# H-Bridge for DC Motor

Discussion in 'Homework Help' started by jegues, Jul 24, 2013.

1. ### jegues Thread Starter Well-Known Member

Sep 13, 2010
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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.

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2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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I would think the buck converter superfluous. With pwm control of the H bidge switching (option 2) you have the means of controlling the motor speed.

3. ### John P AAC Fanatic!

Oct 14, 2008
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You should be OK with this power supply and this motor. In your option 2, it's true that the instantaneous voltage is above the rated level, but 55V isn't a whole lot higher than 48, and I very much doubt if you'd get insulation breakdown from that. And the inductive characteristic of the motor ought to keep the current from getting too large, though you would have to keep the PWM frequency high enough to filter the swings of current. If you can, you'll also most likely want to keep the frequency high in order to limit audible noise--it can be really annoying!