In many motor-driven systems, a shaft that spins freely when the drive is off is a problem. A pump can windmill under back-pressure. A conveyor or actuator can run on under load. A test rig can coast unpredictably after a fault. Mechanical brakes solve this, but they add cost, space and wear.
A simpler option, often overlooked, is to short the motor phases together with a relay. With the windings shorted, the motor brakes itself whenever the shaft turns, with no power and no friction parts. This article explains how the technique works, how to implement it safely, and where its limits are.
How phase shorting works
A spinning permanent magnet motor is also a generator. Short its windings and the back-EMF drives current around the closed loop. That current produces a torque that opposes the rotation. This is usually called dynamic or short-circuit braking.
The braking torque depends on speed, but not in a straight line:
- At standstill the torque is zero. There's no back-EMF, so there's no current. Phase shorting damps motion but cannot hold a load in place.
- At low speed the torque rises roughly in proportion to speed. Winding resistance limits the current, so the motor behaves like a viscous damper.
- Beyond a peak the torque falls. As electrical frequency rises, winding inductance limits the current more than resistance does. The current levels off, and braking torque falls with increasing speed. The peak occurs at around the electrical speed where ωL = R.
In practice, low-inductance motors (common in high-speed and slotless designs) brake strongly across a wide speed range. High-inductance motors brake well at low speed but weakly at high speed. Check the braking curve against your application rather than assuming more speed means more braking.
Why use relays?
Most modern controllers can short the phases electronically by switching on all the low-side MOSFETs, a technique often called active short circuit. That works well while the controller has power. A relay adds three things.
It works with no power at all. With normally closed contacts, the phases short automatically when power is lost, the supply is disconnected or the controller is removed. This is the main reason to use a relay.
Its conduction loss is low. Relay contacts typically have a lower on-resistance than MOSFETs of similar cost, so nearly all the braking energy is dissipated in the motor windings rather than in the controller.
It's independent of the power stage. The relay's coil circuit is isolated from its contacts. The braking function therefore doesn't depend on the gate drivers or MOSFETs working correctly, which matters if the fault you're protecting against is in the power stage itself.
Relay configurations
Three-phase BLDC and PMSM motors. To short all three phases, you need two contacts: one across U–V and one across V–W. A double-pole relay does this. This gives the strongest and smoothest braking.
Shorting just one pair of phases needs only a single contact. The braking torque is lower, though, and it ripples with rotor position. That can be acceptable for anti-windmilling duty, but it's a compromise.
Brushed DC motors. A single contact across the motor terminals gives effective braking. The drive bridge must be disabled first, or the contact will short the bridge output.
Stepper motors. Shorting each winding adds damping on top of the motor's natural detent torque. This is useful where a stepper must not be back-driven when disabled.
Integrating with the controller
The controller needs clearly defined states, and it must never let the relay and the inverter drive the motor at the same time.
Running. The relay is held open, which for a normally closed relay means the coil is energised. The inverter has sole control of the phases.
Disabled, faulted or unpowered. The inverter outputs are switched off (all MOSFETs off). Only then does the relay close, shorting the phases.
The key rule is break before make, in both directions:
- To brake: disable the inverter, wait, then close the relay.
- To run: open the relay, confirm it has opened, then enable the inverter.
The delay must cover the relay's operate or release time plus contact bounce. That's typically 5–20 ms, but check the datasheet rather than assuming. Enforce the interlock in hardware where possible, with firmware as a second layer. If the contacts close while the inverter is switching, you'll short the DC bus through the MOSFETs.
Sizing the relay
The relay is the part most often under-specified.
Making current. If the relay closes while the motor is spinning, the initial current can be very high. It's limited only by winding resistance, inductance and the back-EMF at that speed. Size the contacts for the worst case: maximum speed at engagement, at the highest expected back-EMF.
Breaking current. Opening contacts that are carrying current draws an arc, and this is much harder on the contacts than closing. Where possible, open the relay only when the motor is stationary, or at low speed. If it must open under motion, allow for it explicitly in the relay rating.
DC and low-frequency ratings. In a BLDC motor, the short-circuit current is AC at the electrical frequency, which approaches DC at low speed. In a brushed motor it is DC. Either way, use the relay's DC inductive rating, not its AC resistive rating. Automotive and motor-rated relays are usually the best starting point.
Endurance. If the system brakes frequently, count cycles at your actual current, not the relay's no-load mechanical life figure.
Thermal and magnetic limits
All the braking energy ends up as heat in the motor windings. Short stops are rarely a problem. But a motor that is driven externally for long periods while shorted, such as a pump windmilling against flow, can overheat. Check the continuous duty.
In permanent magnet motors, also check that the short-circuit current stays below the magnet demagnetisation limit. At high speed, the current approaches the motor's characteristic current (flux linkage divided by inductance). In some designs that's high enough to partially demagnetise the rotor.
Failure modes
A normally closed relay fails safe if it loses power, but not in every failure. A welded contact leaves the phases permanently shorted, and the inverter will then drive into a short circuit when it's enabled. For systems where this matters, monitor the contact state (auxiliary contacts or force-guided relays make this easier) and block the inverter from enabling unless the relay has confirmed it's open.
Advantages and limitations
Phase-shorting relays are simple, cheap and self-powered. They fail safe on power loss when normally closed contacts are used, and they have no friction surfaces to wear or adjust.
They have limits, though. They provide no holding torque at standstill, so they cannot hold a vertical load or maintain position. Braking weakens at high speed in high-inductance motors. The relays have a finite life under load. And a phase-shorting relay is not a safety-rated brake: where braking is part of a safety function, it needs to be designed to the relevant functional safety standards, usually alongside a mechanical brake or a safe torque off function.
Typical applications
- Pumps and fans that need to be prevented from windmilling
- Actuators and conveyors that must not run on when disabled
- Test rigs and end-of-line test stations
- Mobile equipment where free rotation is unwanted
- Low-cost braking where a mechanical brake isn't justified
Summary
Shorting motor phases with relays is a proven, low-cost way to stop a motor spinning freely, and it works even with no power. The principle is simple. The engineering is in the details: sequencing the relay against the inverter, rating the contacts for real making and breaking currents, understanding how braking torque varies with speed, and managing the thermal and demagnetisation limits.