QUE-ANS:10-30 words

[1] What is jogging?

It means inching a motor i.e. Make it move a little at a time by constant starting and stopping.

[2] What is meant by plugging?

It means stopping a motor by instantaneously reversing it till it stops.

[3] Mention the factors on which hysteresis loss depends?

1) quality and amount of iron in the core 2) flux density and 3) frequency.

[4] How can eddy current loss be minimized?

By laminating the core.

30-60 words

[1] What is the function of inter-poles and how are inter-pole windings connected?

Inter-poles are small poles placed in between the main poles. Their function: commutation by producing the auxiliary or commutating flux. Consequently brush sparking is practically eliminated. Inter-pole windings are connected in series with the armature winding.

[2] What will happen if a shunt motor is directly connected to the supply line?

Small motors up to 1 kw rating may be line-started without any adverse result being produced. High rating motors must be started through a suitable -starter in order to avoid the huge starting current which will (i) damage the motor itself and (ii) badly affect the voltage regulation of the supply line.

[3] What is back emf in a motor?

When the motor rotates, the armature conductors cuts the magnetic flux. According to the Faraday’s laws of electromagnetic induction, an emf is induced in the conductors. This emf acts in a direction opposite to the applied voltage (Lenz’s law). This is-called as back emf. This back

Emf is always less than the applied voltage.

[4] A motor stops after starting i.e. it fails to carry load. What could be the causes?

Any one of the following:

Hot bearings, which increase the load by excessive friction.
Excessive tension on belt, which causes the bearings to heat.
Failure of short cut-out switch.
Single-phasing on the running position of the starter.

60-100 words

[1] Describe its precautions necessary to ensure that the motor is not started inadvertently.

Precautions necessary to ensure that the motor is not started inadvertently are:

Remove the circuit fuses for the motor concerned.

Motor switch should be locked in open position.

Put sign to motor and switch indicating ‘do not start’ or ‘out of service’.

If necessary, disconnect the terminal at the motor junction box, tagging the terminal correctly and covering the exposed terminal with insulating tapes.

Switch off circuit breaker on the panel switch board in control room or on the respective motor control panel.

[2] Explain why it is important to ensure that supply connections to a three phase motor are made in the correct order.

It is important to ensure that the supply connections to a three phase motors are made in the correct order so that:

To ensure that the direction of motor is correct before couples to machinery, otherwise damage may be done to the driven machinery e.g. electro hydraulic steering pumps.

In case of lifts, hoists, all electric steering gear, the limit switches may not afford protection. It the lift travels up, instead of down, the top limit switch may open control circuit, which is not energized and the motor will not stop.

[3] Explain Regulations regarding Power Supplies for Steering Gears.

The Electrical Power Supply must be from two widely separate supplies i.e. one from the Main Switchboard and the other from the Emergency Board.

Electrical leads and fuses are to allow 100% overload, giving only short circuit protection.

If failure does occur on one system, the stand-by system will cut in within 45 seconds.

The Steering Motors will have sequential starting.

[4] What is ‘cogging’?

This is also known as magnetic locking. When the voltage is low, the motor does not start. This happens when the number of stator teeth is equal to the number of rotor teeth and due to the magnetic locking, the motor does not start. This can be overcome by making the number of rotor slots prime to the number of stator slots.

[5] What happens if single-phasing occurs when the motor is running? And when it is stationary?

If already running and carrying half load phase motor on the remaining single-phase blow normal fuses.
If motor is very heavily loaded, then it will stop under single-phasing since it can neither restart nor blow out the remaining fuses, the burn-out is very prompt.

A stationary motor will not start with one line broken. In fact, due to heavy standstill current, it is likely to burn-out quickly unless immediately disconnected.

[6] What are the causes of faulty starting of a synchronous motor?

It could be due to the following causes:

Voltage may be too low-at least half voltage is required for starting
There may be open-circuit in one phase-due to which motor may heat up.
Static friction may be large-either due to high belt tension or too tight bearings
Stator windings may be incorrectly connected
Field excitation may be too strong.

[7] What happens when a DC motor is connected to AC supply?

Since on ac supply, reactance will come into the picture, The AC supply will be offered impedance (not resistance) by the armature winding. Consequently, with AC supply, current will be much less. The motor will run but it will not carry the same load as it would be on DC supply.

There would be more sparking at the brushes.

Though motor armature is laminated as a rule, the field poles are not. Consequently with AC supply, currents will cause the motor to heat up and eventually burn.

100-150 words

[1] What happens if a motor is operated in star instead of delta?

If the motor is designed to be operated in delta on your local supply voltage, then operating it continuously in star can cause damage to the motor. When a delta motor is connected to star, the voltage across the windings is reduced by the square root of three resulting in reduced flux in the iron. This will reduce the magnetizing current, and will also reduce the torque capacity of the motor.

If you operate at light loads, there will be no problem, however if you operate at higher load the slip of the motor will be increased dramatically and it may stall. The increased slip will result dramatic increase in the power dissipated in the rotor. If the motor begins to stall, the stator will also suffer excess heating causing motor failure.

[2] What are the different types of enclosures available for induction motors?

Totally enclosed, non-ventilated type: such motors have solid frames and end shields, but no openings for ventilation.

Splash proof type: in the frames of such motors, the ventilating openings are so constructed that the liquid drops or dust particles falling on the motor will not be able to enter the motor when the incident angle is not greater than 100^0.

