The Case of the Stuck Freezer Elevator

THE MYSTERY

A beef production facility had a industrial deep freeze, 20 shelves high, complete with an elevator that delivered boxed beef to each shelf. The pusher that automatically dispensed the boxes onto the shelves for freezing had become stuck in the out position, and the whole elevator was stuck, period. The elevator can’t move unless the pusher is in the home position.

The customer had an electrician swap out the motor on the faulty pusher, with no results. The electrician replaced the original motor and the customer called ACES.

THE CLUES

When the ACES CSI called tech support for the deep freeze, the tech said the problem was a bad encoder. Or maybe a bad drive. Or a bad motor.

First the CSI traced out the encoder wiring. When the motor turns, the encoder creates pulses and the servo drive counts the pulses, and halts the motor if it’s running backwards or if something’s not right. Everything was correct with the encoder wiring. Next he hooked up a different servo drive, but the stubborn pusher remained motionless.

Finally he traced the wiring on the motor.

THE PERP

When he investigated the wiring on the motor, the CSI uncovered an anomaly: Two phases of the motor were wired backwards. The customer’s electrician was implicated in the backwards wiring, having switched them when he changed out the motor during the client’s initial troubleshooting.

Then the CSI turned to the gearbox where he discovered another wiring snafu. Encoder wires are about 26 gauge and motor wires are about 10 gauge. Encoder wires run off 24v while motor wires run off 480v. The electrician had laid down encoder wires right next to the high-voltage motor wires, so it was inducing signals into the encoder wires, telling the encoder that the servo drive was getting mixed signals — it was seeing counts but then seeing a lot of static. It would get discombobulated and shut down.

THE SOLUTION

The CSI rewired the motor and the wires in the gearbox, making sure he had adequate separation between motor and encoder wires. Once the electronic signals were straightened out, the pusher swung into action and the deep freeze elevator was humming up and down, delivering boxes to all of the freezer’s shelves.

CASE CLOSED