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7 - THEORY OF OPERATION
7.5.1 DC Injection Braking, Standard Duty
The MX
2
Standard Duty Braking allows up to approximately 250% FLA current to be applied to the motor. The MX
2
Standard Duty
package consists of an extra braking contactor that shorts Motor Terminals 2 & 3 together while braking, as DC current is applied by the
MX
2
starter to provide moderate braking torque.
CAUTION: Contactor MUST NOT short phase T1 and phase T3.
z NOTE: Contactor sizing requires AC1 contactor rating (Motor FLA / 1.6). The three contacts must be paralleled.
7.5.2 DC Injection Braking, Heavy Duty
The MX
2
Heavy Duty Braking allows up to 400% FLA current to be applied to the motor for maximum braking performance. The MX
2
Heavy Duty braking package includes a freewheel current path between phases 1 and 3 that consists of a fuse and a 7
th
SCR with gating
card. In combination with the applied DC current from the MX
2
starter, the freewheeling current path greatly enhances available braking
torque. When Braking, the stop must be counted as another motor start when looking at the motor starts per hour limit.
z NOTE: Semi-Conductor Fuse and 7th SCR supplied by Benshaw.
7.5.3 Braking Output Relay
To utilize DC injection braking, one of the user output Relays needs to be programmed as a Braking relay. (Refer to the Relay Output
Configuration parameters on page 112 for more information). The output of a Braking relay is needed to control the contactor and/or 7
th
SCR gating control card used during braking.
z NOTE: Verify that the correct output relay is programmed to Braking and that the wiring of this relay is correct. Damage to the
starter can result if the braking relay is not programmed and/or wired properly.
7.5.4 Stand Alone Overload Relay for emergency ATL (Across The Line) operation
Due to the currents being drawn on Line 1 and Line 3 for braking, this stand alone overload relay will cause nuisance current imbalance
trips. For a solution consult factory.
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