Why have a DC motor in a mains-powered lawn mower? - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: September 07, 2020, 08:06:32 pm
content: I have a B&D human-propelled electric lawn mower. It has a single horizontal spinning blade connected to the shaft of a DC motor. The 120 VAC power from the switch goes to a full wave rectifier and then to the motor, which has brushes that have to be replaced periodically. The original rectifier was 25 A, 400 V. After the first one toasted, it was replaced with another 25A, 400 V copy. After that toasted, a 35 A, 1000 V version was used. After a couple of those blew, I now have a 50 A, 1000 V version … so far, so good. The switch is a special design (SPST-NC-mom + SPST-NO-mom, sequential) that shorts the motor leads when it disconnects the AC power. I presume this is to prevent someone from being shocked from back-EMF if they should unplug it while the blade is still spinning and touch the plug pins.Why couldn’t a brushless AC motor be used?Mike in California

author: bob91343
author_rank: Super Contributor
timestamp: September 07, 2020, 08:38:26 pm
content: It may be due to safety considerations. Perhaps a dc motor stops in a shorter time, or has a better stall characteristic.I have used many Black and Decker lawn care products and have found them to be minimalistic and very cheaply made. Repair is difficult if at all possible. Nearly every one I have owned has failed.I solved the problem by hiring a gardener. (I am also in California.)

author: Kjelt
author_rank: Super Contributor
timestamp: September 07, 2020, 08:43:17 pm
content: Could be mass production to lower the cost of manufacturing, the most expensive part being the motor.If they manufacture for the whole world and with all the different mains voltages they would need as many different motors.

author: SunnyDay
author_rank: Contributor
timestamp: September 07, 2020, 08:57:30 pm
content: Quote from: bob91343 on September 07, 2020, 08:38:26 pmI think that's the reason. Releasing the operating lever shorts the motor terminals, and uses the back emf to brake the rotation of the blade within about a second. An AC motor wouldn't be able to use that method to stop the spinning blade when the lever is released, so it would require a separate mechanical braking mechanism to meet blade stopping safety requirements.

author: mikerj
author_rank: Super Contributor
timestamp: September 07, 2020, 08:59:02 pm
content: Quote from: calzap on September 07, 2020, 08:06:32 pmThe motor is shorted to create a brake for the rotating blade, stopping it much more quickly than if it were allowed to freewheel. This is one benefit of a permanent magnet DC motor that other motors do not provide. The torque/RPM characteristics of DC motors is also likely to be favourable for a lawnmower, as they are loaded and the RPM drops the provide more torque, maximum torque being acheived at 0 RPM, idealt for rapidly spinning up the blade and cutting through long grass. However being a Black and Decker, cheapness was likely the number 1 priority.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 07, 2020, 08:59:41 pm
content: shorting motor leads is an effective way for braking the motor.A rotating motor generates back EMF, and shorting this generates a high current through the motor, which acts as a brake. This braking action gets less effective at lower RPM.Maybe you can do a simple test to unplug the running motor instead of switching it off normally, and then observe the difference in spin down behavior.Your rectifiers smoking may be due to a missing or damaged snubber circuit. The brushes may generate high voltage peaks that go over the rating of the rectifier.Are you sure this is original?It may be an "universal motor" and someone else may have added the rectifier to get more power out of the motor. This could also explain excessive wear of the brushes, but brushes are normal wear items so hard to tell with the little info I have.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2020, 09:18:40 pm
content: That would be a "universal" motor. It can run on AC or DC because the series or shunt connected rotor and field are both supplied from the same source. They are common on AC powered power tools because they respond well to variable speed control. They are also used in fixed speed applications because of better stall characteristics.https://en.wikipedia.org/wiki/Universal_motor

author: calzap
author_rank: Frequent Contributor
timestamp: September 07, 2020, 09:24:31 pm
content: Thanks for the replies. I should have thought of dynamic braking. It was used in centrifuges in my lab. But I thought for it to work effectively, the current has to go into a resistance. That’s what it did in my centrifuges, and that’s the way it works in diesel/electric locomotives.I bought the lawnmower new many years ago. It hasn‘t been messed with by anyone but me, and I haven’t done anything creative beyond upgrading the rectifier and adding a heat sink. The original rectifier was a full-wave, 25 A, 400V.I haven’t seen the rectifiers smoke, but they are in the motor compartment which has lots of ventilation and takes a while to open. Last one that failed (35A ,1000V rated) failed closed. There was some interesting sparking and smoking at the plug when it was plugged in to the extension cord. Tripped the breaker. It must have been the back EMF that killed the rectifier when the mower was shutdown at the end of the last mowing.Mike in California

author: Zero999
author_rank: Super Contributor
timestamp: September 07, 2020, 09:44:16 pm
content: Does it have a wound, or permanent magnet field? If it's the latter, it's definitely a DC, rather than universal motor.

