Motor Types Overview

Permanent Magnet Motors

In a Permanent Magnet motor a coil of wire (called the armature) is arranged in the magnetic field of a permanent magnet in such a way that it rotates when a current is passed through it. Now, when a coil of wire is moving in a magnetic field a voltage is induced in the coil – so the current (which is caused by applying a voltage to the coil) causes the armature to rotate and so generate a voltage. It is the nature of cause and effect in physics that the effect tends to cancel the cause, so the induced voltage tends to cancel out the applied voltage (indeed were the effects to add, we should have a perpetual motion machine!).

Voltage is electrical pressure. Current is electrical flow. Pressure tends to cause movement, or flow so an electrical pressure is a force which moves electricity – or an ‘electromotive force’ (EMF). The induced voltage caused by the armature’s movement is a ‘back EMF’ – ‘back’ because it tends to cancel out the applied voltage so that the actual voltage (pressure) across the armature is the difference between the applied voltage and the back EMF.

The value of the back emf is determined by the speed of rotation and the strength of the magnet(s) such that if the magnet is strong the back emf increases and if the speed increases, so too does the back emf. It follows from this that if you use a weaker magnet to make a particular motor, you will get a higher speed motor!

If you apply now a load to the armature, it will slow down. The back emf will decrease so the difference between applied voltage and back emf will increase. It is this difference that causes the current in the armature to flow – so the current will increase as you increase the mechanical loading. It should be apparent therefore that an unloaded motor will take little current. It should also be clear that if you apply more voltage the motor will speed up, apply less and it will slow: this is what the motor speed controller does: it varies the voltage applied to the motor.

Shunt Wound Motors

Shunt would motors have two windings (so 4 wires): a high resistance field and a low resisance armature.

If the shunt wound field is permanently energised, it behaves just like a permanent magnet and there is then very little difference between a shunt-wound and a permanent magnet.

If you chose a controller with a parking brake driver, this can be used to energise the field winding (subject to required current) which will then be switched off when the motor is at rest, saving power consumption.

Series Wound Motors

These have totally different characteristics to PM or shunt motors. However most 4QD controllers can be made to work with them.

---

How can I tell what motor I have?

• • The motor has only 2 wires

If the motor wires are swapped (positive motor to negative battery) the motor runs in the other direction, the motor is a brushed permanent magnet type and will work with our controllers.Be aware however that some permanent magnet motors are not made for reversing but are designed to only run in one direction.

If the motor wires are reversed, the motor runs in the same direction, this is likely to be a series-wound motor. Such motors are not good for speed control.

• • The motor has 4 wires.

The motor may be a series-wound or a shunt-wound motors or a compound-wound motor.

• • The motor has 3 wires, or some multiple of 3.

This is probably a brushless motor. 4QD do not manufacture controllers for these at the moment.

---

Nameplate ratings

The nameplate on a PM motor usually quotes a voltage, a rotation speed and a current. But, as we have seen, the current is dependant on the mechanical loading and the rotation speed is proportional to the applied voltage and to the load. So what do the figures on the motor nameplate mean?

Voltage and Speed

For most people, the nameplate figures are misleading. To say a motor is 24v is to misunderstand it. A better was of understanding would be if the motor specifications were that the motor gives so many rpm. per volt, and is suitable for operating at voltages up to 36. As far as I know only the Lynch motor is specified like that. So a ’24v’ motor, rated at 3000 rpm, gives (3000/24) 125rpm per volt. The actual maximum voltage it can stand may well be a lot more than the nameplate says and will either be limited by centrifugal force or by the maximum rate at which the commutator can switch the current or simply because the accelerated brush wear at the higher speed may be unacceptable.

The voltage and speed on the nameplate are simply the speed you can expect from a motor when it is running from the nameplate voltage and loaded so that it draws the rated current.

Current and load

The nameplate current is in fact a safe working continuous load. If you load a motor so that it draws more than this current then, eventually, it may overheat. In practise, most applications only place intermittent loads on the motor.

The motor, unloaded, will run a little faster and will draw very little current. If you lock the rotor a typical motor may draw as much as 20 times the nameplate current – but it will very quickly heat. However most motors will safely take a 300-400% overload for about one minute – if your mechanical loading is enough to cause that much current to be drawn.

There is on site a motor current calculator which enables you to work our the current your own loading will require.

‘Locked armature’ or Stall current

This rating is, frankly, misleading. It is not safe to pass this current through most motors for more than a second or two. It is best to ignore it when choosing a controller – unless you wish to be able to burn out the motor!

If you are not using a controller, then the switchgear used to start the motor will have to be chosen to take the locked armature rating current for an instant as ‘inrush current’. If the controller does not have current limiting (all controllers made by 4QD do!) then it will have to be capable of withstanding the locked armature current as an inrush.

 

Suppression – Motor Capacitor

Motors are electrically noisy. Electrical noise reduces system reliability and can on rare occasion, cause controllers to fail. True, such failure is partly down to MOSFET technology, which improves steadily with time, so such failures are geting less commom. 4QD controllers are as least susceptible to this effect as any controller, but it is advisable chose a motor with internal suppression: many manufacturers can do this on request – a small ceramic capacitor, value aaround 10n, is fitted internally across the motor brushes. If your motor does not include this, fit one externally across the motor connections as near to the motor as possible. There’s more information on this subject in the page Radio Controlled Machines: General wiring hints.

Our information area has a more detailed discussion of motor types.

 

Next page