# Motor Mechanics

I feel like this is a pretty simple question, might be slightly noob, but I have yet to find a direct answer. How does a motor operate in a circuit.

If I have a 12 VDC (11 to 16 Operating) Motor that operates from 6-15 Amps, how do I design my H-Bridge around those specifics. I have enough knowledge on how transistors/diodes/resistors interact, but I have never had to design with a motor and therefore I do not know how to account for it.

Thanks

Design your H-bridge to handle well over 16V (say 20V) and well over 15A (say 20A).

johnwasser:
Design your H-bridge to handle well over 16V (say 20V) and well over 15A (say 20A).

Unfortunately the supply Voltage is limited to a 12VDC line, and that is not negotiable (it is an industrially supplied line). There is a circuit breaker that will blow at 24 Amps for the entire mechanism (granted the power consumption by lights/bells/various other components, and the parts I have been looking at are rated for much higher Amps, I just have no idea how to calculate/design a circuit with a motor in it.

My questions are pretty much these:

1. What determines the voltage drop across the motor (Not internal windings or anything, circuit-wise).
2. What determines how much current it draws, is that determined by the rest of the circuit as well?

I need to to be able to hand calculate stuff.

Voltage drop and current draw are both a result of the motor design. The motor will have a measurable impedance (AC related) and a measurable resistance (DC related).
You can apply less voltage, or limit the current via external means, both will limit the motor's performance.
If you want to control the motor speed, that is typically done by pulsing the voltage to it (via PWM typically).

Select parts that can handle the supply voltage and expected current draw, with a margin of 50% to 100% above that.
The motor will have a stall current (when it gets jammed up for instance, or when it first starts up and electrically looks like its jammed up) that your circuit must be able to supply enough current at the rated voltage to get past to get the motor turning.

Voltage drop and current draw are both a result of the motor design. The motor will have a measurable impedance (AC related) and a measurable resistance (DC related).
You can apply less voltage, or limit the current via external means, both will limit the motor's performance.
If you want to control the motor speed, that is typically done by pulsing the voltage to it (via PWM typically).

Select parts that can handle the supply voltage and expected current draw, with a margin of 50% to 100% above that.
The motor will have a stall current (when it gets jammed up for instance, or when it first starts up and electrically looks like its jammed up) that your circuit must be able to supply enough current at the rated voltage to get past to get the motor turning.

This measurable resistance, can I just, measure it, and then, for design purposes, put a resistor in place of the motor until I get the desired current through the motor or the right voltage across it?

I would like to control the motor using PWM through an H-bridge, that is why I needed some specifics so I could figure out what kind of transistors I need and what resistor values necessary to bias my transistors.

The DC resistance will be pretty low, after all you are just measuring the resistance of wire, but yes, just make sure the resistor has a high enough power rating for the current you put thru it, and you may have to provide some cooling airflow also.
P=I*V, so for 12V, 10A, you need a 120W resistor, or a bunch of lower power rating parts arranged in serial/parallel to achieve the resistance you want.

There's also no Biasing, you are not operating the MOSFETs in their linear region. They will be either Full On or Full Off.
At those kinds of currents you will want to use MOSFET drivers to control the power MOSFETs to ensure they very quickly go from On state with Low Rds to the Off state with very high Rds. Time spent transitioning will allow high current to flow thru some middle resistance range, where the MOSFETs can heat up & fail.

If I wanted to monitor the current to the motor, what would be the easiest way? Would it be to buy a module for this, or simply use some Ohm's Law action?

I'd go with ohms law - If you trying to monitor H -bridge pulses tho, you may need to view the pulses on a scope to see actual voltage levels.