Why have a DC motor in a mains-powered lawn mower?

[calzap]

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

[bob91343]

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.)

[Kjelt]

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.

[SunnyDay]

It may be due to safety considerations.  Perhaps a dc motor stops in a shorter time, or has a better stall characteristic.

I 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.

[mikerj]

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

The 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.

[Doctorandus_P]

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.

[David Hess]

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

[calzap]

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

[Zero999]

Does it have a wound, or permanent magnet field? If it's the latter, it's definitely a DC, rather than universal motor.

[calzap]

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

[Someone]

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.

Load 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.

[NiHaoMike]

Is there a similar looking battery operated version of the mower?

[WattsThat]

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.

[NiHaoMike]

An AC motor needs an inverter. Inverter cost is proportional to amps and amps are proportional to torque.

Not if it's corded only.

[WattsThat]

An AC motor needs an inverter. Inverter cost is proportional to amps and amps are proportional to torque.

Not if it's corded only.

Sure 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.

[NiHaoMike]

Sure but now you’ve got to stop it. Mechanical? Easily defeated by idiots.

The 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.

[Circlotron]

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.

With 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.

[calzap]

Is there a similar looking battery operated version of the mower?

This 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

[NiHaoMike]

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.

I 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.

[SparkyFX]

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.

[TurboTom]

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.

[Circlotron]

A permanent magnet DC motor (similar as a shunt-field motor) has got a more or less fixed, voltage-dependent maximum rpm.

A 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.

[TurboTom]

A 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.

Yes 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.

[Circlotron]

Yeah, I see what you mean with regard to stator saturation with DC. Never thought of that. 

[David Hess]

A permanent magnet DC motor (similar as a shunt-field motor) has got a more or less fixed, voltage-dependent maximum rpm.

A 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.

Exactly, and if the field of a shunt wound motor becomes disconnected, bad things^tm happen.