Why does the voltage measured across a DC motor drop when it is turned on?

Question

In my experiences with DC motors, I have noticed that the voltage measured across the motor when it is off is higher compared to when it is on. Why is that?

I have always paid no attention to it since the motor still spins. Is this a bad indication that supplementary components are needed?

Let us say I have a 6V volt motor and I apply six volts to it via a driver. Upon turning it on it will be just 5.3V. I do not believe that the driver is to blame since I know my drivers are using MOSFETs and the voltage drop should be very minimal.

Should I adjust my input voltage so that upon measuring the motor voltage it goes back up to 6V?

Answer 1

When the DC supply sags with load current , this is a load regulation error.

Vout=Voc-I*R
(Voc-Vout)/Voc * 100% = Load Regulation Error
(Voc-Vout)/I = effective source impedance.

R is the total resistance of your battery or supply and drivers.

Generally if you need 10% supply stability the source impedance must be < 10% of the DC load resistance.

I suspect your supply is unregulated.

Motor RPM is proportional to V.out with no load. Current = torque + losses

You can increase the motor voltage easily 10% as it is the average power that affects heat rise and normally you want to operate less than the rated power.

Answer 2

Every resistive element in the current loop between your motor and the voltage source will develop a voltage dependent upon the current flowing. The circuit will contain several such resistances, and will resemble this its simplest form:

^{simulate this circuit – Schematic created using CircuitLab}

Even the voltage source, the battery, has some internal resistance, shown here as 0.025Ω. You say that the MOSFETs have no resistance, but that's not the case. Here I represented MOSFET resistance at 0.2Ω. Then all the wiring around the loop will have some total resistance too, which I've arbitrarily set to 0.125Ω.

Obviously these values will vary, but they illustrate the problem; each element will develop some voltage that subtracts from the the total 6V, lowering the voltage that actually appears across the load, the motor.

By Kirchhoff's Voltage Law, the sum of voltages across each element must equal the voltage source, regardless of the current flowing:

$$ V_{MOTOR} + V_{WIRE} + V_{MOSFET} + V_{SOURCE} = V_{BAT1} $$

$$ 5.3V + 0.25V + 0.4V + 0.05V = 6.0V $$

Given that these resistances are unavoidable, there's absolutely nothing you can do about this without raising the source's voltage, as you suggested. However, this approach has a problem; the motor's own effective resistance will vary depending on its mechanical load, and the voltage across it will also vary as that load varies.

You must expect some voltage drop, therefore, but you can minimise it by using thicker wires (and better connectors), a battery with lower internal resistance, and a better MOSFET, with lower \$R_{DS(ON)}\$.

If you do increase battery voltage to achieve the full 6V across the motor under no mechanical load, motor voltage will still fall as load increases, so you still haven't fixed the problem. To ensure 6V across the motor at all times, you require both greater battery voltage and also active voltage regulation, using some kind of closed loop (feedback) control.

Answer 3

My prompt

In addition to the constant voltage drop of the supply voltage caused by unwanted resistances, there is also a momentary drop of the motor voltage when it is turned on. I asked Gemini AI about this phenomenon with the following prompt:

"Gemini, have you heard that in a DC motor, as it rotates, there is a back electromotive force (back EMF) that is subtracted from the applied voltage? Because of this, at the moment of the motor's initial connection, it acts like a short circuit, and the current is very high because there is no such back EMF. Am I right?"

Gemini's answer

Back electromotive force (back EMF): When the rotor of a DC motor rotates under the influence of the magnetic field created by the stator, a voltage is induced in it. This voltage is called “back EMF” and has a direction opposite to the applied voltage from the source.

Effect of back EMF on current: So, the back EMF has the effect of opposing the applied voltage. The faster the rotor rotates, the greater the back EMF and the smaller the difference in voltages, which causes current to flow through the motor. This means that at higher speeds, the current through the motor decreases.

Starting current: At the moment the motor is started, the rotor is stationary and there is no back EMF. This means that the entire applied voltage is applied to the motor windings, resulting in a very high starting current. This current can be several times greater than the motor's rated current and can cause overheating if appropriate measures are not taken.