Hydraulic elevator controls and starters

The new controller for Holloway car 2 came pre-wired from the factory in a tall cabinet, with a soft starter for the pump motor in its lower half and a microprocessor board, relays and terminal strips above. Priya spent an evening class on its drawings. A hydraulic controller, she explained, does most of what a traction controller does: it answers calls, picks the direction, runs the doors and keeps the car safe. But it does not control a hoist motor and brake. It controls a pump motor that only runs for up travel and a set of valve solenoids that run both directions. "Learn the sequence of an up run and a down run," she said, "and you can follow any hydraulic controller, relay or computer."

What the controller does

A hydraulic elevator controller has these main jobs:

• Register hall and car calls and decide which direction to travel, using the car position from the landing system.
• For up travel, start the pump motor through its starter, then energize the up valve solenoids in sequence; for down travel, energize the down valve solenoids. Slow down and stop at the floor from landing system signals.
• Run the doors, and prevent the car from moving unless the doors and locks are closed, except in the levelling zone with the doors open under the code's conditions.
• Monitor the safety circuit, the motor's thermal protection, phase loss and reversal, the pump run timer and the oil temperature, and stop or remove the car from service when any of them trips.
• Re-level the car if it drifts away from floor level while parked.
• Provide emergency operations: fire service recall and in-car operation, and emergency lowering on loss of power where required.

The sequence of an up run

Every up run follows the same steps, whether the controller uses relays or software. The car is at a floor with the doors closed and the safety circuit made. The controller has a call above the car and selects up. It closes the motor starter; on a wye-delta starter, it closes the wye contactor first. The motor accelerates, pumping oil through the open bypass back to the tank, so the car does not move. After a short delay, long enough for the motor to reach speed or, on a wye-delta starter, after the transition to delta, the controller energizes the up valve solenoids. The valve begins to close its bypass, the pressure rises to the value needed to lift the car, the check valve opens and the car accelerates up. At the slowdown point for the destination floor, the controller drops the up fast solenoid; the valve slows the car to levelling speed. At floor level, the landing system's stop signal drops the levelling solenoid; the bypass opens fully, the check valve closes and holds the car, and the controller stops the motor a moment later. The doors open.

A down run is simpler, because the pump does not run. With a call below, the controller energizes the down valve solenoids. The lowering valve opens, oil flows from the jack back to the tank, and the car accelerates down. At the slowdown point, one solenoid drops and the car slows to levelling speed; at floor level, the last drops, the lowering valve closes and the car stops.

# Up and down run sequence on a hydraulic controller (simplified, times in seconds)
up_run = [
    (0.0, "run up", "call above, doors locked, safety circuit made: run up selected"),
    (0.1, "motor on", "starter closes; the pump runs through the open bypass"),
    (1.2, "up fast + up level", "after the start delay the valve closes the bypass; car accelerates"),
    (6.5, "up fast off", "slowdown point reached; car slows to low speed"),
    (9.0, "up level off", "floor level reached; bypass opens, check valve holds the car"),
    (9.5, "motor off", "motor stops after a short run-on; doors open"),
]
down_run = [
    (0.0, "run down", "call below: down selected; the motor stays off"),
    (0.2, "down fast + down level", "lowering valve opens; car accelerates down"),
    (6.0, "down fast off", "slowdown point; car slows to low speed"),
    (8.6, "down level off", "floor level; lowering valve closes and the car stops"),
]
for name, steps in (("UP RUN", up_run), ("DOWN RUN", down_run)):
    print(name)
    for t, event, why in steps:
        print(f"  {t:5.1f} s  {event:<27} {why}")
motor_time = up_run[-1][0] - up_run[1][0]
print(f"pump motor runs {motor_time:.1f} s per up run between two floors")

UP RUN
    0.0 s  run up                      call above, doors locked, safety circuit made: run up selected
    0.1 s  motor on                    starter closes; the pump runs through the open bypass
    1.2 s  up fast + up level          after the start delay the valve closes the bypass; car accelerates
    6.5 s  up fast off                 slowdown point reached; car slows to low speed
    9.0 s  up level off                floor level reached; bypass opens, check valve holds the car
    9.5 s  motor off                   motor stops after a short run-on; doors open
DOWN RUN
    0.0 s  run down                    call below: down selected; the motor stays off
    0.2 s  down fast + down level      lowering valve opens; car accelerates down
    6.0 s  down fast off               slowdown point; car slows to low speed
    8.6 s  down level off              floor level; lowering valve closes and the car stops
pump motor runs 9.4 s per up run between two floors

The delay between motor start and up solenoids matters. Too short, and the valve tries to lift the car before the motor is up to speed, causing a slow, labouring start and high current. Too long, and passengers wait for the car to move. The run-on after the stop lets the bypass open fully before the motor stops, so the stop is smooth.

Starting the pump motor

A pump motor started directly across the line, with full voltage applied at once, draws a locked-rotor current of five to seven times its full-load current for the second or two it takes to reach speed. For Holloway's 30 kW (40 hp) motors, with a full-load current of 38 A at 600 V (47 A at 480 V), that is about 250 A (300 A) at each start, many times an hour. The building supply dips, lights may flicker, and the motor and starter contacts are stressed. Because a hydraulic pump motor starts unloaded with the bypass open, it can be started with reduced voltage, which reduces the current.