author: calzap
author_rank: Frequent Contributor
timestamp: September 07, 2020, 10:51:03 pm
content: I'm not sure whether the fields are wound or magnets. I've seen the motor many times and replaced brushes, but never investigated the fields.Next time I have the mower open I'll look.For those who are interested, I've attached a circuit diagram.Mike

author: Someone
author_rank: Super Contributor
timestamp: September 07, 2020, 11:56:32 pm
content: Quote from: calzap on September 07, 2020, 09:24:31 pmLoad banks/brake resistors are a way to have some control over the braking, and dump (most of) the heat somewhere other than the motor/cabling/switch. The motor winding is a lossy element its self and dissipates the power in the example of the mower.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 08, 2020, 01:08:39 am
content: Is there a similar looking battery operated version of the mower?

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 08, 2020, 01:16:06 am
content: Why DC? Because it’s the cheapest way to make gobs of torque and it’s easy to stop which is a lawyer requirement.An AC motor needs an inverter. Inverter cost is proportional to amps and amps are proportional to torque.It’s all about the cost and the use case.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 08, 2020, 01:42:15 am
content: Quote from: WattsThat on September 08, 2020, 01:16:06 amNot if it's corded only.

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 08, 2020, 02:31:11 am
content: Quote from: NiHaoMike on September 08, 2020, 01:42:15 amSure but now you’ve got to stop it. Mechanical? Easily defeated by idiots.It still becomes a dollars and cents decision and the lawyer tax drives the decision. IMO.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 08, 2020, 02:39:47 am
content: Quote from: WattsThat on September 08, 2020, 02:31:11 amThe electrical brake is also trivial to defeat with some duct tape. A mechanical brake released by the torque of the motor would require tinkering to defeat.

author: Circlotron
author_rank: Super Contributor
timestamp: September 08, 2020, 02:42:26 am
content: Quote from: calzap on September 07, 2020, 10:51:03 pmWith the power disconnected (!) and the switch in the run position, just rotate the blades by hand. A permanent magnet motor will have a stepped, cogging feel to it. A motor with a wound field will rotate smoothly with no stepping.

author: calzap
author_rank: Frequent Contributor
timestamp: September 08, 2020, 02:49:12 am
content: Quote from: NiHaoMike on September 08, 2020, 01:08:39 amThis one was bought in the 90's, and I am not sure if there was a battery version then ... probably there was. The same line has been continued to today, and there have been parallel battery versions since at least the early 2000s. Using the same motor might be an explanation for the DC motor except the battery versions have ranged from 24 to 60 V. Rectified 120 VAC is a lot higher.Mike in California

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 08, 2020, 03:49:39 am
content: Quote from: calzap on September 08, 2020, 02:49:12 amI have seen a mower that could be powered either from an 80V battery or 120V plug in, with the 120V labeled as a "boost" setting.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: September 08, 2020, 04:13:10 am
content: The universal motor allows to run without starting/run capacitors as a single phase motor and might be a bit more wear resistant than a motor that might require a centrifugal switch. Starting torque might be sufficient for the application.The DC configuration gives you the dynamic braking.

author: TurboTom
author_rank: Super Contributor
timestamp: September 08, 2020, 10:55:04 am
content: A permanent magnet DC motor (similar as a shunt-field motor) has got a more or less fixed, voltage-dependent maximum rpm. A typical "universal motor" with a series filed hasn't got that characteristic, i.e. at low load, rpm will increase considerably which will be dangerous in case of a lawn mower. This feature, combined with the dynamic braking probably made B&D to chose the permanent magnet DC motor.The braking (or should I say beaking) switch is probably also the reason for the rectifier failures: If you turn the mower off and back on while it's still spinning, you interrupt a considerable current flowing through the shorting portion of the mains switch. Since the motor is inductive, it will induce a voltage spike, easily exceeing 1000V and with enough energy to cause the rectifier to break down.You may try installing a reasonably sized 200VAC MOV parallel to the motor's terminals (or the rectifier's DC terminals which is basically the same) to eliminate that problem.

author: Circlotron
author_rank: Super Contributor
timestamp: September 08, 2020, 12:53:11 pm
content: Quote from: TurboTom on September 08, 2020, 10:55:04 amA permanent magnet motor, yes. A shunt field motor, no. A place I worked at once had a machine for cutting radial capacitors from their ammo pack tape. It had a shunt wound motor. The service dept got the job of making the machine go faster... The guys put a variac on it and found that increasing the mains voltage made no difference. The method that was found to speed it up was to either increase the rotor voltage only, or decrease the field voltage only.

