Two Technologies, One Purpose
Both magnetic powder brakes and hysteresis brakes are non-contact load devices used in motor testing to apply controlled braking torque to a spinning shaft — without mechanical friction, wear pads, or physical contact between stationary and rotating components. Despite this shared principle, they use fundamentally different physics and have very different performance envelopes.
Choosing the wrong technology for your test bench application leads to either inadequate torque control, excessive heat, premature failure, or unnecessarily high cost. This guide provides the technical comparison needed to make the right decision.
How They Work: The Physics
Magnetic Powder Brake
A magnetic powder brake contains a dry magnetic powder (iron particles, typically 50–200 μm in size) suspended in the air gap between the rotating rotor and the stationary coil housing. When DC current is applied to the coil, it creates a magnetic field that chains the powder particles into rigid links between rotor and housing — transmitting torque from the rotating shaft to the stationary frame.
The transmitted torque is proportional to the applied current: T ∝ I. This linear relationship makes magnetic powder brakes extremely easy to control — a simple DC current source is all that is needed.
Key physics implications:
- Torque is set by current — independent of slip speed (at normal operating speeds)
- The powder particles transmit shear forces — moderate slip generates heat in the powder
- Continuous high-slip operation causes powder degradation — limiting heat dissipation capability
- At very low slip speeds (<5 RPM), powder friction can cause torque “stiction”
Hysteresis Brake
A hysteresis brake uses a rotating disk made of a special magnetic alloy (hysteresis material — typically chrome steel or cobalt alloy) moving through the field of permanent magnets or electromagnets in the stationary housing. As the disk rotates, the magnetic domains in the hysteresis material are repeatedly magnetized and demagnetized, creating energy loss proportional to the area of the B-H hysteresis loop.
This hysteresis loss manifests as a braking torque opposing rotation. Crucially, the torque depends only on the magnetic field strength (set by the coil current) — not on the slip speed or shaft speed. The disk has no physical contact with the magnets.
Key physics implications:
- True non-contact operation — no wear surfaces at all
- Torque is constant at all slip speeds from 0 to maximum rpm
- Heat dissipation is distributed through the hysteresis disk — better thermal handling
- No degradation over time — hysteresis material is solid metal
Performance Comparison
| Paramètre | Magnetic Powder Brake | Hysteresis Brake |
|---|---|---|
| Torque range | 0.1 N·m – 3,000 N·m | 0.01 N·m – 500 N·m |
| Contrôle du couple | Linear vs current (±2–5%) | Linear vs current (±1–2%) |
| Torque at zero speed | Full rated torque | Full rated torque |
| Low-speed smoothness | Bien (some stiction below 5 RPM) | Excellent (zero stiction) |
| High-speed limit | Medium (1,000–6,000 rpm typical) | High (jusqu'à 30,000 RPM) |
| Continuous slip power | Medium (powder degrades under sustained high slip) | High (solid metal disk handles continuous heat) |
| Torque ripple | Low (<3% of rated) | Very low (<1% of rated) |
| Service life | Powder requires periodic replacement (1,000–5,000 hours slip operation) | Unlimited (no wear components) |
| Cost (relative) | Inférieur (1×) | Plus haut (2–5×) |
| Maintenance | Powder replacement, seal inspection | Near-zero (bearings only) |
Heat Dissipation: The Critical Difference
The most important practical difference between the two technologies is how they handle heat under continuous slip conditions.
Magnetic Powder Brake Heat Limits
In a magnetic powder brake, heat is generated by shear within the powder layer. The powder particles are the thermal weak point — above approximately 100–120°C, powder particles begin to agglomerate (stick together), permanently degrading torque linearity and control repeatability. This limits continuous slip power.
