Simple Semi H-Bridge circuit for Driving Salvaged Treadmill Motor

Thread Starter

kharrisma

Joined Dec 9, 2019
4
Hi Forum Folke,

Been quite awhile since I've done this kind of work... Tore down a working Nordic Track Treadmill with an MC 2100 motor controller card in it to use as a variable-speed drive for a 1940s vintage Craftsman 4" belt / 8" disc sander. Treadmill was working when it went into storage, but when taken out, the motor would come on at full speed for about half a second, then shut down totally. Found "mouse leavings" across the board, blown caps, fractured power resistor, unsure about the status of the power MosFet... the PWM circuitry and motor driver part of this card is toast, anyway.

The schematic for the circuit, monitoring circuitry aside, seems pretty simple: 90 VDC power directly to motor on one side (switched on via an SCR), and the MosFet between the motor and ground. PWM generated in the console according to user settings, through a microprocessor, down to the MC 2100.

As set up in the treadmill, the motor is unidirectional. I'd like to set it up so I can reverse the direction of rotation. The motor brushes are directly opposed 180 degrees apart, dead perpendicular to the armature, with no 'offset' to smooth things out and reduce brush arcing, so it should do bidirectional rotation without trouble. As I understand it, anyway.

I'd like to cobble up a pseudo H-bridge setup (maybe an "A" bridge?) to drive the motor, and keep the complexity, cost, and parts count low (since it isn't going to be a salvaged-parts operation anymore, other than the motor itself). Since the motor was powered directly from the 90 VDC rail on the 'upstream' side from the factory, is there any particular reason I can't simply put a DPDT high-current toggle switch there and switch (under static conditions, motor stopped, not 'live' reversal) in the direction of rotation I want? Two MosFets, one for each 'lower leg' of the H-Bridge would complete the power circuitry. I'm just going to 'copy' the R/C tank parts of the motor controller board to control oscillating voltage spikes caused by the on-off switching of the active MosFet. Forgive the crudity of the attached rough schematic; KiCad is new to me.

Any ideas on why this is a bad idea, or wouldn't work?SchemoSnip.JPG
 

LowQCab

Joined Nov 6, 2012
2,515
You only need one DPDT-Switch connected to the Motor to reverse the Polarity,
and one FET.
Lots of Spike/RFI Suppression/Protection is in order if You
expect the FET to survive with all that Brush-Arcing going on.

I'd buy a proper Induction-Motor instead.
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