H-Bridge Control of DC Motor: Battery Bank Voltage vs Motor Ratings

[jegues]

If we assume we are using an H-Bridge to control the average voltage across a PMDC motor, is it still necessary that we place a buck converter between the battery bank and the H-Bridge in order to step down the voltage supplying the H-Bridge?

I've attached a schematic of what I'm reffering to.

Perhaps the first question to ask is, what should the battery bank voltage be in relation to the ratings of the motor? Higher? How much higher, and why?

The motor is rated at 48V while the battery bank has a voltage of ~55V. (16 Cells of 3.65V nominal)

Aside from using a buck converter, there are two other ways I can think of in which we could adjust the voltage of the battery bank (if necessary) to better suit the required voltage at the motor:

1) Simply disconnect some of the 3.65V battery cells in order to lower the battery bank voltage to a level more suitable for the ratings of the motor.

2) Limit the duty cycle of the H Bridge switches to a threshold below 100%. Thus, when we are operating at the maximum duty cycle threshold (i.e. this may be say 80% duty cycle) the motor will have a maximum average voltage of say 48V across it.

I think method 2) may cause additional problems because although the switches are only on momentarily providing an average voltage of 48V, the motor is still being exposed to an instantaneous voltage that is larger than its rated voltage. (i.e. 55V) This may cause damage and stress the insulation of the motor.

Any thoughts/comments/suggestions?

 

[Baluncore]

You have not specified the type of switch you are using in your H-bridge. I will assume MOSFETs with built in fly-back diodes. Only a very small ohmic voltage drop (maybe 0.2V) will appear across the diagonal on-pair in the bridge, so to get 48 volts on the motor you need not have more than about 48.4V. There will be ohmic losses between the battery and motor when it is conducting so they may need to be allowed for. Now when all the switches turn off, current continues to flow through the inductive motor and a diagonal pair of fly-back diodes, to recharge the output capacitor of the buck converter if used, or to recharge the battery. The voltage across the motor will then be at a maximum since the voltage drop across the conducting diodes and cable series resistance will be added to the battery voltage.

I don't think the motor voltage rating is an insulation specification, that will be closer to 500V minimum. It is more likely to be there to regulate RPM when running unloaded, or internal power when under torque close to stall.

To be professional, you should not subject a motor to a greater voltage than it's rating. The H-bridge must also handle the supply voltage. As a one-off you will probably get away with 55V on a 48V motor. I certainly would not design for manufacture with that degree of mismatch.

The H-bridge with it's fly-back diodes, coupled with the motor inductance is effectively a buck converter. Why duplicate it. You could put an inductor in series with the motor. The inductor's value would need to be (55-48)/48=15% of the motor inductance. That would drop the excess voltage efficiently in a self-bucking inductive voltage divider.

So now consider the application. Unloaded motor speed will be proportional to average voltage. The motor torque will be proportional to motor current. Will the motor ever run unloaded or be stalled? How will the motor current and speed be regulated? What will the duty cycle and power dissipation in the motor be. Those operating conditions may constrain the motor to remain within it's specifications without the need to limit the voltage.

One critical thing is that 50 volts is usually the legal boundary between low voltage and high voltage. There may be legal implications depending on your solution.