A variable frequency drive and a mechanical brake do two different jobs. The brake holds the load when the machine is stopped. The drive moves the load when it runs. The place they hand the load back and forth, the fraction of a second when one lets go and the other takes over, is where a conveyor lurches, a transfer car misses its position, and a brake wears out early.
This article is about that handoff on horizontal machines, conveyors, trolleys, long travel axes, transfer cars, and traverse drives, where gravity is not pulling the load along the direction of travel. Vertical loads like hoists are a harder and more dangerous version of the same problem, and they get their own Field Note, because on a hoist the same gap that only jerks a trolley will drop a load. Here we stay horizontal. Setting the brake sequence up correctly is a commissioning task, and it lives in the VFD Commissioning Guide.
The Brake Holds, the Drive Moves
A holding brake is spring set and electrically released. Springs clamp the friction disc by default, and it takes power to a coil to pull the brake open. That is deliberate. If power is lost, the springs set the brake and the machine is held. The brake is fail safe because a stopped machine should stay stopped when the plant loses power.
This is why you do not park a load on the drive. A VFD can hold zero speed with torque for a moment, but it is not a parking device, and a power loss on a drive that is holding a load lets the load go. The mechanical brake is the parking device. The drive is the motion device. Even on a forgiving horizontal machine, the brake is what holds the load between moves, and the drive only takes over for the moment the machine is actually running.
How the VFD Commands the Brake
The drive controls the brake through an output, usually a relay or digital output tied to the run logic, energizing the brake coil to release and de-energizing it to set. Wiring the brake to the drive’s own output rather than to a bare contactor is what lets the drive time the release and the set against the torque it is making, which is the whole point of letting the drive run the brake.
The start sequence runs in a fixed order, and the better drive programs break it into named stages you can time. First a premagnetizing step, where the drive builds flux in the motor, because a motor with no flux makes no torque no matter what else happens. Then the drive develops torque against the still set brake. Only then does it energize the brake release output, and the brake takes its own release time to physically pull open, during which the drive is already holding the load so the machine does not jump. Then the drive ramps to speed.
The stop sequence runs the same logic in reverse, with one stage most people skip. The drive decelerates and, rather than dropping straight to zero and slamming the brake on, holds at a low speed for a brake engage delay so the load can settle to a steady state, which matters because the motor does not always track the deceleration ramp exactly. Then it de-energizes the brake output, and the brake takes its engage closing time to physically clamp while the drive holds torque through that whole time. Only after the brake is set does the drive fade its current to zero and drop out, which ends the stop smooth and quiet instead of on a bump. Vendors like KEB and ABB expose these as programmable timers, premagnetize, release, engage delay, closing, and fadeout, and the point of every one of them is the same, hold the load with one device until the other has fully taken over.
Torque Before Release, Even When It Is Forgiving
The rule under all of this is that the drive should be making torque before the brake releases, and the brake should be set before the drive drops out. Skip that order and there is a gap, a window where the drive is not holding the load and the brake is letting go. On a horizontal machine that gap usually shows up as a lurch or a jerk rather than a runaway, because gravity is not standing by to pull the load along the track.
That forgiveness is real, and it is the reason horizontal brake jobs are less nerve wracking than hoists. But forgiving is not the same as free. A lurch at every start hammers the mechanics, upsets the product on the belt, and puts a position error into anything that has to stop in the right place. The goal is still a clean handoff where the load never feels the brake let go.
Where a Horizontal Load Still Bites
A few horizontal cases behave more like a hoist than the flat conveyor suggests, and they are the ones that surprise people.
An inclined conveyor is the big one. The moment you put a grade under the belt, gravity gets a component along the direction of travel, and a loaded incline will roll back when the brake releases if the drive is not already holding it. A steep, loaded incline is a partial hoist, and it needs the drive making holding torque before release for the same reason a hoist does. On a volts per hertz drive that means holding a small output frequency that makes just enough torque, through slip, to carry the load at standstill before the brake opens, the same holding frequency trick a vertical hoist relies on and a subject the vertical Field Note takes up in full. This is the same load that trips on the fully loaded restart, so it is worth reading alongside how a loaded conveyor recovers from a stop.
Positioning machines are the second case. On a transfer car, an indexing table, or any axis that has to stop on a mark, the lurch from a sloppy handoff turns into a repeatability problem, and no amount of tuning the position loop fixes a brake that releases before the drive is holding. High breakaway loads are the third. A conveyor that sits packed and sticky needs the drive to make breakaway torque before the brake opens, or it sits still and then jumps once the drive finally overcomes the stiction. And outdoors, wind on a long gantry bridge can push a horizontal axis, so the handoff on a windy day behaves like a mild rollback.
