Hi All, I have an old but good 36V Cordless Lawnmower running off 20 X 18650 Li Ion cells, 10s-2p. The mower is named Green Tools-PC 1000. I think the motor controller has died and I want to build or buy a replacement.
The main requirement is soft start, with possibly stall current protection. Speed would be, I guess, simply flat out all the time. At this stage I do not have any specs on the brushed PM motor other than 3800 RPM. I guess 20 -30 Amps would be Max current draw from LI Ion cells.
I am happy to try any suggestions, or if someone has already done this any advice, schematics etc. would be greatly appreciated.

Thanks in advance. . . .Brian

 

How many wires to the motor from the controller?

 

Hi crutschow, only 2 wires. . . .Brian

 

If you want a current-limit circuit for the motor then we need to know exactly how much the motor normally draws.
Need a more accurate value than "I guess".

 

There is 2Kw ones on ebay for <$10.00. 221842925195
Max.

 

If you want a current-limit circuit for the motor then we need to know exactly how much the motor normally draws.
Need a more accurate value than "I guess".

I can't run the machine at present but the motor measures at 5.5 Ohms. With full batteries (say 42v) I think that gives me a starting current of under 8 Amps. Maybe I can just start it with a relay ?

I was worried because I have read about inrush currents damaging the Permanent Magnet in the motor. What do you think ?

Brian

 

What makes you think the control module is dead? A lot of these mowers had problems with stretched cables that wouldn't allow it to start. Can you put some pictures of the mower up on the thread?

 

I can't run the machine at present but the motor measures at 5.5 Ohms. With full batteries (say 42v) I think that gives me a starting current of under 8 Amps. Maybe I can just start it with a relay ?

I was worried because I have read about inrush currents damaging the Permanent Magnet in the motor. What do you think ?
Brian

You shouldn't damage the motor, but for large current motors it is always better to slow start it if possible, also easier on the mechanics.
The best/reliable way to measure maximum load is to lock the rotor and apply a small DC voltage and measure current, 2 or 3 reading on different rotor positions is the most accurate way.
Max.

 

Did the original controller seem to generate a slow startup signal?

 

Did the original controller seem to generate a slow startup signal?

Yes, the original controller has a soft start. I have checked the motor with a different Ohm meter and it reads about 1.2 Ohms ?

I will double check the connection from the on/off switch.

 

There is 2Kw ones on ebay for <$10.00. 221842925195
Max.

That would have to be modified to give a soft start, which may not be a trivial modification.
One of these with a built-in soft-start might be better.

 

That would have to be modified to give a soft start, which may not be a trivial modification.
One of these with a built-in soft-start might be better.

Thanks for the eBay link, some of the items look good. I will get some more time next week to follow up

Thanks again . . . .Brian

 

As you are probably aware, some of the things on ebay are junk so caveat emptor.

 

The motor resistance does not really tell you the maximum current the motor can handle. That only tells you that when it stalls, then the maximum current will be VBAT/RMOTOR. However, motor current depends on loading. Free running, current will be super low. While chopping grass, then it will increase directly proportional to load.

What you need is an H Bridge with current chopping capabilities to regulate the current and a simple algorithm to generate acceleration/deceleration profiles. Because there is current regulation, the motor will be well protected even if it locks.

Because of the high currents, you will need something with external FETs. If you want to build it yourself, I would look into the PAC5223 device. I have done plenty of drives with this device during the past 5 years, and I can tell you some garden tools commercially available out there, that is what they use ;-)

 

What you need is an H Bridge

You only need an H-bridge if you need to reverse the motor and that's not the case here.

 

Is this a GreenWorks lawnmower? Pull extra on the cable when you try to start it and see if it will go. This mower is famous for stretched cables and I have a fix for it if that is what is happening.