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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
Table 9.1 For use in a single dynamic braking unit
(4 poles)
Motor: Hitachi standard 3-phase squirrel-cage
totally-enclosed fan-cooled motor
(Common conditions)
Time
Rotational speed
• BRD operation frequency
• …
0.2
(or
• …
0.1 for the EZ2-30K)
t
B
t
C
• T
B
A : Mean braking torque at 3 Hz to 60 (or 50) Hz
(For meanings of A , see Figure 9.2.)
• BRD continuous operation time t
B
• …
2 minutes
(or
• …
10 seconds for the EZ2-30K)
Motor torque ratio %
E2
EZ2
30K
55K
30K
55K
110K
Rated torque of the motor
To: N.m (kgf.m)
4
Ω
2
Ω
10
Ω
6
Ω
3
Ω
Motor
(kW)
Inverter
(kW)
Moment of inertia J
for one motor (GD
2
)
kg.m
2
(kgf.m
2
)
50Hz
60Hz
T
B
A
T
B
A
T
B
A
T
B
A
T
B
A
18.5
22.0
0.16 ( 0.64)
122 ( 12.4)
101 ( 10.3)
170
22.0
22.0
0.19 ( 0.76)
144 ( 14.7)
120 ( 12.2)
150
30.0
30.0
0.30 ( 1.20)
197 ( 20.0)
164 ( 16.7)
110
175
37.0
37.0
0.35 ( 1.40)
243 ( 24.7)
202 ( 20.6)
85
170
142
45.0
45.0
0.40 ( 1.60)
295 ( 30.1)
245 ( 25.0)
75
140
116
195
55.0
55.0
0.55 ( 2.20)
361 ( 36.8)
300 ( 30.6)
60
120
95
160
75.0
75.0
1.03 ( 4.12)
492 ( 50.2)
407 ( 41.5)
70
115
90.0
90.0
1.23 ( 4.92)
588 ( 60.0)
487 ( 49.6)
58
95
190
110.0
110.0
1.83 ( 7.32)
719 ( 73.3)
595 ( 60.7)
80
150
