ELECTRIC MOTOR - Part 3

ELECTRIC MOTOR

Part 3
March 11, 2014

• Motor Protection
• Protecting an electric motor basically involves preventing the motor from getting too hot.
•
• Every 10C above the maximum recommended temperature of the insulation can reduce its working life by half.
• The best way to protect a motor against overheating is to directly monitor the temperature of the motor windings.

• If the temperature exceeds the maximum set value for the motor its contactor is tripped to allow the motor to stop and cool down.
• There are number of temperature dependent protection devices available, but thermistor protection is probably the most common.
• Thermistors are small beads of semiconductor material which are embedded into all 3 of the motor stator windings during manufacture.

• When one of the lines becomes open-circuited a balanced 3- condition no longer exists. Now the currents in the phases are no longer equal to one another.
• Typical values of line and phase currents at various levels of load during a single-phase fault ():
• The current in the winding C is considerably higher than that in the other 2 windings. The local overheating in winding C of the motor will quickly result in damage.

• This is why Y connected motors have, in the past, been preferred to  connected motors for use on ships.
•  connected motors can be protected against this condition using a differential type relay.
• Most modern thermal OCRs for motors have this protection incorporated as a normal feature.
• The 3 thermal elements of the OCR are arranged in such a way that unequal heating of the bi-metal strips causes a differential movement which strips the contactor.

• If single-phasing occurs when the motor is running the motor keeps on running unless the protection trips the contactor.
• If the motor is stopped, it will not restart.
• When the contactor is closed, the motor will take a large starting current but develop no torque.
• The OCR is set to allow the starting current to flow long enough for the motor, under normal conditions, to run up to speed.

• With no ventilation on the stationary motor, this time delay will result in rapid and severe overheating. Worse still, if the operator makes a several attempts to start the motor, it will burn-out. If the motor fails to start after two attempts, you must investigate the cause.
• U/V protection is necessary in a distribution system that supplies motors. If there is a blackout all the motors must be disconnected from the supply.

• This is to prevent all the motors restating together when the supplied voltage is restored, which would result in a huge current surge, tripping the generator again. Motors must be restarted in a controlled sequence after a supply failure.
• U/V protection is provided by the spring loaded motor contactor because it will drop out when the supply voltage is lost.
• When the supply returns the motor will not restart until its contactor coil is re-energized.

• This may require the operator to press the start button or, if it is an essential load, it may be done automatically by a sequence restart system.
• This system ensures that essential services are restarted automatically on restoration of supply following the blackout.
• Timer relays in the starters of essential motor circuits will initiate start-up in a controlled sequence.

• Motor Speed Control
• The standard cage induction motor operates as an almost constant speed drive over its load range.
• This feature is satisfactory for most of the ship’s auxiliary services supplying power to ventilation fans and circulating pumps.
• Variable speed control is necessary for cranes, winches, windlass, capstan, forced-draught fans, etc.

• 2 main forms of speed control:
• 1. Pole-changing to give two or more fixed speeds, e.g. 2-speed FD fans and 3-speed winches
• 2. Continuously variable speed control, e.g. smooth control of deck cranes and winches.
• Fixed set speeds can be obtained from a cage-rotor induction motor by using a dual-wound stator winding; each winding being designed to create a different number of magnetic poles.

• Q: A dual-wound induction motor is arranged to create either 6 poles or 10 poles. Estimate the rated speeds assuming that the rotor slips by 5% and the power supply is 60Hz.
• A: Two fixed speeds in a 2:1 ratio from a cage rotor induction motor is to use a single stator winding which has a center-tap connections available on each phase.
• A continuously variable speed range of motor control involves more complication and expense than that required to obtain a couple of set speeds.

