FORM 160.75-EG1 (519)
JOHNSON CONTROLS
33
Programmable
• Local Motor Current Limit
• Pulldown Demand Limit
• Pulldown Demand Time
Other features include: low line voltage; 115V control transformer; three-leg motor current
sensing overloads; phase rotation and single-phase failure protection; high temperature
safety protection; motor current imbalance and under-voltage safeties; open and shorted
SCR protection; momentary power interruption protection. The Solid-State Starter is air
cooled generating about the same heat as an auto-transformer E-M starter. Ground fault
protection and surge protection are also standard features. The 50,000 amp short circuit
withstand rating is in accordance with UL Standard 508.
ELECTROMECHANICAL STARTER - (FIELD-INSTALLED)
Characteristics - For comparison purposes, here is a description of some of the gen-
eral characteristics of electromechanical starters. Until the development of the Solid-
State Starter, all centrifugal chillers required the use of starters using electromechanical
contactors, which are limited to operating totally ON, or totally OFF. There was no alter-
native to this mechanical equipment with its inability to control applied voltage or power.
This contrasts markedly with the YORK
®
Medium Voltage Solid-State Starter which au-
tomatically maintains a predetermined current during starting, regardless of variations in
line voltage or motor load, to give optimum acceleration without surges. Even with the
addition of transformers, reactors, resistors and additional contactors, timers and relays,
the mechanical controllers offer limited adjustment, no positive control during starting and
impose an objectionable transition spike. Some also require modified motors. A field-
installed, electromechanical compressor motor starter is available, selected for proper
size and type for job requirements and in accordance with Johnson Controls Engineering
Standard (R-1132) for Starters (Form 160.76-PA1).
The most common failure mode of mechanical contactors is OFF. This occurs due to the
coil open-circuiting or failure of a pole to make an electrical contact when it closes. How-
ever, failure in the ON mode is not completely uncommon and can be a more dramatic
type of failure, particularly if this failure mode exists at the same time that equipment
safety controls are demanding a shutdown.
When contacts are “made,” the current builds up to its maximum value from zero, but
when contacts are separated the current tends to flow through the gap thus formed and
causes an arc. This arcing depends upon the voltage between the separating contacts.
For medium voltage the use of vacuum contactors mitigates this problem somewhat by
providing an environment to extinguish the arc. In the alternating current circuit, the sepa-
ration of contacts may take place when the current is zero or maximum or at any value
in between. An alternating current passes through zero and reverses its polarity twice
during each cycle. If two or more contacts, one in each leg of a polyphase system, are
separated simultaneously, the current values in each will vary. In a three-phase system,
if one contact has zero current when opened, the other two contacts will have 86.6% of
their maximum values, as an example. Additionally, when inductive circuits are broken,
the voltage is increased at the contacts due to the counter (induced) EMF of the circuit.
The instant the contacts separate, the voltage between them momentarily rises from zero
to the maximum of the circuit, or higher if inductance is present in the circuit. In practice,
Accessories and Modifications (Cont'd)
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