author: TurboTom
author_rank: Super Contributor
timestamp: September 08, 2020, 09:41:04 pm
content: Quote from: Circlotron on September 08, 2020, 12:53:11 pmYes you are correct - that's what my "more or less" was meant for. An ideal shunt field motor actually should keep a constant RPM, only increasing its possible torque as voltage increases. But saturation effects in the stator (especially if DC-operated) usually also affect the idle RPM of the motor, leading to an increase at higher voltages.

author: Circlotron
author_rank: Super Contributor
timestamp: September 08, 2020, 11:46:47 pm
content: Yeah, I see what you mean with regard to stator saturation with DC. Never thought of that.

author: David Hess
author_rank: Super Contributor
timestamp: September 09, 2020, 12:06:10 am
content: Quote from: Circlotron on September 08, 2020, 12:53:11 pmExactly, and if the field of a shunt wound motor becomes disconnected, bad thingstmhappen.

author: TurboTom
author_rank: Super Contributor
timestamp: September 09, 2020, 05:07:35 am
content: Quote from: David Hess on September 09, 2020, 12:06:10 amI'ld rather say, "things get interesting..."

author: Berni
author_rank: Super Contributor
timestamp: September 09, 2020, 05:50:47 am
content: Yep the reason for a DC motor is to be able to stop it quickly by using it as a generator.The solution is also probably similar in cost to using an universal motor. The only difference between a DC motor and universal motor is that the permanent magnets in the stator are replaced with electromagnets. So you get the same brush wear problem, but shorting it out does not cause braking because the stator field disappears.That being said braking the motor by shorting it out is pretty violent for all of the components. Lawnmowers have a fair bit of inertia in them (so it might take >30 seconds to stop with no braking, hence safety hazard) so once the motor is disconnected from mains it keeps rotating and generates about 120V DC across its terminals and this is what the switch shorts out. Since the only thing limiting the current is the resistance of the whole loop(motor winding, brushes, rectifier, wires..) means that a very large current flows trough everything. To make things worse this current is DC all the way until the motor stops while in normal operation the AC waveform is choppy so it gives the diodes some time to relax. This high DC reverse current produces a very large braking force while turning all that rotational energy into heat on all of those components.This is the reason why such DC motor braking tends to use a resistor as as a load rather than shorting it. That resistor limits the amount of current that can flow in order to protect the components from excessive current. Also since this resistor will typically be much higher in resistance compared to everything else means that most of the mechanical braking energy is burned as heat on the resistor rather than the motors windings.So to fix this my suggestion is to add a large power resistor somewhere between 10 to 30 Ohm across the switch. This will make it take slightly longer to stop, but at the same time significantly reduce the currents during braking to save the rectifier and some brush wear.

author: TurboTom
author_rank: Super Contributor
timestamp: September 09, 2020, 06:14:24 pm
content: With the braking current, the rectifier hasn't got anything to do. But the resistor may, as you suggested, help nevertheless -- since the current through the brushes is very high at a dead short, this tends to cause heavy wear. Just look through the cooling air grills of a (brush DC motor) battery drill at the commutator and observe how it sparks when running high speed and quickly letting go the speed control switch...

author: David Hess
author_rank: Super Contributor
timestamp: September 09, 2020, 08:00:28 pm
content: Quote from: TurboTom on September 09, 2020, 05:07:35 amHilarity ensues.

author: Gyro
author_rank: Super Contributor
timestamp: September 09, 2020, 08:24:49 pm
content: Quote from: David Hess on September 09, 2020, 08:00:28 pmOnce upon a time there used to be compound wound motors that had both shunt and series wound field windings. They used to be used on cranes and the like where they wanted lots of torque available but that would go to constant speed rather than running away when lightly loaded.This talk of field loss reminds me...

