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9.1
Braking Energy without a Dynamic Braking Unit
Usually, the braking energy of a motor is the sum of the motor loss (including mechanical loss)
and inverter loss.
The overall efficiency of an inverter-controlled motor is 65% to 90% depending upon the motor
capacity and power loss, and the braking energy which fluctuates around 10% to 35% when at
frequency of 50 Hz to 60 Hz).
Generally, the braking torque of up to about 11 KW is about 20% of the rated torque of the
motor.
If regeneration energy is greater than the braking energy, the capacitor in the DC bus circuit will
be charged, thus increasing the DC voltage. This may cause the over-voltage protecting
circuit to shut off the output of the inverter. (The motor runs freely.)
9.2
Using the Dynamic Braking Unit
For a load with high inertia, the motor must be decelerated immediately, and the dynamic
braking unit (BRD) will need to absorb the resulting regeneration energy.
In Figure 9.3, when the voltage of the D.C. circuit reaches a preset value, the transistor Tr
switches on and the resistor RB consumes the regeneration energy as heat. In this status, the
V/F characteristics of the inverter, the permissive current (determined by the resistance of RB),
and the capacitance of RB in the dynamic braking unit will determine the braking torque.
This is because the torque generated by the motor is dependent upon the current passing
through Tr (and RB).
Do not combine any items which are blank in Table 9.1 to Table 9.3.
(1) For use in one dynamic braking unit
(Common conditions)
Moment of inertia J
Motor
Inverter
for one motor (GD
(kW)
(kW)
18.5
22.0
22.0
22.0
30.0
30.0
37.0
37.0
45.0
45.0
55.0
55.0
75.0
75.0
90.0
90.0
110.0
110.0
Table 9.1 For use in a single dynamic braking unit
Motor: Hitachi standard 3-phase squirrel-cage
totally-enclosed fan-cooled motor
Time
Rated torque of the motor
2
)
kg . m
2
(kgf . m
2
)
50Hz
0.16 ( 0.64)
122 ( 12.4)
0.19 ( 0.76)
144 ( 14.7)
0.30 ( 1.20)
197 ( 20.0)
0.35 ( 1.40)
243 ( 24.7)
0.40 ( 1.60)
295 ( 30.1)
0.55 ( 2.20)
361 ( 36.8)
1.03 ( 4.12)
492 ( 50.2)
1.23 ( 4.92)
588 ( 60.0)
1.83 ( 7.32)
719 ( 73.3)
• BRD operation frequency
• BRD continuous operation time t
• T
A : Mean braking torque at 3 Hz to 60 (or 50) Hz
B
(For meanings of A , see Figure 9.2.)
To: N . m (kgf . m)
30K
4
T
60Hz
B
101 ( 10.3)
170
120 ( 12.2)
150
164 ( 16.7)
110
202 ( 20.6)
245 ( 25.0)
300 ( 30.6)
407 ( 41.5)
487 ( 49.6)
595 ( 60.7)
- 21 -
(4 poles)
t
B
• ... 0.2
t
C
(or • ... 0.1 for the EZ2-30K)
• ... 2 minutes
B
(or • ... 10 seconds for the EZ2-30K)
Motor torque ratio %
E2
55K
30K
2
10
A
T
A
T
T
A
B
B
175
85
170
142
75
140
116
60
120
95
70
58
EZ2
55K
110K
6
3
A
T
A
B
B
195
160
115
95
190
80
150
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