Why a 0.5 HP Grinder Shuts Down an 800 W Inverter
A tailoring shop runs a 12 V, 800 W modified-sine-wave inverter with a 12 V, 150 Ah tubular lead-acid battery. The backup load is normally 6 LED lamps of 15 W, 4 fans of 75 W and a 120 W television. The shop also has a 0.5 HP (373 W) bench grinder with a rated efficiency of 0.72 and power factor 0.70, which the owner now wants to run during power cuts by plugging it into the inverter. Whenever the grinder is switched on, the inverter shuts down and restarts after a few seconds. Fan speed is normal on the backup load. The battery is two years old and the backup time has fallen from about 2 hours when new to 40 minutes. Inverter efficiency is 85 percent and the battery's recommended depth of discharge is 80 percent. The battery's internal resistance is about 8 milliohm.
1. Calculate the DC current drawn by the normal backup load and estimate the backup time the battery should give. Use this to judge whether the 40-minute figure points to a battery problem or an inverter problem.
Read the worked answer
AC load: 6 x 15 = 90 W; 4 x 75 = 300 W; TV 120 W. Total = 510 W.
DC current from the battery = 510 / (12 x 0.85) = 510 / 10.2 = 50 A.
C-rate = 50 / 150 = 0.33 C. That is a fairly hard discharge for a tubular battery, but within its capability if the cables and terminations are sound.
Usable energy = 12 x 150 x 0.8 = 1440 Wh. At the load, after inverter losses, 1440 x 0.85 = 1224 Wh, so 1224 / 510 = 2.4 hours ideally.
But the 2.4 hours assumes the battery delivers its full 150 Ah at the 10-hour rate. At 50 A, the Peukert effect and internal resistance mean it delivers noticeably less, perhaps 100-110 Ah, so the realistic figure when new is about 1.6 to 1.8 hours, not 2.4. A fall from about 1.7 hours to 40 minutes means the battery has lost roughly 60 percent of its capacity, which is typical of two years of daily deep cycling without equalisation.
Conclusion: the 40-minute backup is a battery ageing problem, not an inverter fault. The grinder shutdown is a separate issue of surge rating and DC current, and the two must not be confused.
2. Explain, with numbers, why the grinder trips the inverter even though its running power is below the inverter's 800 W rating.
Read the worked answer
The grinder's input power is not 373 W. Input = output / (efficiency x power factor) = 373 / (0.72 x 0.70) = 373 / 0.504 = 740 W. So the grinder alone consumes almost the whole 800 W continuous rating, and the inverter still has the fans, lights and TV on it.
DC current for the grinder alone = 740 / (12 x 0.85) = 72.5 A, and with the 510 W backup load the total is about 122 A.
Now add the start. A capacitor-start induction motor takes roughly 3 to 5 times running current for a fraction of a second, so 220 to 370 A from the battery. The inverter's peak rating is usually only about twice its continuous rating, around 1600 W, which at 12 V is 157 A. The start surge is well above that, so the inverter shuts down on overload.
The battery adds a second failure mode: at 250 A, a terminal voltage sag of 250 x 0.008 = 2 V occurs, dragging 12.6 V down to about 10.5 V, which is exactly the inverter's low-voltage cut-off. So the shutdown may be triggered by voltage collapse rather than by the overload circuit.
Finally, a modified sine wave is a stepped square wave. It raises eddy-current and copper losses in the motor, reduces starting torque, and makes the grinder hotter and noisier, so the motor draws more current for the same work. Running a motor on a modified sine inverter is a compatibility issue, not just a size issue.
3. Give the owner a prioritised set of recommendations and explain why you reject the obvious cheap options.
Read the worked answer
Reject: (a) running two inverters in parallel, which needs synchronised outputs and is impossible between two ordinary off-the-shelf units, and risks back-feeding one from the other; (b) plugging the grinder into a larger fuse or bypassing the shutdown, which removes the protection and can start a fire in the inverter's transformer; (c) keeping the grinder on the modified sine inverter whatever its rating, because the waveform itself damages the motor windings over time.
Recommend, in order:
- Keep the grinder on mains only. It is a 30-second job, costs nothing, and removes the whole problem.
- If the grinder must run during cuts, use a pure sine wave inverter rated at least three times the motor's input power, say 2.5 kVA, and move to a 24 V or 48 V battery so the DC current at 740 W falls to about 36 A or 18 A, well within the battery and cable ratings. Add a soft starter or a VFD with ramp-up if the inverter supports it; that alone can halve the surge.
- Consider instead a small petrol generator for the grinder, which is often cheaper than a larger sine-wave inverter and battery bank.
- Test the ageing battery (load test, specific gravity of every cell) and replace it if it fails; set the low-voltage cut-off at 10.8-11 V so it is not deep cycled, and equalise monthly.
- Check cable size and terminations between battery and inverter: at 50 A a loose terminal is both a fire risk and a source of extra voltage sag.