author: calzap
author_rank: Frequent Contributor
timestamp: September 09, 2020, 11:38:03 pm
content: Had a chance to do some testing on the mower today. Couldn’t do it yesterday. A windstorm slammed a big eucalyptus tree on to a road on our ranch. Was able to pull it into a field with a tractor. Next will be the hard work of slicing it into firewood.With the mower unplugged and the switch in the off position (motor leads shorted), I tried turning the blade with my finger. Turned with a bit of resistance, and no cogging was felt. With the switch held in the on position, the blade turned more easily. Again, no cogging was felt. So, it appears there is some dynamic braking even at low RPM. Whether the lack of cogging means the field is a coil rather than permanent magnets, I don’t know.Next were stopping time tests. When mower was running and switch was released, stopping time was perceived as not instant but very quick. Timed at 1-2 seconds but closer to 1. If the switch was held on and the plug was pulled, stopping time was 7 seconds. So there was significant dynamic braking when the motor leads were shorted.Next were voltage tests. Done with an Instek GDM-461 because it has both VAC RMS and VAC maximum peak modes. There was no way I was going to use my scope. Plugged mower into extension cord with plug blades a bit exposed and connected meter leads to plug blades. Did RMS measurements first. Line voltage with mower off was 122; it dropped a volt when the mower was running. If the switch was released, the voltage instantly became line voltage. If extension cord was unplugged with switch held in the on position, the voltage went to zero over 3-4 seconds. It was never observed to go up. I didn’t try putting my hand on the plug blades to see if I could feel a shock, but it should have been possible in the first couple of seconds.Then the meter was put in maximum peak mode. The line reading was 175 V max peak. This is slightly higher than the theoretical of 173 (=122*0.707). But it would take only one very brief higher excursion of the waveform to nudge the max up. Short story on this is that it stayed at 175 V max peak when mower was running and with either way of shutting it down. There was no spike detected. However, the time resolution of the meter is nowhere close to that of a scope, so there could have been brief high voltage spikes.Conclusion is that main purpose of shorting mower leads on shutdown is dynamic braking as a number of forum members have stated. Prevention of shock for someone touching plug blades is likely secondary.Mike in California

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: September 10, 2020, 01:40:48 am
content: Quote from: bob91343 on September 07, 2020, 08:38:26 pmI didn't think maintaining your own lawn in California was even legal.

author: Berni
author_rank: Super Contributor
timestamp: September 10, 2020, 05:34:10 am
content: Don't think anyone was thinking of the possibility of someone unplugging the mower while someone else was using it and touching the prongs. While for stopping it quickly there is probably some sort of formal requirement for it so they designed for it.Similar reason why washing machines have heater+bimetal operated door locks. The heater heats up the bimetal strip, making it bend and lock the door. Then if the machine is unplugged while in the spin cycle the bimetal strip stays hot and keeps the door locked. All of it timed so that the washing machines own friction slows the drum down before the strip cools off and unlocks the door again. This was a cheaper and easier solution compared to installing a brake on the drum. Still meets the safety standards since it prevents the user from hurting themselves with the spinning drum.My dad got a bit of a close encounter with a mower blade once. The gas powered mower we had got the pair of coupling pins snapped off long ago (They hold the blade from rotating on the mount) likely from hitting rocks and such. As a result the bolt holding the blade would gradually tighten overtime and become a huge pain to get off once it was time to sharpen the blade. As a result you don't tend to tighten it super tight when reinstalling it (in hopes of getting the blade off not involving 15 minutes of hammering at it next time). Then after a few weeks that bolt instead got loosened during use. So the blade started slipping and not cutting properly. He stopped to check what is wrong, reached down to tip the mower on the side and suddenly felt a pain on his finger. Turns out the blade got so loose that it kept freely spinning around that loose bolt, continuing to spin for a good while after the engine was off. Luckily all he got was a bit of a cut on one finger. So at that point we realized the actual purpose of those two locating pins on the blade mount. Later on drilled them out and put in new pins.

author: calzap
author_rank: Frequent Contributor
timestamp: September 10, 2020, 03:07:20 pm
content: Must be a new safety requirement. Our washer is at least 10 years old. When it’s spinning, the door does not lock. If you open it, the motor cuts out, but there is no brake.Speaking of difficult to remove mower blades, we have a large mower (1.5 m blade) used on a tractor. The blade is attached to a main shaft, but about 1/3 of the way in from each tip there is a joint with a nut and bolt securing the tips to the main blade. Nut and bolt are designed to allow each tip to pivot. Purpose is to lessen the shock if the tips hit something really solid, like a rock or tree stump. Because of dirt and corrosion, one of the tips froze at an angle, unbalancing the whole blade. Solution was to remove old nut and bolt, clean main blade and tip, and reassemble with new nut and bolt. Nut and bolt were large (25 mm shaft) and frozen solidly together. In addition, access to the nut was limited because it was on top of the blade, and there was not much room between it and the mower housing. There was a small access hole in the top of the housing, which was useful, but not ideal. The bolt head was round and relatively thin.We tried penetrating oil and all sorts of wrenches … open-end, clamping, slip-joint, manual socket, impact socket. Tried thermal shocking as well. Nothing worked. Finally drilled a 12 mm hole down the middle of the bolt using left-hand bits. Required a lot of bit sharpening to keep going and time extracting one broken bit. Idea was to allow inward flexibility of the bolt steel when torque was applied. It would also allow better heating and cooling for thermal shocking. Another round of thermal shocking and penetrating oil and the nut came off with a socket wrench. A picture of the bolt is attached. We pay a lot more attention to maintenance of the pivot joints now.Mike in California

author: Gyro
author_rank: Super Contributor
timestamp: September 10, 2020, 08:01:26 pm
content: Copper anti-seize paste is useful for hindsight avoidance.