Typical magnetic powder brake heat ratings:
- Intermittent slip: 100% rated torque × slip speed (any duration)
- Continuous slip: 20–40% du couple nominal × slip speed for most units
- Forced-air cooled units: 60–80% rated torque × slip speed continuous
Hysteresis Brake Heat Limits
In a hysteresis brake, heat is generated uniformly throughout the hysteresis disk volume. The solid chromium-cobalt or cobalt-vanadium disk is an excellent thermal conductor, transferring heat to the housing fins efficiently. The disk can sustain much higher temperatures without degradation — it is solid metal with no wear surface.
Typical hysteresis brake heat ratings:
- Continuous slip: 100% rated torque × slip speed (within power rating)
- Some units: Liquid-cooled for sustained high-power slip operations
Application Decision Guide
Choose a Magnetic Powder Brake When:
- Motor torque range exceeds 500 N·m (powder brakes scale to 3,000 N·m; hysteresis tops out at ~500 N·m)
- Budget is limited and intermittent slip is acceptable
- La plage de vitesse est inférieure 6,000 RPM
- Testing is primarily steady-state (cartographie de l'efficacité, Courbes T-N) with limited continuous slip time
- Application is production line testing with short test cycles (<2 minutes per motor)
Choose a Hysteresis Brake When:
- High-speed testing above 6,000 RPM (hysteresis up to 30,000 RPM)
- Continuous high-slip testing (cogging torque measurement, back-EMF analysis at varying speeds)
- Torque control accuracy below ±1% est requis
- Zero-speed and near-zero-speed torque must be smooth (precision servo, medical, cobot testing)
- Maintenance-free operation is important (no powder to replace)
- Motor power below 10 kw (hysteresis is most competitive in this range)
Combined Systems: Powder Brake + Hysteresis Brake
For test benches covering a wide motor range (par ex., 0.1 N·m to 200 N·m), some laboratories install both technologies in series on the same shaft:
- Hysteresis brake handles the precision low-torque, high-speed tests (below 50 N·m)
- Magnetic powder brake engages for high-torque tests (above 50 N·m)
Both units are connected in series with a torque sensor between them. The appropriate unit is activated for each test. This provides the best of both technologies across the full motor range.
Foire aux questions
Can a magnetic powder brake be repaired when the powder degrades?
Oui. Magnetic powder replacement is a routine maintenance procedure. The unit is disassembled, the degraded powder is removed, and fresh powder (matched to the OEM specification by particle size and magnetic properties) is installed. Rebuild kits are available from most manufacturers. Rebuild cost is typically 15–25% of new unit cost.
Is there a middle ground between powder and hysteresis in terms of cost?
Eddy current brakes offer a third option: a copper or aluminum disk rotating in a magnetic field, with braking torque generated by induced eddy currents. They are more cost-effective than hysteresis brakes for the 1–100 kW range, handle continuous slip well, and have no wear components. Cependant, eddy current torque varies with speed — they cannot apply significant torque at zero or very low speed (below approximately 100 RPM), limiting their use for cogging torque or stall torque tests.
What current supply is needed to control a powder or hysteresis brake?
Both technologies are controlled by DC current. Typical coil voltages are 12V–90V DC; current range is 0.1–5A depending on brake size. A simple programmable DC power supply or a dedicated brake controller (0–10V analog input, ±0.1% current regulation) provides the control signal. Most brake controllers include an analog current output proportional to the torque setpoint.
Conclusion
Magnetic powder brakes offer cost-effective load control for steady-state testing across a wide torque range. Hysteresis brakes provide superior smoothness, infinite life, and high-speed capability for precision applications where torque ripple, continuous slip, and maintenance downtime matter most. For most servo motor and small EV motor test benches in the 0.1–50 N·m range, the hysteresis brake’s advantages justify its higher cost. Au-dessus de 100 N·m, the magnetic powder brake’s scale and cost economics make it the practical choice.
EconoTest supplies both magnetic powder and hysteresis brake test benches, configured to the motor range and test requirements of each customer.
→ Contact us for a recommendation based on your specific motor testing requirements.
