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Force/Nominal Force
%
100
80
60
40
20
0
v
v
/
v
v
v
/
v
-~
~
0
10
20
30
40
50
60
70
80
90
100
110
Mains Voltage/Nominal Mains Voltage%
27/02()
,
Fig.3. 15.
Force capability
system failure is unnecessary. The exciter field coils and field supply com-
ponents will cool adequately at reduced mains voltage, and are still protect-
ed against cooling system failure by the thermostat on the rectifier heat
sink.
The force capability of the exciter is reduced at low mains voltages. The
force generated by the driver coil is proportional to the product of the flux
density and the driver coil current. The lowered voltage reduces the field
current and thus the flux density, and the useable moving coil current is
reduced because of reduced cooling. See Fig.3.15 for typical Type 4801
force capability. Displacement capability is also reduced in the low frequen-
cy region where the force required to deflect the suspension is dominant
over the force required to accelerate the moving mass. As well as lowered
background noise at the table, low voltage operation may be required to
reduce power demand from the mains, reduce heat dissipated from the
exciter, etc.
For the 4801 Exciter Body the power input to the field supply at
nominal mains voltage is .about 1200 watts: this power is dissipated to the
environment after the exciter has attained thermal equilibrium. The
~issipat
ed power can be reduced by operating at lower mains voltages, and can
reasonably be approximated to:
Power Dissipated
=
1200 x
mams
t
E
·
]2
En om ina I
Thus operation at 70% of nominal voltage cuts the power to 600 watts:
operation at 50% of nominal voltage reduces the power to 300 watts.
28
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