What are the consequences of connecting two motors in parallel?

Question

I am using two DC motors rated at 36V and maximum current draw of 5A. To reduce the number of motor-controllers, I am planning to connect them in parallel.

1. The two motors are required to run at the same speed, so wiring them in parallel will ensure same PWM signal to both these motors.

2. Both the motors are unidirectional.

3. Both the motors are identical (electrical rating, manufacturer, load).
4. Also, they need not rotate at exactly the same speed, there can be slight variation (15%).
5. The motors will be driven in an open-loop, there is no feedback.

6. The motors will be connected to brushes.

   My concern is:

1. What happens if one of the motor stalls? Theoretically, this should not affect the other motor as they are in parallel and the UN-stalled motor will receive the required voltage and also assuming that the batteries are able to supply sufficient current.
2. What happens if one of the motors produces back EMF?
3. Is it possible for a stalled motor to produce back EMF?
4. Should I be using a circuit-breaker/PTC fuse/fast-acting fuse in series with the motors?

Answer 1

A simple PWM controller will cause each of two similar motors connected in parallel to run at approximately the same speed (with some minor change on each due to loading). That is because the resistance of the rotor winding is small, back EMF is proportional to speed and torque is proportional to applied voltage minus back-EMF (divided by rotor winding resistance). To a first approximation, speed is proportional to applied voltage, and a PWM source behaves like a constant voltage (not like a series resistance).

However, a more sophisticated controller with IR compensation or back EMF measurement will cause the motors to interact and loading up one motor more will cause the other motor to increase in speed, while not providing optimal control for the first motor. A closed-loop controller with feedback of some kind (tacho) will act similarly- the motor with feedback will be controlled well but the other will speed up and slow down in sympathy with the loading on the controlled motor.

It really depends on the nature of your motor controller.

In answer to your specific questions:

1. If one motor stalls the current will increase. If the motor controller can supply the full stall current plus operating current for the other motor then the non-stalled motor will be unaffected. Otherwise, it will slow down or may stall.

2. All DC motors produce back EMF- it's proportional to the rotational speed.

3. No, since it's proportional to rotational speed, at speed = 0, back EMF = 0. There's going to be inductive 'kick' at 0 RPM which is unrelated to motor RPM, but related to motor inductance (and current, which is high- perhaps even high-est when the motor is stalled).

4. You should follow the protection and fusing recommendations of the motor controller and the motor manufacturer and seek help if there is a conflict between the two.

Answer 2

1. If one motor stalls, you have to consider not only if the battery can supply the current, but what will happen to the controller.

2. Back EMF is what determines the current the motor draws to produce the torque necessary to drive the load at a given speed. Each motor will produce back EMF according to the load it sees. If one motor sees more load, it will produce less back EMF, reduce its speed and draw more current.

3. A stalled motor will produce no back EMF and the current that it draws will be limited only by the resistance of the armature.

4. A fuse would protect a stalled motor from burning up. It shouldn't be fast-acting. The motor needs to be able to power through a brief overload without blowing a fuse every time.

Answer 3

If it is a open loop without feed back, in case of stalling of one motor the current will increase to very high value causing damage of the motor / the power source. So overload / current cutoff protection should be used in the circuit.