Page 197 - Industrial Power Engineering and Applications Handbook

Special-purpose motors 7/177
geometrical axes of the motor are not concentri
as tooth ripples and magneto-striction. Aerodynamic noise
is caused by the flow of cooling air at a higher peripheral
speed over the cooling fins and the rotor bars ax well as
unbalanced rotating masses, aerodynamic loads and some
secondary effects such as noise from the bearings.
instability of the shaft in the bearings. passive resistance
and aerodynamic expansion.
In an electric motor, although such parameters are
inherent, they can be tolerated only to a certain extent.
IEC 60034-14 gives these levels as indicated in Table
1 I .3. For applications such as household appliances.
escalators in residential buildings, offices and hospitals,
and for machine tools, the vibration levels must be even
0 lower to eliminate the transmission of these as far as
Soeed
possible through the driven structure to the building or
Figure 7.11 Characteristic of a torque motor to the cutting tool in the case of a machine tool. Unless
the vibrations are reduced to a reasonably low level. they
may cause a noise nuisance to the occupants of a building
be required to operate frequently and a high I,, would or affect the accuracy of the cutting tool and the machine.
be a deterrent. Research on the effect of noise on a human body has
The speed-torque characteristics are almost linear, with revealed that noise level (sound pressure level) must be
torque falling with speed (Figure 7.1 I). limited to 85 dB for a human body to work safely without
There is no Tpo region, thus no unstable region. The fatigue, for eight hours a day and without causing a health
motor can operate at any speed up to the rated one hazard on a continuous basis. In the OSHA (Occupational
without any stalling region. Safety and Health Administration, USA) regulations the
The locked rotor thermal withstand time is much higher maximum industrial noise level is 90 dB. Table 7.9 shows
than for a standard motor. the likely noise levels and their sources of origin.
Vibrations, when transmitted from the source to the
connected appliance or structure or a machine tool, become
Likely applications
magnified, depending upon the contact surface area and
create noise. In fact, vibrations are the primary source of
Wire-winding machine.; noise. In machine tools, the vibrations are transmitted
Roller tables from the motor body to the cutting tool and affect its
Frequent starts and reverses accuracy. For precision work and for sophisticated machine
Material handling tools, a vibration level as low as 2 microns is sometimes
Valve actuator preferred.
Vane control Vibrations and noise levels are mainly associated with
Bagging machines the mechanical construction and electrical design of the
Fa\t tapping machines motor. Sound mechanical construction and a balanced
Clamping and positioning devices rotor, a tightly fastened core and smooth bearings can
eliminate vibrations and mechanical noise to a large extent.
Also a better electrical design can eliminate electrical
7.9 Vibration and noise level vibrations in the stampings. due to magnetic forces and
higher harmonics. A better electrical and mechanical
Excessive vibrations according to international codes can design will thus mean:
cause mechanical failure in the insulation by loosening
wedges, overhangs, blocks and other supports that hold 1 Rigid fastening of the stator core to the housing
the stator and the rotor windings or rotor bars in their 2 Smooth and frictionless bearings with proper greasing
slots. Vibrations also tend to harden and embrittle copper 3 Precision dynamically balanced rotating parts. The
windings and may eventually break them when they leading Indian manufacturers recommend and maintain
become loose (see also Sections 11.4.6 and 11.4.7). a vibration level, peak to peak as shown in Table 7.3.
There are some essential features that a good motor See also Table 11.3, according to IEC 60034.14
should possess, and among these are vibrations and noise 4 A uniform air gap between the stator and the rotor
level. The noise may be magnetic or aerodynamic, 5 Judicious selection of stator and rotor slots, with angle
assuming that there are no frictional or other noises of skew and glue density
emanating from the motor. Magnetic noise is due to 6 Magnetic loading, i.e. tlux density
resonant excitation of the stator core by slot harmonics, 7 Windage noise (noise at the suction and the exhaust
caused by the electro-magnetic circuit of the machine of the cooling airj. This makes a large contribution to
(Section 23.6(a)) and loose clamping of the stator’s steel the total noise emanating from a motor. To suppress
stampings and the motor’s rotor core. Or loose rotor bars this, some manufacturers provide the following
and magnetic unbalance, when the magnetic and additional features or noise-suppression devices in