Various methods:
• 1. Electro-hydraulic drive
• 2. Wound-rotor resistance control of induction motor
• 3. Ward-Leonard dc motor drive
• 4. Variable frequency induction motor control
• Electro-Hydraulic Drive
• Electro-hydraulic drive, often used for deck crane control, has a relatively simple electrical section.
• This is a constant single-speed induction motor supplied from a DOL or Y- starter.
• The motor runs continuously to maintain oil pressure to the variable-speed hydraulic motors.
• Wound Rotor Induction Motor
• A crude form of speed control is provided by the wound rotor induction motor.
• The rotor has a 3- winding (similar to its stator winding) which is connected to 3 slip rings mounted on the shaft.
• An external 3- resistor bank is connected to brushes on the rotor slip rings.
• A set of contactors or a slide wiper (for small motors) varies the amount of resistance added to the rotor circuit.
• Increasing the value of external resistance decreases the rotor speed.
• Generally, the starter of wound-rotor motors are interlocked to allow start-up only when maximum rotor resistance is in the circuit.

• This has the benefits of reducing the starting current surge while providing a high starting torque.
• The wound-rotor arrangement is more expensive than an equivalent cage-rotor machine.
• It requires more maintenance on account of the slip rings and the external resistor bank which may require special cooling facilities.
• Ward-Leonard DC Motor Drive
• Where continuously variable speed has to be combined with high torque, smooth acceleration, including inching control and regenerative braking, it is necessary to consider the merits of a dc motor drive.
• Speed and torque control of a dc motor is basically simple requiring the variation of armature voltage and field current.
The problem is: where does the necessary dc power supply come from a ship with an ac electrical system

• A popular method for lifts, cranes and winches is found in the Ward-Leonard drive.
• Here a constant speed induction motor drives a dc generator which in turn supplies one or more dc motors.
• The generator output voltage is controlled by adjusting its small excitation current via the speed regulator.
• The dc motor speed is directly controlled by the generator voltage.

• The Motor-Generator (M-G) set requires space and maintenance.
• An alternative is to replace the rotary M-G set with a static electronic thyristor controller which is supplied with constant ac voltage and delivers a variable dc output voltage to the drive motor.

• Variable Frequency Induction Motor Control
• Although the Ward-Leonard dc scheme provides an excellent power drive, the maintenance problems are high due to the commutator and brushgear aspects of the dc machines.
To eliminate these problems means returning to the simplicity of the cage-rotor induction motor.
The only way to achieve a continuously variable speed output by electrical control is to vary the input supply frequency.

A static electronic thyristor controller may be used to generate such a variable frequency output to directly control the speed of the motor. The result is relatively maintenance free system with a lot of electronic circuitry

Motor and Starter Maintenance
Maintenance for cage-rotor induction motor:
Keep insulation resistances high and contact resistances low. Lubricate correctly and maintain an even air gap. Ensure both the interior and exterior are always clean and dry. Provided these requirements are met, an induction motor should give trouble-free service during its long life.

Q: What is the most common cause of induction motor failure?
A: Failure of stator insulation due to dampness is a major problem with marine motors.
Open ventilated motors are most at risk, particularly when they are not used for long periods.
Anti-condensation heaters should be regularly checked to see that they are actually working and keeping the motor dry.

A regular cleaning routine is required to remove harmful deposits of dust, dirt, grease and oil from both inside and outside of the motor. The cleaning of the external surface is especially important for totally enclosed motors which run continuously.

A thick layer of dust will reduce the heat dissipation and result in very high temperatures.

Internal dust and dirt in open ventilated motors must be regularly removed by blowing or extraction and ventilation screens and ducks cleared out.

If motors are to be blown out, the air used must b absolutely dry and the pressure should not be more than 1.75 bar.
If the pressure is higher than this it forces the dust into the winding insulation rather than removing it.

When blowing out a motor remember to cover up other machines in the area to protect them from flying dust.
Suction cleaning is better than blowing out.

Q: How often should a motor be cleaned?
A: Basically this will be determined by the local conditions and the type of ventilation.
Contamination by oil and grease from motor bearings is often a cause of insulation failure.
The insulation should be cleaned by brushing or spraying with one of the many propriety brands of cleaning fluid which are available.

Badly contaminated motors may require total immersion of the stator windings in cleaning fluid.

Broken or missing bearing covers must be repaired or replaced to prevent grease escaping.
When a motor has been dismantled for cleaning and overhaul it should be thoroughly inspected.
In this way faults can be detected before they lead to breakdowns.

Carefully examine the stator core for signs of rubbing with the rotor caused by worn bearings.
Even slight rubbing of the rotor against the stator will generate enough heat to destroy the stator insulation.

