CAIR
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Electrical Connection
PR-2009-0096-GB • Subject to modifications • R8-10/2015
51
Motor full protection electronics
For motor full protection, we recommend to use 912.MVS 1.0 tripping units. These are
specially designed for PTC thermistors of motors. Full motor protection is based on the
closed-circuit principle and is intrinsically safe. Motor protection tripping units 12.MVS
1.0 generally enable isolated disconnection and a restart following a power outage.
Electronic motor protection tripping units deactivate motors in the following situations:
– Overload
– Ambient temperatures exceed allowed limits
– Locked fan wheel
– Insufficient cooling
– Short circuit in winding
– Wire breakage and loose contacts in the measuring line
Extended motor protection
With frequent start-ups, intermittent operation, high switching frequency and to ensure
optimum protection against 2-phase run, DencoHappel recommends to protect motors
using thermal overload relays/motor circuit breakers in addition to the PTC thermistor
protection.
Disconnection in the event of
an error
In case of fault shutdown investigate the above-mentioned causes.
Motor protection with thermal contact
Thermal contacts are temperature-dependent operating elements for automatic moni-
toring of the motor winding temperature. Thermal contacts are integrated in motor
windings and enable additional protection as compared to thermal cut-out devices and
motor circuit breakers:
– Insufficient cooling
– High ambient temperature
– Extreme speed reduction in course of control and regulation operations
– Motor disconnection in case of control-voltage line failure
– Faulty contacts and cable breakages in the control circuit.
A thermal contact opens if the allowed winding temperature is exceeded. As a result
the mains circuit of the control unit is interrupted. The fan contactor drops out and dis-
connects the motor from the mains. Motor protection is intrinsically safe.
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Temperature T
Operating point of
tripping unit
