Braking Controls
7.5 Braking Controls
Overview When the Stop Mode parameter is set to DC Brake, the MX
2
starter provides DC injection braking for fast and
frictionless braking of a three-phase motor. The MX
2
starter applies a controlled DC current to the motor in
order to induce a stationary magnetic field that then exerts a braking torque on the motor's rotating rotor. The
braking current level and braking time required depends on the motor characteristics, the load inertia, and the
friction in the system.
The MX
2
starter supports two different levels of DC injection braking:
1.Standard Duty Brake - For less than 6 x motor inertia.
2.Heavy Duty Brake - For NEMA specified inertia and two motor current feedback methods:
a) Standard Current Transformers (CTs)
b) Optional Hall Effect Current Sensor (LEM)
The optional Hall Effect Current sensor can be used when a more precise measurement of braking current is
necessary. This can occur if the DC injection braking is applied when the source supply has a very high short
circuit capability (very stiff) or in special instances when more precise braking current control is required.
The appropriate brake type and feedback method is preset from the factory. Please consult Benshaw for more
information if changes need to be made.
Maximum Load Inertia The following table shows maximum load inertia, NEMA MG1 parts 12 and 20. A thermostat, thermistor or
RTD MUST be installed to protect the motor from overheating.
151
7 - THEORY OF OPERATION
Speed - RPM
3600 1800 1200 900 720 600 514
HP Inertia (lb-ft2)
2 2.4 11 30 60 102 158 228
3 3.5 17 44 87 149 231 335
5 5.7 27 71 142 242 375 544
71/2 8.3 39 104 208 356 551 798
10 11 51 137 273 467 723 1048
15 16 75 200 400 685 1061 1538
20 21 99 262 525 898 1393 2018
25 26 122 324 647 1108 1719 2491
30 31 144 384 769 1316 2042 2959
40 40 189 503 1007 1725 2677 3881
50 49 232 620 1241 2127 3302 4788
60 58 275 735 1473 2524 3819 5680
75 71 338 904 1814 3111 4831 7010
100 92 441 1181 2372 4070 6320 9180
125 113 542 1452 2919 5010 7790 11310
150 133 640 1719 3456 5940 9230 -
200 172 831 2238 4508 7750 12060 -
250 210 1017 2744 5540 9530 14830 -
300 246 1197 3239 6540 11270 - -
350 281 1373 3723 7530 - - -
400 315 1546 4199 8500 - - -
450 349 1714 4666 9460 - - -
500 381 1880 5130 - - - -
600 443 2202 6030 - - - -
700 503 2514 - - - - -
800 560 2815 - - - - -
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