Specifications

Doc Number: VIS-ANA-VER-01001-9002
Date: 27 May 2004
Issue: 5.0
Page: 49 of 80
Author: J. Delgadillo
Controls Preliminary Design Report Issue 5.doc
AZ 594 W
CASS 71 W
The above tracking dissipation values are well below the continuous rating (15% of AZ, 8%
for CASS) of the regeneration resistors and thus will hardly produce minimal heat emissions,
if any.
The instance where the most heat will be dissipated will be when the telescope is slewing
from target to target. Per AD01, Technical Specification, the worse case scenario would be
45° slews on sky, with a 10-minute frequency. Therefore the acceleration/deceleration
durations would be 2 seconds for AZ and ALT and 1.4 sec for CASS and the slew durations
would be approximately 23 seconds for ALT, 66 seconds for AZ and 36 seconds for CASS.
Therefore the duty cycles between changing targets would be approximately 4.5% for ALT,
12% for AZ and 6.5% for CASS. The low duty cycles result in negligible average power
dissipated in the resistors, thus the above-mentioned dissipated tracking power is still valid
when slewing from target to target.
The transient (peak) power seen by the resistors is the most when the motors are commanded
to decelerate (since for accelerations, the AZ, CASS back-driven motors’ regeneration levels
would drop due to the high current command needed to accelerate). Therefore the transient
power that the resistors would see would be:
AZ 8.6A = 1,479 W (37% of rated)
CASS 1.09 A = 142 W (17% of rated)
The above values are still way below the rated values, and well below the thermal switch trip
threshold. Therefore, conditions much more severe (having higher duty cycle) than those
listed for this application would be needed in order to have an over temperature condition.
4.2.5.11 Isolation Transformer
The Isolation Transformer houses a 3-φ to 1-φ 5 KVA and a 3-Phase 50 KVA electro
statically shielded transformers. The transformers secondary feed the AZ/ALT and the CASS
PDU cabinets. This power is used to drive the brushless DC motors and the Azimuth bearing
lube pump and other utility powered equipment.