A brake motor with its brake wired to the motor terminals will run fine across the line and fail the moment you put it on a variable frequency drive. The motor will hum, strain, overheat, and trip, or it will sit there fighting a brake that never let go. The cause is not the drive and not the brake. It is that the brake was getting its release voltage from the motor leads, and a VFD does not hold those leads at full voltage the way the line did.
This one catches people because the motor worked perfectly yesterday on a contactor. Understanding why it stops working on a drive comes down to how a brake motor is wired and how a VFD makes voltage. Getting the wiring right is an installation decision, and it lives in the VFD Installation Guide.
Two Ways a Brake Motor Is Wired
A brake motor is an induction motor with a spring set, electrically released holding brake built onto the back. The springs clamp the brake by default, and a coil pulls it open when energized. The only question that matters here is where that coil gets its power.
In the common and cheaper arrangement, the brake coil is connected internally to the motor terminals. When the motor is energized, the brake taps that same voltage and releases. Wire the motor, and the brake comes along for free. This is why an across the line brake motor is so simple, one contactor powers the motor and releases the brake in the same stroke.
In the other arrangement, the brake coil leads are brought out separately, so the brake can be powered and controlled on its own, independent of the motor connection. Inverter duty brake motors come this way for a reason. The separate leads are the whole difference between a motor that works on a drive and one that does not.
Why the Internally Wired Brake Fails on a Drive
A VFD controls motor speed by changing frequency, and below base speed it lowers the output voltage in proportion to the frequency. That is the volts per hertz relationship, and it is fundamental to how the drive works. At 30 hertz on a 60 hertz motor the drive is putting out roughly half voltage. At the moment of start, near zero hertz, it is putting out almost no voltage at all.
Now put the brake coil on those terminals. The coil needs its full rated voltage to pull the brake open, and at start the drive is giving it almost nothing. The brake stays clamped. The drive ramps up trying to turn a motor whose brake is still set, the current climbs, and you get an overload trip, an overheated motor, or a brake dragging against the disc until something burns. Even part way up the ramp, at half voltage, the brake may only partly release, which is worse in its own way because now the motor turns against a dragging brake and cooks it slowly.
There is a second problem underneath the first. A VFD output is not a clean sine wave. It is pulse width modulated with steep switching edges, and it is meant to feed a motor and nothing else. Hanging a brake coil, or the rectifier that feeds a DC brake coil, on that output subjects it to a waveform it was never designed to see. The rule that comes out of both problems is the same. Nothing but the motor belongs on the VFD output.
What the Failure Looks Like in the Field
The symptoms are consistent once you know the cause. The motor will not start, or starts hard and trips on overload almost immediately. The motor runs hot for no obvious load reason. The brake smells burnt or shows a glazed, overheated friction surface. Sometimes the motor will run once it finally gets up to speed and voltage, then fail to start cleanly the next time from zero. Every one of those traces back to a brake that is not getting reliable release voltage, and it belongs in the VFD Troubleshooting Guide under faults that look like the drive but are really the wiring.
The Fix: Power the Brake From Its Own Source
The brake coil has to be brought out and powered from a source that is at full voltage regardless of what the motor is doing. That means a constant voltage supply taken ahead of the drive, off the incoming line or a dedicated control transformer, never off the drive output. The brake then gets its full rated voltage to release cleanly at any motor speed, including zero.
That constant supply is switched by the drive’s brake control output so the brake still releases and sets in proper sequence, the drive making torque before the brake opens and the brake setting before the drive drops out. This is the same sequencing covered for an external brake, now applied to a motor mounted brake. The drive controls when the brake operates through its relay, and the line provides the voltage that actually pulls it open. Setting that sequence up is a commissioning step in the VFD Commissioning Guide.
If the brake motor you have has the brake internally tied to the motor terminals, you either separate those brake leads inside the terminal box and feed them from their own supply, or you replace the motor with one that has the brake separately terminated. On some motors the leads are accessible and the conversion is straightforward. On others it is not, and the honest answer is that the motor was not built for drive duty.
What to Specify Next Time
The way to avoid the whole problem is at selection. Specify an inverter duty brake motor with the brake coil separately terminated, and confirm the brake voltage and whether it is AC or DC so you can provide the right constant supply and rectifier. This is part of matching the motor to the drive, which the VFD Selection and Sizing Guide treats alongside inverter rated insulation and bearing protection. A brake motor ordered without asking the question is a coin flip, and half the time the brake is wired to the terminals and you are rewiring it in the field.
The rule to carry out of here is short, and it is a wiring rule, which is why it sits in the VFD Installation Guide. The drive powers the motor. Something else powers the brake, at full voltage, switched by the drive. Never the same leads, and never off the drive output. Motor and brake control, and the selection of a motor built for drive duty, 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
Will a standard brake motor work on a VFD?
Only if the brake coil is powered separately from the motor. If the brake is wired internally to the motor terminals, it taps the motor voltage to release, and a VFD lowers that voltage at low speed, so the brake never gets enough voltage to open at start. The motor then fights a set brake and trips or overheats.
Why does the brake on my motor not release when it runs on a drive?
Because the brake is drawing its release voltage from the motor leads, and the drive puts out very little voltage at low frequency. At start the drive is near zero volts, so the brake coil cannot pull the brake open. The brake only releases correctly when it is fed full rated voltage from a source independent of motor speed.
Can I power the brake from the VFD output?
No. The VFD output is a pulse width modulated waveform intended for the motor alone, and its voltage varies with speed, so it cannot reliably release the brake and can damage the brake coil or its rectifier. Power the brake from a constant voltage source taken ahead of the drive, and switch it with the drive's brake control output.
How do I make an existing brake motor work on a VFD?
Bring the brake coil leads out separately in the terminal box and feed them from their own constant voltage supply, switched by the drive's brake relay. On some motors this separation is straightforward, and on others the brake is not accessible for it, in which case the motor needs to be replaced with one that has the brake separately terminated.
What kind of brake motor should I order for a VFD?
Order an inverter duty brake motor with the brake coil separately terminated, and confirm the brake voltage and whether it is AC or DC. That lets you power the brake from the correct constant source and control it in sequence with the drive, and it avoids field rewiring. Asking the wiring question at the order stage is the whole fix.
Why did the motor work across the line but not on the drive?
Across the line, the motor terminals sit at full voltage the instant the contactor closes, so a brake tapped off those terminals releases immediately. A VFD ramps the voltage up with frequency, so at start the terminals are near zero volts and the brake stays clamped. The same wiring that was fine on a contactor is exactly what fails on a drive.