Electronic – Why is the DC motor losing torque when over voltaged

First of all, Vref on the chip controls maximum power = maximum torque. When S1 on eval board is in TRQ mode, then VREF pin on CN4 is connected directly to Vref of the drived. In this mode, it will have very little effect if the motor is unloaded.

In order to control the speed of the motor while maintaining the torque you will need to wrap the whole thing into a control loop -- something measures motor speed, and changes power to motor correspondingly.

For the best performance, you will want to do it per-phase basis, delaying control inputs if speed is exceeded. However, this is quite involved, so I would instead use manufacturer-recommended analog way (see section 9, 'Tachometer'). This is already assembled for you on the eval board -- set S1 to SPEED mode, and VREF should vary motor voltage smoothly between 0 and max speed.

The DC motor parameter you are interested in maximizing is variously called "starting torque" or "stall torque" or "locked rotor torque", or similar. It is the torque (measured in foot-lbs or inch-lbs) created by the motor when the rotor of the motor is prevented from moving. (This is done in a test bench by jamming the motor shaft mechanically and making the torque measurement - e.g. with a torque wrench.)

This "locked rotor" condition is what the motor will experience momentarily when it starts from a completely stopped condition. E.g. when your electric bike is at a standstill and you press the accelerator.

Some DC motor data sheets will state Locked Rotor Torque directly, others leave it to a simple calculation you must perform. In this case you must know the "torque constant" of the motor, which is usually present in the data sheet. The torque constant is a fixed value for a motor which tells you how many ft-lbs of torque the motor produces for each ampere of current passing thru its armature winding. In other words, ft-lbs per amp.

So, if you know the torque constant all you have to do is measure the current flowing thru the motor at stall (e.g. at the instant you apply power to it) and you will know the resultant torque by simple multiplication ( torque constant x amps).

Typically, you will use the same voltage supply for starting the motor that you do for running the motor. In your case this seems to be 24 Volts. So, in most applications the stopped motor will be started by applying the supply voltage directly across the motor's terminals. What is the starting torque in this common condition? Very simple, use Ohm's Law to figure this out. The motor's data sheet will usually list the motor's "internal resistance" or "winding resistance". In a motor like yours it will likely be less than one Ohm. According to Ohm's Law divide the terminal voltage (24 V) by the Winding Resistance and you will get the starting current in Amps. Now, multiply this calculated starting current by the Torque Constant and you get Starting Torque - the torque the motor will produce while the rotor is not moving - exactly what happens for the first instant you apply voltage to the motor before the rotor actually starts to turn. See? It's a pretty straightforward calculation. If you can make a non-intrusive amperage reading in the motor leads (e.g. with a DC current clamp) you can verify the electrical part of the calculation pretty simply.

As a practical matter, typical DC motors will perform closely to the theoretical calculation I described above. The problem comes in actually delivering the full voltage and amperage to the stalled motor. The amperage will be high for a motor like yours. You state "15-20 Amps", but it may be even greater when you do the calculations from the data sheet values. This means you have to use very heavy gauge wire to allow enough voltage to actually reach the motor without significant voltage drop in the supply wires. Plus, you've got to have a voltage supply that can actually deliver that much instantaneous current.

So that, if you are trying to select a DC motor to get the maximum starting torque at the starting condition, you will look for one with the highest Torque Constant and the lowest Winding Resistance. It's that simple.