Control Design – May 2019

Originally, we used star-delta motor

controls to reduce starting currents

because most countries’ power infra-

structures could not handle the current

draw from across-the-line starting. In

the late 1 990s, we switched to soft-

start motor controls, which worked

well, allowing us to buy three-wire

motors instead of the six-wire ones

needed for star-delta starting.

In the early 2 010s, we went to VFDs

for the pump motors, chiefly so we could

network the drives and get all the bene-

fits of start-stop commands, drive status,

drive-speed manipulation and actual

motor speeds via the network. The only

control wiring we do to the drives is the

safe-stop input circuits controlled by the

emergency-stop safety relays.

By using the VFDs we have an even

greater advantage. Originally the mo-

tors always ran at the rated speed. This

means they were always pushing the oil,

even though, while not performing work,

they were still putting heat into the oil.

So, we would also supply an oil cooling

means, either oil/air with a fan or oil/

water, usually by the customer.

When the oil reached 105 °F, we would

energize the fan or open the water valve.

If it reached 140 °F, we would warn at the

HMI that the oil was getting hot. When

it reached 155 °F, we would shut down,

because the oil would chemically break

down at temperatures higher than that.

Now, with the VFD, as soon as the

hydraulics finished doing any part of the

job, the pump motors are dropped to 20

Hz or so in 1 .5 seconds, which is just fast

enough to keep a decent air flow over

the totally enclosed, fan-cooled (TEFC)

motors and keep oil flowing through the

cooling loop.

Three more benefits are reduced noise,

reduced power consumption and cooler

motors and oil. After doing this on the

China job, the cooling system might

come on near the end of the day and go

right back off after a few minutes, and

you could put your hand on the motor

and feel it at about 95 °F, which was awe-

some. I have the VFD accel rate set to 1 .0

seconds and the decel rate set to 0 .5 sec-

onds, so the response is really fast when

the next hydraulic operation is needed.

If the press has been idle for more

than five minutes, I set the speed to 0 .00

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