• Across-the-line (direct-on-line) starters close one contactor and apply full voltage. They are simple and give full starting torque but the highest current. They are found on small units and older cars like the Marlowe.
• Wye-delta starters connect the motor's windings in wye for starting, which applies about 58 percent of the line voltage to each winding and cuts the starting current and torque to about a third, then switch to delta after a timed interval. The motor must have six leads brought out. In an open-transition starter, the motor is briefly disconnected during the switch, which causes a current spike; closed-transition starters avoid it with a resistor bank. A wrong transition time, or a failed timer, is a common cause of starting trouble.
• Solid-state soft starters use thyristors to ramp the voltage up over a few seconds, so the current rises smoothly and is limited to a set value, often around three times full-load current. Many also provide motor overload protection, phase loss and reversal detection and a bypass contactor that takes over once the motor is at speed, so the thyristors do not run hot. The new Holloway units use soft starters.
• Variable-frequency drives on the pump motor, used on some modern power units, start the motor at low frequency and vary its speed during the run. The valve then does less throttling, which saves energy and heats the oil less, and the drive can shape the up run directly.

# Starting current of the Holloway pump motor by starter type (illustrative)
fla = 38.0                      # full-load current at 600 V
locked = 6.5 * fla               # locked-rotor current, direct on line
starters = [
    ("across the line", locked, 1.00),
    ("wye-delta (open transition)", locked / 3, 1 / 3),
    ("soft starter, 3 x FLA limit", 3.0 * fla, (3.0 * fla / locked) ** 2),
]
for name, amps, torque in starters:
    print(f"{name:<29} starting current about {amps:4.0f} A, starting torque about {100 * torque:3.0f} % of full-voltage value")
print("a pump that starts against an open bypass needs little starting torque, so all three can start it")

across the line               starting current about  247 A, starting torque about 100 % of full-voltage value
wye-delta (open transition)   starting current about   82 A, starting torque about  33 % of full-voltage value
soft starter, 3 x FLA limit   starting current about  114 A, starting torque about  21 % of full-voltage value
a pump that starts against an open bypass needs little starting torque, so all three can start it

Phase loss and phase reversal

A three-phase motor will run backwards if two of its supply lines are swapped, and a screw pump turning backwards does not pump; it may run noisily, draw oil the wrong way or damage itself. A phase swap can happen when the utility or an electrician works on the building supply. A motor that loses one phase, single-phasing, will not start and overheats if it is left trying. The code requires protection that prevents the car from starting if a phase is lost or reversed. Older controllers use a phase monitor relay; soft starters and electronic controllers usually include it. When a car will not run after an electrical shutdown in the building, Ray's first check is the phase monitor's indicator.

Protecting the motor and the oil

The starter's overload protection, set from the motor's nameplate current, opens the motor circuit if the current stays too high for too long. Many submersible motors also have thermal sensors embedded in the windings, wired to the controller, which stop the car at the next landing and hold it there until the windings cool. The pump run timer, the low oil protection described in Lesson 4, stops the motor if an up run takes too long and lowers the car to the bottom landing. An oil temperature sensor may hold the car out of service if the oil is too hot, or switch on the cooler or heater. Each of these protections leaves a trace: an indicator on the controller, a fault code in the event log, or a tripped relay. Reading them before resetting is how the mechanic learns what happened.

Emergency lowering and fire service

When the building loses power, a hydraulic car stops wherever it is, possibly between floors. Because lowering needs only an open valve, many hydraulic cars have a battery lowering device, which on loss of power lowers the car at levelling speed to the lowest landing, or the next landing below, and opens the doors so passengers can leave. In many jurisdictions, this or an emergency power supply is required on new installations. The device has its own battery, which must be tested and replaced on schedule, and its operation is part of the periodic tests.

On fire alarm, fire service Phase I recall brings the car to the designated landing, opens its doors and takes it out of normal service, so firefighters can use it under Phase II in-car control. Hydraulic cars are recalled like traction cars. Because a hydraulic's machine room is often next to the hoistway at the bottom, the designated and alternate landings and the machine room's smoke detection are coordinated so that the car does not travel to a floor where smoke could enter the hoistway.

Manual lowering is the last resort. The valve has a manual lowering screw or button that opens a small lowering path. With power off and the car between floors, a mechanic can lower the car slowly to a landing by opening it, under the controlled rescue procedure set by the company and the code. Tamsin practised it with Ray on the Marlowe car during a scheduled shutdown, with the car empty, a mechanic at the landing to watch the car approach, and radio contact between them.

Re-levelling

A parked hydraulic car slowly sinks as oil leaks past the check valve, the lowering valve and the packing, and it may also rise or fall a little as the oil cools or warms, or as a heavy load enters or leaves the car. The controller watches the landing system. If the car drifts beyond a set distance below floor level, it starts the pump and runs the up levelling solenoid to bring the car back; if it is above floor level, it opens the down levelling solenoid. The doors may be open during re-levelling, within the code's limits. Frequent re-levelling is a symptom, and Lesson 10 shows how to measure the leak behind it.