Protecting the Brake Itself
Sequencing is not only about the load. It is about the brake. Set the brake while the machine is still turning and you drag the friction surface against a moving disc, which wears the lining and heats the brake. Release the drive against a brake that has not finished setting and the load creeps. Both are the drive and the brake fighting each other because the timing is off. A brake that is set at zero speed and released only after the drive is holding lasts far longer than one that gets used as a stopping device on every cycle, and brake wear that traces back to sequencing belongs in the VFD Maintenance and Reliability Guide.
Wiring and Practical Notes
A few field points decide whether the sequence actually works. The brake coil is often DC, fed through a rectifier, and it is an inductive load, so it needs suppression across the coil or the collapsing field will hammer the drive’s brake output contact and couple noise into the control wiring. Check the drive output’s contact rating against the brake coil inrush, and add an interposing relay if the coil pulls more than the output can switch.
The emergency stop should set the brake, not just remove the run command, so an E stop actually holds the machine instead of leaving it on drive torque. And the brake belongs to the drive’s sequence, not to a separate operator switch that can release it while the drive is not holding the load. When lurching, drift, or brake wear show up as symptoms, the sequence is almost always where the trouble lives, which is why it appears in the VFD Troubleshooting Guide.
The Vertical Case Is Coming
Everything here gets less forgiving the moment the load hangs. On a hoist, gravity acts straight down the direction of motion and never stops pulling, so the same gap that only jerks a trolley drops a load. Vertical loads need the drive holding torque before release with no slip at all, which means pre torque, torque proving, closed loop feedback, and dedicated hoist firmware. That is a bigger subject and a safety critical one, so it gets its own Field Note rather than a paragraph here. If you are commissioning a hoist, do not treat it as a horizontal brake job with a steeper grade. It is a different animal.
Setting It Up
The order is the whole thing, and it does not change with the machine. The drive makes torque, then the brake releases. The brake sets, then the drive lets go. On a flat conveyor a mistake in that order costs you a lurch and some brake life. On an incline or a positioning axis it costs you rollback or a missed stop. Get the sequence right once during commissioning and the brake and the drive stop fighting each other for the life of the machine.
Brake sequencing, breakaway torque, and the selection of a drive that can hold torque at low speed get their full treatment in the VFD training program and in Before the First Fault.
Author: Dr. Carl Lee Tolbert, PhD, CMRP, Wayward Leaders LLC, waywardleaders.com
Frequently Asked Questions
Why does the VFD need to control the brake on a horizontal machine?
The drive has to release and set the brake in step with the torque it is making, so the load is always held by one or the other and the handoff is smooth. If a separate contactor controls the brake without knowing what the drive is doing, the brake can release before the drive is holding, and the machine lurches. Wiring the brake to the drive's brake output keeps the timing coordinated.
Do horizontal loads need torque before the brake releases like hoists do?
In principle yes, but they are far more forgiving. On a flat horizontal load a brief gap at brake release causes a lurch rather than a runaway, because gravity is not pulling the load along the track. The exceptions are inclined conveyors and high breakaway loads, where the drive really does need to be holding torque before the brake opens.
Why does my conveyor lurch or jerk when it starts or stops?
A lurch at start usually means the brake released before the drive built torque, and a jerk at stop usually means the brake set while the drive was still turning or the drive dropped out before the brake finished setting. Both are brake sequencing problems. Setting the drive to build torque before releasing, and to hold torque until the brake is set, removes the lurch.
Does an inclined conveyor behave like a hoist?
Partly. Once there is a grade, gravity gains a component along the direction of travel, so a loaded incline can roll back when the brake releases if the drive is not already holding it. It is a partial hoist, and it needs holding torque before release for the same reason, even though it is not a fully vertical load.
Can bad brake sequencing damage the brake?
Yes. Setting the brake while the machine is still moving drags the friction lining against a turning disc, which wears and overheats the brake. Using the mechanical brake as the stopping device on every cycle, rather than letting the drive stop the machine and then setting the brake at zero speed, shortens brake life significantly.
Are hoists and vertical loads handled the same way?
No. A vertical load has gravity pulling straight down the direction of motion, so the handoff gap that only jerks a horizontal machine will drop a hoist load. Hoists need pre torque, torque proving, closed loop control, and dedicated firmware, which is a safety critical subject covered in a separate Field Note.