Totally enclosed, fan cooled type

Protected type: end covers have perforated covers.

Drip proof motors: the frames are so constructed that liquid drops or dust particles, falling on the machine at any angle greater than l50 from the vertical, cannot enter the motor, either directly or by striking and running along a horizontal or inwardly inclined smooth surface.

Self-ventilated type: consists of enclosed shields with provision for pipe connection on both the shields. The motor fan circulates sufficient air through pipes which are of ample section.

Separately (forced) ventilated type.

[3] What is meant by ‘single-phasing’ and what are its causes?

Single-phasing is meant the opening of one wire (or leg) of a three-phase circuit there upon the remaining leg at once becomes single-phase. When a three-phase circuit functions normally, there are three distinct currents flowing in the circuit. As is known, any two of these currents use the third wire as the return path i.e. one of the three phase acts as a return path for the other two. Obviously, an open circuit in one leg kills two of the phases and there will be only one current or phase working, even though two wires are left intact. The remaining phase attempts to carry the entire load. The usual cause of single-phasing is, what is generally referred to as running fuse, which is a fuse whose current carrying capacity is equal to the full-load current of the motor connected in the circuit. This fuse will blow-out whenever there is overload (either momentary or sustained on the motor.

[4] Briefly describe the effect on a 3 phase AC motor if the supply voltage was at a lower frequency than the motor design frequency.

At frequency lower than that for which motor is designed, the stator flux increases and magnetizing current increased, so the heating on winding and iron will increase. Motor will run hotter, unless reducing in speed, also bring about reduction in load for example centrifugal pump, fan etc.

An increase in magnetizing current, power factor will be lower which indicating increasing in air gap. Uneven air gap can cause bearing worn out. The possibility rotor and stator rub each other occur causing overheating which can destroy the coil insulation. Life of bearing will reduce due to unbalanced torque and vibration can occur as a result of increase in magnetic flux. Excessive heat generating to the bearing may cause seizure resulting noise sound in the motor. Directly the damage on the moving part of the pump occurs and seizure takes place which reduce efficiency of the pump performance.

[5] Explain why an AC motor which is stopped due to interruption of supply should be disconnected from the supply.

AC motors which are stopped due to interruption of supply should be disconnected from supply because on restarting of supply it could not be immediately switched on for the following reasons:

Operating personnel may be injured due to motor starting unexpectedly, e.g. engine room overhead cranes, lathe machines, portable electric appliances, tools and cargo winches.

With certain AC motors, a definite sequence of restarting motor operation must be gone through.

If all motors were to restart simultaneously, the generators might be seriously overloaded.

Certain adjustment to the driven machinery may be necessary before restarting.

If the motor is driving some heavy loads, it may be necessary to make adjustments to the driven machinery such as controlling valves, regulating the load so as not to overload the motor if it were to restart immediately, thus preventing damage to motor.

[6] Describe the effect of running a motor with the stator windings connected in star when the motor is designed for running with delta connected windings.

The effect of running a motor with stator windings connected in star when the motor is designed for running with delta connected windings are:

A star delta motor starter gets stuck in star position, malfunction in control timer, contactors, etc.
On full load, each stator winding will be carrying an overload of 1.73 x rated line current.
This will cause overheating, because of I² R effect. The motor will run 3 times hotter.
But the line current will be normal and thermal overload protection will not protect the motor. This will cause overheating and eventually burn out.

[7] Briefly state the points on which attention should be given if a large induction motor is to be shut down for an extended period.

The points on which attention should be given if a large induction motor is to be shut down for an extended period are:

Motor should be cleaned and windings free of oil. This can be done by cleaning with chemical using carbon tetrachloride.

Heavy deposits of dirt will have to be removed by brushing or scraping using a suitable scraper or by compressed air, reusing carbon tetrachloride for final cleaning of surface.

Excess fluid should be wiped off and deposits of fluid in crevices should be avoided.

Unpainted iron and steel shafting, coupling etc. should be given a light of coat to prevent rusting.

Motor should be covered if possible so as to keep the dirt and dust from entering the motors and to reduce dampness in the windings.

If heaters are installed in the motor, it should be kept ‘on’ to prevent accumulation of moisture.

[8] State what investigations would need to be made before starting a large induction motor which had been shut down for an extended period?

Investigation that should be done before starting large induction motor which had been shut down for an extended period are:

Check insulation with megger. If reading is low, dry out winding with heating lamp.

Check heaters, if installed.

If it has brushes, check, clear and be sure they are free in the holders. Check spring pressure.

Check cooling fan if installed.

Check lubrication.

Check temperature recording instrument.

If necessary, remove rotor from stator, clean any dirt accumulated and revarnished if necessary.

Ventilation ducts need to be cleaned for free flow of ventilating air.

Check air gap after assembled.

[9] State the requisite steps of starting up operation mentioning any precautions

Requisite steps of starting up operation are:

Uncoupled motor from load, turn rotor by hand and ensure for free moving and indicating air gaps are correct and bearings are in good condition.

Replace the fuses with correct rating of the motor.

Start motor for short period for direction of rotation and rectified any abnormal noise.

Coupled back motor to the load, check for alignment of the coupling flanges.

Turn the coupling with hand and ensure smooth movement of the motor and pump.

Start motor, observe current reading on the ammeter of motor.

Monitor the temperature of the motor and observed any abnormality for some period after putting the motor into service.