How to determine DC motor voltage

dc motorpwmvoltage

I am building a new controller for an existing robot. I need to know the voltage to send the motors. I don't want to remove the motors from the robot, but am able to take measurements while it operates using the old controller.

The power supply feeding the robot is 24 volts, so I assumed that I should send the robot 24 volts. I measured the current consumed by the motors at various speeds, and then used my controller to send PWM frequencies until the current consumed matched that of the original controller. What I saw was that the motors seemed to spin faster.

Does this make sense? Would the same current cause the motors to spin at different speeds, if the voltage was different? If so, how do I figure out what voltage to use?

Or could it be that I'm sending the same voltage, but the difference in PWM frequency from my controller and the existing controller is the problem?

Thanks!

Best Answer

What type of machine brushed?

Does this make sense? Would the same current cause the motors to spin at different speeds, if the voltage was different? If so, how do I figure out what voltage to use?

The speed of the machine is proportional to the voltage applied

The torque the machine can oppose is proportional to the current it sinks.

How to determine? if the previous controller used 24V, for now I would not consider using a higher voltage. Once you can determine the specific's of the machine (partnumber) you may find you can apply higher voltage as it can support higher speeds.

Or could it be that I'm sending the same voltage, but the difference in PWM frequency from my controller and the existing controller is the problem?

Frequency or duty? for a fixed duty, increasing the frequency will actually reduce the voltage (as the % of the switching time & interlock becomes a bigger contributor). For a different duty then yes as the average voltage will be different.

As to the actual title question How to determine DC motor voltage

Either from the datasheet of the part in question or Backdriving the machine to determine the \$k_e\$ constant (although without knowing the top speed you could damage the bearings)

\$V = K_e * \omega\$

Related Solutions

Electronic – Motor Control with PMW or RPM on heavy robot

Your second problem is probably closely related to the first. When you write the calculation:

Left Wheel = ( FrontError - SideError ) * Acceleration

what you really mean is:

Left Motor Input = ( FrontError - SideError ) * Acceleration

If there is a significant difference between the input to the motor and what the wheel actually does, perhaps because there is an unexpected load on the robot, then it is going to be difficult or impossible to tune those FrontError and SideError PID controllers in combination. If the wheel response is not linear you cannot simply add together two PID outputs and expect to have both work at once.

You have two general solutions here -- either develop a model so you can compensate for the difference, or make it so that the wheels actually respond fairly linearly to their input. This second option is probably the best one and it is what you will get if you use encoders and separate well-tuned PID controllers for each wheel.

Depending on all the other variables involved, the latency may prevent your control system from being stable. A simple two-pole PID controller might not be able to compensate adequately for it, but if you're experimenting it would be worth a shot, especially if you can fix up your wheel drives.

Electronic – How to tell whether a motor is an AC or DC motor

I haven't even bothered watching after "only DC motors can be used as a generator".

As far as I am aware, a motor can be of the following families:

Permanent magnet DC brushed. DC back emf.
Coiled stator DC brushed (as a separate winding, or internally wound as series or parallel). DC back emf IF the stator is powered. Single phase universal motors are a subset of series connection types for which, regardless of the polarity of the voltage, torque is always generated (needs moveable brushes or a different wiring to change direction though).
Permanent magnet AC synchronous (three phases). Three phase AC back emf.
Coiled rotor AC synchronous. I think those generally are not brushed but rather rectify the current induced by the stator. If brushed, no back emf unless the rotor is powered.
DC brushless. This one is basically a permanent magnet AC synchronous with hall sensors built in, to be able to electronically switch the phases. The back emf is however square or trapezoidal to maximise flux linkage.
Stepper motor (2, 3, 5 phases). Close to the PM AC synchronous in its construction, except that the motor is made to maximise the number of stable equilibrium positions of the rotor (many alternating magnetic poles at the rotor or variable reluctance). Back emf depends on how it's driven.
AC asynchronous (3 phases). The rotor is a closed loop (a coil, or a squirrel cage made of bars) which creates its field from currents induced by the stator. Can only be used as a generator beyond the synchronous rpm (+voltage at stator). AC back emf (TBC).
AC asynchronous (single phase). The motor cannot be self-started unless an out-of-phase auxiliary supply is created via a reacting capacitor, and fed to windings 90° from the main windings. Can only be used as a generator beyond the synchronous rpm (+voltage at stator). AC back emf (TBC).

There are many more (e.g. hybrids), but I think those represent 95% of the production. I'm sure I've missed a few important ones, please feel free to comment and I'll update the list.

The biggest clue to the type of a motor is the number of wires, but as you can see this is not enough. Some motors cannot generate power without an excitation, some not at all, and even if they do, the back emf is funny sometimes (trapezoidal for example) depending on its construction.

You could plan to try the various types of supplies on the motor, ramping up the voltage, and see if it does anything, but what's your "OK that's not it, better cut the power before I smoke it" point? If you don't know what type of motor it is, I assume you don't know anything about it. Including the voltage and current ratings, Max rpm. You could get that from eyeballing it, but there is no guarantee then.

For your specific problem though, if you are certain your motor is a DC bruhless but you don't know if the inverter+control circuit are integrated, look at the number of wires. Generally the motor does not have a circuit built in, and an ESC must be connected to it. You will have to identify which wires are the hall sensors.

ESC might or might not be used for current generation, it depends on how they are made. I don't think there can be any harm in hooking up a resistive load compatible with its current range at the input and test it.

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Related Solutions

Electronic – Motor Control with PMW or RPM on heavy robot

Electronic – How to tell whether a motor is an AC or DC motor

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