Page 17 - MetalForming November 2010

sense,” says Bisbee. “This is because they can shut the motor off during that idle time, and then program it for a slow startup, rather than the more typical inrush of current experienced without aVFD.”
Bisbee offers this summary of VFD versus no VFD:
A hydraulic press without a VFD will typically:
• Turn its motor on when power is applied;
• Only turn the motor off when either the power is turned off or when detecting an emergency-stop condition. The motor will continue to run even when the press is idle (the platen not moving).
A hydraulic press with a VFD will typically:
• Start the motor when the press is ready to do some work;
• Turn off the motor when move-
ment of the platen is not required;
• Keep the motor off during press idle
times; and
• Run the motor at slower speeds
when the press is idle, to allow for accu- mulators to be charged or the oil to be cooled and filtered.
The Benefits are Numerous
With a VFD on a hydraulic press, energy-cost savings by only running the motor when it’s needed can accu- mulate quickly (see the sidebar). More energy savings can be realized by pro- gramming the VFD to ramp the motor up to speed (more than 3 to 5 sec. typ- ically), which eliminates the large elec- trical inrush of current. This can allow a metalformer to minimize its surge (or demand) amperage. Also, the abili- ty to control the speed of the motor enables the motor to run at slower speeds while idle to filter and cool the
oil, charge accumulators, or satisfy other small-hp demand requirements.
“With a VFD, the motor can com- municate through Ethernet (or with discrete inputs/outputs) to a PLC,” adds Bisbee, “allowing touchscreen (and potentially recipe) control of motor on/off timing, idle speeds and fault indications. Also, the VFD will generate its own AC voltage to drive the motor. This naturally isolates the motor from most voltage fluctuations or spikes, pro- longing its life.”
A Poor-Man’s Transformer
Bisbee shared a recent customer suc- cess story where a VFD installed on a press destined for an offshore company acted as what he calls a “poor-man’s transformer.”
“The customer needed to run a press on 415 VAC at 50 Hz, and the motor was rated at 460 VAC, 60 Hz ,” he notes. “A
How Much Money You Can Save, and the Resulting ROI
...depends on the price paid for electricity, how many hours the press runs and idle time. Assumptions made for this exercise: • Price for electricity = $0.1299/kW
• Press operates 2080 hr./yr.
• 30 percent of time press operates at full-hp rating
• 70 percent of time press operates at idle, at 30 percent full rated hp
hp
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
hp at idle
3 6 9
12
15
18
21
24
27
30
33
36
39
42
45
kW at
idle hp
2.238 4.476 6.714 8.952
11.19 13.428 15.666
17.904
20.142
22.38
24.618
26.856
29.094
31.332
33.57 76.5
kW, based on
previous column
3.8046 7.6092 11.4138 15.2184
19.023 22.8276 26.6322 30.4368 34.2414 38.046 41.8506 45.6552 49.4598 53.2644
57.069
Cost/yr., based on
previous column
$1,027.97 $2,055.94 $3,083.92 $4,111.89 $5,139.86 $6,167.83 $7,195.81 $8,223.78 $9,251.75
$10,279.72 $11,307.70 $12,335.67 $13,363.64 $14,391.61 $15,419.59
kW at
full hp
7.46 14.92 22.38 29.84
37.3 44.76 52.22 59.68
67.14 74.6 82.06 89.52 96.98
104.44 111.9
Average effective hp, based on %
of time at full hp
Savings if VFD turns motor off when idle
$423.28
$846.57 $1,269.85 $1,693.13 $2,116.41 $2,539.70 $2,962.98 $3,386.26 $3,809.55 $4,232.83 $4,656.11 $5,079.39 $5,502.68 $5,925.96 $6,349.24
5.1 10.2 15.3 20.4 25.5 30.6 35.7 40.8 45.9
51 56.1 61.2 66.3 71.4
For this exercise, the calculations do not account for the power required to move the platen up and down; we assume that the press always uses full tonnage in production and that the VFD shuts off the motor and does not drop to an idle speed.
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