synchronous speed, the PFR pulls in, opening the NC contact. This prevents the DC field from being applied to the rotor.