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Speed Stability

A motor which is heavily loaded will always run slower than an unloaded motor. This statement seems so obvious that it is hardly worth saying – yet there are motors (stepper motors and synchronous motors) where it is not true and speed is independent of load. However, for normal d.c. motors it most certainly is true. This article discusses the speed stability you can expect.

Consider a motor whose rating plate states 2000 rpm, 20 amps and 24v. This means that the motor will run at 2000rpm when working from 24v and when loaded such that it is drawing 20 amps. Moreover the 20 amps quoted on the name plate will normally be the continuous ‘safe working current’ – in other words if you run the motor continuously at more current than this then it may eventually get too hot.

But such a motor may take, perhaps 120 amps if you stall it. Knowing V, the motor voltage (24v), and I, the stall current, we can, by Ohm’s law work out the armature resistance, V/I. So our stall current shows that the armature resistance is 0.3 ohms. We can now draw a graph:


This shows that the expected no-load speed is nearly 2600 rpm and that, at 50 amps, the motor will run at about 1000 rpm. Note however that the name plate working point (NP) is quite near the top of the curve and a properly loaded motor won’t use three quarters of the curve.

It should be evident from this that a larger motor, with a higher stall current, will have a flatter slope and therefore better speed stability. The rule is – the larger the motor, the better the stability against load variations. You would also expect a slower motor to have a flatter slope, except that a slower motor (of the same size) will be wound with more turns of thinner wire, so its stall current will be less.

This graph is for a motor operated from a fixed supply voltage. A motor speed controller reduces the available voltage to reduce the speed, shown by the pecked lines parallel to the main graph. So reducing the top speed with a simple controller also reduces the available stall torque.

It is evident from this that, if your gear ratio is such that you never operate the motor above half speed, then you are losing a lot of maximum torque and performance. It also follows that, if you start a motor under heavy load, you have to put a large voltage across it before it moves. Then, if the load suddenly reduces, the motor will suddenly spring to life at high speed.

In most battery applications the operator automatically adjusts the controller’s output voltage (the motor speed) to compensate for load variations so there is no problem. The times when this is not true are either when the speed is not operator controlled and the load is varying or when there is a sudden change in load such that the operator finds it difficult to react fast enough and the vehicle surges when the load reduces suddenly (this is a situation which occurs frequently in Robot Wars). In these cases some form of closed loop control may be required.