Renew bearings before putting back into service.
Core plates which have been badly scored may cause a local hot spot to be generated when the motor is running.

This is because the iron losses will increase in the damaged area. After the motor has been put back into service with new bearings check the motor running temperature.

After a short period of service dismantle the motor and check for discoloration at the core damage which will indicate local heating.

If you suspect core hot spots then the motor core will need to be dismantled for the laminations to be cleaned and re-insulated – a shore job.

The insulation resistance reading is the best indication as to the presence of moisture in the motor windings.

Breakdowns due to insulation failure usually result in an E/F, short-circuited turns in a phase or phase-to-phase faults.

Q: How do you check the insulation resistance between phases on an induction motor?
A: Large motors are usually 6 terminals, which means that all 6 ends of the stator windings are brought out to the terminal block. Links between the terminals are used to Y or  connected motors. Disconnect the supply leads and remove links. Test between phases with an insulation resistance tester. A problem can arise on small 3 terminals motors where the Y or  connection is made inside the motor. Only one end of each winding is available at the terminal block.
Phase-to-phase insulation resistance cannot be checked. If a 3 terminal motor is to be rewound ask the repairer to convert it to a 6 terminal arrangement.

BEARINGS
Induction motors are fitted with ball and/or roller bearings.
These bearings are tough and reliable and should give very little trouble provided they are properly fitted, kept absolutely clean and lubricated correctly.

ROTOR
Cage-rotors require little or no special care in normal service. Inspect for sign of damage and overheating in the cage winding and laminated core.

Make sure that all core ventilating ducts are clean and clear. If an internal fan is fitted it must be in good condition if it is to provide adequate cooling.

Q: A cage-rotor induction motor has been flooded with seawater. Insulation resistance is down to zero M. What is the procedure for putting the motor back into service?

A: The main problem is to restore the insulation value of the stator winding to a high value. This is achieved in 3 stages:
1. Cleaning
2. Drying
3. Re-varnishing

Salt contamination can be removed by washing with clean, fresh water. Any grease or oil on the windings has to be removed using a degreasant liquid such as Armaclean.

Dry the stator windings with low power electric heaters or lamps with plenty of ventilation to allow the dampness to escape. Alternatively, the windings can be heated by current injection from a welding set or from a special injection transformer.

Be sure to keep the injected current level well below the motor’s full load rating
With the windings clean and dry, and if the IR test remains high over a few hours, apply a couple of coats of good quality air-drying insulating varnish.

Starter and other motor control gear should be regularly inspected to check and maintain the following items:

1. Enclosure – Check for accumulation of dirt and rust. Any corroded parts must be cleaned and repainted. Examine the starter fixing bolts and its earth bonding connection – particularly where high vibration is present, e.g. in the steering flat and the foc’sle.

2. Contactors and relays – Check for any signs of overheating and loose connections.
Remove any dust and grease from insulating components to prevent voltage breakdown by surface tracking. Ensure that the magnet armature of contactors moves freely. Remove any dirt or rust from magnet faces which may prevent correct closing.

3. Contacts – Examine the excessive pittings and roughness due to burning.
When contacts have to be replaced, always replace both fixed and moving contacts in pairs. Check contact spring pressure and compare adjacent contacts sets for equal pressure. Check power and control fuse contacts for signs of overheating – lubricate moving contacts on fuse-holder.

4. Connections – Examine all power and control connections for tightness and signs of overheating. Check flexible leads for fraying and brittleness.

5. Overcurrent relays – Check for proper size (relate to motor full load current).
Inspect for dirt, grease and corrosion and for freedom of movement. A thorough OCR performance test can only be carried out by calibrated current injection.

6. Control operation – Watch the sequence of operation during a normal start-up, control and shut- down of the motor. Particularly look for excessive contact sparking. Check operation of emergency stop and auto restart functions

References:
Practical Marine Electrical Knowledge- By Hall
Siskind Charles , Electrical Machine
Preventive Maintenance of Electrical Machine by Hubert, Charles (2nd Edition)
www.electricalmachine.com

These(3 articles will be one of the sources of the questions for our mastery test number two. Please read thoroughly over and over.