Limit a flywheel’s speed by building the drive so it cannot exceed it, and add an independent overspeed trip on top: an induction motor at 50 Hz cannot drive a 4-pole machine above 1,500 rpm, a fixed-field DC machine on a fixed-voltage bus cannot motor it above the speed where its EMF equals the bus, and a spring-loaded bolt of 50 g at 40 mm radius pulls 49.3 N at 1,500 rpm but 59.7 N at 1,650 rpm, so a spring set between those releases the drive at 110% of working speed. A governor that must work at wheel speed fails for geometry reasons.
Part of Flywheel Energy Storage
Ways to keep a flywheel under its design speed: drives with a natural cap, a calibrated overspeed trip, speed indication, and braking.
Layer 1: A Drive That Cannot Overspeed
- Induction motor. It cannot drive above its synchronous speed; the corpus Alternator Design gives 1,500 rpm for four poles at 50 Hz. A wheel designed for under 1,500 rpm cannot be motored past it. If the wheel is driven by something else and the motor is connected, it generates, which is a load, not a drive.
- Fixed-field DC machine on a fixed bus. As worked in Motor-Generator Set, it motors the wheel only until its back-EMF equals the bus voltage, 750 rpm in that example.
- Never a series motor. The corpus DC Motors says a series motor should never run unloaded; a flywheel is nearly unloaded once spinning.
- A mechanical prime mover. A water wheel or turbine whose load is removed accelerates (Load Control); put the load switch, not the wheel, behind a governor and keep a dump load.
Layer 2: A Governor Cannot Work At Wheel Speed
Centrifugal Governor gives the ball height of a pendulum governor as h = g ÷ ω². At 1,500 rpm, ω = 157.1 rad/s, so h = 9.81 ÷ 24,674 = 0.0004 m, 0.4 mm. The balls barely move, and no linkage can use that. Geared down to 150 rpm (ω = 15.7 rad/s) the height is 9.81 ÷ 246.7 = 39.8 mm, which a linkage can use. So drive a governor spindle through a 10:1 reduction. See Governor Design for damping so it does not hunt.
Layer 3: An Overspeed Trip You Can Calibrate
A mass m on a spring at radius r on the wheel shaft feels a centrifugal force F = m × r × ω². The spring holds it in until that force exceeds the spring preload, then it flies out and strikes a latch that releases the drive, applies the brake, or dumps the load.
Worked example: m = 0.05 kg, r = 0.04 m.
- At 1,500 rpm (ω² = 24,674): F = 0.05 × 0.04 × 24,674 = 49.3 N.
- At 1,650 rpm, 110% of working speed (ω = 172.8 rad/s, ω² = 29,855): F = 59.7 N.
- Force goes as speed squared, so 110% of the speed is 1.21 times the force; set the spring preload to 59.7 N and the bolt moves out at 1,650 rpm.
Calibrate it, don’t trust the arithmetic alone: spin the shaft up with the wheel off (the trip bolt on a test shaft in a small drill or lathe spindle) and compare the trip with a counted rpm. If the trip speed is wrong by even 5%, the force is wrong by 10%. Re-test after any adjustment, and after each service.
Layer 4: Speed Indication
Couple a small permanent-magnet dynamo to the shaft. Its voltage is proportional to speed (the EMF equation in DC Motors), so a voltmeter with a red line painted at the limit becomes a tachometer. A relay on the same signal gives an overspeed trip. The corpus carrier suggests estimating speed from the pitch of the wheel’s hum: with eight spokes at 1,500 rpm the pitch is 8 × 25 = 200 Hz and goes up with speed, a useful cross-check and not a measurement.
Layer 5: Stopping
A trip that opens the drive does not stop the wheel; see the stop times in Safety & Containment. Practical brake options:
- Dump load. Load the generator into a resistor in a water tank. The 2.22 MJ heats 2,221 kJ ÷ (4.18 kJ/kg·K × 40 K) = 13.3 kg of water through 40 K, so a 100 kg tank absorbs it with a 5 K rise. A 10-minute stop is 3.7 kW.
- Friction brake. A band or shoe brake of 47 N·m stops the wheel in 10 minutes, 471 N·m in a minute. The brake absorbs 2.2 MJ as heat in its surfaces.
Only a brake that engages by spring, not by power, will work after a power failure. Fit one.
Common Mistakes
- Relying on one device. A trip, a drive cap, and a speed indication should be independent.
- Mounting a governor on the wheel shaft. The geometry makes it useless.
- A trip with its latch bolted to the same shaft that fails. Anchor it to the frame.
Safety
Overspeed is the one failure you design out
A wheel driven past the speed calculated in Burst Energy Calculation can burst in seconds. Combine a drive that cannot overspeed with a calibrated trip and a spring-applied brake; keep the stop reachable from outside the plane of the wheel; and never test the trip with the wheel in the working pit at full speed until it has been shown to work on a test shaft.
What’s Next
- Learn next: Safety & Containment — all the layers together
- Combine with: Housing — the last layer if the others fail