1. Introduction
In many motor-driven systems, uncontrolled or free motor movement is undesirable or unsafe. Applications such as pumps, actuators, conveyors, robotic joints, and test rigs may require the motor shaft to resist motion when the controller is disabled, unpowered, or in a fault state.
One effective and often overlooked technique for achieving this is electrically locking the motor phases together using relays. By shorting the motor phases, the motor itself generates resistive torque when the shaft attempts to turn, providing a passive braking effect without the need for mechanical brakes.
This article explains the principle, implementation considerations, and trade-offs of using relays to lock motor phases together on a motor controller.
2. The Principle: Why Phase Locking Works
When the phases of a motor are shorted together, any attempt to rotate the motor shaft forces the motor to act as a generator. The generated current circulates within the shorted windings, producing:
- Electromagnetic resistance to motion
- Damping proportional to speed
- No external power requirement
This effect is often referred to as dynamic braking or short-circuit braking.
Key characteristics:
- Torque increases with speed (very effective at preventing free spinning)
- Torque drops to near zero at standstill
- No holding torque at zero speed (unlike a mechanical brake)
3. Why Use Relays Instead of Semiconductors?
Relays are well suited to this function for several reasons:
3.1 Fail-Safe Behaviour
- Relays can be configured normally closed (NC) so that phases are locked when power is lost.
- This makes the system inherently safer in power-fail conditions.
3.2 Electrical Isolation
- Relays provide galvanic isolation between the motor and control electronics.
- This reduces risk during faults, ESD events, or high-energy transients.
3.3 Low Conduction Loss
- Mechanical contacts have very low on-resistance compared to MOSFETs.
- Heat generation during braking is therefore dominated by the motor, not the controller.
4. Typical Relay Configurations
4.1 Three-Phase BLDC / PMSM Motors
For three-phase motors, the most common approaches are:
Option A: All Phases Shorted Together
- A relay shorts U, V, and W together
- Simple and effective
- Produces strong braking torque
Option B: Pairwise Phase Shorting
- Phases shorted in pairs (U–V, V–W, W–U)
- Slightly more complex
- Can reduce peak currents in some designs
4.2 Brushed DC Motors
- The motor terminals are shorted together
- Very effective dynamic braking
- Requires only a single relay
5. Integration with a Motor Controller
5.1 Normal Operation
During active motor control:
- Relays must be open
- Motor phases are driven exclusively by the inverter (MOSFET bridge)
5.2 Disable / Fault / Power-Off States
When the controller is disabled or faulted:
- Inverter outputs are tri-stated or disabled
- Relay closes, shorting motor phases
- Motor resists rotation automatically
This sequencing is critical to avoid:
- Shorting active drive outputs
- Welding relay contacts
- Damaging MOSFETs
6. Control Strategy and Timing Considerations
6.1 Break-Before-Make
- Ensure inverter outputs are fully disabled before closing the relay
- A delay of a few milliseconds is typically sufficient
6.2 Relay Switching Under Motion
- Closing the relay while the motor is spinning will cause a current surge
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Relay contact ratings must account for:
- Motor inductance
- Maximum speed at engagement
- Repeated braking cycles
6.3 Release Timing
- Opening the relay before enabling the inverter is equally important
- Interlocks in firmware or hardware are strongly recommended
7. Thermal and Electrical Considerations
7.1 Motor Heating
- All braking energy is dissipated in the motor windings
- Extended braking at high speed can cause overheating
7.2 Relay Contact Stress
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Contacts must be rated for:
- DC current (often more demanding than AC)
- Inductive loads
- Automotive or motor-rated relays are preferred
7.3 Current Magnitude
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Short-circuit current is limited by:
- Winding resistance
- Motor back-EMF
- Peak currents can still be very high at speed
8. Advantages of Phase Locking via Relays
- ✔ No external power required
- ✔ Simple, robust, and cost-effective
- ✔ Fail-safe when designed with NC contacts
- ✔ No mechanical wear mechanisms like friction brakes
- ✔ Works even when controller is completely unpowered
9. Limitations and When Not to Use It
- ✖ No holding torque at zero speed
- ✖ Not suitable where position must be maintained under load
- ✖ Relay lifetime must be considered
- ✖ Not a substitute for safety-rated mechanical brakes in critical applications
In applications requiring static holding torque, a mechanical brake or active motor control is still necessary.
10. Typical Applications
- Pump and fan systems to prevent windmilling
- Test rigs and end-of-line motor testing
- Mobile equipment where free rotation is undesirable
- Low-cost alternatives to mechanical brakes
- Fail-safe braking in power-loss scenarios
11. Summary
Using relays to lock motor phases together is a proven and effective method for preventing free motor movement. When implemented correctly, it offers a simple, passive, and fail-safe braking solution that integrates well with modern motor controllers.
For designers, the key challenges lie not in the concept itself, but in:
- Correct sequencing
- Relay selection
- Thermal management
- Clear definition of operating states
When these factors are addressed, phase-locking relays can significantly improve system safety and robustness with minimal complexity.