Simple Performance Curve Torque/RPM Question?

[Mike99]

I just have a simple question regarding DC motor performance curves.
For example taking the performance curves from this link: http://www.thunderstruck-ev.com/Manuals/PMG132curve.pdf
This shows RPM and torque on different curves. Is RPM and torque independent from each other?

What I'm confused about is that when you compare the torque and RPM, the torque is only shown for very high RPM.
Does this mean that the RPM is maximum RPM (ie. steady state RPM) for that specific current input and the torque is instantaneous torque for that specific current input?
Also is RPM solely dependent upon voltage input and torque solely dependent upon current input?

Sorry if the question is confusing. I have a mechanical background and am having a bit of confusion trying to understand the electrical side.

Thanks for the help.

 

[Jeremy Harris]

For a permanent magnet motor, torque is a function of current and rpm is a function of voltage. In essence, PM motors have a constant torque over their whole normal operating rpm range, that is determined by the current flowing through the motor.

It's fairly normal to plot performance with either torque or current on the X axis for PM motors, because of this fairly constant torque with current characteristic. The net result is that, for any given PM motor, more rpm = more power, because the motor limitations are usually dominated by the current it will take before overheating.

Jeremy

 

[Mike99]

Thanks for the helpful reply Jeremy,

So just to confirm your response.

For a given current input the torque will remain constant at all times. It does not matter at what speed the motor is spinning at.
Is this statement correct?

 

[Jeremy Harris]

Thanks for the helpful reply Jeremy,

So just to confirm your response.

For a given current input the torque will remain constant at all times. It does not matter at what speed the motor is spinning at.
Is this statement correct?

More or less, yes. There are limits at the extreme end of the rpm range that cause the motor to deviate from the constant torque per unit current characteristic, but over the motors normal working range it holds up.

The rpm that a PM DC (either brushed or brushless) motor runs at is determined by the actual voltage across the motor windings (not the supply voltage) and the motors speed constant, Kv (in rpm per volt).

The torque that any PM DC motor produces is determined by the current flowing through the windings and the value of Kt, the torque constant (usually in N-m per amp, but it may also be given in US or Imperial units).

If you know the motor Kv (it's usually quoted by the manufacturer) then you can calculate the value of Kt.

Kt (in N-m per amp) = ((60 / (2 x Pi )) / Kv

For example, if you have a motor like that Perm, with a Kv of around 50, then it will have a Kt of about 0.191 N-m per amp. If you ran that motor at 100A, at pretty much any rpm, then you would get a torque at the shaft of around 19.1 N-m.

The limit to the torque that a motor will deliver is usually given by the maximum current that it will handle before overheating. The limit to the rpm that the motor will run at is usually a function of the mechanical strength of the rotor, or possibly the maximum frequency at which the stator magnetic field can be switched due to core losses getting to be too great.

Jeremy

 

[Mike99]

Thanks Jeremy,

What I am confused about is that the link below explains the as the motor speed increases there is a decrease in torque (going from the stall torque at 0 RPM to the no load torque at maximum RPM).

http://lancet.mit.edu/motors/motors4.html

Are these the same characteristics as PM motors? Is T=kt*I true at all times for a PM motor (except when it approaches maximum speed)?

 

[Jeremy Harris]

If you look at the start of that article it explains that they are running these motors at a constant voltage and allowing the motor back EMF and winding resistance to control the current. The small motors they are using have a high winding resistance, so as they are loaded the winding resistance acts together with the back EMF to reduce the effective voltage at the windings as torque increases. This reduction in effective voltage results in lower rpm, hence the apparent speed/torque curve they are talking about.

The motors we use would do the same thing if you loaded them up with a high enough torque and ran then on a constant voltage supply, but the current they would draw would be pretty high and the motors would start to heat up fairly quickly. Some high resistance hub motors do start to limit in this way, but it's not the case for a lot of motors we use.

What's important is the ratio of motor winding resistance voltage losses to the back EMF of the motor at any given speed and current. If the winding resistance is very low, then even at maximum torque (i.e. winding current) the voltage drop due to resistance will still be far less than the motor back EMF.

For example, a typical ebike motor, with a winding resistance of around 0.05 ohms, running from a 50V supply at a current of 10A (so 500W input), will lose 0.5V because of winding resistance, so the motor will still see 49.5V, even at 500 watts, and the rpm will drop by just 1% from the no load maximum rpm.

Now, if you had a motor with a winding resistance of 5 ohms, with the same supply voltage and running at the same current, then the voltage drop due to winding resistance would increase to 50V and the motor would be stalled, with a 100% rpm drop from no load speed. Obviously the motor would get hot, because it would be dissipating the whole 500W of input power without delivering any output.

A perfect motor would have no winding resistance at all, so would have a completely flat rpm torque curve, at least until the point where it was spinning so fast that other losses started having an effect.

Jeremy