Motors connected in parallel

Projects General Guidance
1

Hi, so I was wondering if you could help me. I've kind of hit a brick wall.
Something fishy about my circuit;

First:

Two motors connected in parallel (and a transistor to control the RPM). Supplying a 5v from a power supply, unlocked current.

I tried connecting just one of the motors up, with 5 volts, pulling about (0.7 +-2) A. The second motor, pretty much identical model, pulls (0.65 +-2) A with 5V. I'd expect when in parallel they should pull a total of (1.35 +-2) A. Though I'm only getting 1.1 A?

I'm not sure of the behaviour of the motors, but when connecting in parallel, I hear an overall decrease in RPM of one as the other motor increases (I connect them individually). Should this happen? I'd have thought that the motors have the same voltage applied, and their individual RMP should be the same when connected in parallel?)

Previously, when testing the system, I've been getting 1.3-1.4A for both motors, and the RPM is around 9000. But the RPM has dropped to 6000-7000??

In the actual set-up, I've got a transistor to vary the RPM, and controlled using the Arduino 101 (3.3v logic). The transistor is this:

Is something fishy?

2

At 5v on the the gate, it should have low resistance so should not be a problem.

I suspect the power supply may not be able to supply the current required. What happens when you put the motors straight across the supply? Connect one at a time and compare to what happens in circuit.

Weedpharma

3

Hi, thanks for your reply. I have tried connecting the motors directly to the PSU, one at a time, and they both pull ~ 0.65 -0.70 A each. Together, directly to the PSU, independent of the system itself, they produce 1.1A. I'm assuming this is a problem with the power supply then? The power supply can supply up to about 30V I believe (has a dial to control it).

4

moodydog:
The power supply can supply up to about 30V I believe (has a dial to control it).

The important question is "how many amps can it supply?"

...R

5

an important question is what is the measured voltage when you connect both motors ?
sounds like the voltage is dropping.

also, instead of putting them on one FET, it would be better to put each on a separate FET, then use one signal for both FETs.