Siemens Level 1 Mechanical #1

Which component has to be maintained most often in a small DC machine?

• Armature

• Brushes

• Permanent Magnets

• Bearings

• Commutator

ow could you change the direction of rotation for an asynchronous machine (ASM) with squirrel cage?

• Use secondary shaft on opposite side.

• Exchange three terminals with each other.

• Exchange two terminals with each other

• Possible only with mechanical solutions (gear box and clutch).

• Apply negative voltage to the terminals.

A pump is connected with a clutch to a series-wound DC machine. What happens with the machine if the clutch is suddenly opened during operation?

• The speed could get very high and probably destroy the machine.

• The machine gets very hot.

• The speed breaks down to zero.

• The motor’s voltage will directly increase.

• Nothing; the machine will continue to run at the rated speed.

An electrical DC machine gets too hot during operation. What could be the cause? Two answers are correct.

• There is malfunction in the cooling system.

• The load increased.

• The supply voltage broke down.

• The load decreased.

• The brushes broke and have to be maintained.

The switching frequency of a power electronic for a DC machine is set to 2 kHz. This could cause:

• Torque ripples.

• Annoying noise.

• More losses than necessary.

• High armature currents.

• Nothing; 2 kHz is an appropriate switching frequency and often used in industries.

The ideal rotating magnetic field in the stator of a three-phase machine is deformed by: Two answers are correct.

• High no-load speed.

• Non-symmetrical stator windings.

• Wrong direction of rotation.

• Non-ideal net situation.

• Alternating voltages and currents.

The speed of a synchronous machine is higher than written in the data sheet. What could be the reason?

• The amplitude of the three-phase voltage is too high.

• The frequency of the three-phase voltage is too high.

• The load connected to the machine is too small.

• The load connected to the machine is too high.

• The excitation of the machine is too low.

A frequency inverter has a malfunction: one low-side switch is broken and as a result always opens. Result for the driven asynchronous machine: Two answers are correct.

• Half of the speed.

• More torque ripple and more noise.

• Less maximum torque.

• Much less no-load speed.

• The maximum torque will increase and possibly break the machine.

Which components are used for energy transport between different machine units of a system? Two answers are correct.

• Timing Belts

• Washers

• Bearings

• Screws

• Power Screws

Which of the following rolling contact bearings are able to absorb thrust forces at least from one side?

• Bearing b and e.

• Bearing a, c and d.

• Bearing a and d.

• All bearings.

• None of the answers given.