Need a good DC motor controller for 50A or greater

[adauphin]

I'm looking to upgrade the motor controller in my kids ride on Power Wheels. I currently have a 1000W scooter controller and I feel it's not sufficient for the two motors it's controlling. The controller works but after one went bad after 6 months, I was looking at the Kelly 100A controllers but hear mixed reviews.

The controller will need to use a 5V hall-effect throttle and possibly a brake but the brake isn't a deal brekler as I have a paired DPDT relay for the brake. The vehicle is currently running 24V, motors wired in parallel... I might go 36V in the future and run the motors in series for an 18V load but that's down the road.

One other question I have is, with the current 24V setup, if I wire the motors in series, will that increase the amp draw or lower it? I think it will raise it due to the lower volts but not sure. I might go to a faster motor which on 12V may run better than the current motors on 24V....and last longer.

Right now the toy pulls 35-45A easy going up the driveway which is steep, I would like a better controller for the added amperage just to be safe.

Thanks and all input and criticism appreciated.

[Siwastaja]

Always use a separate controller per motor. Neither parallel nor series works properly.

[MagicSmoker]

...One other question I have is, with the current 24V setup, if I wire the motors in series, will that increase the amp draw or lower it? I think it will raise it due to the lower volts but not sure. I might go to a faster motor which on 12V may run better than the current motors on 24V....and last longer.

Right now the toy pulls 35-45A easy going up the driveway which is steep, I would like a better controller for the added amperage just to be safe. ...

These are likely permanent magnet field DC motors, so RPM will be linearly proportional to applied voltage and torque will be linearly proportional to current (up until the field magnets start to demagnetize or the armature/commutator burns up, etc.).

This behavior is not unlike a voltage source, and so it is best to not wire PM DC motors in parallel even if the shafts are locked together because small differences in RPM per volts (usually specified as Kv) can result in large currents circulating between the two motors as they fight each other. Needless to say, when the two motors are fighting each other they are not doing useful work and tend to overheat much quicker. Of lesser concern is that the effective inductance seen by the controller will be cut in half, and pretty much all modern motor controllers (AC or DC) rely on the inductance of the motor to smooth out the current waveform produced by PWM.

It is perfectly fine to wire two PM DC motors in series, however, with the caveat that top speed will be cut in half, more or less. Torque from each motor will be the same as each motor sees the same current. Effective inductance is doubled, so that is even better as far as the controller is concerned.

Finally, modern PWM DC motor controllers are step down converters (aka "buck converters"), which chop up the input voltage to create a lower average voltage at a higher average current output. Note the emphasis - input current is not a reliable indicator of semiconductor current stress in a buck converter, only output current is! If the input voltage is 24V and the controller is drawing 30A, the output current will be 300A if the output voltage is 2.4V!*

Motor controllers for traction applications typically (should) make motor current proportional to throttle position, but cheap motor controllers might just make PWM duty cycle (ie - output voltage) proportional to throttle position and really cheap controller might not even have overcurrent limiting, relying on the relatively high internal resistance of gel-cell batteries to limit the current. 

A few years ago I made a replacement motor controller (aka "ESC") for my nephew's "Razor" scooter and I thought it might make for a good product to sell on the side until I saw what the Chinese competition costs... I can't even get the bare boards made for the cost of a complete Chinese ESC.


* - this is neglecting all losses and voltage drops for the sake of making the example simple, but note that any inefficiencies such as voltage drops show up as increases in the input current for a given output voltage to input voltage ratio.